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<title>The Boost Parameter Library</title>
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<div class="document" id="the-boost-parameter-library">
<h1 class="title">The Boost Parameter Library</h1>

<p><a class="reference external" href="../../../../index.htm"><img alt="Boost" src="../../../../boost.png" /></a></p>
<hr class="docutils" />
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Abstract:</th><td class="field-body"><p class="first">Use this library to write functions and class templates
that can accept arguments by name:</p>
<pre class="literal-block">
new_window(&quot;alert&quot;, <strong>_width=10</strong>, <strong>_titlebar=false</strong>);

smart_ptr&lt;
   Foo
 , <strong>deleter&lt;Deallocate&lt;Foo&gt; &gt;</strong>
 , <strong>copy_policy&lt;DeepCopy&gt;</strong> &gt; p(new Foo);
</pre>
<p class="last">Since named arguments can be passed in any order, they are
especially useful when a function or template has more than one
parameter with a useful default value.  The library also supports
<em>deduced</em> parameters; that is to say, parameters whose identity
can be deduced from their types.</p>
</td>
</tr>
</tbody>
</table>
<!-- @jam_prefix.append('''
project test : requirements <include>. <implicit-dependency>/boost//headers ;''') -->
<!-- @example.prepend('''
#include <boost/parameter.hpp>

namespace test
{
  BOOST_PARAMETER_NAME(title)
  BOOST_PARAMETER_NAME(width)
  BOOST_PARAMETER_NAME(titlebar)

  BOOST_PARAMETER_FUNCTION(
     (int), new_window, tag, (required (title,*)(width,*)(titlebar,*)))
  {
     return 0;
  }

  BOOST_PARAMETER_TEMPLATE_KEYWORD(deleter)
  BOOST_PARAMETER_TEMPLATE_KEYWORD(copy_policy)

  template <class T> struct Deallocate {};
  struct DeepCopy {};

  namespace parameter = boost::parameter;

  struct Foo {};
  template <class T, class A0, class A1>
  struct smart_ptr
  {
      smart_ptr(Foo*);
  };
}
using namespace test;
int x = '''); -->
<!-- @test('compile') -->
<hr class="docutils" />
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Authors:</th><td class="field-body">David Abrahams, Daniel Wallin</td>
</tr>
<tr class="field"><th class="field-name">Contact:</th><td class="field-body"><a class="reference external" href="mailto:dave&#64;boost-consulting.com">dave&#64;boost-consulting.com</a>, <a class="reference external" href="mailto:daniel&#64;boostpro.com">daniel&#64;boostpro.com</a></td>
</tr>
<tr class="field"><th class="field-name">organization:</th><td class="field-body"><a class="reference external" href="http://www.boostpro.com">BoostPro Computing</a></td>
</tr>
<tr class="field"><th class="field-name">date:</th><td class="field-body">$Date: 2005/07/17 19:53:01 $</td>
</tr>
<tr class="field"><th class="field-name">copyright:</th><td class="field-body">Copyright David Abrahams, Daniel Wallin
2005-2009. Distributed under the Boost Software License,
Version 1.0. (See accompanying file LICENSE_1_0.txt
or copy at <a class="reference external" href="http://www.boost.org/LICENSE_1_0.txt">http://www.boost.org/LICENSE_1_0.txt</a>)</td>
</tr>
</tbody>
</table>
<hr class="docutils" />
<p>[Note: this tutorial does not cover all details of the library.  Please see also the <a class="reference external" href="reference.html">reference documentation</a>]</p>
<div class="contents topic" id="table-of-contents">
<p class="topic-title first"><strong>Table of Contents</strong></p>
<ul class="auto-toc simple">
<li><a class="reference internal" href="#motivation" id="id22">1&nbsp;&nbsp;&nbsp;Motivation</a><ul class="auto-toc">
<li><a class="reference internal" href="#named-function-parameters" id="id23">1.1&nbsp;&nbsp;&nbsp;Named Function Parameters</a></li>
<li><a class="reference internal" href="#deduced-function-parameters" id="id24">1.2&nbsp;&nbsp;&nbsp;Deduced Function Parameters</a></li>
<li><a class="reference internal" href="#class-template-parameter-support" id="id25">1.3&nbsp;&nbsp;&nbsp;Class Template Parameter Support</a></li>
</ul>
</li>
<li><a class="reference internal" href="#tutorial" id="id26">2&nbsp;&nbsp;&nbsp;Tutorial</a><ul class="auto-toc">
<li><a class="reference internal" href="#parameter-enabled-functions" id="id27">2.1&nbsp;&nbsp;&nbsp;Parameter-Enabled Functions</a></li>
<li><a class="reference internal" href="#parameter-enabled-member-functions" id="id28">2.2&nbsp;&nbsp;&nbsp;Parameter-Enabled Member Functions</a></li>
<li><a class="reference internal" href="#parameter-enabled-constructors" id="id29">2.3&nbsp;&nbsp;&nbsp;Parameter-Enabled Constructors</a></li>
<li><a class="reference internal" href="#parameter-enabled-class-templates" id="id30">2.4&nbsp;&nbsp;&nbsp;Parameter-Enabled Class Templates</a></li>
</ul>
</li>
<li><a class="reference internal" href="#advanced-topics" id="id31">3&nbsp;&nbsp;&nbsp;Advanced Topics</a><ul class="auto-toc">
<li><a class="reference internal" href="#fine-grained-name-control" id="id32">3.1&nbsp;&nbsp;&nbsp;Fine-Grained Name Control</a></li>
<li><a class="reference internal" href="#more-argumentpacks" id="id33">3.2&nbsp;&nbsp;&nbsp;More <span class="concept">ArgumentPack</span>s</a></li>
</ul>
</li>
<li><a class="reference internal" href="#best-practices" id="id34">4&nbsp;&nbsp;&nbsp;Best Practices</a><ul class="auto-toc">
<li><a class="reference internal" href="#keyword-naming" id="id35">4.1&nbsp;&nbsp;&nbsp;Keyword Naming</a></li>
<li><a class="reference internal" href="#namespaces" id="id36">4.2&nbsp;&nbsp;&nbsp;Namespaces</a></li>
<li><a class="reference internal" href="#documentation" id="id37">4.3&nbsp;&nbsp;&nbsp;Documentation</a></li>
</ul>
</li>
<li><a class="reference internal" href="#portability-considerations" id="id38">5&nbsp;&nbsp;&nbsp;Portability Considerations</a><ul class="auto-toc">
<li><a class="reference internal" href="#no-sfinae-support" id="id39">5.1&nbsp;&nbsp;&nbsp;No SFINAE Support</a></li>
<li><a class="reference internal" href="#no-support-for-result-of" id="id40">5.2&nbsp;&nbsp;&nbsp;No Support for <tt class="docutils literal">result_of</tt></a></li>
<li><a class="reference internal" href="#compiler-can-t-see-references-in-unnamed-namespace" id="id41">5.3&nbsp;&nbsp;&nbsp;Compiler Can't See References In Unnamed Namespace</a></li>
</ul>
</li>
<li><a class="reference internal" href="#python-binding" id="id42">6&nbsp;&nbsp;&nbsp;Python Binding</a></li>
<li><a class="reference internal" href="#reference" id="id43">7&nbsp;&nbsp;&nbsp;Reference</a></li>
<li><a class="reference internal" href="#glossary" id="id44">8&nbsp;&nbsp;&nbsp;Glossary</a></li>
<li><a class="reference internal" href="#acknowledgements" id="id45">9&nbsp;&nbsp;&nbsp;Acknowledgements</a></li>
</ul>
</div>
<hr class="docutils" />
<div class="section" id="motivation">
<h1><a class="toc-backref" href="#id22">1&nbsp;&nbsp;&nbsp;Motivation</a></h1>
<p>In C++, <a class="reference internal" href="#arguments">arguments</a> are normally given meaning by their positions
with respect to a <a class="reference internal" href="#parameter">parameter</a> list: the first argument passed maps
onto the first parameter in a function's definition, and so on.
That protocol is fine when there is at most one parameter with a
default value, but when there are even a few useful defaults, the
positional interface becomes burdensome:</p>
<ul>
<li><div class="first compound">
<p class="compound-first">Since an argument's meaning is given by its position, we have to
choose an (often arbitrary) order for parameters with default
values, making some combinations of defaults unusable:</p>
<pre class="compound-middle literal-block">
window* new_window(
   char const* name,
   <strong>int border_width = default_border_width,</strong>
   bool movable = true,
   bool initially_visible = true
   );

const bool movability = false;
window* w = new_window(&quot;alert box&quot;, movability);
</pre>
<p class="compound-middle">In the example above we wanted to make an unmoveable window
with a default <tt class="docutils literal">border_width</tt>, but instead we got a moveable
window with a <tt class="docutils literal">border_width</tt> of zero.  To get the desired
effect, we'd need to write:</p>
<pre class="compound-last literal-block">
window* w = new_window(
   &quot;alert box&quot;, <strong>default_border_width</strong>, movability);
</pre>
</div>
</li>
<li><div class="first compound">
<p class="compound-first">It can become difficult for readers to understand the meaning of
arguments at the call site:</p>
<pre class="compound-middle literal-block">
window* w = new_window(&quot;alert&quot;, 1, true, false);
</pre>
<p class="compound-last">Is this window moveable and initially invisible, or unmoveable
and initially visible?  The reader needs to remember the order
of arguments to be sure.</p>
</div>
</li>
<li><p class="first">The author of the call may not remember the order of the
arguments either, leading to hard-to-find bugs.</p>
</li>
</ul>
<!-- @ignore(3) -->
<div class="section" id="named-function-parameters">
<h2><a class="toc-backref" href="#id23">1.1&nbsp;&nbsp;&nbsp;Named Function Parameters</a></h2>
<div class="compound">
<p class="compound-first">This library addresses the problems outlined above by associating
each parameter name with a keyword object.  Now users can identify
arguments by name, rather than by position:</p>
<pre class="compound-last literal-block">
window* w = new_window(&quot;alert box&quot;, <strong>movable_=</strong>false); // OK!
</pre>
</div>
<!-- @ignore() -->
</div>
<div class="section" id="deduced-function-parameters">
<h2><a class="toc-backref" href="#id24">1.2&nbsp;&nbsp;&nbsp;Deduced Function Parameters</a></h2>
<div class="compound">
<p class="compound-first">A <strong>deduced parameter</strong> can be passed in any position <em>without</em>
supplying an explicit parameter name.  It's not uncommon for a
function to have parameters that can be uniquely identified based
on the types of arguments passed.  The <tt class="docutils literal">name</tt> parameter to
<tt class="docutils literal">new_window</tt> is one such example.  None of the other arguments,
if valid, can reasonably be converted to a <tt class="docutils literal">char const*</tt>.  With
a deduced parameter interface, we could pass the window name in
<em>any</em> argument position without causing ambiguity:</p>
<pre class="compound-middle literal-block">
window* w = new_window(movable_=false, <strong>&quot;alert box&quot;</strong>); // OK!
window* w = new_window(<strong>&quot;alert box&quot;</strong>, movable_=false); // OK!
</pre>
<p class="compound-last">Appropriately used, a deduced parameter interface can free the
user of the burden of even remembering the formal parameter
names.</p>
</div>
<!-- @ignore() -->
</div>
<div class="section" id="class-template-parameter-support">
<h2><a class="toc-backref" href="#id25">1.3&nbsp;&nbsp;&nbsp;Class Template Parameter Support</a></h2>
<div class="compound">
<p class="compound-first">The reasoning we've given for named and deduced parameter
interfaces applies equally well to class templates as it does to
functions.  Using the Parameter library, we can create interfaces
that allow template arguments (in this case <tt class="docutils literal">shared</tt> and
<tt class="docutils literal">Client</tt>) to be explicitly named, like this:</p>
<pre class="compound-middle literal-block">
smart_ptr&lt;<strong>ownership&lt;shared&gt;</strong>, <strong>value_type&lt;Client&gt;</strong> &gt; p;
</pre>
<p class="compound-middle">The syntax for passing named template arguments is not quite as
natural as it is for function arguments (ideally, we'd be able to
write <tt class="docutils literal"><span class="pre">smart_ptr&lt;ownership=shared,…&gt;</span></tt>).  This small syntactic
deficiency makes deduced parameters an especially big win when
used with class templates:</p>
<pre class="compound-last literal-block">
// <em>p and q could be equivalent, given a deduced</em>
// <em>parameter interface.</em>
smart_ptr&lt;<strong>shared</strong>, <strong>Client</strong>&gt; p;
smart_ptr&lt;<strong>Client</strong>, <strong>shared</strong>&gt; q;
</pre>
</div>
<!-- @ignore(2) -->
</div>
</div>
<div class="section" id="tutorial">
<h1><a class="toc-backref" href="#id26">2&nbsp;&nbsp;&nbsp;Tutorial</a></h1>
<p>This tutorial shows all the basics—how to build both named- and deduced-parameter
interfaces to function templates and class templates—and several
more advanced idioms as well.</p>
<div class="section" id="parameter-enabled-functions">
<h2><a class="toc-backref" href="#id27">2.1&nbsp;&nbsp;&nbsp;Parameter-Enabled Functions</a></h2>
<p>In this section we'll show how the Parameter library can be used to
build an expressive interface to the <a class="reference external" href="../../../graph/index.html">Boost Graph library</a>'s
<a class="reference external" href="../../../graph/doc/depth_first_search.html"><tt class="docutils literal">depth_first_search</tt></a> algorithm.<a class="footnote-reference" href="#old-interface" id="id3"><sup>1</sup></a></p>
<!-- Revisit this

After laying some groundwork
and describing the algorithm's abstract interface, we'll show you
how to build a basic implementation with keyword support.  Then
we'll add support for default arguments and we'll gradually refine the
implementation with syntax improvements.  Finally we'll show how to
streamline the implementation of named parameter interfaces,
improve their participation in overload resolution, and optimize
their runtime efficiency. -->
<div class="section" id="headers-and-namespaces">
<h3>2.1.1&nbsp;&nbsp;&nbsp;Headers And Namespaces</h3>
<p>Most components of the Parameter library are declared in a
header named for the component.  For example,</p>
<pre class="literal-block">
#include &lt;boost/parameter/keyword.hpp&gt;
</pre>
<p>will ensure <tt class="docutils literal"><span class="pre">boost::parameter::keyword</span></tt> is known to the
compiler.  There is also a combined header,
<tt class="docutils literal">boost/parameter.hpp</tt>, that includes most of the library's
components.  For the the rest of this tutorial, unless we say
otherwise, you can use the rule above to figure out which header
to <tt class="docutils literal">#include</tt> to access any given component of the library.</p>
<!-- @example.append('''
using boost::parameter::keyword;
''') -->
<!-- @test('compile') -->
<p>Also, the examples below will also be written as if the
namespace alias</p>
<pre class="literal-block">
namespace parameter = boost::parameter;
</pre>
<!-- @ignore() -->
<p>has been declared: we'll write <tt class="docutils literal"><span class="pre">parameter::xxx</span></tt> instead of
<tt class="docutils literal"><span class="pre">boost::parameter::xxx</span></tt>.</p>
</div>
<div class="section" id="the-abstract-interface-to-dfs">
<h3>2.1.2&nbsp;&nbsp;&nbsp;The Abstract Interface to <tt class="docutils literal">depth_first_search</tt></h3>
<p>The Graph library's <tt class="docutils literal">depth_first_search</tt> algorithm is a generic function accepting
from one to four arguments by reference.  If all arguments were
required, its signature might be as follows:</p>
<pre class="literal-block">
template &lt;
    class Graph, class DFSVisitor, class Index, class ColorMap
&gt;
void depth_first_search(
  , Graph const&amp; graph
  , DFSVisitor visitor
  , typename graph_traits&lt;g&gt;::vertex_descriptor root_vertex
  , IndexMap index_map
  , ColorMap&amp; color);
</pre>
<!-- @ignore() -->
<p>However, most of the parameters have a useful default value, as
shown in the table below.</p>
<table border="1" class="docutils" id="default-expressions">
<span id="parameter-table"></span><caption><tt class="docutils literal">depth_first_search</tt> Parameters</caption>
<colgroup>
<col width="17%" />
<col width="11%" />
<col width="35%" />
<col width="37%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Parameter Name</th>
<th class="head">Dataflow</th>
<th class="head">Type</th>
<th class="head">Default Value (if any)</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="docutils literal">graph</tt></td>
<td>in</td>
<td>Model of <a class="reference external" href="../../../graph/doc/IncidenceGraph.html"><span class="concept">Incidence Graph</span></a> and
<a class="reference external" href="../../../graph/doc/VertexListGraph.html"><span class="concept">Vertex List Graph</span></a></td>
<td>none - this argument is required.</td>
</tr>
<tr><td><tt class="docutils literal">visitor</tt></td>
<td>in</td>
<td>Model of <a class="reference external" href="../../../graph/doc/DFSVisitor.html"><span class="concept">DFS Visitor</span></a></td>
<td><tt class="docutils literal"><span class="pre">boost::dfs_visitor&lt;&gt;()</span></tt></td>
</tr>
<tr><td><tt class="docutils literal">root_vertex</tt></td>
<td>in</td>
<td><tt class="docutils literal">graph</tt>'s vertex descriptor
type.</td>
<td><tt class="docutils literal"><span class="pre">*vertices(graph).first</span></tt></td>
</tr>
<tr><td><tt class="docutils literal">index_map</tt></td>
<td>in</td>
<td>Model of <a class="reference external" href="../../../property_map/doc/ReadablePropertyMap.html"><span class="concept">Readable Property Map</span></a>
with key type := <tt class="docutils literal">graph</tt>'s
vertex descriptor and value type
an integer type.</td>
<td><tt class="docutils literal"><span class="pre">get(boost::vertex_index,graph)</span></tt></td>
</tr>
<tr><td><tt class="docutils literal">color_map</tt></td>
<td>in/out</td>
<td>Model of <a class="reference external" href="../../../property_map/doc/ReadWritePropertyMap.html"><span class="concept">Read/Write Property Map</span></a>
with key type := <tt class="docutils literal">graph</tt>'s
vertex descriptor type.</td>
<td>an <tt class="docutils literal">iterator_property_map</tt>
created from a <tt class="docutils literal"><span class="pre">std::vector</span></tt> of
<tt class="docutils literal">default_color_type</tt> of size
<tt class="docutils literal">num_vertices(graph)</tt> and using
<tt class="docutils literal">index_map</tt> for the index map.</td>
</tr>
</tbody>
</table>
<p>Don't be intimidated by the information in the second and third
columns above.  For the purposes of this exercise, you don't need
to understand them in detail.</p>
</div>
<div class="section" id="defining-the-keywords">
<h3>2.1.3&nbsp;&nbsp;&nbsp;Defining the Keywords</h3>
<p>The point of this exercise is to make it possible to call
<tt class="docutils literal">depth_first_search</tt> with named arguments, leaving out any
arguments for which the default is appropriate:</p>
<pre class="literal-block">
graphs::depth_first_search(g, <strong>color_map_=my_color_map</strong>);
</pre>
<!-- @ignore() -->
<p>To make that syntax legal, there needs to be an object called
“<tt class="docutils literal">color_map_</tt>” whose assignment operator can accept a
<tt class="docutils literal">my_color_map</tt> argument.  In this step we'll create one such
<strong>keyword object</strong> for each parameter.  Each keyword object will be
identified by a unique <strong>keyword tag type</strong>.</p>
<!-- Revisit this

We're going to define our interface in namespace ``graphs``.  Since
users need access to the keyword objects, but not the tag types,
we'll define the keyword objects so they're accessible through
``graphs``, and we'll hide the tag types away in a nested
namespace, ``graphs::tag``.  The library provides a convenient
macro for that purpose. -->
<p>We're going to define our interface in namespace <tt class="docutils literal">graphs</tt>.  The
library provides a convenient macro for defining keyword objects:</p>
<pre class="literal-block">
#include &lt;boost/parameter/name.hpp&gt;

namespace graphs
{
  BOOST_PARAMETER_NAME(graph)    // Note: no semicolon
  BOOST_PARAMETER_NAME(visitor)
  BOOST_PARAMETER_NAME(root_vertex)
  BOOST_PARAMETER_NAME(index_map)
  BOOST_PARAMETER_NAME(color_map)
}
</pre>
<!-- @test('compile') -->
<p>The declaration of the <tt class="docutils literal">graph</tt> keyword you see here is
equivalent to:</p>
<pre class="literal-block">
namespace graphs
{
  namespace tag { struct graph; } // keyword tag type

  namespace // unnamed
  {
    // A reference to the keyword object
    boost::parameter::keyword&lt;tag::graph&gt;&amp; _graph
    = boost::parameter::keyword&lt;tag::graph&gt;::get();
  }
}
</pre>
<!-- @example.prepend('#include <boost/parameter/keyword.hpp>') -->
<!-- @test('compile') -->
<p>It defines a <em>keyword tag type</em> named <tt class="docutils literal"><span class="pre">tag::graph</span></tt> and a <em>keyword
object</em> reference named <tt class="docutils literal">_graph</tt>.</p>
<p>This “fancy dance” involving an unnamed namespace and references
is all done to avoid violating the One Definition Rule (ODR)<a class="footnote-reference" href="#odr" id="id5"><sup>2</sup></a> when the named parameter interface is used by function
templates that are instantiated in multiple translation
units (MSVC6.x users see <a class="reference internal" href="#compiler-can-t-see-references-in-unnamed-namespace">this note</a>).</p>
</div>
<div class="section" id="writing-the-function">
<h3>2.1.4&nbsp;&nbsp;&nbsp;Writing the Function</h3>
<p>Now that we have our keywords defined, the function template
definition follows a simple pattern using the
<tt class="docutils literal">BOOST_PARAMETER_FUNCTION</tt> macro:</p>
<pre class="literal-block">
#include &lt;boost/parameter/preprocessor.hpp&gt;

namespace graphs
{
  BOOST_PARAMETER_FUNCTION(
      (void),                // 1. parenthesized return type
      depth_first_search,    // 2. name of the function template

      tag,                   // 3. namespace of tag types

      (required (graph, *) ) // 4. one required parameter, and

      (optional              //    four optional parameters, with defaults
        (visitor,           *, boost::dfs_visitor&lt;&gt;())
        (root_vertex,       *, *vertices(graph).first)
        (index_map,         *, get(boost::vertex_index,graph))
        (in_out(color_map), *,
          default_color_map(num_vertices(graph), index_map) )
      )
  )
  {
      // ... body of function goes here...
      // use graph, visitor, index_map, and color_map
  }
}
</pre>
<!-- @example.prepend('''
#include <boost/parameter/name.hpp>

BOOST_PARAMETER_NAME(graph)
BOOST_PARAMETER_NAME(visitor)
BOOST_PARAMETER_NAME(root_vertex)
BOOST_PARAMETER_NAME(index_map)
BOOST_PARAMETER_NAME(color_map)

namespace boost {

template <class T = int>
struct dfs_visitor
{};

int vertex_index = 0;

}''') -->
<!-- @test('compile') -->
<p>The arguments to <tt class="docutils literal">BOOST_PARAMETER_FUNCTION</tt> are:</p>
<ol class="arabic simple">
<li>The return type of the resulting function template.  Parentheses
around the return type prevent any commas it might contain from
confusing the preprocessor, and are always required.</li>
<li>The name of the resulting function template.</li>
<li>The name of a namespace where we can find tag types whose names
match the function's parameter names.</li>
<li>The function signature.</li>
</ol>
</div>
<div class="section" id="function-signatures">
<h3>2.1.5&nbsp;&nbsp;&nbsp;Function Signatures</h3>
<p>Function signatures are described as one or two adjacent
parenthesized terms (a <a class="reference external" href="../../../preprocessor/index.html">Boost.Preprocessor</a> <a class="reference external" href="http://boost-consulting.com/mplbook/preprocessor.html#sequences">sequence</a>) describing
the function's parameters in the order in which they'd be expected
if passed positionally.  Any required parameters must come first,
but the <tt class="docutils literal">(required … )</tt> clause can be omitted when all the
parameters are optional.</p>
<div class="section" id="required-parameters">
<h4>2.1.5.1&nbsp;&nbsp;&nbsp;Required Parameters</h4>
<div class="compound">
<p class="compound-first">Required parameters are given first—nested in a <tt class="docutils literal">(required … )</tt>
clause—as a series of two-element tuples describing each parameter
name and any requirements on the argument type.  In this case there
is only a single required parameter, so there's just a single
tuple:</p>
<pre class="compound-middle literal-block">
(required <strong>(graph, *)</strong> )
</pre>
<p class="compound-last">Since <tt class="docutils literal">depth_first_search</tt> doesn't require any particular type
for its <tt class="docutils literal">graph</tt> parameter, we use an asterix to indicate that
any type is allowed.  Required parameters must always precede any
optional parameters in a signature, but if there are <em>no</em>
required parameters, the <tt class="docutils literal">(required … )</tt> clause can be omitted
entirely.</p>
</div>
<!-- @example.prepend('''
#include <boost/parameter.hpp>

BOOST_PARAMETER_NAME(graph)

BOOST_PARAMETER_FUNCTION((void), f, tag,
''') -->
<!-- @example.append(') {}') -->
<!-- @test('compile') -->
</div>
<div class="section" id="optional-parameters">
<h4>2.1.5.2&nbsp;&nbsp;&nbsp;Optional Parameters</h4>
<div class="compound">
<p class="compound-first">Optional parameters—nested in an <tt class="docutils literal">(optional … )</tt> clause—are given
as a series of adjacent <em>three</em>-element tuples describing the
parameter name, any requirements on the argument type, <em>and</em> and an
expression representing the parameter's default value:</p>
<pre class="compound-last literal-block">
(optional <strong>    (visitor,           *, boost::dfs_visitor&lt;&gt;())
    (root_vertex,       *, *vertices(graph).first)
    (index_map,         *, get(boost::vertex_index,graph))
    (in_out(color_map), *,
      default_color_map(num_vertices(graph), index_map) )</strong>
)
</pre>
</div>
<!-- @example.prepend('''
#include <boost/parameter.hpp>

namespace boost
{
  int vertex_index = 0;

  template <class T = int>
  struct dfs_visitor
  {};
}

BOOST_PARAMETER_NAME(graph)
BOOST_PARAMETER_NAME(visitor)
BOOST_PARAMETER_NAME(root_vertex)
BOOST_PARAMETER_NAME(index_map)
BOOST_PARAMETER_NAME(color_map)

BOOST_PARAMETER_FUNCTION((void), f, tag,
  (required (graph, *))
''') -->
<!-- @example.append(') {}') -->
<!-- @test('compile') -->
</div>
<div class="section" id="handling-out-parameters">
<h4>2.1.5.3&nbsp;&nbsp;&nbsp;Handling “Out” Parameters</h4>
<div class="compound">
<p class="compound-first">Within the function body, a parameter name such as <tt class="docutils literal">visitor</tt> is
a <em>C++ reference</em>, bound either to an actual argument passed by
the caller or to the result of evaluating a default expression.
In most cases, parameter types are of the form <tt class="docutils literal">T const&amp;</tt> for
some <tt class="docutils literal">T</tt>.  Parameters whose values are expected to be modified,
however, must be passed by reference to <em>non</em>-<tt class="docutils literal">const</tt>.  To
indicate that <tt class="docutils literal">color_map</tt> is both read and written, we wrap
its name in <tt class="docutils literal"><span class="pre">in_out(…)</span></tt>:</p>
<pre class="compound-last literal-block">
(optional
    (visitor,            *, boost::dfs_visitor&lt;&gt;())
    (root_vertex,        *, *vertices(graph).first)
    (index_map,          *, get(boost::vertex_index,graph))
    (<strong>in_out(color_map)</strong>, *,
      default_color_map(num_vertices(graph), index_map) )
)
</pre>
</div>
<!-- @example.prepend('''
#include <boost/parameter.hpp>

namespace boost
{
  int vertex_index = 0;

  template <class T = int>
  struct dfs_visitor
  {};
}

BOOST_PARAMETER_NAME(graph)

BOOST_PARAMETER_NAME(visitor)
BOOST_PARAMETER_NAME(root_vertex)
BOOST_PARAMETER_NAME(index_map)
BOOST_PARAMETER_NAME(color_map)

BOOST_PARAMETER_FUNCTION((void), f, tag,
  (required (graph, *))
''') -->
<!-- @example.append(') {}') -->
<!-- @test('compile') -->
<p>If <tt class="docutils literal">color_map</tt> were strictly going to be modified but not examined,
we could have written <tt class="docutils literal">out(color_map)</tt>.  There is no functional
difference between <tt class="docutils literal">out</tt> and <tt class="docutils literal">in_out</tt>; the library provides
both so you can make your interfaces more self-documenting.</p>
</div>
<div class="section" id="positional-arguments">
<h4>2.1.5.4&nbsp;&nbsp;&nbsp;Positional Arguments</h4>
<p>When arguments are passed positionally (without the use of
keywords), they will be mapped onto parameters in the order the
parameters are given in the signature, so for example in this
call</p>
<pre class="literal-block">
graphs::depth_first_search(x, y);
</pre>
<!-- @ignore() -->
<p><tt class="docutils literal">x</tt> will always be interpreted as a graph and <tt class="docutils literal">y</tt> will always
be interpreted as a visitor.</p>
</div>
<div class="section" id="default-expression-evaluation">
<h4>2.1.5.5&nbsp;&nbsp;&nbsp;Default Expression Evaluation</h4>
<div class="compound">
<p class="compound-first">Note that in our example, the value of the graph parameter is
used in the default expressions for <tt class="docutils literal">root_vertex</tt>,
<tt class="docutils literal">index_map</tt> and <tt class="docutils literal">color_map</tt>.</p>
<pre class="compound-middle literal-block">
(required (<strong>graph</strong>, *) )
(optional
  (visitor,           *, boost::dfs_visitor&lt;&gt;())
  (root_vertex,       *, *vertices(<strong>graph</strong>).first)
  (index_map,         *, get(boost::vertex_index,<strong>graph</strong>))
  (in_out(color_map), *,
    default_color_map(num_vertices(<strong>graph</strong>), index_map) )
)
</pre>
<!-- @ignore() -->
<p class="compound-last">A default expression is evaluated in the context of all preceding
parameters, so you can use any of their values by name.</p>
</div>
<div class="compound">
<p class="compound-first">A default expression is never evaluated—or even instantiated—if
an actual argument is passed for that parameter.  We can actually
demonstrate that with our code so far by replacing the body of
<tt class="docutils literal">depth_first_search</tt> with something that prints the arguments:</p>
<pre class="compound-middle literal-block">
#include &lt;boost/graph/depth_first_search.hpp&gt; // for dfs_visitor

BOOST_PARAMETER_FUNCTION(
    (void), depth_first_search, tag
    <em>…signature goes here…</em>
)
{
   std::cout &lt;&lt; &quot;graph=&quot; &lt;&lt; graph &lt;&lt; std::endl;
   std::cout &lt;&lt; &quot;visitor=&quot; &lt;&lt; visitor &lt;&lt; std::endl;
   std::cout &lt;&lt; &quot;root_vertex=&quot; &lt;&lt; root_vertex &lt;&lt; std::endl;
   std::cout &lt;&lt; &quot;index_map=&quot; &lt;&lt; index_map &lt;&lt; std::endl;
   std::cout &lt;&lt; &quot;color_map=&quot; &lt;&lt; color_map &lt;&lt; std::endl;
}

int main()
{
    depth_first_search(1, 2, 3, 4, 5);

    depth_first_search(
        &quot;1&quot;, '2', _color_map = '5',
        _index_map = &quot;4&quot;, _root_vertex = &quot;3&quot;);
}
</pre>
<p class="compound-last">Despite the fact that default expressions such as
<tt class="docutils literal"><span class="pre">vertices(graph).first</span></tt> are ill-formed for the given <tt class="docutils literal">graph</tt>
arguments, both calls will compile, and each one will print
exactly the same thing.</p>
</div>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <iostream>

BOOST_PARAMETER_NAME(graph)
BOOST_PARAMETER_NAME(visitor)
BOOST_PARAMETER_NAME(root_vertex)
BOOST_PARAMETER_NAME(index_map)
BOOST_PARAMETER_NAME(color_map)''') -->
<!-- @example.replace_emphasis('''
, (required
    (graph, *)
    (visitor, *)
    (root_vertex, *)
    (index_map, *)
    (color_map, *)
  )
''') -->
<!-- @test('compile') -->
</div>
<div class="section" id="signature-matching-and-overloading">
<h4>2.1.5.6&nbsp;&nbsp;&nbsp;Signature Matching and Overloading</h4>
<p>In fact, the function signature is so general that any call to
<tt class="docutils literal">depth_first_search</tt> with fewer than five arguments will match
our function, provided we pass <em>something</em> for the required
<tt class="docutils literal">graph</tt> parameter.  That might not seem to be a problem at first;
after all, if the arguments don't match the requirements imposed by
the implementation of <tt class="docutils literal">depth_first_search</tt>, a compilation error
will occur later, when its body is instantiated.</p>
<p>There are at least three problems with very general function
signatures.</p>
<ol class="arabic simple">
<li>By the time our <tt class="docutils literal">depth_first_search</tt> is instantiated, it has
been selected as the best matching overload.  Some other
<tt class="docutils literal">depth_first_search</tt> overload might've worked had it been
chosen instead.  By the time we see a compilation error, there's
no chance to change that decision.</li>
<li>Even if there are no overloads, error messages generated at
instantiation time usually expose users to confusing
implementation details.  For example, users might see references
to names generated by <tt class="docutils literal">BOOST_PARAMETER_FUNCTION</tt> such as
<tt class="docutils literal"><span class="pre">graphs::detail::depth_first_search_with_named_params</span></tt> (or
worse—think of the kinds of errors you get from your STL
implementation when you make a mistake).<a class="footnote-reference" href="#conceptcpp" id="id7"><sup>4</sup></a></li>
<li>The problems with exposing such permissive function template
signatures have been the subject of much discussion, especially
in the presence of <a class="reference external" href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#225">unqualified calls</a>.  If all we want is to
avoid unintentional argument-dependent lookup (ADL), we can
isolate <tt class="docutils literal">depth_first_search</tt> in a namespace containing no
types<a class="footnote-reference" href="#using" id="id8"><sup>6</sup></a>, but suppose we <em>want</em> it to found via ADL?</li>
</ol>
<p>It's usually a good idea to prevent functions from being considered
for overload resolution when the passed argument types aren't
appropriate.  The library already does this when the required
<tt class="docutils literal">graph</tt> parameter is not supplied, but we're not likely to see a
depth first search that doesn't take a graph to operate on.
Suppose, instead, that we found a different depth first search
algorithm that could work on graphs that don't model
<a class="reference external" href="../../../graph/doc/IncidenceGraph.html"><span class="concept">Incidence Graph</span></a>?  If we just added a simple overload,
it would be ambiguous:</p>
<pre class="literal-block">
// new overload
BOOST_PARAMETER_FUNCTION(
    (void), depth_first_search, (tag), (required (graph,*))( … ))
{
    // new algorithm implementation
}

…

// ambiguous!
depth_first_search(boost::adjacency_list&lt;&gt;(), 2, &quot;hello&quot;);
</pre>
<!-- @ignore() -->
<div class="section" id="adding-type-requirements">
<h5>2.1.5.6.1&nbsp;&nbsp;&nbsp;Adding Type Requirements</h5>
<p>We really don't want the compiler to consider the original version
of <tt class="docutils literal">depth_first_search</tt> because the <tt class="docutils literal">root_vertex</tt> argument,
<tt class="docutils literal">&quot;hello&quot;</tt>, doesn't meet the <a class="reference internal" href="#parameter-table">requirement</a> that it match the
<tt class="docutils literal">graph</tt> parameter's vertex descriptor type.  Instead, this call
should just invoke our new overload.  To take the original
<tt class="docutils literal">depth_first_search</tt> overload out of contention, we need to tell
the library about this requirement by replacing the <tt class="docutils literal">*</tt> element
of the signature with the required type, in parentheses:</p>
<pre class="literal-block">
(root_vertex,
     <strong>(typename boost::graph_traits&lt;graph_type&gt;::vertex_descriptor)</strong>,
     *vertices(graph).first)
</pre>
<!-- @ignore() -->
<p>Now the original <tt class="docutils literal">depth_first_search</tt> will only be called when
the <tt class="docutils literal">root_vertex</tt> argument can be converted to the graph's vertex
descriptor type, and our example that <em>was</em> ambiguous will smoothly
call the new overload.</p>
<div class="note">
<p class="first admonition-title">Note</p>
<p class="last">The <em>type</em> of the <tt class="docutils literal">graph</tt> argument is available in the
signature—and in the function body—as <tt class="docutils literal">graph_type</tt>.  In
general, to access the type of any parameter <em>foo</em>, write <em>foo</em><tt class="docutils literal">_type</tt>.</p>
</div>
</div>
<div class="section" id="predicate-requirements">
<h5>2.1.5.6.2&nbsp;&nbsp;&nbsp;Predicate Requirements</h5>
<p>The requirements on other arguments are a bit more interesting than
those on <tt class="docutils literal">root_vertex</tt>; they can't be described in terms of simple
type matching.  Instead, they must be described in terms of <a class="reference external" href="../../../mpl/doc/refmanual/metafunction.html">MPL
Metafunctions</a>.  There's no space to give a complete description
of metafunctions or of graph library details here, but we'll show
you the complete signature with maximal checking, just to give you
a feel for how it's done.  Each predicate metafunction is enclosed
in parentheses <em>and preceded by an asterix</em>, as follows:</p>
<pre class="literal-block">
// We first need to define a few metafunction that we use in the
// predicates below.

template &lt;class G&gt;
struct traversal_category
{
    typedef typename boost::graph_traits&lt;G&gt;::traversal_category type;
};

template &lt;class G&gt;
struct vertex_descriptor
{
    typedef typename boost::graph_traits&lt;G&gt;::vertex_descriptor type;
};

template &lt;class G&gt;
struct value_type
{
    typedef typename boost::property_traits&lt;G&gt;::value_type type;
};

template &lt;class G&gt;
struct key_type
{
    typedef typename boost::property_traits&lt;G&gt;::key_type type;
};

template&lt;class Size, class IndexMap&gt;
boost::iterator_property_map&lt;
    boost::default_color_type*, IndexMap
  , boost::default_color_type, boost::default_color_type&amp;
&gt;
default_color_map(Size num_vertices, IndexMap const&amp; index_map)
{
   std::vector&lt;boost::default_color_type&gt; colors(num_vertices);
   return &amp;colors[0];
}

BOOST_PARAMETER_FUNCTION(
    (void), depth_first_search, graphs

  , (required
      (graph
       , <strong>*(boost::mpl::and_&lt;
               boost::is_convertible&lt;
                   traversal_category&lt;_&gt;, boost::incidence_graph_tag
               &gt;
             , boost::is_convertible&lt;
                   traversal_category&lt;_&gt;, boost::vertex_list_graph_tag
               &gt;
           &gt;)</strong> ))

    (optional
      (visitor, *, boost::dfs_visitor&lt;&gt;()) // not checkable

      (root_vertex
        , (vertex_descriptor&lt;graphs::graph::_&gt;)
        , *vertices(graph).first)

      (index_map
        , <strong>*(boost::mpl::and_&lt;
              boost::is_integral&lt;value_type&lt;_&gt; &gt;
            , boost::is_same&lt;
                  vertex_descriptor&lt;graphs::graph::_&gt;, key_type&lt;_&gt;
              &gt;
          &gt;)</strong>
        , get(boost::vertex_index,graph))

      (in_out(color_map)
        , <strong>*(boost::is_same&lt;
              vertex_descriptor&lt;graphs::graph::_&gt;, key_type&lt;_&gt;
          &gt;)</strong>
       , default_color_map(num_vertices(graph), index_map) )
    )
)
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/depth_first_search.hpp>

BOOST_PARAMETER_NAME((_graph, graphs) graph)
BOOST_PARAMETER_NAME((_visitor, graphs) visitor)
BOOST_PARAMETER_NAME((_root_vertex, graphs) root_vertex)
BOOST_PARAMETER_NAME((_index_map, graphs) index_map)
BOOST_PARAMETER_NAME((_color_map, graphs) color_map)

using boost::mpl::_;
''') -->
<!-- @example.append('''
{}

int main()
{
    typedef boost::adjacency_list<boost::vecS, boost::vecS, boost::directedS> G;

    enum {u, v, w, x, y, z, N};
    typedef std::pair<int, int> E;
    E edges[] = {E(u, v), E(u, x), E(x, v), E(y, x), E(v, y), E(w, y),
                 E(w,z), E(z, z)};
    G g(edges, edges + sizeof(edges) / sizeof(E), N);

    depth_first_search(g);
    depth_first_search(g, _root_vertex = (int)x);
}
''') -->
<!-- @test('compile') -->
<p>Note the use of the nested <cite>tag::_</cite>. This is a shortcut for:</p>
<pre class="literal-block">
value_type&lt;boost::mpl::_2, tag&gt;
</pre>
<!-- @ignore() -->
<p>Intended to be used to access preceding arguments types in the
predicates.</p>
<p>We acknowledge that this signature is pretty hairy looking.
Fortunately, it usually isn't necessary to so completely encode the
type requirements on arguments to generic functions.  However, it
is usally worth the effort to do so: your code will be more
self-documenting and will often provide a better user experience.
You'll also have an easier transition to an upcoming C++ standard
with <a class="reference external" href="http://www.generic-programming.org/software/ConceptGCC/">language support for concepts</a>.</p>
</div>
</div>
<div class="section" id="deduced-parameters">
<h4>2.1.5.7&nbsp;&nbsp;&nbsp;Deduced Parameters</h4>
<p>To illustrate deduced parameter support we'll have to leave behind
our example from the Graph library.  Instead, consider the example
of the <a class="reference external" href="../../../python/doc/v2/def.html"><tt class="docutils literal">def</tt></a> function from <a class="reference external" href="../../../python/doc/index.html">Boost.Python</a>.  Its signature is
roughly as follows:</p>
<pre class="literal-block">
template &lt;
  class Function, Class KeywordExpression, class CallPolicies
&gt;
void def(
    // Required parameters
    char const* name, Function func

    // Optional, deduced parameters
  , char const* docstring = &quot;&quot;
  , KeywordExpression keywords = no_keywords()
  , CallPolicies policies = default_call_policies()
);
</pre>
<!-- @ignore() -->
<p>Try not to be too distracted by the use of the term “keywords” in
this example: although it means something analogous in Boost.Python
to what it means in the Parameter library, for the purposes of this
exercise you can think of it as being completely different.</p>
<p>When calling <tt class="docutils literal">def</tt>, only two arguments are required.  The
association between any additional arguments and their parameters
can be determined by the types of the arguments actually passed, so
the caller is neither required to remember argument positions or
explicitly specify parameter names for those arguments.  To
generate this interface using <tt class="docutils literal">BOOST_PARAMETER_FUNCTION</tt>, we need
only enclose the deduced parameters in a <tt class="docutils literal">(deduced …)</tt> clause, as
follows:</p>
<pre class="literal-block">
namespace mpl = boost::mpl;

BOOST_PARAMETER_FUNCTION(
    (void), def, tag,

    (required (name,(char const*)) (func,*) )   // nondeduced

    <strong>(deduced</strong>
      (optional
        (docstring, (char const*), &quot;&quot;)

        (keywords
           , *(is_keyword_expression&lt;mpl::_&gt;) // see<a class="footnote-reference" href="#is-keyword-expression" id="id13"><sup>5</sup></a>
           , no_keywords())

        (policies
           , *(mpl::not_&lt;
                 mpl::or_&lt;
                     boost::is_convertible&lt;mpl::_, char const*&gt;
                   , is_keyword_expression&lt;mpl::_&gt; // see<a class="footnote-reference" href="#is-keyword-expression" id="id14"><sup>5</sup></a>
                 &gt;
             &gt;)
           , default_call_policies()
         )
       )
     <strong>)</strong>
 )
 {
    <em>…</em>
 }
</pre>
<!-- @example.replace_emphasis('') -->
<!-- @example.prepend('''
#include <boost/parameter.hpp>

BOOST_PARAMETER_NAME(name)
BOOST_PARAMETER_NAME(func)
BOOST_PARAMETER_NAME(docstring)
BOOST_PARAMETER_NAME(keywords)
BOOST_PARAMETER_NAME(policies)

struct default_call_policies
{};

struct no_keywords
{};

struct keywords
{};

template <class T>
struct is_keyword_expression
  : boost::mpl::false_
{};

template <>
struct is_keyword_expression<keywords>
  : boost::mpl::true_
{};

default_call_policies some_policies;

void f()
{}

''') -->
<div class="admonition-syntax-note admonition">
<p class="first admonition-title">Syntax Note</p>
<p class="last">A <tt class="docutils literal">(deduced …)</tt> clause always contains a <tt class="docutils literal">(required …)</tt>
and/or an <tt class="docutils literal">(optional …)</tt> subclause, and must follow any
<tt class="docutils literal">(required …)</tt> or <tt class="docutils literal">(optional …)</tt> clauses indicating
nondeduced parameters at the outer level.</p>
</div>
<p>With the declaration above, the following two calls are equivalent:</p>
<pre class="literal-block">
def(&quot;f&quot;, &amp;f, <strong>some_policies</strong>, <strong>&quot;Documentation for f&quot;</strong>);
def(&quot;f&quot;, &amp;f, <strong>&quot;Documentation for f&quot;</strong>, <strong>some_policies</strong>);
</pre>
<!-- @example.prepend('''
int main()
{''') -->
<p>If the user wants to pass a <tt class="docutils literal">policies</tt> argument that was also,
for some reason, convertible to <tt class="docutils literal">char const*</tt>, she can always
specify the parameter name explicitly, as follows:</p>
<pre class="literal-block">
def(
    &quot;f&quot;, &amp;f
   , <strong>_policies = some_policies</strong>, &quot;Documentation for f&quot;);
</pre>
<!-- @example.append('}') -->
<!-- @test('compile', howmany='all') -->
</div>
</div>
</div>
<div class="section" id="parameter-enabled-member-functions">
<h2><a class="toc-backref" href="#id28">2.2&nbsp;&nbsp;&nbsp;Parameter-Enabled Member Functions</a></h2>
<p>The <tt class="docutils literal">BOOST_PARAMETER_MEMBER_FUNCTION</tt> and
<tt class="docutils literal">BOOST_PARAMETER_CONST_MEMBER_FUNCTION</tt> macros accept exactly the
same arguments as <tt class="docutils literal">BOOST_PARAMETER_FUNCTION</tt>, but are designed to
be used within the body of a class:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(arg1)
BOOST_PARAMETER_NAME(arg2)

struct callable2
{
    BOOST_PARAMETER_CONST_MEMBER_FUNCTION(
        (void), call, tag, (required (arg1,(int))(arg2,(int))))
    {
        std::cout &lt;&lt; arg1 &lt;&lt; &quot;, &quot; &lt;&lt; arg2 &lt;&lt; std::endl;
    }
};
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <iostream>
using namespace boost::parameter;
''') -->
<!-- @test('compile') -->
<p>These macros don't directly allow a function's interface to be
separated from its implementation, but you can always forward
arguments on to a separate implementation function:</p>
<pre class="literal-block">
struct callable2
{
    BOOST_PARAMETER_CONST_MEMBER_FUNCTION(
        (void), call, tag, (required (arg1,(int))(arg2,(int))))
    {
        call_impl(arg1,arg2);
    }
 private:
    void call_impl(int, int); // implemented elsewhere.
};
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>

BOOST_PARAMETER_NAME(arg1)
BOOST_PARAMETER_NAME(arg2)
using namespace boost::parameter;
''') -->
<!-- @test('compile') -->
<div class="section" id="static-member-functions">
<h3>2.2.1&nbsp;&nbsp;&nbsp;Static Member Functions</h3>
<p>To expose a static member function, simply insert the keyword
“<tt class="docutils literal">static</tt>” before the function name:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(arg1)

struct somebody
{
    BOOST_PARAMETER_MEMBER_FUNCTION(
        (void), <strong>static</strong> f, tag, (optional (arg1,(int),0)))
    {
        std::cout &lt;&lt; arg1 &lt;&lt; std::endl;
    }
};
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <iostream>
using namespace boost::parameter;
''') -->
<!-- @test('compile') -->
</div>
</div>
<div class="section" id="parameter-enabled-constructors">
<h2><a class="toc-backref" href="#id29">2.3&nbsp;&nbsp;&nbsp;Parameter-Enabled Constructors</a></h2>
<p>The lack of a “delegating constructor”
feature in C++
(<a class="reference external" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1986.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1986.pdf</a>)
limits somewhat the quality of interface this library can provide
for defining parameter-enabled constructors.  The usual workaround
for a lack of constructor delegation applies: one must factor the
common logic into a base class.</p>
<p>Let's build a parameter-enabled constructor that simply prints its
arguments.  The first step is to write a base class whose
constructor accepts a single argument known as an <a class="reference external" href="reference.html#argumentpack"><span class="concept">ArgumentPack</span></a>:
a bundle of references to the actual arguments, tagged with their
keywords.  The values of the actual arguments are extracted from
the <span class="concept">ArgumentPack</span> by <em>indexing</em> it with keyword objects:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(name)
BOOST_PARAMETER_NAME(index)

struct myclass_impl
{
    template &lt;class ArgumentPack&gt;
    myclass_impl(ArgumentPack const&amp; args)
    {
        std::cout &lt;&lt; &quot;name = &quot; &lt;&lt; args[_name]
                  &lt;&lt; &quot;; index = &quot; &lt;&lt; args[_index | 42]
                  &lt;&lt; std::endl;
    }
};
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <iostream>''') -->
<p>Note that the bitwise or (“<tt class="docutils literal">|</tt>”) operator has a special
meaning when applied to keyword objects that are passed to an
<span class="concept">ArgumentPack</span>'s indexing operator: it is used to indicate a
default value.  In this case if there is no <tt class="docutils literal">index</tt> parameter in
the <span class="concept">ArgumentPack</span>, <tt class="docutils literal">42</tt> will be used instead.</p>
<p>Now we are ready to write the parameter-enabled constructor
interface:</p>
<pre class="literal-block">
struct myclass : myclass_impl
{
    BOOST_PARAMETER_CONSTRUCTOR(
        myclass, (myclass_impl), tag
      , (required (name,*)) (optional (index,*))) // no semicolon
};
</pre>
<p>Since we have supplied a default value for <tt class="docutils literal">index</tt> but not for
<tt class="docutils literal">name</tt>, only <tt class="docutils literal">name</tt> is required.  We can exercise our new
interface as follows:</p>
<pre class="literal-block">
myclass x(&quot;bob&quot;, 3);                     // positional
myclass y(_index = 12, _name = &quot;sally&quot;); // named
myclass z(&quot;june&quot;);                       // positional/defaulted
</pre>
<!-- @example.wrap('int main() {', '}') -->
<!-- @test('run', howmany='all') -->
<p>For more on <span class="concept">ArgumentPack</span> manipulation, see the <a class="reference internal" href="#advanced-topics">Advanced Topics</a>
section.</p>
</div>
<div class="section" id="parameter-enabled-class-templates">
<h2><a class="toc-backref" href="#id30">2.4&nbsp;&nbsp;&nbsp;Parameter-Enabled Class Templates</a></h2>
<p>In this section we'll use Boost.Parameter to build <a class="reference external" href="../../../python/doc/index.html">Boost.Python</a>'s <a class="reference external" href="http://www.boost.org/libs/python/doc/v2/class.html#class_-spec">class_</a> template, whose “signature” is:</p>
<pre class="literal-block">
template class&lt;
    ValueType, BaseList = bases&lt;&gt;
  , HeldType = ValueType, Copyable = void
&gt;
class class_;
</pre>
<!-- @ignore() -->
<p>Only the first argument, <tt class="docutils literal">ValueType</tt>, is required.</p>
<div class="section" id="named-template-parameters">
<h3>2.4.1&nbsp;&nbsp;&nbsp;Named Template Parameters</h3>
<p>First, we'll build an interface that allows users to pass arguments
positionally or by name:</p>
<pre class="literal-block">
struct B { virtual ~B() = 0; };
struct D : B { ~D(); };

class_&lt;
     <strong>class_type&lt;B&gt;</strong>, <strong>copyable&lt;boost::noncopyable&gt;</strong>
&gt; …;

class_&lt;
    <strong>D</strong>, <strong>held_type&lt;std::auto_ptr&lt;D&gt; &gt;</strong>, <strong>base_list&lt;bases&lt;B&gt; &gt;</strong>
&gt; …;
</pre>
<!-- @ignore() -->
<div class="section" id="template-keywords">
<h4>2.4.1.1&nbsp;&nbsp;&nbsp;Template Keywords</h4>
<p>The first step is to define keywords for each template parameter:</p>
<pre class="literal-block">
namespace boost { namespace python {

BOOST_PARAMETER_TEMPLATE_KEYWORD(class_type)
BOOST_PARAMETER_TEMPLATE_KEYWORD(base_list)
BOOST_PARAMETER_TEMPLATE_KEYWORD(held_type)
BOOST_PARAMETER_TEMPLATE_KEYWORD(copyable)

}}
</pre>
<!-- @example.prepend('#include <boost/parameter.hpp>') -->
<!-- @test('compile') -->
<p>The declaration of the <tt class="docutils literal">class_type</tt> keyword you see here is
equivalent to:</p>
<pre class="literal-block">
namespace boost { namespace python {

namespace tag { struct class_type; } // keyword tag type
template &lt;class T&gt;
struct class_type
  : parameter::template_keyword&lt;tag::class_type,T&gt;
{};

}}
</pre>
<!-- @example.prepend('#include <boost/parameter.hpp>') -->
<!-- @test('compile') -->
<p>It defines a keyword tag type named <tt class="docutils literal"><span class="pre">tag::class_type</span></tt> and a
<em>parameter passing template</em> named <tt class="docutils literal">class_type</tt>.</p>
</div>
<div class="section" id="class-template-skeleton">
<h4>2.4.1.2&nbsp;&nbsp;&nbsp;Class Template Skeleton</h4>
<p>The next step is to define the skeleton of our class template,
which has three optional parameters.  Because the user may pass
arguments in any order, we don't know the actual identities of
these parameters, so it would be premature to use descriptive names
or write out the actual default values for any of them.  Instead,
we'll give them generic names and use the special type
<tt class="docutils literal"><span class="pre">boost::parameter::void_</span></tt> as a default:</p>
<pre class="literal-block">
namespace boost { namespace python {

template &lt;
    class A0
  , class A1 = parameter::void_
  , class A2 = parameter::void_
  , class A3 = parameter::void_
&gt;
struct class_
{
    <em>…</em>
};

}}
</pre>
<!-- @example.prepend('#include <boost/parameter.hpp>') -->
<!-- @example.replace_emphasis('') -->
<!-- @test('compile') -->
</div>
<div class="section" id="class-template-signatures">
<h4>2.4.1.3&nbsp;&nbsp;&nbsp;Class Template Signatures</h4>
<p>Next, we need to build a type, known as a <a class="reference external" href="reference.html#parameterspec"><span class="concept">ParameterSpec</span></a>,
describing the “signature” of <tt class="docutils literal"><span class="pre">boost::python::class_</span></tt>.  A
<a class="reference external" href="reference.html#parameterspec"><span class="concept">ParameterSpec</span></a> enumerates the required and optional parameters in
their positional order, along with any type requirements (note that
it does <em>not</em> specify defaults -- those will be dealt with
separately):</p>
<pre class="literal-block">
namespace boost { namespace python {

using boost::mpl::_;

typedef parameter::parameters&lt;
    required&lt;tag::class_type, boost::is_class&lt;_&gt; &gt;
  , parameter::optional&lt;tag::base_list, mpl::is_sequence&lt;_&gt; &gt;
  , parameter::optional&lt;tag::held_type&gt;
  , parameter::optional&lt;tag::copyable&gt;
&gt; class_signature;

}}
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <boost/mpl/is_sequence.hpp>
#include <boost/noncopyable.hpp>
#include <boost/type_traits/is_class.hpp>
#include <memory>

using namespace boost::parameter;

namespace boost { namespace python {

BOOST_PARAMETER_TEMPLATE_KEYWORD(class_type)
BOOST_PARAMETER_TEMPLATE_KEYWORD(base_list)
BOOST_PARAMETER_TEMPLATE_KEYWORD(held_type)
BOOST_PARAMETER_TEMPLATE_KEYWORD(copyable)

template <class B = int>
struct bases
{};

}}''') -->
</div>
<div class="section" id="argument-packs-and-parameter-extraction">
<span id="binding-intro"></span><h4>2.4.1.4&nbsp;&nbsp;&nbsp;Argument Packs and Parameter Extraction</h4>
<p>Next, within the body of <tt class="docutils literal">class_</tt> , we use the <span class="concept">ParameterSpec</span>'s
nested <tt class="docutils literal">::bind&lt; … &gt;</tt> template to bundle the actual arguments into an
<a class="reference external" href="reference.html#argumentpack"><span class="concept">ArgumentPack</span></a> type, and then use the library's <tt class="docutils literal">value_type&lt; … &gt;</tt>
metafunction to extract “logical parameters”.  <tt class="docutils literal">value_type&lt; … &gt;</tt> is
a lot like <tt class="docutils literal">binding&lt; … &gt;</tt>, but no reference is added to the actual
argument type.  Note that defaults are specified by passing it an
optional third argument:</p>
<pre class="literal-block">
namespace boost { namespace python {

template &lt;
    class A0
  , class A1 = parameter::void_
  , class A2 = parameter::void_
  , class A3 = parameter::void_
&gt;
struct class_
{
    // Create ArgumentPack
    typedef typename
      class_signature::bind&lt;A0,A1,A2,A3&gt;::type
    args;

    // Extract first logical parameter.
    typedef typename parameter::value_type&lt;
      args, tag::class_type&gt;::type class_type;

    typedef typename parameter::value_type&lt;
      args, tag::base_list, bases&lt;&gt; &gt;::type base_list;

    typedef typename parameter::value_type&lt;
      args, tag::held_type, class_type&gt;::type held_type;

    typedef typename parameter::value_type&lt;
      args, tag::copyable, void&gt;::type copyable;
};

}}
</pre>
</div>
</div>
<div class="section" id="exercising-the-code-so-far">
<h3>2.4.2&nbsp;&nbsp;&nbsp;Exercising the Code So Far</h3>
<div class="compound">
<p class="compound-first">Revisiting our original examples,</p>
<pre class="compound-middle literal-block">
typedef boost::python::class_&lt;
    class_type&lt;B&gt;, copyable&lt;boost::noncopyable&gt;
&gt; c1;

typedef boost::python::class_&lt;
    D, held_type&lt;std::auto_ptr&lt;D&gt; &gt;, base_list&lt;bases&lt;B&gt; &gt;
&gt; c2;
</pre>
<!-- @example.prepend('''
using boost::python::class_type;
using boost::python::copyable;
using boost::python::held_type;
using boost::python::base_list;
using boost::python::bases;

struct B {};
struct D {};''') -->
<p class="compound-middle">we can now examine the intended parameters:</p>
<pre class="compound-last literal-block">
BOOST_MPL_ASSERT((boost::is_same&lt;c1::class_type, B&gt;));
BOOST_MPL_ASSERT((boost::is_same&lt;c1::base_list, bases&lt;&gt; &gt;));
BOOST_MPL_ASSERT((boost::is_same&lt;c1::held_type, B&gt;));
BOOST_MPL_ASSERT((
     boost::is_same&lt;c1::copyable, boost::noncopyable&gt;
));

BOOST_MPL_ASSERT((boost::is_same&lt;c2::class_type, D&gt;));
BOOST_MPL_ASSERT((boost::is_same&lt;c2::base_list, bases&lt;B&gt; &gt;));
BOOST_MPL_ASSERT((
    boost::is_same&lt;c2::held_type, std::auto_ptr&lt;D&gt; &gt;
));
BOOST_MPL_ASSERT((boost::is_same&lt;c2::copyable, void&gt;));
</pre>
</div>
<!-- @test('compile', howmany='all') -->
</div>
<div class="section" id="deduced-template-parameters">
<h3>2.4.3&nbsp;&nbsp;&nbsp;Deduced Template Parameters</h3>
<p>To apply a deduced parameter interface here, we need only make the
type requirements a bit tighter so the <tt class="docutils literal">held_type</tt> and
<tt class="docutils literal">copyable</tt> parameters can be crisply distinguished from the
others.  <a class="reference external" href="../../../python/doc/index.html">Boost.Python</a> does this by requiring that <tt class="docutils literal">base_list</tt> be
a specialization of its <tt class="docutils literal">bases&lt; … &gt;</tt> template (as opposed to
being any old MPL sequence) and by requiring that <tt class="docutils literal">copyable</tt>, if
explicitly supplied, be <tt class="docutils literal"><span class="pre">boost::noncopyable</span></tt>.  One easy way of
identifying specializations of <tt class="docutils literal">bases&lt; … &gt;</tt> is to derive them all
from the same class, as an implementation detail:</p>
<pre class="literal-block">
namespace boost { namespace python {

namespace detail { struct bases_base {}; }

template &lt;class A0 = void, class A1 = void, class A2 = void <em>…</em> &gt;
struct bases <strong>: detail::bases_base</strong>
{};

}}
</pre>
<!-- @example.replace_emphasis('') -->
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <boost/mpl/is_sequence.hpp>
#include <boost/noncopyable.hpp>
#include <memory>

using namespace boost::parameter;
using boost::mpl::_;

namespace boost { namespace python {

BOOST_PARAMETER_TEMPLATE_KEYWORD(class_type)
BOOST_PARAMETER_TEMPLATE_KEYWORD(base_list)
BOOST_PARAMETER_TEMPLATE_KEYWORD(held_type)
BOOST_PARAMETER_TEMPLATE_KEYWORD(copyable)

}}''') -->
<p>Now we can rewrite our signature to make all three optional
parameters deducible:</p>
<pre class="literal-block">
typedef parameter::parameters&lt;
    required&lt;tag::class_type, is_class&lt;_&gt; &gt;

  , parameter::optional&lt;
        deduced&lt;tag::base_list&gt;
      , is_base_and_derived&lt;detail::bases_base,_&gt;
    &gt;

  , parameter::optional&lt;
        deduced&lt;tag::held_type&gt;
      , mpl::not_&lt;
            mpl::or_&lt;
                is_base_and_derived&lt;detail::bases_base,_&gt;
              , is_same&lt;noncopyable,_&gt;
            &gt;
        &gt;
    &gt;

  , parameter::optional&lt;deduced&lt;tag::copyable&gt;, is_same&lt;noncopyable,_&gt; &gt;

&gt; class_signature;
</pre>
<!-- @example.prepend('''
#include <boost/type_traits/is_class.hpp>
namespace boost { namespace python {''') -->
<!-- @example.append('''
template <
    class A0
  , class A1 = parameter::void_
  , class A2 = parameter::void_
  , class A3 = parameter::void_
>
struct class_
{
    // Create ArgumentPack
    typedef typename
      class_signature::bind<A0,A1,A2,A3>::type
    args;

    // Extract first logical parameter.
    typedef typename parameter::value_type<
      args, tag::class_type>::type class_type;

    typedef typename parameter::value_type<
      args, tag::base_list, bases<> >::type base_list;

    typedef typename parameter::value_type<
      args, tag::held_type, class_type>::type held_type;

    typedef typename parameter::value_type<
      args, tag::copyable, void>::type copyable;
};

}}''') -->
<p>It may seem like we've added a great deal of complexity, but the
benefits to our users are greater.  Our original examples can now
be written without explicit parameter names:</p>
<pre class="literal-block">
typedef boost::python::class_&lt;<strong>B</strong>, <strong>boost::noncopyable</strong>&gt; c1;

typedef boost::python::class_&lt;<strong>D</strong>, <strong>std::auto_ptr&lt;D&gt;</strong>, <strong>bases&lt;B&gt;</strong> &gt; c2;
</pre>
<!-- @example.prepend('''
struct B {};
struct D {};

using boost::python::bases;''') -->
<!-- @example.append('''
BOOST_MPL_ASSERT((boost::is_same<c1::class_type, B>));
BOOST_MPL_ASSERT((boost::is_same<c1::base_list, bases<> >));
BOOST_MPL_ASSERT((boost::is_same<c1::held_type, B>));
BOOST_MPL_ASSERT((
     boost::is_same<c1::copyable, boost::noncopyable>
));

BOOST_MPL_ASSERT((boost::is_same<c2::class_type, D>));
BOOST_MPL_ASSERT((boost::is_same<c2::base_list, bases<B> >));
BOOST_MPL_ASSERT((
    boost::is_same<c2::held_type, std::auto_ptr<D> >
));
BOOST_MPL_ASSERT((boost::is_same<c2::copyable, void>));''') -->
<!-- @test('compile', howmany='all') -->
</div>
</div>
</div>
<div class="section" id="advanced-topics">
<h1><a class="toc-backref" href="#id31">3&nbsp;&nbsp;&nbsp;Advanced Topics</a></h1>
<p>At this point, you should have a good grasp of the basics.  In this
section we'll cover some more esoteric uses of the library.</p>
<div class="section" id="fine-grained-name-control">
<h2><a class="toc-backref" href="#id32">3.1&nbsp;&nbsp;&nbsp;Fine-Grained Name Control</a></h2>
<p>If you don't like the leading-underscore naming convention used
to refer to keyword objects, or you need the name <tt class="docutils literal">tag</tt> for
something other than the keyword type namespace, there's another
way to use <tt class="docutils literal">BOOST_PARAMETER_NAME</tt>:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(<strong>(</strong><em>object-name</em><strong>,</strong> <em>tag-namespace</em><strong>)</strong> <em>parameter-name</em>)
</pre>
<!-- @ignore() -->
<p>Here is a usage example:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME((<strong>pass_foo</strong>, <strong>keywords</strong>) <strong>foo</strong>)

BOOST_PARAMETER_FUNCTION(
  (int), f,
  <strong>keywords</strong>, (required (<strong>foo</strong>, *)))
{
    return <strong>foo</strong> + 1;
}

int x = f(<strong>pass_foo</strong> = 41);
</pre>
<!-- @example.prepend('#include <boost/parameter.hpp>') -->
<!-- @example.append('''
int main()
{}''') -->
<!-- @test('run') -->
<p>Before you use this more verbose form, however, please read the
section on <a class="reference internal" href="#keyword-naming">best practices for keyword object naming</a>.</p>
</div>
<div class="section" id="more-argumentpacks">
<h2><a class="toc-backref" href="#id33">3.2&nbsp;&nbsp;&nbsp;More <span class="concept">ArgumentPack</span>s</a></h2>
<p>We've already seen <span class="concept">ArgumentPack</span>s when we looked at
<a class="reference internal" href="#parameter-enabled-constructors">parameter-enabled constructors</a> and <a class="reference internal" href="#binding-intro">class templates</a>.  As you
might have guessed, <span class="concept">ArgumentPack</span>s actually lie at the heart of
everything this library does; in this section we'll examine ways to
build and manipulate them more effectively.</p>
<div class="section" id="building-argumentpacks">
<h3>3.2.1&nbsp;&nbsp;&nbsp;Building <span class="concept">ArgumentPack</span>s</h3>
<p>The simplest <span class="concept">ArgumentPack</span> is the result of assigning into a
keyword object:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(index)

template &lt;class ArgumentPack&gt;
int print_index(ArgumentPack const&amp; args)
{
    std::cout &lt;&lt; &quot;index = &quot; &lt;&lt; args[_index] &lt;&lt; std::endl;
    return 0;
}

int x = print_index(_index = 3);  // prints &quot;index = 3&quot;
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <iostream>''') -->
<p>Also, <span class="concept">ArgumentPack</span>s can be composed using the comma operator.
The extra parentheses below are used to prevent the compiler from
seeing two separate arguments to <tt class="docutils literal">print_name_and_index</tt>:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(name)

template &lt;class ArgumentPack&gt;
int print_name_and_index(ArgumentPack const&amp; args)
{
    std::cout &lt;&lt; &quot;name = &quot; &lt;&lt; args[_name] &lt;&lt; &quot;; &quot;;
    return print_index(args);
}

int y = print_name_and_index((_index = 3, _name = &quot;jones&quot;));
</pre>
<p>To build an <span class="concept">ArgumentPack</span> with positional arguments, we can use a
<a class="reference external" href="reference.html#parameterspec"><span class="concept">ParameterSpec</span></a>.  As introduced described in the section on <a class="reference internal" href="#class-template-signatures">Class
Template Signatures</a>, a <span class="concept">ParameterSpec</span> describes the positional
order of parameters and any associated type requirements.  Just as
we can build an <span class="concept">ArgumentPack</span> <em>type</em> with its nested <tt class="docutils literal">::bind&lt; …
&gt;</tt> template, we can build an <span class="concept">ArgumentPack</span> <em>object</em> by invoking
its function call operator:</p>
<pre class="literal-block">
parameter::parameters&lt;
    required&lt;tag::name, is_convertible&lt;_,char const*&gt; &gt;
  , optional&lt;tag::index, is_convertible&lt;_,int&gt; &gt;
&gt; spec;

char const sam[] = &quot;sam&quot;;
int twelve = 12;

int z0 = print_name_and_index( <strong>spec(</strong>sam, twelve<strong>)</strong> );

int z1 = print_name_and_index(
   <strong>spec(</strong>_index=12, _name=&quot;sam&quot;<strong>)</strong>
);
</pre>
<!-- @example.prepend('''
namespace parameter = boost::parameter;
using parameter::required;
using parameter::optional;
using boost::is_convertible;
using boost::mpl::_;''') -->
<!-- @example.append('''
int main()
{}''') -->
<!-- @test('run', howmany='all') -->
<p>Note that because of the <a class="reference external" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n1385.htm">forwarding problem</a>, <tt class="docutils literal"><span class="pre">parameter::parameters::operator()</span></tt>
can't accept non-const rvalues.</p>
</div>
<div class="section" id="extracting-parameter-types">
<h3>3.2.2&nbsp;&nbsp;&nbsp;Extracting Parameter Types</h3>
<p>If we want to know the types of the arguments passed to
<tt class="docutils literal">print_name_and_index</tt>, we have a couple of options.  The
simplest and least error-prone approach is to forward them to a
function template and allow <em>it</em> to do type deduction:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(name)
BOOST_PARAMETER_NAME(index)

template &lt;class Name, class Index&gt;
int deduce_arg_types_impl(Name&amp; name, Index&amp; index)
{
    Name&amp; n2 = name;  // we know the types
    Index&amp; i2 = index;
    return index;
}

template &lt;class ArgumentPack&gt;
int deduce_arg_types(ArgumentPack const&amp; args)
{
    return deduce_arg_types_impl(args[_name], args[_index|42]);
}
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <cassert>''') -->
<!-- @example.append('''
int a1 = deduce_arg_types((_name = "foo"));
int a2 = deduce_arg_types((_name = "foo", _index = 3));

int main()
{
    assert(a1 == 42);
    assert(a2 == 3);
}''') -->
<!-- @test('run') -->
<p>Occasionally one needs to deduce argument types without an extra
layer of function call.  For example, suppose we wanted to return
twice the value of the <tt class="docutils literal">index</tt> parameter?  In that
case we can use the <tt class="docutils literal">value_type&lt; … &gt;</tt> metafunction introduced
<a class="reference internal" href="#binding-intro">earlier</a>:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(index)

template &lt;class ArgumentPack&gt;
typename parameter::value_type&lt;ArgumentPack, tag::index, int&gt;::type
twice_index(ArgumentPack const&amp; args)
{
    return 2 * args[_index|42];
}

int six = twice_index(_index = 3);
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <boost/type_traits/remove_reference.hpp>
#include <cassert>

namespace parameter = boost::parameter;
''') -->
<!-- @example.append('''
int main()
{
    assert(six == 6);
}''') -->
<!-- @test('run', howmany='all') -->
<p>Note that if we had used <tt class="docutils literal">binding&lt; … &gt;</tt> rather than <tt class="docutils literal">value_type&lt; …
&gt;</tt>, we would end up returning a reference to the temporary created in
the <tt class="docutils literal">2 * …</tt> expression.</p>
</div>
<div class="section" id="lazy-default-computation">
<h3>3.2.3&nbsp;&nbsp;&nbsp;Lazy Default Computation</h3>
<p>When a default value is expensive to compute, it would be
preferable to avoid it until we're sure it's absolutely necessary.
<tt class="docutils literal">BOOST_PARAMETER_FUNCTION</tt> takes care of that problem for us, but
when using <span class="concept">ArgumentPack</span>s explicitly, we need a tool other than
<tt class="docutils literal">operator|</tt>:</p>
<pre class="literal-block">
BOOST_PARAMETER_NAME(s1)
BOOST_PARAMETER_NAME(s2)
BOOST_PARAMETER_NAME(s3)

template &lt;class ArgumentPack&gt;
std::string f(ArgumentPack const&amp; args)
{
    std::string const&amp; s1 = args[_s1];
    std::string const&amp; s2 = args[_s2];
    typename parameter::binding&lt;
        ArgumentPack,tag::s3,std::string
    &gt;::type s3 = args[_s3|(s1+s2)]; // always constructs s1+s2
    return s3;
}

std::string x = f((_s1=&quot;hello,&quot;, _s2=&quot; world&quot;, _s3=&quot;hi world&quot;));
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <string>

namespace parameter = boost::parameter;''') -->
<!-- @example.append('''
int main()
{}''') -->
<!-- @test('run') -->
<p>In the example above, the string <tt class="docutils literal">&quot;hello, world&quot;</tt> is constructed
despite the fact that the user passed us a value for <tt class="docutils literal">s3</tt>.  To
remedy that, we can compute the default value <em>lazily</em> (that is,
only on demand), by using <tt class="docutils literal"><span class="pre">boost::bind()</span></tt> to create a function
object.</p>
<!-- danielw: I'm leaving the text below in the source, because we might -->
<!-- want to change back to it after 1.34, and if I remove it now we -->
<!-- might forget about it. -->
<!-- by combining the logical-or (“``||``”) operator -->
<!-- with a function object built by the Boost Lambda_ library: [#bind]_ -->
<pre class="literal-block">
typename parameter::binding&lt;
    ArgumentPack, <a class="reference external" href="tag::s3">tag::s3</a>, std::string
&gt;::type s3 = args[_s3
    <strong>|| boost::bind(std::plus&lt;std::string&gt;(), boost::ref(s1), boost::ref(s2))</strong> ];
</pre>
<!-- @example.prepend('''
#include <boost/bind.hpp>
#include <boost/ref.hpp>
#include <boost/parameter.hpp>
#include <string>
#include <functional>

namespace parameter = boost::parameter;

BOOST_PARAMETER_NAME(s1)
BOOST_PARAMETER_NAME(s2)
BOOST_PARAMETER_NAME(s3)

template <class ArgumentPack>
std::string f(ArgumentPack const& args)
{
    std::string const& s1 = args[_s1];
    std::string const& s2 = args[_s2];''') -->
<!-- @example.append('''
    return s3;
}

std::string x = f((_s1="hello,", _s2=" world", _s3="hi world"));

int main()
{}''') -->
<!-- @test('run') -->
<!-- .. _Lambda: ../../../lambda/index.html -->
<div class="sidebar">
<p class="first sidebar-title">Mnemonics</p>
<p class="last">To remember the difference between <tt class="docutils literal">|</tt> and <tt class="docutils literal">||</tt>, recall that
<tt class="docutils literal">||</tt> normally uses short-circuit evaluation: its second
argument is only evaluated if its first argument is <tt class="docutils literal">false</tt>.
Similarly, in <tt class="docutils literal"><span class="pre">color_map[param||f]</span></tt>, <tt class="docutils literal">f</tt> is only invoked if
no <tt class="docutils literal">color_map</tt> argument was supplied.</p>
</div>
<p>The expression <tt class="docutils literal"><span class="pre">bind(std::plus&lt;std::string&gt;(),</span> ref(s1), ref(s2))</tt> yields
a <em>function object</em> that, when invoked, adds the two strings together.
That function will only be invoked if no <tt class="docutils literal">s3</tt> argument is supplied by
the caller.</p>
<!-- The expression ``lambda::var(s1)+lambda::var(s2)`` yields a -->
<!-- *function object* that, when invoked, adds the two strings -->
<!-- together.  That function will only be invoked if no ``s3`` argument -->
<!-- is supplied by the caller. -->
</div>
</div>
</div>
<div class="section" id="best-practices">
<h1><a class="toc-backref" href="#id34">4&nbsp;&nbsp;&nbsp;Best Practices</a></h1>
<p>By now you should have a fairly good idea of how to use the
Parameter library.  This section points out a few more-marginal
issues that will help you use the library more effectively.</p>
<div class="section" id="keyword-naming">
<h2><a class="toc-backref" href="#id35">4.1&nbsp;&nbsp;&nbsp;Keyword Naming</a></h2>
<p><tt class="docutils literal">BOOST_PARAMETER_NAME</tt> prepends a leading underscore to the names
of all our keyword objects in order to avoid the following
usually-silent bug:</p>
<pre class="literal-block">
namespace people
{
  namespace tag { struct name; struct age;  }

  namespace // unnamed
  {
    boost::parameter::keyword&lt;tag::name&gt;&amp; <strong>name</strong>
    = boost::parameter::keyword&lt;tag::name&gt;::instance;
    boost::parameter::keyword&lt;tag::age&gt;&amp; <strong>age</strong>
    = boost::parameter::keyword&lt;tag::age&gt;::instance;
  }

  BOOST_PARAMETER_FUNCTION(
      (void), g, tag, (optional (name, *, &quot;bob&quot;)(age, *, 42)))
  {
      std::cout &lt;&lt; name &lt;&lt; &quot;:&quot; &lt;&lt; age;
  }

  void f(int age)
  {
  <span class="vellipsis">     .
     .
     .
   </span>
     g(<strong>age</strong> = 3); // whoops!
  }
}
</pre>
<!-- @ignore() -->
<p>Although in the case above, the user was trying to pass the value
<tt class="docutils literal">3</tt> as the <tt class="docutils literal">age</tt> parameter to <tt class="docutils literal">g</tt>, what happened instead
was that <tt class="docutils literal">f</tt>'s <tt class="docutils literal">age</tt> argument got reassigned the value 3,
and was then passed as a positional argument to <tt class="docutils literal">g</tt>.  Since
<tt class="docutils literal">g</tt>'s first positional parameter is <tt class="docutils literal">name</tt>, the default value
for <tt class="docutils literal">age</tt> is used, and g prints <tt class="docutils literal">3:42</tt>.  Our leading
underscore naming convention that makes this problem less likely
to occur.</p>
<p>In this particular case, the problem could have been detected if
f's <tt class="docutils literal">age</tt> parameter had been made <tt class="docutils literal">const</tt>, which is always a
good idea whenever possible.  Finally, we recommend that you use
an enclosing namespace for all your code, but particularly for
names with leading underscores.  If we were to leave out the
<tt class="docutils literal">people</tt> namespace above, names in the global namespace
beginning with leading underscores—which are reserved to your C++
compiler—might become irretrievably ambiguous with those in our
unnamed namespace.</p>
</div>
<div class="section" id="namespaces">
<h2><a class="toc-backref" href="#id36">4.2&nbsp;&nbsp;&nbsp;Namespaces</a></h2>
<p>In our examples we've always declared keyword objects in (an
unnamed namespace within) the same namespace as the
Boost.Parameter-enabled functions using those keywords:</p>
<pre class="literal-block">
namespace lib
{
  <strong>BOOST_PARAMETER_NAME(name)
  BOOST_PARAMETER_NAME(index)</strong>

  BOOST_PARAMETER_FUNCTION(
    (int), f, tag,
    (optional (name,*,&quot;bob&quot;)(index,(int),1))
  )
  {
      std::cout &lt;&lt; name &lt;&lt; &quot;:&quot; &lt;&lt; index &lt;&lt; std::endl;
      return index;
  }
}
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <iostream>''') -->
<!-- @namespace_setup = str(example) -->
<!-- @ignore() -->
<p>Users of these functions have a few choices:</p>
<ol class="arabic simple">
<li>Full qualification:</li>
</ol>
<blockquote>
<pre class="literal-block">
int x = <strong>lib::</strong>f(<strong>lib::</strong>_name = &quot;jill&quot;, <strong>lib::</strong>_index = 1);
</pre>
<p>This approach is more verbose than many users would like.</p>
</blockquote>
<!-- @example.prepend(namespace_setup) -->
<!-- @example.append('int main() {}') -->
<!-- @test('run') -->
<ol class="arabic simple" start="2">
<li>Make keyword objects available through
<em>using-declarations</em>:</li>
</ol>
<blockquote>
<pre class="literal-block">
<strong>using lib::_name;
using lib::_index;</strong>

int x = lib::f(_name = &quot;jill&quot;, _index = 1);
</pre>
<p>This version is much better at the actual call site, but the
<em>using-declarations</em> themselves can be verbose and hard-to
manage.</p>
</blockquote>
<!-- @example.prepend(namespace_setup) -->
<!-- @example.append('int main() {}') -->
<!-- @test('run') -->
<ol class="arabic simple" start="3">
<li>Bring in the entire namespace with a <em>using-directive</em>:</li>
</ol>
<blockquote>
<pre class="literal-block">
<strong>using namespace lib;</strong>
int x = <strong>f</strong>(_name = &quot;jill&quot;, _index = 3);
</pre>
<p>This option is convenient, but it indiscriminately makes the
<em>entire</em> contents of <tt class="docutils literal">lib</tt> available without qualification.</p>
</blockquote>
<!-- @example.prepend(namespace_setup) -->
<!-- @example.append('int main() {}') -->
<!-- @test('run') -->
<p>If we add an additional namespace around keyword declarations,
though, we can give users more control:</p>
<pre class="literal-block">
namespace lib
{
  <strong>namespace keywords
  {</strong>
     BOOST_PARAMETER_NAME(name)
     BOOST_PARAMETER_NAME(index)
  <strong>}</strong>

  BOOST_PARAMETER_FUNCTION(
    (int), f, <strong>keywords::</strong>tag,
    (optional (name,*,&quot;bob&quot;)(index,(int),1))
  )
  {
      std::cout &lt;&lt; name &lt;&lt; &quot;:&quot; &lt;&lt; index &lt;&lt; std::endl;
      return index;
  }
}
</pre>
<!-- @example.prepend('''
#include <boost/parameter.hpp>
#include <iostream>''') -->
<p>Now users need only a single <em>using-directive</em> to bring in just the
names of all keywords associated with <tt class="docutils literal">lib</tt>:</p>
<pre class="literal-block">
<strong>using namespace lib::keywords;</strong>
int y = lib::f(_name = &quot;bob&quot;, _index = 2);
</pre>
<!-- @example.append('int main() {}') -->
<!-- @test('run', howmany='all') -->
</div>
<div class="section" id="documentation">
<h2><a class="toc-backref" href="#id37">4.3&nbsp;&nbsp;&nbsp;Documentation</a></h2>
<p>The interface idioms enabled by Boost.Parameter are completely new
(to C++), and as such are not served by pre-existing documentation
conventions.</p>
<div class="note">
<p class="first admonition-title">Note</p>
<p class="last">This space is empty because we haven't settled on any
best practices yet.  We'd be very pleased to link to your
documentation if you've got a style that you think is worth
sharing.</p>
</div>
</div>
</div>
<div class="section" id="portability-considerations">
<h1><a class="toc-backref" href="#id38">5&nbsp;&nbsp;&nbsp;Portability Considerations</a></h1>
<p>Use the <a class="reference external" href="http://www.boost.org/regression/release/user/parameter.html">regression test results</a> for the latest Boost release of
the Parameter library to see how it fares on your favorite
compiler.  Additionally, you may need to be aware of the following
issues and workarounds for particular compilers.</p>
<div class="section" id="no-sfinae-support">
<h2><a class="toc-backref" href="#id39">5.1&nbsp;&nbsp;&nbsp;No SFINAE Support</a></h2>
<p>Some older compilers don't support SFINAE.  If your compiler meets
that criterion, then Boost headers will <tt class="docutils literal">#define</tt> the preprocessor
symbol <tt class="docutils literal">BOOST_NO_SFINAE</tt>, and parameter-enabled functions won't be
removed from the overload set based on their signatures.</p>
</div>
<div class="section" id="no-support-for-result-of">
<h2>5.2&nbsp;&nbsp;&nbsp;No Support for <a class="reference external" href="../../../utility/utility.htm#result_of"><tt class="docutils literal">result_of</tt></a></h2>
<p><a class="reference internal" href="#lazy-default-computation">Lazy default computation</a> relies on the <tt class="docutils literal">result_of</tt> class
template to compute the types of default arguments given the type
of the function object that constructs them.  On compilers that
don't support <tt class="docutils literal">result_of</tt>, <tt class="docutils literal">BOOST_NO_RESULT_OF</tt> will be
<tt class="docutils literal">#define</tt>d, and the compiler will expect the function object to
contain a nested type name, <tt class="docutils literal">result_type</tt>, that indicates its
return type when invoked without arguments.  To use an ordinary
function as a default generator on those compilers, you'll need to
wrap it in a class that provides <tt class="docutils literal">result_type</tt> as a <tt class="docutils literal">typedef</tt>
and invokes the function via its <tt class="docutils literal">operator()</tt>.</p>
<!-- Can't Declare |ParameterSpec| via ``typedef``
=============================================

In principle you can declare a |ParameterSpec| as a ``typedef``
for a specialization of ``parameters<…>``, but Microsoft Visual C++
6.x has been seen to choke on that usage.  The workaround is to use
inheritance and declare your |ParameterSpec| as a class:

.. parsed-literal::

     **struct dfs_parameters
       :** parameter::parameters<
           tag::graph, tag::visitor, tag::root_vertex
         , tag::index_map, tag::color_map
     > **{};**


Default Arguments Unsupported on Nested Templates
=================================================

As of this writing, Borland compilers don't support the use of
default template arguments on member class templates.  As a result,
you have to supply ``BOOST_PARAMETER_MAX_ARITY`` arguments to every
use of ``parameters<…>::match``.  Since the actual defaults used
are unspecified, the workaround is to use
|BOOST_PARAMETER_MATCH|_ to declare default arguments for SFINAE.

.. |BOOST_PARAMETER_MATCH| replace:: ``BOOST_PARAMETER_MATCH`` -->
</div>
<div class="section" id="compiler-can-t-see-references-in-unnamed-namespace">
<h2><a class="toc-backref" href="#id41">5.3&nbsp;&nbsp;&nbsp;Compiler Can't See References In Unnamed Namespace</a></h2>
<p>If you use Microsoft Visual C++ 6.x, you may find that the compiler
has trouble finding your keyword objects.  This problem has been
observed, but only on this one compiler, and it disappeared as the
test code evolved, so we suggest you use it only as a last resort
rather than as a preventative measure.  The solution is to add
<em>using-declarations</em> to force the names to be available in the
enclosing namespace without qualification:</p>
<pre class="literal-block">
namespace graphs
{
  using graphs::graph;
  using graphs::visitor;
  using graphs::root_vertex;
  using graphs::index_map;
  using graphs::color_map;
}
</pre>
</div>
</div>
<div class="section" id="python-binding">
<h1><a class="toc-backref" href="#id42">6&nbsp;&nbsp;&nbsp;Python Binding</a></h1>
<p>Follow <a class="reference external" href="python.html">this link</a> for documentation on how to expose
Boost.Parameter-enabled functions to Python with <a class="reference external" href="../../../python/doc/index.html">Boost.Python</a>.</p>
</div>
<div class="section" id="reference">
<h1><a class="toc-backref" href="#id43">7&nbsp;&nbsp;&nbsp;Reference</a></h1>
<p>Follow <a class="reference external" href="reference.html">this link</a> to the Boost.Parameter reference
documentation.</p>
</div>
<div class="section" id="glossary">
<h1><a class="toc-backref" href="#id44">8&nbsp;&nbsp;&nbsp;Glossary</a></h1>
<table class="docutils field-list" frame="void" id="arguments" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Argument (or “actual argument”):</th></tr>
<tr><td>&nbsp;</td><td class="field-body">the value actually passed to a
function or class template</td>
</tr>
</tbody>
</table>
<table class="docutils field-list" frame="void" id="parameter" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name" colspan="2">Parameter (or “formal parameter”):</th></tr>
<tr><td>&nbsp;</td><td class="field-body"><p class="first">the name used to refer to an
argument within a function or class template.  For example, the
value of <tt class="docutils literal">f</tt>'s <em>parameter</em> <tt class="docutils literal">x</tt> is given by the <em>argument</em>
<tt class="docutils literal">3</tt>:</p>
<pre class="last literal-block">
int f(int x) { return x + 1 }
int y = f(3);
</pre>
</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="acknowledgements">
<h1><a class="toc-backref" href="#id45">9&nbsp;&nbsp;&nbsp;Acknowledgements</a></h1>
<p>The authors would like to thank all the Boosters who participated
in the review of this library and its documentation, most
especially our review manager, Doug Gregor.</p>
<hr class="docutils" />
<table class="docutils footnote" frame="void" id="old-interface" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id3">[1]</a></td><td>As of Boost 1.33.0 the Graph library was still
using an <a class="reference external" href="../../../graph/doc/bgl_named_params.html">older named parameter mechanism</a>, but there are
plans to change it to use Boost.Parameter (this library) in an
upcoming release, while keeping the old interface available for
backward-compatibility.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="odr" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id5">[2]</a></td><td>The <strong>One Definition Rule</strong> says that any given entity in
a C++ program must have the same definition in all translation
units (object files) that make up a program.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="vertex-descriptor" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label">[3]</td><td>If you're not familiar with the Boost Graph
Library, don't worry about the meaning of any
Graph-library-specific details you encounter.  In this case you
could replace all mentions of vertex descriptor types with
<tt class="docutils literal">int</tt> in the text, and your understanding of the Parameter
library wouldn't suffer.</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="conceptcpp" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id7">[4]</a></td><td>This is a major motivation behind <a class="reference external" href="http://www.generic-programming.org/software/ConceptGCC/">ConceptC++</a>.</td></tr>
</tbody>
</table>
<!-- .. [#bind] The Lambda library is known not to work on `some -->
<!-- less-conformant compilers`__.  When using one of those you could -->
<!-- use `Boost.Bind`_ to generate the function object:: -->
<!-- boost::bind(std::plus<std::string>(),s1,s2) -->
<table class="docutils footnote" frame="void" id="is-keyword-expression" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label">[5]</td><td><em>(<a class="fn-backref" href="#id13">1</a>, <a class="fn-backref" href="#id14">2</a>)</em> Here we're assuming there's a predicate
metafunction <tt class="docutils literal">is_keyword_expression</tt> that can be used to
identify models of Boost.Python's KeywordExpression concept.</td></tr>
</tbody>
</table>
<!-- .. __ http://www.boost.org/regression/release/user/lambda.html -->
<table class="docutils footnote" frame="void" id="using" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id8">[6]</a></td><td><p class="first">You can always give the illusion that the function
lives in an outer namespace by applying a <em>using-declaration</em>:</p>
<pre class="last literal-block">
  namespace foo_overloads
  {
    // foo declarations here
    void foo() { ... }
    ...
  }
  using foo_overloads::foo;

This technique for avoiding unintentional argument-dependent
lookup is due to Herb Sutter.
</pre>
</td></tr>
</tbody>
</table>
<table class="docutils footnote" frame="void" id="sfinae" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label">[7]</td><td>This capability depends on your compiler's support for SFINAE.
<strong>SFINAE</strong>: <strong>S</strong>ubstitution <strong>F</strong>ailure <strong>I</strong>s
<strong>N</strong>ot <strong>A</strong>n <strong>E</strong> rror.  If type substitution during the
instantiation of a function template results in an invalid type,
no compilation error is emitted; instead the overload is removed
from the overload set. By producing an invalid type in the
function signature depending on the result of some condition,
we can decide whether or not an overload is considered during overload
resolution.  The technique is formalized in
the <a class="reference external" href="../../../utility/enable_if.html"><tt class="docutils literal">enable_if</tt></a> utility.  Most recent compilers support SFINAE;
on compilers that don't support it, the Boost config library
will <tt class="docutils literal">#define</tt> the symbol <tt class="docutils literal">BOOST_NO_SFINAE</tt>.
See
<a class="reference external" href="http://www.semantics.org/once_weakly/w02_SFINAE.pdf">http://www.semantics.org/once_weakly/w02_SFINAE.pdf</a> for more
information on SFINAE.</td></tr>
</tbody>
</table>
</div>
</div>
<div class="footer">
<hr class="footer" />
Generated on: 2011-11-08 21:40 UTC.
Generated by <a class="reference external" href="http://docutils.sourceforge.net/">Docutils</a> from <a class="reference external" href="http://docutils.sourceforge.net/rst.html">reStructuredText</a> source.

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