ceph/src/boost/libs/proto/doc/reference/transform/pass_through.xml

   1 <?xml version="1.0" encoding="utf-8"?>
   2 <!--
   3   Copyright 2012 Eric Niebler
   4
   5   Distributed under the Boost
   6   Software License, Version 1.0. (See accompanying
   7   file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
   8   -->
   9 <header name="boost/proto/transform/pass_through.hpp">
  10   <para>Definition of the
  11     <computeroutput><classname alt="boost::proto::pass_through">proto::pass_through&lt;&gt;</classname></computeroutput>
  12     transform, which is the default transform of all of the expression generator metafunctions such as
  13     <computeroutput><classname alt="boost::proto::unary_plus">proto::unary_plus&lt;&gt;</classname></computeroutput>,
  14     <computeroutput><classname alt="boost::proto::plus">proto::plus&lt;&gt;</classname></computeroutput> and
  15     <computeroutput><classname alt="boost::proto::nary_expr">proto::nary_expr&lt;&gt;</classname></computeroutput>.</para>
  16   <namespace name="boost">
  17     <namespace name="proto">
  18       <struct name="pass_through">
  19         <template>
  20           <template-type-parameter name="Grammar"/>
  21           <template-type-parameter name="Domain">
  22             <default><classname>proto::deduce_domain</classname></default>
  23           </template-type-parameter>
  24         </template>
  25         <inherit><type><classname>proto::transform</classname>&lt; pass_through&lt;Grammar, Domain&gt; &gt;</type></inherit>
  26         <purpose>A <conceptname>PrimitiveTransform</conceptname> that transforms the child expressions of an expression
  27           node according to the corresponding children of a Grammar. The resulting expression is in the specified domain.</purpose>
  28         <description>
  29           <para>
  30             Given a Grammar such as <computeroutput><classname>proto::plus</classname>&lt;T0, T1&gt;</computeroutput>,
  31             an expression type that matches the grammar such as
  32             <computeroutput><classname>proto::plus</classname>&lt;E0, E1&gt;::type</computeroutput>, a state
  33             <computeroutput>S</computeroutput> and a data <computeroutput>D</computeroutput>, the result of applying
  34             the <computeroutput>proto::pass_through&lt;<classname>proto::plus</classname>&lt;T0, T1&gt; &gt;</computeroutput>
  35             transform is: <programlisting><classname>proto::plus</classname>&lt;
  36   boost::result_of&lt;T0(E0, S, D)&gt;::type,
  37   boost::result_of&lt;T1(E1, S, D)&gt;::type
  38 &gt;::type</programlisting>
  39           </para>
  40           <para>
  41             The above demonstrates how child transforms and child expressions are applied pairwise, and how the
  42             results are reassembled into a new expression node with the same tag type as the original.
  43           </para>
  44           <para>
  45             The <code>Domain</code> template parameter determines which domain the resulting expression should
  46             be in. If it is <code><classname>proto::deduce_domain</classname></code>, which is the default,
  47             the resulting expression is in the same domain as the expression passed in. Otherwise, the resulting
  48             expression is in the specified domain. Practically, that means the specified domain's generator is
  49             used to post-process the resulting expression.
  50           </para>
  51           <para>
  52             The explicit use of <computeroutput>proto::pass_through&lt;&gt;</computeroutput> is not usually
  53             needed, since the expression generator metafunctions such as
  54             <computeroutput><classname>proto::plus</classname>&lt;&gt;</computeroutput> have
  55             <computeroutput>proto::pass_through&lt;&gt;</computeroutput> as their default transform. So,
  56             for instance, these are equivalent:
  57             <itemizedlist>
  58               <listitem>
  59                 <computeroutput>
  60                   <classname>proto::when</classname>&lt; <classname>proto::plus</classname>&lt;X, Y&gt;, proto::pass_through&lt; <classname>proto::plus</classname>&lt;X, Y&gt; &gt; &gt;
  61                 </computeroutput>
  62               </listitem>
  63               <listitem>
  64                 <computeroutput>
  65                   <classname>proto::when</classname>&lt; <classname>proto::plus</classname>&lt;X, Y&gt;, <classname>proto::plus</classname>&lt;X, Y&gt; &gt;
  66                 </computeroutput>
  67               </listitem>
  68               <listitem>
  69                 <computeroutput>
  70                   <classname>proto::when</classname>&lt; <classname>proto::plus</classname>&lt;X, Y&gt; &gt; // because of proto::when&lt;class X, class Y=X&gt;
  71                 </computeroutput>
  72               </listitem>
  73               <listitem>
  74                 <computeroutput>
  75                   <classname>proto::plus</classname>&lt;X, Y&gt;  // because plus&lt;&gt; is both a grammar and a transform
  76                 </computeroutput>
  77               </listitem>
  78             </itemizedlist>
  79           </para>
  80           <para>
  81             For example, consider the following transform that promotes all
  82             <computeroutput>float</computeroutput> terminals in an expression to
  83             <computeroutput>double</computeroutput>.
  84             <programlisting>// This transform finds all float terminals in an expression and promotes
  85 // them to doubles.
  86 struct Promote :
  87   <classname>proto::or_</classname>&lt;
  88     <classname>proto::when</classname>&lt;<classname>proto::terminal</classname>&lt;float&gt;, <classname>proto::terminal</classname>&lt;double&gt;::type(<classname>proto::_value</classname>) &gt;,
  89     // terminal&lt;&gt;'s default transform is a no-op:
  90     <classname>proto::terminal</classname>&lt;<classname>proto::_</classname>&gt;,
  91     // nary_expr&lt;&gt; has a pass_through&lt;&gt; transform:
  92     <classname>proto::nary_expr</classname>&lt;<classname>proto::_</classname>, <classname>proto::vararg</classname>&lt;Promote&gt; &gt;
  93   &gt;
  94 {};</programlisting>
  95           </para>
  96         </description>
  97         <struct name="impl">
  98           <template>
  99             <template-type-parameter name="Expr"/>
 100             <template-type-parameter name="State"/>
 101             <template-type-parameter name="Data"/>
 102           </template>
 103           <inherit><type><classname>proto::transform_impl</classname>&lt;Expr, State, Data&gt;</type></inherit>
 104           <typedef name="GN">
 105             <purpose>For each N in [0,Expr arity), for exposition only</purpose>
 106             <type>typename proto::result_of::child_c&lt;Grammar, N&gt;::type</type>
 107           </typedef>
 108           <typedef name="EN">
 109             <purpose>For each N in [0,Expr arity), for exposition only</purpose>
 110             <type>typename proto::result_of::child_c&lt;Expr, N&gt;::type</type>
 111           </typedef>
 112           <typedef name="RN">
 113             <purpose>For each N in [0,Expr arity), for exposition only</purpose>
 114             <type>typename boost::result_of&lt;GN(EN,State,Data)&gt;::type</type>
 115           </typedef>
 116           <typedef name="T">
 117             <purpose>For exposition only</purpose>
 118             <type>typename Expr::proto_tag</type>
 119           </typedef>
 120           <typedef name="Deduce">
 121             <purpose>For exposition only</purpose>
 122             <type>boost::is_same&lt;Domain, <classname>deduce_domain</classname>&gt;</type>
 123           </typedef>
 124           <typedef name="DD">
 125             <purpose>For exposition only</purpose>
 126             <type>typename Expr::proto_domain</type>
 127           </typedef>
 128           <typedef name="D">
 129             <purpose>For exposition only</purpose>
 130             <type>typename mpl::if_&lt;Deduce, DD, Domain&gt;::type</type>
 131           </typedef>
 132           <typedef name="G">
 133             <purpose>For exposition only</purpose>
 134             <type>typename D::proto_generator</type>
 135           </typedef>
 136           <typedef name="A">
 137             <purpose>For exposition only</purpose>
 138             <type><classname>proto::listN</classname>&lt;R0,...RN&gt;</type>
 139           </typedef>
 140           <typedef name="E">
 141             <purpose>For exposition only</purpose>
 142             <type><classname>proto::expr</classname>&lt;T, A&gt;</type>
 143           </typedef>
 144           <typedef name="BE">
 145             <purpose>For exposition only</purpose>
 146             <type><classname>proto::basic_expr</classname>&lt;T, A&gt;</type>
 147           </typedef>
 148           <typedef name="expr_type">
 149             <purpose>For exposition only</purpose>
 150             <type>typename mpl::if_&lt;<classname>proto::wants_basic_expr</classname>&lt;G&gt;, BE, E&gt;::type</type>
 151           </typedef>
 152           <typedef name="result_type">
 153             <type>typename boost::result_of&lt;D(expr_type)&gt;::type</type>
 154           </typedef>
 155           <method-group name="public member functions">
 156             <method name="operator()" cv="const">
 157               <type>result_type</type>
 158               <parameter name="expr">
 159                 <paramtype>typename impl::expr_param</paramtype>
 160               </parameter>
 161               <parameter name="state">
 162                 <paramtype>typename impl::state_param</paramtype>
 163               </parameter>
 164               <parameter name="data">
 165                 <paramtype>typename impl::data_param</paramtype>
 166               </parameter>
 167               <requires>
 168                 <para>
 169                   <computeroutput>
 170                     <classname>proto::matches</classname>&lt;Expr, Grammar&gt;::value
 171                   </computeroutput> is <computeroutput>true</computeroutput>.
 172                 </para>
 173               </requires>
 174               <returns>
 175                 <para>
 176                   <programlisting>D()(expr_type::make(
 177   G0()(<functionname>proto::child_c</functionname>&lt;0&gt;(expr), state, data),
 178   ...
 179   GN()(<functionname>proto::child_c</functionname>&lt;N&gt;(expr), state, data)
 180 ))</programlisting>
 181                 </para>
 182               </returns>
 183             </method>
 184           </method-group>
 185         </struct>
 186       </struct>
 187     </namespace>
 188   </namespace>
 189 </header>