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[ceph.git] / ceph / src / boost / libs / tti / doc / tti_detail_has_function.qbk

[/ 
  (C) Copyright Edward Diener 2012
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]

[section:tti_detail_has_function Introspecting an inner function]

The TTI macro [macroref BOOST_TTI_HAS_FUNCTION] introspects
an inner function of a class. The function can be either a member 
function or a static member function.

BOOST_TTI_HAS_FUNCTION takes a single
parameter which is the name of an inner function whose existence
the programmer wants to check. The macro generates a metafunction
called 'has_function_'name_of_inner_function'. 

The metafunction can be invoked by passing it the enclosing type to introspect and a 
signature for the function as separate template arguments. The signature for the 
function consists of a return type, optional parameter types in the form of a boost::mpl 
forward sequence of types, and an optional Boost FunctionTypes tag type. A 
typical boost::mpl forward sequence of types is a boost::mpl::vector<>.

The metafunction returns a single type called 'type', which is a 
boost::mpl::bool_. As a convenience the metafunction 
returns the value of this type directly as a compile time bool constant 
called 'value'. This is true or false depending on whether the inner 
function, of the specified signature, exists or not.

[heading Generating the metafunction]

You generate the metafunction by invoking the macro with the name 
of an inner function:

  BOOST_TTI_HAS_FUNCTION(AnInnerFunction)
  
generates a metafunction called 'has_function_AnInnerFunction' in the current scope.

[heading Invoking the metafunction]

You invoke the metafunction by instantiating the template with an enclosing 
type to introspect and the signature of the function as a series of template
parameters.

A return value called 'value' is a compile time bool constant.

  has_function_AnInnerFunction
    <
    Enclosing_Type,
    Function_ReturnType,
    boost::mpl::vector<Function_ParameterTypes>, // optional, can be any mpl forward sequence
    boost::function_types::SomeTagType           // optional, can be any FunctionTypes tag type
    >::value

[heading Examples]

First we generate metafunctions for various inner function names: 

 #include <boost/tti/has_function.hpp>
 
 BOOST_TTI_HAS_FUNCTION(function1)
 BOOST_TTI_HAS_FUNCTION(function2)
 BOOST_TTI_HAS_FUNCTION(function3)
 
Next let us create some user-defined types we want to introspect. 

 struct AClass { };
 struct Top
   {
   static int function1();
   AClass function2(double,short *);
   };
 struct Top2
   {
   long function2(Top &,int,bool,short,float);
   static Top * function3(long,int,AClass &);
   };
   
Finally we invoke our metafunction and return our value.
This all happens at compile time, and can be used by 
programmers doing compile time template metaprogramming.
  
 has_function_function1<Top,int>::value; // true
 has_function_function1<Top2,int>::value; // false
 
 has_function_function2<Top,AClass,boost::mpl::vector<double,short *> >::value; // true
 has_function_function2<Top2,AClass,boost::mpl::vector<double,short *> >::value; // false
 
 has_function_function3<Top2,int>::value; // false
 has_function_function3<Top2,Top *,boost::mpl::vector<long,int,AClass &> >::value; // true;
   
[heading Metafunction re-use]

The macro encodes only the name of the function 
for which we are searching and the fact that we are 
introspecting for a function within an enclosing type.

Because of this, once we create our metafunction for 
introspecting a function by name, we can reuse the 
metafunction for introspecting any enclosing type, having any 
function, for that name.

[endsect]