third_party/boost/tti/doc/tti_detail_has_member_data.qbk

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  (C) Copyright Edward Diener 2011,2012
  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).
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[section:tti_detail_has_member_data Introspecting member data]

The TTI macro [macroref BOOST_TTI_HAS_MEMBER_DATA] introspects
member data of a class.

BOOST_TTI_HAS_MEMBER_DATA macro takes a single
parameter which is the name of an inner member data whose existence
the programmer wants to check. The macro generates a metafunction
called "has_member_data_'name_of_inner_member_data'". 

The metafunction can be invoked in two different ways.

The first way is by passing it two parameters. The first parameter
is the enclosing type to introspect and the second parameter is the
type of the member data.

The second way is by passing it a single parameter, which is a pointer
to member type. This type has the form of:

 MemberData_Type Enclosing_Type::*

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 value is true or false depending on whether the 
inner member data, of the specified type, exists or not.

[heading Generating the metafunction]

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

  BOOST_TTI_HAS_MEMBER_DATA(AMemberData)
  
generates a metafunction called 'has_member_data_AMemberData' in the current scope.

[heading Invoking the metafunction]

You invoke the metafunction by instantiating the template with an enclosing 
type to introspect and the type of the member data, or by instantiating the
template with a pointer to member data type. The return value called 'value' 
is a compile time bool constant telling you whether or not the member data .
exists.

  has_member_data_AMemberData<Enclosing_Type,MemberData_Type>::value
  
  OR
  
  has_member_data_AMemberData<MemberData_Type Enclosing_Type::*>::value
  
[heading Examples]

First we generate metafunctions for various inner member data names: 

 #include <boost/tti/has_member_data.hpp>
 
 BOOST_TTI_HAS_MEMBER_DATA(data1)
 BOOST_TTI_HAS_MEMBER_DATA(data2)
 BOOST_TTI_HAS_MEMBER_DATA(data3)
 
Next let us create some user-defined types we want to introspect. 
 
 struct AClass 
   { 
   };
 struct Top
   {
   int data1;
   AClass * data2;
   };
 struct Top2
   {
   long data1;
   Top data3;
   };
   
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.
  
We will show both forms in the following examples. 
Both forms are completely interchangeable as to the result
desired.

 has_member_data_data1<Top,int>::value; // true
 has_member_data_data1<Top,long>::value; // false
 
 has_member_data_data1<Top2,int>::value; // false
 has_member_data_data1<long Top2::*>::value; // true
 
 has_member_data_data2<AClass * Top::*>::value; // true
 has_member_data_data2<Top,int *>::value; // false
 
 has_member_data_data3<int Top2::*>::value; // false
 has_member_data_data3<Top Top2::*>::value; // true;
   
[heading Metafunction re-use]

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

Because of this, once we create our metafunction for 
introspecting an inner member data by name, we can reuse the 
metafunction for introspecting any enclosing type, having any 
inner member data type, for that name.

[endsect]