third_party/boost/tti/doc/tti_detail_has_member_function_template.qbk

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  (C) Copyright Edward Diener 2020
  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_function_template Introspecting member function template]

We can introspect a member function template of a user-defined type
using the TTI functionality we shall now explain.

A member function template is a function template that is a non-static memmber
of a user-defined type. An example of a member function template would be:

  struct AType 
    {
    template<class X,class Y,class Z> double AFuncTemplate(X x,Y * y,Z & z) 
      { ...some code using x,y,z; return 0.0; }
    };
    
In order to introspect the function template we use some theoretical valid instantiation of
the member function template `AFuncTemplate`. An instantiation of a function template
was previously explained in the topic [link sectti_function_templates "Introspecting function templates technique"].

For the purposes of illustration the instantiation we will use is:

  double AFuncTemplate<int,long,bool>(int,long *,bool &)

What we have now which the TTI will need in order to introspect the member function
template `template<class X,class Y,class Z> double AFuncTemplate(X,Y *,Z &)`
within the `AType` struct is:

* The name of `AFuncTemplate`
* The template parameters of `int,long,bool`
* The enclosing type of `AType`
* The return type of `double`
* The function parameters of `int,long *,bool &`

[heading Generating the metafunction]

As with all TTI functionality for introspecting entities within a user-defined
type introspecting a member function template is a two step process. The first
process is using a macro to generate a metafunction. The macro for
member function templates is [macroref BOOST_TTI_HAS_MEMBER_FUNCTION_TEMPLATE].
This macro takes the name of the member function template and the instantiated 
template parameters, the first two items in our list above:

  BOOST_TTI_HAS_MEMBER_FUNCTION_TEMPLATE(AFuncTemplate,int,long,bool)
  
An alternative form for compilers which do not support variadic macros, and which will
also work with compilers which do support variadic macros, is to specify
the template parameters of the instantiation as a single macro argument using a 
Boost PP array:
  
  BOOST_TTI_HAS_MEMBER_FUNCTION_TEMPLATE(AFuncTemplate,(3,(int,long,bool)))
  
The macro generates a metafunction based on the pattern of
"has_member_function_template_'name_of_inner_member_function_template'",
which in our example case would be `has_member_function_template_AFuncTemplate`.

[heading Invoking the metafunction]

To use this macro to test whether our member function template exists
the metafunction the macro creates is invoked with the enclosing type, the instantiated return type,
and the instantiated function parameters, with the resulting `value` being a compile time
boolean constant which is `true` if the member function template exists,
or `false` otherwise. There are two ways to do this. We can either
use each of our needed types as separate parameters, with the function parameters
being enclosed in an MPL forward sequence, or we can compose our needed type
in the form of a pointer to member function type. In the first case we would have:

  has_member_function_template_AFuncTemplate<AType,double,boost::mpl::vector<int,long *,bool &> >::value
  
and in the second case we would have:

  has_member_function_template_AFuncTemplate<double (AType::*) (int,long *,bool &)>::value
  
Both invocations are equivalent in functionality.

[heading Other considerations]

The macro for generating the metafunction for introspecting member function templates
also has, like other macros in the TTI library, a complex macro form where the
end-user can directly specify the name of the metafunction to be generated. The
corresponding macro is BOOST_TTI_TRAIT_HAS_MEMBER_FUNCTION_TEMPLATE, 
where the first parameter is the name of the metafunction to be generated,
the second parameter is the member function template name, and the remaining parameters
are the instantiated template parameters. For our example we could have

  BOOST_TTI_TRAIT_HAS_MEMBER_FUNCTION_TEMPLATE(AMetafunctionName,AFuncTemplate,int,long,bool)
  
or for the non-variadic macro form

  BOOST_TTI_TRAIT_HAS_MEMBER_FUNCTION_TEMPLATE(AMetafunctionName,AFuncTemplate,(3,(int,long,bool)))
  
which generates a metafunction whose name would be `AMetafunctionName`.
  
In all other respects the resulting metafunction generated works exactly the same
as when using the simpler macro form previously illustrated.

If you do use the simple macro form, which generates the metafunction name
from the name of the function template you are introspecting, you can use
a corresponding macro, taking the name of the member function template as a single
parameter, to create the appropriate metafunction name if you do not want to
remember the pattern for generating the metafunction name. This macro name is
`BOOST_TTI_HAS_MEMBER_FUNCTION_TEMPLATE_GEN` as in

  BOOST_TTI_HAS_MEMBER_FUNCTION_TEMPLATE_GEN(AFuncTemplate)
  
which would generate the name `has_member_function_template_AFuncTemplate`.

When invoking the appropriate metafunction using the long form of an enclosing
type, instantiated return type, and instantiated function parameters, a fourth
template argument may optionally be given which holds a Boost FunctionTypes tag
type to specify cv-qualification. This means you can add 'const', 'volatile', or
both by specifying an appropriate tag type. An alternate to using the tag type 
is to specify the enclosing type as 'const', 'volatile', or both.
As an example if you specify the tag type as 
'boost::function_types::const_qualified' or if you specify the enclosing
type as 'const YourEnclosingType', the member function template which you are
introspecting must be a const function template to match correctly.

When invoking the metafunction using the shorter form of a pointer to member function
you can simply add a possible cv-qualification, such as `const`, to the end of the
pointer to member function syntax.

[endsect]