third_party/boost/tti/doc/tti_detail_has_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_function_template Introspecting function template]

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

A function template of a user-defined type can either be a member function
template or a static member function template. An example 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; }
    template<class X,int Y> static int AFuncTemplate(X x) 
      { ...some code using x; return Y; }
    };
    
A function template `AFuncTemplate` is a member function template of the `AType`
user-defined type and a different function template also called `AFuncTemplate`
is a static member function template of the `AType` user-defined type.

In order to introspect either function template we use some theoretical valid
instantiations of `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<char,bool,int>(char,bool *,int &)
  
What we have now which the TTI will need in order to introspect the 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 `char,bool,int`
* The enclosing type of `AType`
* The return type of `double`
* The function parameters of `char,bool *,int &`

[heading Generating the metafunction]

As with all TTI functionality for introspecting entities within a user-defined
type introspecting a function template is a two step process. The first
process is using a macro to generate a metafunction. The macro for
function templates is [macroref BOOST_TTI_HAS_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_FUNCTION_TEMPLATE(AFuncTemplate,char,bool,int)
  
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_FUNCTION_TEMPLATE(AFuncTemplate,(3,(char,bool,int)))
  
The macro generates a metafunction based on the pattern of
"has_function_template_'name_of_inner_function_template'",
which in our example case would be `has_function_template_AFuncTemplate`.

[heading Invoking the metafunction]

To use this macro to test whether our 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 function template exists,
or `false` otherwise. We use each of our needed types as separate parameters, with the
function parameters being enclosed in an MPL forward sequence. We would have:

  has_function_template_AFuncTemplate<AType,double,boost::mpl::vector<char,bool *,int &> >::value

[heading Introspecting the other function template]

If we chose to try to introspect the second `AFuncTemplate` within `AType` we might
choose an instantiation of:

  int AFuncTemplate<long,7435>(long)
  
Our generation of the metafunction would then be:

  BOOST_TTI_HAS_FUNCTION_TEMPLATE(AFuncTemplate,long,7435)
  
or

  BOOST_TTI_HAS_FUNCTION_TEMPLATE(AFuncTemplate,(2,(long,7435)))
  
and our invocation of the metafunction would now be:
  
  has_function_template_AFuncTemplate<AType,int,boost::mpl::vector<long> >::value
  
[heading Other considerations]

In our two examples above we could not introspect both function templates
in the same namespace using the BOOST_TTI_HAS_FUNCTION_TEMPLATE macro as we would
be generating two metafunctions with the same name, which would be
`has_function_template_AFuncTemplate`, thus violating the One Definition Rule.
The solution to this is the use of the complex macro form.

The macro for generating the metafunction for introspecting 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_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 first example we could have

  BOOST_TTI_TRAIT_HAS_FUNCTION_TEMPLATE(FirstMetafunction,char,bool,int)
  
or for the non-variadic macro form

  BOOST_TTI_TRAIT_HAS_FUNCTION_TEMPLATE(FirstMetafunction,(3,(char,bool,int)))
  
which generates a metafunction whose name would be `FirstMetafunction`.
  
For our second example we could have

  BOOST_TTI_TRAIT_HAS_FUNCTION_TEMPLATE(SecondMetafunction,AFuncTemplate,long,7435)
  
or for the non-variadic macro form

  BOOST_TTI_TRAIT_HAS_FUNCTION_TEMPLATE(SecondMetafunction,AFuncTemplate,(2,(long,7435)))
  
which generates a metafunction whose name would be `SecondMetafunction`.
  
In all other respects the resulting metafunctions 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 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_FUNCTION_TEMPLATE_GEN` as in

  BOOST_TTI_HAS_FUNCTION_TEMPLATE_GEN(AFuncTemplate)
  
which would generate the name `has_function_template_AFuncTemplate`.

When invoking the appropriate metafunction a fourth
template argument may optionally be given which holds a Boost FunctionTypes tag
type. This optional template argument is of much less use for 
function templates than for non-static member function templates since static
member function templates, like static member functions, can not have
cv-qualifications. which a number of Boost FunctionTypes tags provide.
Nonetheless this optional Boost FunctionTypes tag is available for
end-user use and may come in handy in certain rare cases, as when some calling
convention qualification for the function template needs to be
specified. If you do use a Boost FunctionTypes tag type for cv-qualification,
such as `boost::function_types::const_qualified` to look for a function template
it will be applied when introspecting for the member function template side of
the match, but will ensure that introspecting for the static member function 
template side of the match will always fail. In either case no compiler error
will be generated.

[endsect]