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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_static_member_function_template Introspecting static member function template]

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

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

  struct AnotherType
    {
    template<class X,int Y> static int AStaticFuncTemplate(X x) { ...some code using x; return Y; }
    };

In order to introspect the function template we use some theoretical valid instantiation of
the static member function template `AStaticFuncTemplate`. 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:

  int AStaticFuncTemplate<char,483>(char)

What we have now which the TTI will need in order to introspect the static member
function template `template<class X,int Y> static int AStaticFuncTemplate(X)`
within the `AnotherType` struct is:

* The name of `AStaticFuncTemplate`
* The template parameters of `char,483`
* The enclosing type of `AnotherType`
* The return type of `int`
* The function parameters of `char`

[heading Generating the metafunction]

As with all TTI functionality for introspecting entities within a user-defined
type introspecting a static member function template is a two step process. The first
process is using a macro to generate a metafunction. The macro for
static member function templates is [macroref BOOST_TTI_HAS_STATIC_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_STATIC_MEMBER_FUNCTION_TEMPLATE(AStaticFuncTemplate,char,483)

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_STATIC_MEMBER_FUNCTION_TEMPLATE(AStaticFuncTemplate,(2,(char,483)))

The macro generates a metafunction based on the pattern of
"has_static_member_function_template_'name_of_inner_static_member_function_template'",
which in our example case would be `has_static_member_function_template_AStaticFuncTemplate`.

[heading Invoking the metafunction]

To use this macro to test whether our static 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 static 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 instantiated function parameters
being enclosed in an MPL forward sequence, or we can compose our needed type
in the form of an enclosing type and a function type. In the first case we would have:

  has_static_member_function_template_AStaticFuncTemplate<AnotherType,int,boost::mpl::vector<char> >::value

and in the second case we would have:

  has_static_member_function_template_AStaticFuncTemplate<AnotherType,int (char) >::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_STATIC_MEMBER_FUNCTION_TEMPLATE,
where the first parameter is the name of the metafunction to be generated,
the second parameter is the static member function template name, and the remaining parameters
are the instantiated template parameters. For our example we could have

  BOOST_TTI_TRAIT_HAS_STATIC_MEMBER_FUNCTION_TEMPLATE(AMetafunctionName,AStaticFuncTemplate,char,483)

or for the non-variadic macro form

  BOOST_TTI_TRAIT_HAS_STATIC_MEMBER_FUNCTION_TEMPLATE(AMetafunctionName,AStaticFuncTemplate,(2,(char,483)))

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 static 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_STATIC_MEMBER_FUNCTION_TEMPLATE_GEN` as in

  BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION_TEMPLATE_GEN(AStaticFuncTemplate)

which would generate the name `has_static_member_function_template_AStaticFuncTemplate`.

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. This optional template argument is of much less use for static member
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 static member function template needs to be
specified.

When invoking the metafunction using the shorter form of an enclosing type and a
function type any possibly needed qualification can be added to the end of the
function type.

[endsect]