Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for a snapshot of the develop branch, built from commit 3bdf7dba54.

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 "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:

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 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:


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:


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.

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.

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


or for the non-variadic macro form


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


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.