Template Metafunction Has Function

Useful with StaticReflection and StaticPolymorphism. The type has_function receives information about the function signature and stores this in a typedef. Function names are provided in a derived type. SubstitutionFailureIsNotAnError (SFINAE) performs the magic.

The following implementation supports up to 8 parameters - this may be extended as required.


        namespace framework {

        namespace meta

        {

        template <

        typename T, typename R,

        typename A0 = unspecified, 

        typename A1 = unspecified, 

        typename A2 = unspecified,

        typename A3 = unspecified,

        typename A4 = unspecified,

        typename A5 = unspecified,

        typename A6 = unspecified> 

        struct has_function

        {

        // 0-7 arg member function signature

        template <

        typename U,

        typename if_::value,

        typename if_::value,

        typename if_::value,

        typename if_::value,

        typename if_::value,

        typename if_::value,

        typename if_::value,

        /* 0 parameter(s) */ R(U::*)(),

        /* 1 parameter(s) */ R(U::*)(A0)>::value,

        /* 2 parameter(s) */ R(U::*)(A0, A1)>::value,

        /* 3 parameter(s) */ R(U::*)(A0, A1, A2)>::value,

        /* 4 parameter(s) */ R(U::*)(A0, A1, A2, A3)>::value,

        /* 5 parameter(s) */ R(U::*)(A0, A1, A2, A3, A4)>::value,

        /* 6 parameter(s) */ R(U::*)(A0, A1, A2, A3, A4, A5)>::value,

        /* 7 parameter(s) */ R(U::*)(A0, A1, A2, A3, A4, A5, A6)>::value

        > 

        struct signature;

        };


        #define FW_HAS_FUNCTION(name)\ 

        template < \ 

        typename T,\ 

        typename R,\ 

        typename A0 = framework::meta::unspecified,\ 

        typename A1 = framework::meta::unspecified,\ 

        typename A2 = framework::meta::unspecified,\ 

        typename A3 = framework::meta::unspecified,\ 

        typename A4 = framework::meta::unspecified,\ 

        typename A5 = framework::meta::unspecified,\ 

        typename A6 = framework::meta::unspecified \ 

        >\ 

        struct has_function_##name : framework::meta::has_function\ 

        {\ 

        template  static framework::meta::no check(...);\ 

        template  static framework::meta::yes check(signature*);\ 

        enum { value = sizeof(check(0)) == sizeof(framework::meta::yes) };\ 

        };

        } // namespace meta

        } // namespace framework

For an example, see StaticReflection.

See CeePlusPlusIdioms, TemplateMetaprogrammingTechniques, TemplateMetafunctionIdentity, TemplateMetafunctionAnd, TemplateMetafunctionIf.

CategoryCppTemplates CategoryMetaprogramming CategoryCpp