[/ (C) Copyright Edward Diener 2011,2012,2020 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). ] [section:tti_usingMM An example using the macro metafunctions] [#sectti_usingMM] Using the macro metafunctions can be illustrated by first creating some hypothetical user-defined type with corresponding nested types and other inner elements. With this type we can illustrate the use of the macro metafunctions. This is just meant to serve as a model for what a type T might entail from within a class or function template where 'T' is a type passed to the template. // An enclosing type, this could also be a class or a union struct AType { // Type typedef int AnIntType; // as a typedef // Class/Struct struct BType // as a nested struct { struct CType { }; }; class AClass // as a nested class { }; // Enumeration enum EColor // as a nested enumeration { Red, Blue, Green }; // Union union AUnion // as a nested union { int i; long l; }; // Class Template template struct AMemberTemplate { }; template struct AnotherMemberTemplate { }; template class,class,long> struct ManyParameters { }; template class,class> struct MoreParameters { }; // Data BType IntBT; // Function int IntFunction(short) { return 0; } // Const Function long AnotherFunction(bool,double) const { return 0L; } // Function Template template int MyFunctionTemplate(X *, Y &) { return Z; } // Const Function Template template int AnotherFunctionTemplate(X &, Z **) const { return Y; } // Static Data static short DSMember; // Static Function static int SIntFunction(long,double) { return 2; } // Static Function Template template static long SFunctionTemplate(long &,A **,B,float) { return 20L; } }; I will be using the type above just to illustrate the sort of metaprogramming questions we can ask of some type T which is passed to the template programmer in a class template. Here is what the class template might look like: #include template struct OurTemplateClass { // compile-time template code regarding T }; Now let us create and invoke the macro metafunctions for each of our inner element types, to see if type T above corresponds to our hypothetical type above. Imagine this being within 'OurTemplateClass' above. In the examples below the same macro is invoked just once to avoid ODR violations. Also in these examples we are showing results which return the compile time boolean result of 'true' to illustrate how our macro metafunctions work. We could just as well create examples which return the compile time boolean result of 'false' if we introspect for constructs that do not exist in our hypothetical type of T. We can validly return 'true' or 'false' without generating compiler errors as long as the types using in our macro metafunction invocations exist at the time we invoke the metafunction. [heading Type] C++ named types can be typedefs or type aliases, class, structs, unions, or enumerations Does T have a nested type called 'AnIntType' ? BOOST_TTI_HAS_TYPE(AnIntType) has_type_AnIntType < T > Does T have a nested type called 'BType' ? BOOST_TTI_HAS_TYPE(BType) has_type_BType < T > Does T have a nested type called 'AClass' ? BOOST_TTI_HAS_TYPE(AClass) has_type_ACLass < T > Does T have a nested type called 'EColor' ? BOOST_TTI_HAS_TYPE(EColor) has_type_EColor < T > Does T have a nested type called 'AUnion' ? BOOST_TTI_HAS_TYPE(AUnion) has_type_AUnion < T > [heading Type checking the typedef using an MPL lambda expression] Does T have a nested typedef called 'AnIntType' whose type is an 'int' ? #include > [heading Class/Struct] Does T have a nested class called 'AClass' ? BOOST_TTI_HAS_CLASS(AClass) has_class_AClass < T > Does T have a nested struct called 'BType' ? BOOST_TTI_HAS_CLASS(BType) has_class_BType < T > You can also use an MPL lambda expression with a class/struct just as you can with a general type. [heading Enumeration] Does T have a nested enumeration called 'EColor' ? BOOST_TTI_HAS_ENUM(EColor) has_enum_EColor < T > You can also use an MPL lambda expression with an enumeration just as you can with a general type. [heading Union] Does T have a nested union called 'AUnion' ? BOOST_TTI_HAS_UNION(AUnion) has_union_AUnion < T > You can also use an MPL lambda expression with a union just as you can with a general type. [heading Template] Does T have a nested class template called 'AMemberTemplate' whose template parameters are all types ('class' or 'typename') ? BOOST_TTI_HAS_TEMPLATE(AMemberTemplate,BOOST_PP_NIL) has_template_AMemberTemplate < T > [heading Template using variadic macros] Does T have a nested class template called 'AMemberTemplate' whose template parameters are all types ('class' or 'typename') ? BOOST_TTI_HAS_TEMPLATE(AnotherMemberTemplate) has_template_AnotherMemberTemplate < T > [heading Template with params] Does T have a nested class template called 'MoreParameters' whose template parameters are specified exactly ? BOOST_TTI_HAS_TEMPLATE(MoreParameters,(8,(class,class,int,short,class,template class,class))) has_template_MoreParameters < T > [heading Template with params using variadic macros] Does T have a nested class template called 'ManyParameters' whose template parameters are specified exactly ? BOOST_TTI_HAS_TEMPLATE(ManyParameters,class,class,int,class,template class,class,long) has_template_ManyParameters < T > [heading Member data] Does T have a member data called 'IntBT' whose type is 'AType::BType' ? BOOST_TTI_HAS_MEMBER_DATA(IntBT) has_member_data_IntBT < T, AType::BType > [heading Member data with composite type] Does T have a member data called 'IntBT' whose type is 'AType::BType' ? BOOST_TTI_HAS_MEMBER_DATA(IntBT) has_member_data_IntBT < AType::BType T::* > [heading Member function with individual types] Does T have a member function called 'IntFunction' whose type is 'int (short)' ? BOOST_TTI_HAS_MEMBER_FUNCTION(IntFunction) has_member_function_IntFunction < T, int, boost::mpl::vector > [heading Member function with composite type] Does T have a member function called 'IntFunction' whose type is 'int (short)' ? BOOST_TTI_HAS_MEMBER_FUNCTION(IntFunction) has_member_function_IntFunction < int (T::*) (short) > [heading Const member function with individual types] Does T have a member function called 'AnotherFunction' whose type is 'long (bool,double) const' ? BOOST_TTI_HAS_MEMBER_FUNCTION(AnotherFunction) has_member_function_AnotherFunction < T, long, boost::mpl::vector, boost::function_types::const_qualified > [heading Const member function with composite type] Does T have a member function called 'AnotherFunction' whose type is 'long (bool,double) const' ? BOOST_TTI_HAS_MEMBER_FUNCTION(AnotherFunction) has_member_function_AnotherFunction < long (T::*) (bool,double) const > [heading Member function template with individual types] Does T have a member function template called 'MyFunctionTemplate' woth an instantiated signature of 'int MyFunctionTemplate(short *,float &)' ? BOOST_TTI_HAS_MEMBER_FUNCTION_TEMPLATE(MyFunctionTemplate,short,float,579) has_member_function_MyFunctionTemplate < T, int, boost::mpl::vector > [heading Member function template with composite type] Does T have a member function template called 'MyFunctionTemplate' woth an instantiated signature of 'int MyFunctionTemplate(int *,long &)' ? BOOST_TTI_HAS_MEMBER_FUNCTION_TEMPLATE(MyFunctionTemplate,int,long,183) has_member_function_template_MyFunctionTemplate < int (T::*)(int *,long &) > [heading Const member function template with individual types] Does T have a member function template called 'AnotherFunctionTemplate' with an instantiated signature of 'int AnotherFunctionTemplate(bool &,double **) const' ? BOOST_TTI_HAS_MEMBER_FUNCTION_TEMPLATE(AnotherFunctionTemplate,bool,8359,double) has_member_function_AnotherFunctionTemplate < T, int, boost::mpl::vector, boost::function_types::const_qualified > [heading Const member function template with composite type] Does T have a member function template called 'AnotherFunctionTemplate' with an instantiated signature of 'int AnotherFunctionTemplate(bool &,double **) const' ? BOOST_TTI_HAS_MEMBER_FUNCTION_TEMPLATE(AnotherFunctionTemplate,bool,8359,double) has_member_function_template_MyFunctionTemplate < int (T::*)(bool &,double **) const > [heading Static member data] Does T have a static member data called 'DSMember' whose type is 'short' ? BOOST_TTI_HAS_STATIC_MEMBER_DATA(DSMember) has_static_member_data_DSMember < T, short > [heading Static member function with individual types] Does T have a static member function called 'SIntFunction' whose type is 'int (long,double)' ? BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION(SIntFunction) has_static_member_function_SIntFunction < T, int, boost::mpl::vector > [heading Static member function with composite type] Does T have a static member function called 'SIntFunction' whose type is 'int (long,double)' ? BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION(SIntFunction) has_static_member_function_SIntFunction < T, int (long,double) > [heading Static member function template with individual types] Does T have a static member function template called 'SFunctionTemplate' with an instantiated signature of 'long SFunctionTemplate(long &,char **,unsigned,float)' ? BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION_TEMPLATE(SFunctionTemplate,char,unsigned) has_static_member_function_template_SFunctionTemplate < T, long, boost::mpl::vector > [heading Static member function template with composite type] Does T have a static member function template called 'SFunctionTemplate' with an instantiated signature of 'long SFunctionTemplate(long &,bool **,int,float)' ? BOOST_TTI_HAS_STATIC_MEMBER_FUNCTION_TEMPLATE(SFunctionTemplate,bool,int) has_static_member_function_template_SFunctionTemplate < T, long (long &,bool **,int,float) > [heading Data] Does T have a member data or static member data called 'DSMember' whose type is 'short' ? BOOST_TTI_HAS_DATA(DSMember) has_static_member_data_DSMember < T, short > [heading Function] Does T have a member function or a static member function called 'IntFunction' whose type is 'int (short)' ? BOOST_TTI_HAS_FUNCTION(IntFunction) has_function_IntFunction < T, int, boost::mpl::vector > [heading Function Template] Does T have a member function template or a static member function template called 'MyFunctionTemplate' with an instantiated signature of 'int MyFunctionTemplate(bool *,short &)' ? BOOST_TTI_HAS_FUNCTION_TEMPLATE(MyFunctionTemplate,bool,short,3487) has_function_template_MyFunctionTemplate < T, int, boost::mpl::vector > [heading Member type] Create a nested type T::BType::CType without creating a compiler error if T does not have the nested type BType::CType ? BOOST_TTI_MEMBER_TYPE(BType) BOOST_TTI_MEMBER_TYPE(CType) typename member_type_CType < typename member_type_BType < T >::type >::type [heading Member type existence] Does a nested type T::BType::CType, created without creating a compiler error if T does not have the nested type BType::CType, actually exist ? BOOST_TTI_MEMBER_TYPE(BType) BOOST_TTI_MEMBER_TYPE(CType) typedef typename member_type_CType < typename member_type_BType < T >::type >::type AType; boost::tti::valid_member_type < AType > [endsect]