...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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 <class> struct AMemberTemplate { }; template <class,class,class> struct AnotherMemberTemplate { }; template <class,class,int,class,template <class> class,class,long> struct ManyParameters { }; template <class,class,int,short,class,template <class,int> 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<class X, class Y, int Z> int MyFunctionTemplate(X *, Y &) { return Z; } // Const Function Template template<class X, int Y, class Z> 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<class A, class B> 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 <boost/tti/tti.hpp> template<class T> 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.
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 >
Does T have a nested typedef called 'AnIntType' whose type is an 'int' ?
#include <boost/mpl/placeholders.hpp #include <boost/type_traits/is_same.hpp using namespace boost::mpl::placeholders; has_type_AnIntType < T, boost::is_same<_1,int> >
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.
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.
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.
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 >
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 >
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,int> class,class))) has_template_MoreParameters < T >
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,class,long) has_template_ManyParameters < T >
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 >
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::* >
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<short> >
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) >
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<bool,double>, boost::function_types::const_qualified >
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 >
Does T have a member function template called 'MyFunctionTemplate' woth an instantiated signature of 'int MyFunctionTemplate<short,float,579>(short *,float &)' ?
BOOST_TTI_HAS_MEMBER_FUNCTION_TEMPLATE(MyFunctionTemplate,short,float,579) has_member_function_MyFunctionTemplate < T, int, boost::mpl::vector<short *,float &> >
Does T have a member function template called 'MyFunctionTemplate' woth an instantiated signature of 'int MyFunctionTemplate<int,long,183>(int *,long &)' ?
BOOST_TTI_HAS_MEMBER_FUNCTION_TEMPLATE(MyFunctionTemplate,int,long,183) has_member_function_template_MyFunctionTemplate < int (T::*)(int *,long &) >
Does T have a member function template called 'AnotherFunctionTemplate' with an instantiated signature of 'int AnotherFunctionTemplate<bool,8359,double>(bool &,double **) const' ?
BOOST_TTI_HAS_MEMBER_FUNCTION_TEMPLATE(AnotherFunctionTemplate,bool,8359,double) has_member_function_AnotherFunctionTemplate < T, int, boost::mpl::vector<bool &,double **>, boost::function_types::const_qualified >
Does T have a member function template called 'AnotherFunctionTemplate' with an instantiated signature of 'int AnotherFunctionTemplate<bool,8359,double>(bool &,double **) const' ?
BOOST_TTI_HAS_MEMBER_FUNCTION_TEMPLATE(AnotherFunctionTemplate,bool,8359,double) has_member_function_template_MyFunctionTemplate < int (T::*)(bool &,double **) const >
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 >
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<long,double> >
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) >
Does T have a static member function template called 'SFunctionTemplate' with an instantiated signature of 'long SFunctionTemplate<char,unsigned>(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<long &,char **,unsigned,float> >
Does T have a static member function template called 'SFunctionTemplate' with an instantiated signature of 'long SFunctionTemplate<bool,int>(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) >
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 >
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<short> >
Does T have a member function template or a static member function template called 'MyFunctionTemplate' with an instantiated signature of 'int MyFunctionTemplate<bool,short,3487>(bool *,short &)' ?
BOOST_TTI_HAS_FUNCTION_TEMPLATE(MyFunctionTemplate,bool,short,3487) has_function_template_MyFunctionTemplate < T, int, boost::mpl::vector<bool *,short &> >
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
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 >