c++ - How to determine if a class contains a subclass / type? -

can have sfinae trick know, if class has subclass/type. like,

template<typename type> // searches "my_type" struct has_inner_type {   enum { value = <???> }; }; 

following examples:

struct {   class my_type {};  // has_inner_type::value = true  }; struct b { }; // has_inner_type::value = false struct c { typedef int my_type; }; // has_inner_type::value = true 

i tried few tricks, falling short expected compiler errors. usage:

bool b = has_inner_type<a>::value;  // respect "my_type" 

edit: have re-edited question, seems it's impossible pass my_type second parameter has_inner_type. so, of question find specific type my_type. have tried code, doesn't work.

following answer present in wikipedia link posted in question !! (thanks @n.m.)

template <typename t>  struct has_inner_type {   typedef char yes[1];   typedef char no[2];    template <typename c> static yes& test(typename c::my_type*);   template <typename> static no& test(...);    static const bool value = sizeof(test<t>(0)) == sizeof(yes); }; 

here the demo.