Specializing C++ template based on presence/absense of a class member?

Question

Consider the following:

struct A {
  typedef int foo;
};

struct B {};

template
struct C {};

I wa

Answer 1

Something like this might help: has_member.

typedef char (&no_tag)[1]; 
typedef char (&yes_tag)[2];

template< typename T > no_tag has_member_foo_helper(...);

template< typename T > yes_tag has_member_foo_helper(int, void (T::*)() = &T::foo);

template< typename T > struct has_member_foo {
    BOOST_STATIC_CONSTANT(bool
        , value = sizeof(has_member_foo_helper(0)) == sizeof(yes_tag)
        ); }; 

template::value> 
struct C {};