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<T>(0)) == sizeof(yes_tag)
        ); }; 

template<class T, bool has_foo = has_member_foo<T>::value> 
struct C {};

Answer 2

Another (C++03) approach:

template<typename T>
struct has_foo
{
private:
    typedef char no;
    struct yes { no m[2]; };

    static T* make();
    template<typename U>
    static yes check(U*, typename U::foo* = 0);
    static no check(...);

public:
    static bool const value = sizeof(check(make())) == sizeof(yes);
};

struct A
{
    typedef int foo;
};

struct B { };

template<typename T, bool HasFooB = has_foo<T>::value>
struct C
{
    // T has foo
};

template<typename T>
struct C<T, false>
{
    // T has no foo
};