c++ - template overloading and SFINAE working only with functions but not classes

Question

can someone explain why the compiler accepts only this code

template<typename L, size_t offset, typename enable_if< (offset<sizeof(L)), int >::type =0>
void a_function(){}

template<typename L, size_t offset, typename enable_if< (offset==sizeof(L)), int >::type =0>
void a_function(){}

but not this:

template<typename L, size_t offset, typename enable_if< (offset<sizeof(L)), int >::type =0>
class a_class{};

template<typename L, size_t offset, typename enable_if< (offset==sizeof(L)), int >::type =0>
class a_class{};

The compiler sees the second class template as a redefinition of the first.

Answer 1

You have to use specialization for classes. Typically, it is done with an extra parameter:

template <class P, class dummy = void>
class T;

template <class P>
class T<P, typename enable_if<something, void>::type> {
   the real thing
};

Two class (or class template) declarations with the same name should always declare the same class or class template (or be a specialization, in which case it is still the same template).