How to detect existence of a class using SFINAE?

Question

Is it possible to detect if a class exists in C++ using SFINAE? If possible then how?

Suppose we have a class that is provided only by some versions of a library. I\

Answer 1

Still did not find a satisfying answer in this post...

Mike Kinghan started the answer right and told a smart thing:

So the problem is to determine whether T is a defined class type.

But

sizeof(T) is no help here

is not correct...

Here is how you can do it with sizeof(T):

template 
struct is_defined
{
    static constexpr bool value = false;
};

template 
struct is_defined 0)>>
{
    static constexpr bool value = true;
};