std::function with non-static member functions

I think the problem you are having is that a member function requires not only a function pointer, but a pointer to the calling object. In other words, member functions have an additional implicit argument that is the pointer to the calling object.

To set a member function to a std::function, you need to use std::bind like this:

std::function<void(int)> testFunc(std::bind(&A::func, this, _1));

This binds the this pointer of the current A instance to the function so it has the function pointer and the object instance, which is enough information to properly call the function. The _1 argument indicates that the first explicit argument will be provided when the function is called.

With c++11 you can also use lambdas which are slightly easier to read than std::bind:

index[WM_CREATE] = [this](HWND h, UINT u, WPARAM w, LPARAM l)
{
  create(h, u, w, l);
}