C-callback to function template: explicitly instantiate template

Question

Premise

I’m using a C library (from C++) which provides the following interface:

void register_callback(void* f, void* data);
void invoke_callback(         


        

Answer 1

This should work:

template 
void do_register_callback(T& value) {
   void (*callback)(void*) = my_callback;
   register_callback(reinterpret_cast(callback), &value);
}

The first line forces the compiler to instantiate that function to generate the address - which you can then happily pass.

EDIT: Let me throw another option in to this mix. Make my_callback a static member of a class template - something like the following:

template 
struct foo
{
static void my_callback(void* data) {
    T& x = *static_cast(data);
    std:: cout << "Call[T] with " << x << std::endl;
}
};

Now, in your register guy, you don't even need the "cast".

template 
void do_register_callback(T& value) {
   register_callback(reinterpret_cast(&foo::my_callback), &value);
}

It appears that the rule for instantiating class templates is different to function templates - i.e. to take the address of a member of a class, the type is instantiated.