How to check if template argument is a callable with a given signature

Question

Basically, what I want to achieve is compile-time verification (with possibly nice error message) that registered callable (either a function, a lambda, a struct with call o

Answer 1

If you accept to transform A in a variadic template class, you can use decltype(), to activare Register only if callable is compatible, as follows

template 
struct A
 {
   using Signature = R(Args...);

   template 
   auto Register (Callable && callable)
      -> decltype( callable(std::declval()...), void() )
    { callback = callable; }

   std::function callback;
 };

This way, if you prefer, calling Register() with a incompatible function, you can obtain a soft error and activate another Register() function

void Register (...)
 { /* do something else */ };