Cast lambda to std::function with parameter pack

Question

There are several questions on SO that relate to casting lambdas to std::functions, but I have yet to see one that uses a parameter pack for the argument list.

Answer 1

It is very rarely a good idea to cast a lambda to a std::function in a template if you are just going to call it. std::function is type-erasure, and templated type erasure only makes sense if you are going to "pass it on" somewhere else and/or return it.

In any case, try this:

template 
void Functor(std::function f) {}

int main(int argc, char * argv[]) {
  auto x = [] (int a, int b) { return a * b; };
  Functor(x);
  return 0;
}

but you should really just do

template 
void Functor(F f) {}

which is perfectly type-safe.

If you want early type checking, you could write

template
struct signature_compatible;
template
struct signature_compatible :
  std::is_consructible< R, std::result_of_t>
{};

then do

template 
void Functor(F f) {
  static_assert( signature_compatible::value, "bad signature" );
}

but only if you really need to.