c++ - Check if class has function with signature

Question

There are other answers on this site using SFINAE but with non C++11 code, and there are others using C++11 code like decltypes to make this process easier. However, I am not sure how to check if a class has a function with a specific signature.

I want to check if a class has the function receive(const Event &) where Event is a class type that is specified when calling the check function.

Answer 1

The best way I know of is checking if you can actually call the function and if it returns the type you expect. Here's an example of how to detect if a class C has a receive method which takes const Event& as a parameter and "returns" void. The detection does not care whether the method is implemented in the class C directly or in some base class that C derives from, neither does it care whether there are further defaulted parameters. Adapt as needed.

template< typename C, typename Event, typename = void >
struct has_receive
  : std::false_type
{};

template< typename C, typename Event >
struct has_receive< C, Event, typename std::enable_if<
    std::is_same<
        decltype( std::declval<C>().receive( std::declval<const Event&>() ) ),
        void
    >::value
>::type >
  : std::true_type
{};