Check if class has function with signature

问题

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.

回答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
{};

回答2:

You may use the following to match exact signature:

template <typename U, typename Event>
class has_receive
{
private:
    template<typename T, T> struct helper;
    template<typename T>
    static std::uint8_t check(helper<void (T::*)(const Event &), &T::receive>*);
    template<typename T> static std::uint16_t check(...);
public:
    static
    constexpr bool value = sizeof(check<U>(0)) == sizeof(std::uint8_t);
};

回答3:

There is help with macros:

#define DEFINE_METHOD_CHECKER(RETURN_TYPE, METHOD_NAME, PARAMETERS)     \
template<typename T>                                                    \
struct Is ## METHOD_NAME ## MemberFunctionExists                        \
{                                                                       \
private:                                                                \
    typedef char True;                                                  \
    typedef char (&False)[2];                                           \
    template<typename U, RETURN_TYPE (U::*)PARAMETERS = &U::METHOD_NAME>\
    struct Checker                                                      \
    {                                                                   \
        typedef True Type;                                              \
    };                                                                  \
    template<typename U>                                                \
    static typename Checker<U>::Type Tester(const U*);                  \
    static False Tester(...);                                           \
public:                                                                 \
    enum { value = (sizeof(Tester(static_cast<const T*>(0))) == sizeof(True)) }; \
}

// IsMethodMemberFunctionExists<T>::value
DEFINE_METHOD_CHECKER(int, Method, (bool));
// IsTestMemberFunctionExists<T>::value
DEFINE_METHOD_CHECKER(int*, Test, (int&, char));

#include <iostream>

class Exists
{
public:
    int Method(bool);
    int* Test(int&, char);

};

class NotExists
{
};

int main()
{
    std::cout << IsMethodMemberFunctionExists<Exists>::value << std::endl;
    std::cout << IsTestMemberFunctionExists<Exists>::value << std::endl;

    std::cout << IsMethodMemberFunctionExists<NotExists>::value << std::endl;
    std::cout << IsTestMemberFunctionExists<NotExists>::value << std::endl;
}

So, in your case you need to define a few checkers - one checker for one type of Event:

// void recieve(const Event1&)
DEFINE_METHOD_CHECKER(void, recieve, (const Event1&));

// void recieve(const Event2&)
DEFINE_METHOD_CHECKER(void, recieve, (const Event2&));

来源：https://stackoverflow.com/questions/24975147/check-if-class-has-function-with-signature

标签

c++

c++11

sfinae