获取类成员函数中的参数数量

#include <iostream>
#include <functional>

template<typename Class, typename ReturnType, typename... Args>
struct FunctionTraits
{
    static constexpr std::size_t arity = sizeof...(Args);

    using FunctionType = ReturnType(Class::*)(Args...);
};

class MyClass
{
public:
    void foo(int x, float y)
    {
        std::cout << "foo() called with " << x << " and " << y << std::endl;
    }

    int bar(double z)
    {
        std::cout << "bar() called with " << z << std::endl;
        return 42;
    }
};

int main()
{
    std::cout << "foo() arity: " << FunctionTraits<MyClass, void, int, float>::arity << std::endl;
    std::cout << "bar() arity: " << FunctionTraits<MyClass, int, double>::arity << std::endl;

    return 0;
}