Ensure derived class implements static method

问题

I want to ensure, that a derived class implements a specific static method. I think doing so should be possible using static_assert, std::is_same, decltype, CRTP and maybe making use of SFINAE. However, similar code I found so far is quite complex and it seems I do not yet fully understand it making me unable to adopt it to my needs.

What I tried so far is this

template <class T>
class Base 
{
    static_assert(std::is_same<decltype(T::foo(1)), int>::value, "ERROR STRING");
};

class Derived : public Base <Derived>
{
public:
    static int foo(int i) { return 42; };
};

However, it does not compile telling me, that Derived does no have an element named foo even if the method is correctly implemented. Furthermore providing actual parameters for foo in the expression inside static_assert feels wrong.

Searching SO revealed a similar question which finally lead me to this piece of code where it is checked that a type has methods begin() and end() returning iterators. So I tried to adopt this code to my needs.

template <class T>
class Base 
{
    template<typename C>
    static char(&g(typename std::enable_if<std::is_same<decltype(static_cast<int(C::*)(int)>(&C::foo)), int(C::*)(int)>::value, void>::type*))[1];

    template<typename C>
    static char(&g(...))[2];

    static_assert(sizeof(g<T>(0)) == 1, "ERROR STRING");
};

But this code does not compile because the assertion fires.

So my questions are

1. Why does the compiler cannot find Derived::foo in my first example?
2. What exactly does typename C::const_iterator(C::*)() const in the example code mean? Isn't it a const function returning C::const_iterator and taking no arguments? What exactly does C::* mean? So why is int(C::*)(int) then wrong in my case?
3. How to correctly solve my problem?

I'am using MSVC 12 but if possible the code should be portable.

回答1:

This is a common problem when using CRTP: Base<Derived> is instantiated at the point where it is encountered in Derived's list of bases, at which time Derived is not yet a complete type since the rest of its declaration hasn't been parsed yet. There are various workarounds. For static_assert, you need to delay instantiation of the assertion until Derived is complete. One way to do so is to put the assertion in a member function of Base that you know must be instantiated - the destructor is always a good choice (Live at Coliru):

template <class T>
class Base 
{
public:
    ~Base() {
        static_assert(std::is_same<decltype(T::foo(1)), int>::value, "ERROR STRING");
    }
};

class Derived : public Base<Derived>
{
public:
    static int foo(int) { return 42; };
};

Addressing question #2: C::* is the syntax for "pointer to member of class C." So int(*)(int) is "pointer to function taking a single int parameter and returning int", and int(C::*)(int) is analogously "pointer to member function of C taking a single int parameter and returning int." The monstrosity

typename C::const_iterator(C::*)() const

would translate to "pointer to constant member function of C taking no parameters and returning C::const_iterator" where of course the typename is necessary to indicate that the dependent name C::const_iterator is a type.

来源：https://stackoverflow.com/questions/23367221/ensure-derived-class-implements-static-method