CRTP refer to typedef in derived class from base class

问题

I have following code:

template <typename T>
class A
{
    typedef typename T::Type MyType;
};

template <typename T>
class B : public A<B<T>>
{
    typedef T Type;
};

When I try to instantiate B, I get following error message using MSVS 2015:

'Type': is not a member of 'B<int>'

Is this code valid C++ or is MSVS right?

回答1:

The problem is at this point

template <typename T>
class A
{
    typedef typename T::Type MyType;
                     ^^^
};

T needs to be a complete type. But in your case, when A<T> is instantiated here:

template <typename T>
class B : public A<B<T>>
                 ^^^^^^^

B<T> is not yet a complete type! So this cannot work unfortunately.

The simple solution is just to pass in Type separately:

template <typename T, typename Type>
class A
{
    typedef Type MyType;
};    

template <typename T>
class B : public A<B<T>, T>
{

};

来源：https://stackoverflow.com/questions/36268400/crtp-refer-to-typedef-in-derived-class-from-base-class