使用继承的类模板

Question

In the following excerpt, how can I access the inherited Sub class template?

As far as I understand the problem with the constellation below is that the base class Base is itself a dependent class. Accessing it using typename / template works but is cumbersome if Sub is needed frequently.

template<int B>
struct Base {
    template<int S>
    class Sub { };
};

template<int C>
struct Class: public Base<C> {
    // (1) Error: 'Sub' does not name a type
    using S2 = Sub<2>;

    // (2) Error: 'Base' used without template argument list
    using S3 = Base::Sub<3>;

    // (3) Error: 'Class' is incomplete here
    using S4 = Class::Sub<4>

    // (4) Works, but complicated
    using S1 = typename Class::template Sub<1>;
};

using Class0 = Class<0>;
int main() { }

Fruther caveats:

• Is there a way to refer to Sub without having to duplicate the specialization for Base ? Ie, consider Base having numerous/complex template arguments. This is basically why option #3 does not work and why I chose Class as qualification in option #4.

Answer 1

If you need the template Sub , then you can go with option 5, an alias template:

template<int I>
using Sub = typename Base<C>::template Sub<I>;

Now Sub is found inside Class during unqualified name lookup, and is known to be a template. It will also stand exactly for the same type obtained from the base class member template, when a specialization is referred to.

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