专门化模板类的功能

Question

This is legal in C++:

template <int N> class A {
    void bar() {std::cout << N << '\n';}
};

template<>
void A<2>::bar() {std::cout << "Two\n";}  // This is ok.

Now consider this class:

template <int...> struct B;

template <int First, int... Rest>
struct B<First, Rest...> : B<Rest...> {
    static void foo() {
        std::cout << First << ' ';
        B<Rest...>::foo();  
    }
    static void bar() {/*Bunch of code*/}
    static void baz() {/*Bunch of code*/}
};

template <>
struct B<> {
    static void foo() {}
    static void bar() {}
    static void baz() {}
};

Then why is the following illegal (placed after the above):

template <int... Rest>
void B<2, Rest...>::foo() {  // Illegal.
    std::cout << "Two ";
    B<Rest...>::foo();
}

I don't see why B<2, Rest...> is an incomplete type, as the error message states. So apparently, the only way to achieve what I want is through this?

template <int... Rest>
struct B<2, Rest...> : B<Rest...> {
    static void foo() {
        std::cout << "Two ";
        B<Rest...>::foo();  
    }
    static void bar() {/*Same bunch of code as above*/}
    static void baz() {/*Same bunch of code as above*/}
};

Thus repeating all the code in bar() and baz()?

Answer 1

What you are trying to achieve is called partial template specialization, and is allowed only for classes, not for functions. See for example Why function template cannot be partially specialized?