Defining an Inner class member function template with a (non type) enum argument

问题

I'm having difficulty defining and specializing a member function update() of an inner class Outer<T1>::Inner that is templated on a non-type (enum) argument.

#include <cstdlib>

template<typename T1>
struct Outer
{
    struct Inner
    {
        enum Type{ A , B , C };

        template<Type T2>
        void update();
    };
};

// Definition
template<typename T1>
template<Outer<T1>::Inner::Type T2>
void Outer<T1>::Inner::update()
{
}

// Specialization
template<typename T1>
template<Outer<T1>::Inner::A >
void Outer<T1>::Inner::update()
{
}

int main()
{
    return EXIT_SUCCESS;
}

I'm getting the following error message in GCC 4.5.3

prog.cpp:17:28: error: ‘Outer::Inner::Type’ is not a type
prog.cpp:18:6: error: prototype for ‘void Outer<T1>::Inner::update()’ does not match any in class ‘Outer<T1>::Inner’
prog.cpp:11:15: error: candidate is: template<class T1> template<Outer<T1>::Inner::Type T2> void Outer<T1>::Inner::update()
prog.cpp:24:28: error: ‘Outer::Inner::A’ is not a type
prog.cpp:25:6: error: prototype for ‘void Outer<T1>::Inner::update()’ does not match any in class ‘Outer<T1>::Inner’
prog.cpp:11:15: error: candidate is: template<class T1> template<Outer<T1>::Inner::Type T2> void Outer<T1>::Inner::update()

BTW, unlike GCC, Visual Studio 2008 is unable to compile the following

template<typename T1>
struct Outer
{
    struct Inner
    {
        enum Type{ A , B , C };

        template<Type T2>
        struct Deep;
    };
};

template<typename T1>
template<typename Outer<T1>::Inner::Type T2>
struct Outer<T1>::Inner::Deep
{
};

回答1:

First of all, you're missing a typename before Outer<T1>::Inner::Type. You have to have it, even in a template type list, because Type is a dependent type.

Secondly, your specialisation syntax is wrong (the type goes in <> after the function name before the parentheses, not in the template<>), but even if it was correct, it would not be legal. You have to specialise the outer template Outer before you can fully specialise update, according to an unfortunate rule regarding explicit template specialisation.

来源：https://stackoverflow.com/questions/14776699/defining-an-inner-class-member-function-template-with-a-non-type-enum-argument

标签

c++

templates

inner-classes

specialization

non-type