存储成员函数模板的实例时出错

Question

我正在尝试存储指向成员函数模板Derived::initialize 实例的指针，如下所示（另请参阅rextester.com。对于后代，我创建了一个simpler version 的问题。）：

class Base
{
public:
    typedef void (Base::*setterFunction)( unsigned );

    template<unsigned N>
    struct SetterInterface
    {
        static Base::setterFunction Function;
    };

protected:
    template<unsigned N>
    void setterImpl( unsigned )
    {
    }
};

template<unsigned N>
Base::setterFunction Base::SetterInterface<N>::Function = &Base::setterImpl<N>;

class Derived : public Base
{
public: 
    typedef void (Derived::*Initializer)();

    template<typename T , void (T::*F)( unsigned ) >
    void initialize()
    {
    }

    template<typename C>
    Derived( C* )
    {
        Initializer initializer = &Derived::initialize<C, C::template SetterInterface<0>::Function>;    // NOT OK
        //Initializer initializer = &Derived::initialize<C, C::template setterImpl<0> >;    // OK
    }
};

int main()
{
    Derived derived( (Base*)0 );
}

但我在 GCC 5.4.0（和 6.4.0）上收到错误消息

Test.cpp: In instantiation of ‘Derived::Derived(C*) [with C = Base]’:
Test.cpp:45:28:   required from here
Test.cpp:37:39: error: no matches converting function ‘initialize’ to type ‘Derived::Initializer {aka void (class Derived::*)()}’
   Initializer initializer = &Derived::initialize<C, C::template SetterInterface<0>::Function>;
                                       ^
Test.cpp:30:7: note: candidate is: template<class T, void (T::* F1)(unsigned int)> void Derived::initialize()
  void initialize()

问题似乎出在成员函数模板参数上，因为C::template setterImpl<0> 有效，而C::template SetterInterface<0>::Function（我认为它是前者的别名）无效。例如：

Base::setterFunction f1 = &Base::setterImpl<0>;
Base::setterFunction f2 = Base::template SetterInterface<0>::Function;

Answer 1

问题是initialize()non-type-template-parameter应该是常量表达式；在你的代码中它是一个静态成员，所以它不能工作；它可以，如果你将它声明为 static constexpr （但你至少需要一个 C++11 编译器）：

class Base
{
protected:
    template<unsigned N>
    void setterImpl( unsigned );
public:
    typedef void (Base::*setterFunction)( unsigned );

    template<unsigned N>
    struct SetterInterface
    {
        static constexpr Base::setterFunction Function = &Base::setterImpl<N>;
    };
};