c++ - 显式特化的成员也需要其包含的类模板也被显式特化

Question

我读过很多文章，专门针对类模板 成员模板，包含专门成员模板的类也应明确专门化。标准中是否有关于它的观点，是否有任何理由有这样的限制？ 我的意思是在幕后。

为什么不允许这样做。

template <typename T>
class A
{
   template <typename U>
   void foo()
   {}
};

template <typename T>
 template <>
void A<T>::foo<int>()
{}

Answer 1

[temp.expl.spec]/16:

In an explicit specialization declaration for a member of a class template or a member template that appears in namespace scope, the member template and some of its enclosing class templates may remain unspecialized, except that the declaration shall not explicitly specialize a class member template if its enclosing class templates are not explicitly specialized as well. [ Example:

template <class T1> class A {
    template<class T2> class B {
        template<class T3> void mf1(T3);
        void mf2();
    };
};

template <> template <class X>
class A<int>::B {
    template <class T> void mf1(T);
};

template <> template <> template<class T>
void A<int>::B<double>::mf1(T t) { }
template <class Y> template <>
void A<Y>::B<double>::mf2() { } // ill-formed; B<double> is specialized 
                                // but its enclosing class template A is not

— end example ]