在哪里声明类成员模板的部分特化？

问题描述

这里stackoverflow我发现几个评论（例如，在问题），说明在非命名空间范围允许类成员模板的部分专门化（相反如下例所示：

Here on stackoverflow I've found several comments (see for example the comment by jrok on this question) stating that partial specializations of class member templates are allowed at non-namespace scope (in contrast to explicit specializations) like in the example below:

class A {
    template <class T, class U>
    class B {};
    template <class U>
    class B<void, U> {};
};

此外，本示例使用gcc和clang进行编译。但是在c ++ 03标准文本中，我只能找到关于这个问题的14.5.4 [temp.class.spec]§6（或者14.5.5§5）：

Also, this example compiles just fine with both gcc and clang. However in the c++03 standard text I can only find 14.5.4 [temp.class.spec] §6 (or 14.5.5 §5 in c++11) about this issue:

与以下示例一起：

template<class T> struct A {
    class C {
        template<class T2> struct B { };
    };
};

// partial specialization of A<T>::C::B<T2>
template<class T> template<class T2>
struct A<T>::C::B<T2*> { };

那么，在非命名空间范围的类模板部分特化他们是否被标准允许？

So, how about class template partial specializations at non-namespace scope? Are they allowed by the standard? And where can I find the relevant text?

具体来说，我的示例是有效的（如果封闭类是模板，它仍然有效）？

Specifically, is my example valid (and would it still be valid if the enclosing class would be a template)? If not, are current compilers wrong to compile my example as stated above?

推荐答案

这似乎是有点混乱，因为我同意您引用的段落似乎禁止在类定义内部的部分专门化。但是，有[temp.class.spec.mfunc] / 2：

This seems to be a bit confusing, as I agree the paragraph you quoted seems to disallow partial specializations inside class definitions. However, there's [temp.class.spec.mfunc]/2:

template<class T> struct A {
    template<class T2> struct B {}; // #1
    template<class T2> struct B<T2*> {}; // #2
};

template<> template<class T2> struct A<short>::B {}; // #3

A<char>::B<int*> abcip; // uses #2
A<short>::B<int*> absip; // uses #3
A<char>::B<int> abci; // uses #1

- end example b $ b

— end example ]

IMHO这不是很明确;它不会在类定义内允许部分专门化，而是（我似乎）指定如何处理成员模板特化。

IMHO this isn't very explicit; it doesn't allow partial specialization inside the class definition but rather (to me, seems to) specifies how member template specializations are treated.

另请参阅和。

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