是否可以将别名模板标记为朋友? [英] Is it possible to mark an alias template as a friend?

问题描述

假设我们有以下代码:

template <class, class>
class Element
{};

template <class T>
class Util
{
public:
   template <class U>
   using BeFriend = Element<T, U>;
};

是否可以将BeFriend标记为朋友? (属于Util或任何其他类).

Is it possible to mark BeFriend as a friend ? (Of Util, or any other class).

显而易见"的曾尝试使用语法，但都因Clang 3.6失败.

The "obvious" syntax were tried, but both failed with Clang 3.6.

template <class> friend class BeFriend;
template <class> friend BeFriend;

我不知道第二种语法，但是在此答案中找到了它.对于非模板别名，它似乎可以正常工作(并且是必需的)，但是在使用别名作为模板的情况下，这无济于事.

I did not know about the second syntax, but found it in this answer. It seems to work (and be required) for non-template aliases, but does not help in this case where the alias is templated.

(注意:正如一些可以从最小示例中推断出的，我正在寻找一种方法来解决C ++不允许将部分模板专业化作为朋友的局限性)

(Note: as some could infer from the minimal example, I am looking for a way to work-around the limitation that C++ does not allow to friend a partial template specialization)

推荐答案

我认为您不能这样做，因为无法将部分专业化声明为好友.

I think you can't do that because partial specializations could not be declared as friend.

从标准禽中， [temp.friend]/7

朋友声明不应声明部分专业化. [ 示例:

Friend declarations shall not declare partial specializations. [ Example:

template<class T> class A { };
class X {
  template<class T> friend class A<T*>; // error
};

-结束示例]

您必须指定一个更通用的版本，例如:

You have to specify a more generic version such as:

template <class, class> friend class Element;

或完整的指定版本，例如:

or a full specified version, such as:

using BeFriend = Element<T, int>;
friend BeFriend;

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