具有默认模板参数的朋友功能模板

问题描述

是否允许在好友声明中为模板参数提供默认值?

Is it allowed to provide a default to a template argument in a friend declaration?

class A {
    int value;
public:
    template<class T = int> friend void foo();
};

Visual Studio 2015似乎允许它. gcc拒绝了.我在 cppreference 页上找不到任何内容.

Visual Studio 2015 seems to allow it. gcc refuses it. I couldn't find anything on it on the cppreference page.

推荐答案

从C ++ 11开始，该规则在14.1[temp.param]/9

As of C++11, the rule is, specified in 14.1[temp.param]/9

在C ++ 11之前，当然，14.1/9表示默认的模板参数不能在好友模板声明中指定."

Until C++11, of course, 14.1/9 said "A default template-argument shall not be specified in a friend template declaration."

(以上内容几乎逐字复制，由cppreference在默认模板参数，现在也在模板朋友)

(the above is copied pretty much verbatim, by cppreference at Default template parameters, now also mentioned at Template friends)

因此，要使您的程序成为有效的C ++，请在类中定义您的朋友模板，而不仅仅是声明.

So, to make your program valid C++, define your friend template inside the class, don't just declare.

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