警告:模板在不同名称空间中的特化 [英] warning: specialization of template in different namespace

问题描述

通过以下代码，我得到警告:

With the following piece of code I get the warning:

警告:"template< class _Iterator>不同名称空间中的struct std :: iterator_traits [-fpermissive]

template<> class std::iterator_traits<Token_ptr>{
public:
    typedef Word difference_type;
    typedef Word value_type;
    typedef Token_ptr pointer;
    typedef Word& reference ;
    typedef std::bidirectional_iterator_tag iterator_category ;
};

尽管一切正常，但是没有人知道确切的含义以及发出警告的原因.(g ++发出警告，而clang ++则不发出警告.)

While everything works correctly, does any body know what exactly means and why is issued the warning. ( g++ issues the warning while clang++ doesn't ).

推荐答案

假设您正在C ++ 11模式下编译此代码(因为clang没有给出警告)，并且此专业化位于全局名称空间中，那么没有任何问题与您的代码.这是一个g ++错误.§14.7.3[temp.expl.spec]/p2:

Assuming that you are compiling this in C++11 mode (since clang gave no warning) and that this specialization is in the global namespace, then there's nothing wrong with your code. It's a g++ bug. §14.7.3 [temp.expl.spec]/p2:

显式专业化应在封闭的名称空间中声明专业模板.一个显式的专业，其 declarator-id 不合格，应在模板的最近封闭名称空间中声明，或者如果该名称空间是内联的，则声明该声明(7.3.1)，其封闭名称空间集中的任何名称空间.这样的声明也可以是定义.如果声明不是定义，以后可以定义专业化(7.3.1.2).

An explicit specialization shall be declared in a namespace enclosing the specialized template. An explicit specialization whose declarator-id is not qualified shall be declared in the nearest enclosing namespace of the template, or, if the namespace is inline (7.3.1), any namespace from its enclosing namespace set. Such a declaration may also be a definition. If the declaration is not a definition, the specialization may be defined later (7.3.1.2).

全局命名空间是一个包含专用模板的命名空间"，并且您的 declarator-id 用 std :: 限定，因此第二句话不适用.解决方法是，您可以执行 cdhowie 的答案所建议的-即打开 namespace std 阻止并将专业化放置在那里.

The global namespace is a "namespace enclosing the specialized template", and your declarator-id is qualified with std::, so the second sentence doesn't apply. As a workaround, you can do what cdhowie's answer suggests - i.e., opening a namespace std block and putting the specialization there.

请参见 CWG问题374 和 GCC错误56480 .

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