C ++声明中的显式限定 [英] explicit qualification in C++ declaration

问题描述

当第一个声明被注释掉时，以下名称空间定义无法编译.如果foo的第一个声明没有注释，则可以正常编译.

The following namespace definition fails to compile when the first declaration is commented out. If the first declaration of foo is uncommented, then it compiles just fine.

namespace Y
{
    //void foo();
    void ::Y::foo(){}
}

标准(§8.3¶1)的相关部分说:

The relevant part in the standard (§8.3¶1) says:

当声明者ID合格时，声明应引用先前声明的成员

When the declarator-id is qualified, the declaration shall refer to a previously declared member

我知道此规则可以防止将名称引入其他名称空间.我想知道是否可以放宽该规则，以允许 qualified-id 引用当前名称空间.

I understand that this rule prevents the introduction of names into other namespaces. I wonder if that rule could be relaxed to allow for qualified-ids referring to the current namespace.

推荐答案

CWG#482 是相关的:

CWG #482 is relevant:

根据8.3 [dcl.意义]第1款，[…]
此限制禁止出现以下示例:

According to 8.3 [dcl.meaning] paragraph 1, […]
This restriction prohibits examples like the following:

void f();
void ::f();        // error: qualified declarator

namespace N {
  void f();
  void N::f() { }  // error: qualified declarator
}

似乎没有任何充分的理由禁止这种行为 声明，尽管有许多实现仍接受它们 标准的禁止.是否应更改标准以允许 他们吗?

There doesn't seem to be any good reason for disallowing such declarations, and a number of implementations accept them in spite of the Standard's prohibition. Should the Standard be changed to allow them?

2006年4月的会议记录:

Notes from the April, 2006 meeting:

在讨论问题548 ，CWG同意禁止 应该删除其命名空间中的合格声明符.

In discussing issue 548, the CWG agreed that the prohibition of qualified declarators inside their namespace should be removed.

因此，如果存在foo的第一个声明，则您的代码有效(自2012年起； GCC具有

So your code is valid if the first declaration of foo is present (as of about 2012; GCC has an open bug report). If not, however, your quoted wording still applies and renders the qualified declaration ill-formed. I see no reason to permit that case; it intuitively implies that the name has been declared already, since qualified name lookup must determine what it refers to.

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