用户声明的名称空间成员 [英] User-declared namespace member

问题描述

从3.4.1/14开始存在

There is from 3.4.1/14:

如果命名空间的变量成员在以下范围之外定义 它的名称空间，然后出现在定义中的任何名称 查找成员(在declarator-id之后)，好像 该成员出现在其命名空间中.

If a variable member of a namespace is defined outside of the scope of its namespace then any name that appears in the definition of the member (after the declarator-id) is looked up as if the definition of the member occurred in its namespace.

如果该名称被视为成员名称的定义，那么声明的重点是什么?

以及以下示例为何起作用:

And why the following example will works:

namespace N 
{
    extern int j;
}
int i = 2;
int N::j = i; //N::j=2

int N::j=i实际出现在名称空间范围内.因此，对于不合格的名称查找，声明int i=2不可见. 为什么找到此声明?

int N::j=i actual appears into the namespace scope. Hence the declarationint i=2 is not visible for unqualified name lookup. Why does this declaration found?

推荐答案

您的问题:

int N::j=i实际出现在名称空间范围内.因此，对于不合格的名称查找，声明int i=2不可见.为什么找到此声明?

int N::j=i actual appears into the namespace scope. Hence the declaration int i=2 is not visible for unqualified name lookup. Why does this declaration found?

答案:

由于在N命名空间中找不到i，因此将在全局命名空间中进行查找.如果在N名称空间中有一个i，该名称将用于初始化N::j.

Since i is not found in the N namespace, it is looked up in the global namespace. Had an i been there in the N namespace, that would have been used to initialize N::j.

希望以下程序可以澄清您的疑问.

Hope the following program clarifies your doubt.

#include <iostream>

namespace N 
{
   extern int j;
   extern int k;

   int x = 3;
}

int x = 2;
int y = 10;

int N::j = x; // N::x is used to initialize N::j
int N::k = y; // ::y is used to initialize N::k

int main()
{
   std::cout << N::j << std::endl;
   std::cout << N::k << std::endl;
}

输出:

3
10

更新，以回应OP的评论

标准所说的是:

namespace N 
{
   extern int j;
}

int x = 2;

int N::j = x;

等效于:

namespace N 
{
   extern int j;
}

int x = 2;

namespace N 
{
   int j = x;
}

x的查找逻辑是相同的.如果在同一名称空间N中找到它，则使用它.如果在名称空间N中找不到x，则会在封闭的名称空间中向外搜索.

The logic for lookup ofx is same. If it is found within the same namespace N, it is used. If x is not found in namespace N, it is searched for outward in the enclosing namespaces.

这篇关于用户声明的名称空间成员的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！