外部链接是什么意思 [英] What does external linkage mean

问题描述

请考虑以下代码：

#include <stdio.h>

namespace EnclosingNmspc
{
    namespace Nmspc
    {
        extern int a;//This a and the a defined above denote the same entity
        int a=5;
    }
}

extern int a;

int main()
{ 
    printf("%d\n",a);
}

有来自3.5 / 2的报价：

There is the quote from 3.5/2:

当名称具有外部链接时，其表示的实体可以是由其他翻译单位范围的名称引用的
或从
其他范围

When a name has external linkage , the entity it denotes can be referred to by names from scopes of other translation units or from other scopes of the same translation unit.

我不明白为什么这条规则在我的案例中不起作用？我有未定义的引用链接器错误。

I dont understand why this rule doesn't work in my case? I have undefined reference linker error.

推荐答案

那么， a>。

Your question is already answered there canonically.

您在其他编译单元中缺少对 :: a 的定义。

You've been missing to have a definition for ::a in a different compilation unit.

int a = 5; 实际上定义 extern int a; 在同一范围内。但这不能通过

int a=5; actually defines extern int a; in the same scope. But that's not accessed with

printf("%d\n",a);

。要检查您的命名空间中的内容，请尝试

in your main program. To check for the stuff from your namespace try

printf("%d\n",EnclosingNmspc::Nmspc::a);

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