为什么“extern const int n;"?没有按预期工作? [英] Why does "extern const int n;" not work as expected?

问题描述

我的项目只包含两个源文件:

My project consists of only two source files:

a.cpp:

const int n = 8;

b.cpp:

extern const int n;

int main()
{
    // error LNK2001: unresolved external symbol "int const n" (?n@@3HB)
    int m = n; 
}

我知道有几种方法可以让它发挥作用；但是，我只是想知道为什么它不起作用?

I know there are several methods to make it work; however, I just wonder WHY it does't work?

推荐答案

因为const默认隐含内部链接，所以您的定义"在翻译单元之外不可见它出现在哪里.

It's because const implies internal linkage by default, so your "definition" isn't visible outside of the translation unit where it appears.

在这种情况下，到目前为止，最好的解决方案是将声明(extern int const n;) 在头文件中，并将其包含在a.cpp 和 b.cpp.链接由编译器看到的第一个声明，所以后面的定义a.cpp 将具有正确的(外部)链接.

In this case, by far the best solution is to put the declaration (extern int const n;) in a header file, and include that in both a.cpp and b.cpp. The linkage is determined by the first declaration the compiler sees, so the later definition in a.cpp will have the correct (external) linkage.

或者，您可以在定义中强制链接:

Alternatively, you can force the linkage in the definition:

extern int const n = 8;

尽管有extern，这仍然是一个定义；任何有类定义之外的初始化器是定义.

Despite the extern, this is still a definition; anything with an initializer outside of a class definition is a definition.

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