这里需要extern关键字，cpp文件中的const [英] Is extern keyword required here, const in cpp file

问题描述

如果我有头文件

namespace Bob
{
    extern const T x;
};

并在源文件中

extern const T Bob::x = 123;

是源文件中的第二个 extern 必要或可选？

Is the second extern in the source file required or optional?

我搜索过并发现了冲突的信息。

I've searched and found conflicting information.

b http://msdn.microsoft.com/en-us/library/357syhfh。 aspx

但是为了在C ++中得到相同的行为，你必须在源文件中声明你的const变量： / p>

But to get the same behavior in C++, you must declare your const variable [in source file] as:

extern const int i = 2;

推荐答案

通常， extern 关键字告诉编译器不要定义一个符号，因为它将被定义在别的地方。例如

Usually, the extern keyword tells the compiler not to define a symbol, because it will be defined somewhere else. So writing e.g.

namespace Bob {
    extern T x;
}

未定义变量 x ，而是声明它。您可以根据需要拥有尽可能多的 extern 声明。但是，如果您不提供定义，链接将失败。所以你必须定义

does not define the variable x, but rather declares it. You can have as many extern declarations as you like. However, if you do not provide a definition the linking will fail. So you have to define

T Bob::x;

在代码中的某处提供定义。

somewhere in the code in order to provide the definition.

const 关键字在这里有点特别，因为它意味着内部链接。这意味着 x 的定义在定义它的特定编译单元之外是不可见的。要改变这种行为，你需要写

The const keyword is a little special here, because it implies internal linkage. This means, that the definition of x will not be visible outside the specific compilation unit where it was defined. To alter this behavior you do need to write

    extern const T Bob::x = 123;

如果您要 x 为 const 并从其他编译单元中引用它。

if you want x to be const and also reference it from other compilation units.

----另一个编辑

绝对清楚：
如果一个 const 变量应该在其编译单元之外被引用，那么您必须显式声明 extern 。

----yet another edit----
Just to be absolutely clear: If a const variable is supposed to be referenced outside of its compilation unit, then you must explicitly declare it extern.

与定义分开，则定义不一定需要再次指定关键字 extern 。另一个演示示例：

However, if the declaration is given separately from the definition, then the definition does not necessarily need to specify the keyword extern again. Yet another example for demonstration:

myheader.h

extern const int i;

这会声明 i a const 整数与外部链接，但不定义它。

This declares i a const integer with external linkage, but does not define it.

main.cpp，版本1 / p>

main.cpp, version 1

#include "myheader.h" //Through the include, the above declaration of `i` comes before its definition.

const int i=123; // Although the `extern` modifier is omitted here,
                 // it's still in effect because we already declared `i` as `extern`
                 // Therefore it makes no difference here, whether or not you specify `extern` again.
                 // The compiler knows this is a definition either way, because of the initialization.

main.cpp，版本2

//#include "myheader.h"

extern const int i=123; // this line is declaration and definition in one, because we did not include
                        // the declaration from myheader.h. Thus, to tell the compiler that you want
                        // `i` with external linkage, you MUST specify `extern`. Otherwise you'll get
                        // internal linkage.

我希望这一切对你来说都有意义。

I hope all this now makes sense to you.

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