如何在C ++中声明一个全局变量 [英] How to declare a global variable in C++

问题描述

我知道一个人不应该使用全局变量，但我需要他们。我已经读过任何在函数之外声明的变量是一个全局变量。我这样做，但在另一个* .cpp文件中找不到该变量。所以这不是真正的全球。不是这样的，必须创建一个头文件GlobalVariabels.h并将该文件包含到使用它的任何其他* cpp文件中？

I know one should not use global variables but I have a need for them. I have read that any variable declared outside a function is a global variable. I have done so, but in another *.cpp File that variable could not be found. So it was not realy global. Isn't it so that one has to create a header file GlobalVariabels.h and include that file to any other *cpp file that uses it?

推荐答案

我读过任何在函数之外声明的变量是一个全局变量。我这样做，但在另一个* .cpp文件中找不到该变量。

I have read that any variable declared outside a function is a global variable. I have done so, but in another *.cpp File that variable could not be found. So it was not realy global.

根据范围的概念，变量 全局。

According to the concept of scope, your variable is global. However, what you've read/understood is overly-simplified.

您可能忘了在其他翻译单元（TU）中声明变量。下面是一个例子：

Perhaps you forgot to declare the variable in the other translation unit (TU). Here's an example:

int x = 5; // declaration and definition of my global variable

b.cpp

b.cpp

// I want to use `x` here, too.
// But I need b.cpp to know that it exists, first:
extern int x; // declaration (not definition)

void foo() {
   cout << x;  // OK
}

通常你会放置 extern int包含在 b.cpp 中的头文件中的 ，以及最终需要使用 x 。

Typically you'd place extern int x; in a header file that gets included into b.cpp, and also into any other TU that ends up needing to use x.

变量可能具有内部链接，这意味着它不会跨翻译单元公开。如果变量被标记为 const （ [C ++ 11：3.5 / 3] ）：

Additionally, it's possible that the variable has internal linkage, meaning that it's not exposed across translation units. This will be the case by default if the variable is marked const ([C++11: 3.5/3]):

const int x = 5; // file-`static` by default, because `const`

b.cpp

b.cpp

extern const int x;    // says there's a `x` that we can use somewhere...

void foo() {
   cout << x;    // ... but actually there isn't. So, linker error.
}

您可以通过应用 extern 到定义：

extern const int x = 5;

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这整个malarky大致相当于通过使功能在TU边界之间可见/可用，但在如何处理它有一些差异。

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This whole malarky is roughly equivalent to the mess you go through making functions visible/usable across TU boundaries, but with some differences in how you go about it.

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