在什么情况下可以使用extern变量定义？ [英] Under what circumstances can an extern variable be used in definition?

问题描述

我很抱歉。我不知道我不完整的代码附件会创建这样的混乱。我很高兴看到这么多真诚的帮助。

I am very very sorry. I didn't know my incomplete code attachment would create such a mess. I am very glad to see so many sincere helps.

此代码将编译：

int myadd(int, int);
static int main_stat = 5;

int main()
{
    int i, j;
    main_stat = 13;
    j = myadd(-1,7);
    i = main_stat;

    cout << j << i;     //  3  and 13
    return 0;

}

myadd.cpp

myadd.cpp

extern int main_stat = -3;
int myadd(int x,int y)
{
    int t = main_stat;
    t = x + y;
    y = t +main_stat;
    return y;    // will return 3
}

请参阅我定义和外部链接main_stat。为什么是合法的？我以为你只能链接和不定义。

See I defined and extern linking main_stat. Why is that legal? I thought you could only link and not define.

存储是否分配在myadd函数调用的堆栈帧中？全局静态是在堆上分配的，我相信是对吗？

Is storage allocated in the stack frame of myadd function call? Global static are allocated on heap, I believe, right?

对不起，我想这次我会缩小我的问题：

I am sorry, but I think this time I will narrow down my questions:

来自C ++ Primer 4ed

From C++ Primer 4ed

extern 声明可能包含一个初始化程序（当组合
时变为定义），只要它出现在函数之外。

An extern declaration may include an initializer (when combined becomes definition) only if it appears outside a function.

我清楚单一定义规则。

I am clear about one-definition rule.

Q1。 myadd（int，int）在调用main_stat时使用哪个副本？与main相同的副本，但具有不同的值（我可以测试）？或者每个函数都有自己的静态全局副本？

Q1. Which copy of main_stat does myadd(int,int) uses when it is called? The same copy as the main has, but with a different value (which I can test) ? Or does each function has its own static global copy?

Q2。在堆上为这些全局静态变量分配内存吗？我知道很多事情是由实现，但不是堆用于静态变量？

Q2. Is memory allocated on the heap for these global static variables? I know many things are up to implementation, but isn't heap used for static variables?

Q3。我知道以下两个是有效的

Q3. I know the followings two are valid

extern int x;    // means int x is defined elsewhere
extern int x = 3;  // declared and defined 

为什么我们想要第二个，如果我们可以声明一个静态全局变量在myadd的命名空间内？ 像aschepler说的那样？

Why do we want the second one if we can just declare a static global variable within the namespace of myadd ? How does it make things clear like aschepler said?

推荐答案

使用初始化程序的所有变量声明也是定义;
这是一个重要的规则。无论 extern 。还有
的情况下，你需要一个 extern 定义：你只能
使用具有外部链接的变量实例化模板。和
const 默认情况下具有内部链接，所以你需要
类似：

All variable declarations with an initializer are also definitions; that's an overriding rule. Regardless of extern. There are even cases where you need an extern on a definition: you can only instantiate a template using a variable which has external linkage. And const variables have internal linkage by default, so you need something like:

extern int const i = 42;

如果你想使用它来实例化一个模板< int const *> ; 。

if you want to use it to instantiate a template<int const*>.

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