extern关键字使用 [英] extern keyword usage

问题描述

我有三个程序，其中我使用 extern 关键字。我不能理解结果。下面是三个例子：

I have three programs in which I am using extern keyword. I am not able to understand the result. Below are three examples:

示例1：我期望下面的代码会给出编译错误，多个声明 k 。但它工作正常？

Example 1: I was expecting that below code will give compilation error that multiple declaration of k. But it works fine?

int k; //works fine
extern int k = 10;

void main()
{
    cout<<k<<endl;
    getchar();
}

给出错误。为什么？

Example 2: When I am trying to initialize "k" in above example compiler gives error. Why?

int k = 20; //error
extern int k = 10;

void main()
{
    cout<<k<<endl;
    getchar();
}

示例3：在此示例中，我更改了示例1 。当我编译这个代码，我得到错误。为什么？

Example 3: In this example I changed the order of definitions mentioned in example 1. When I compile this code I am getting errors. Why?

extern int k = 10;
int k;   //error

void main()
{
    cout<<k<<endl;
    getchar();
}

推荐答案

/ strong>：您尝试初始化一个全局变量两次，有两个不同的值。这是错误。

Example 2: You are trying to initialize a global variable twice, with two different values. This is the error.

示例3 ：首先声明一个 extern 然后在同一个编译单元中定义具有相同名称的变量。这不是可能的。

Example 3: You first declare an extern variable, and then define a variable with the same name in the same compilation unit. This is not possible.

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