在C和C静态和extern全局变量++ [英] static and extern global variables in C and C++

问题描述

我做了2个项目，第一个C和第二个在C ++中，具有相同的行为都工作。

I made 2 projects, the first one in C and the second one in C++, both work with same behavior.

C ++项目：

header.h

int varGlobal=7;

的main.c

#include <stdio.h>
#include <stdlib.h>
#include "header.h"

void function(int i)
{
    static int a=0;
    a++;
    int t=i;
    i=varGlobal;
    varGlobal=t;
    printf("Call #%d:\ni=%d\nvarGlobal=%d\n\n",a,i,varGlobal,t);
}

int main() {
    function(4);
    function(6);
    function(12);
    return 0;
}

C ++项目：

C++ project:

header.h

int varGlobal=7;

的main.cpp

#include <iostream>
#include "header.h"
using namespace std;

void function(int i)
{
    static int a=0;
    int t=i;
    a++;
    i=varGlobal;
    varGlobal=t;
    cout<<"Call #"<<a<<":"<<endl<<"i="<<i<<endl<<"varGlobal="<<varGlobal<<endl<<endl; 
}

int main() {
    function(4);
    function(6);
    function(12);
    return 0;
}

我读了全局变量的extern 默认并用C和静态默认用C ++;那么，为什么C ++ code ++工程？

I read that global variables are extern by default and in C and static by default in C++; so why does the C++ code works?

我的意思的 INT varGlobal = 7; 的是一样的静态INT varGlobal = 7; 的，如果它是静态的话，就只能在它声明的文件中使用，对吧？

I mean int varGlobal=7; is same as static int varGlobal=7; and if it's static then it can be used only in the file it was declared, right?

推荐答案

全局变量是不是的extern 也不静态默认情况下，C和C ++。
当一个变量声明为静态，你把它限制为当前的源文件。如果你把它声明为的extern ，你说该变量存在，但声明别的地方，如果你没有它的其他地方声明（没有的extern 关键字），你会得到一个链接错误（未找到符号）。

Global variables are not extern nor static by default on C and C++. When you declare a variable as static, you are restricting it to the current source file. If you declare it as extern, you are saying that the variable exists, but are declared somewhere else, and if you don't have it declared elsewhere (without the extern keyword) you will get a link error (symbol not found).

您code将打破，当你有多个源文件包括报头，在链接时，你将不得不 varGlobal 多次引用。如果你把它声明为静态，那么它将与多个源工作（我的意思是，它会编译和链接），但是每个源都会有自己的 varGlobal 。

Your code will break when you have more source files including that header, on link time you will have multiple references to varGlobal. If you declare it as static, then it will work with multiple sources (I mean, it will compile and link), but each source will have its own varGlobal.

您可以在C ++做什么，你不能在C，是变量声明为常量的头，像这样的：

What you can do in C++, that you can't in C, is to declare the variable as const on the header, like this:

const int varGlobal = 7;

和包括多个来源，无需在链接时摔东西。我们的想法是，以取代旧的C风格的的#define 常量。

And include in multiple sources, without breaking things at link time. The idea is to replace the old C style #define for constants.

如果你需要一个全局变量在多个来源可见，而不是常量，其声明为的extern 的头，并再次声明，这次没有EXTERN keywork，对源文件：

If you need a global variable visible on multiple sources and not const, declare it as extern on the header, and declare it again, this time without the extern keywork, on a source file:

由多个文件包含头文件：

Header included by multiple files:

extern int varGlobal;

在源文件中的一个：

int varGlobal = 7;

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