[英]Why should I use the extern keyword to declare variables in a namespace scope?
[英]Should we declare extern variables in all the files included in a project?
我一直在用'extern'关键字尝试一些事情。 我写了这个基本功能,我不知道为什么我的打印功能不起作用。 请帮助我理解它。
test1.h
#pragma once
#include<iostream>
using namespace std;
extern int a;
extern void print();
test1.cpp
#include "test1.h"
extern int a = 745;
extern void print() {
cout << "hi "<< a <<endl;
}
test2.cpp
#include"test1.h"
extern int a;
extern void print();
int b = ++a;
int main()
{
cout << "hello a is " << b << endl;
void print();
return 0;
}
Actual output :
hello a is 746
Expected output:
hello a is 746
hi 746
test1.cpp
#include "test1.h"
int a = 745; //< don't need extern here
void print() { //< or here
cout << "hi "<< a <<endl;
}
测试2.cpp
#include"test1.h"
/* we don't need to redefine the externs here - that's
what the header file is for... */
int b = ++a;
int main()
{
cout << "hello a is " << b << endl;
print(); //< don't redeclare the func, call it instead
return 0;
}
只有在声明变量/函数时才需要使用extern,并在其中一个cpp文件中定义变量,其中包括标题。
所以,你想要做的是
test1.h
#pragma once
#include<iostream>
using namespace std;
extern int a;
extern void print();
test1.cpp
#include "test1.h"
int a = 745;
void print() {
cout << "hi "<< a <<endl;
}
测试2.cpp
#include"test1.h"
int b = ++a;
int main()
{
cout << "hello a is " << b << endl;
print();
return 0;
}
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