多个，包括带有可变定义的头文件 [英] Multiple including a head file with a variable definition

问题描述

我只是构建一个简单的C ++项目.代码如下所示:

-------- head.h --------

  #ifndef _HEAD_H_#定义_HEAD_H_int my_var = 100;#万一 

-------- src1.cpp --------

  #include"head.h" 

-------- src2.cpp --------

  #include"head.h"int main(){返回0;} 

然后用这些命令构建二进制文件:

 <代码> g ++ -c src1.cpp -o scr1.og ++ -c src2.cpp -o src2.og ++ -o a.out src2.o src1.o 

但是它在链接步骤中失败，并告诉我我有"my_var"的多个定义.我不只是在头文件中使用安全保护宏吗?我不明白.有人知道为什么吗?>

谢谢.

ps:如果我将my_var定义为静态变量，则代码可以通过链接.我也不明白.

解决方案

按以下方式更改标题

  #ifndef _HEAD_H_#定义_HEAD_H_extern int my_var;#万一 

例如在主模块中添加行

  #include"head.h"int my_var = 100;int main(){返回0;} 

问题在于，由于标头包含在两个模块中，因此每个模块都包含一个具有外部链接的变量，其名称与其他模块中的变量相同.而且链接器不知道要使用哪个变量.

I just build a simple C++ project. The codes are shown in the follows:

-------- head.h --------

#ifndef _HEAD_H_
#define _HEAD_H_

int my_var = 100;

#endif

-------- src1.cpp --------

#include "head.h"

-------- src2.cpp --------

#include "head.h"
int main() { return 0; }

And I build the binary with those command:

g++ -c src1.cpp -o scr1.o
g++ -c src2.cpp -o src2.o
g++ -o a.out src2.o src1.o

But it fails in the linking step and tells me that i have "multiple definition of `my_var'. Did't I just use safe guard macros in the head file? I don't understand. Anyone know why?

thanks.

ps: If I define my_var as a static variable, then the code can pass linking. I don't understand it either.

解决方案

Change the header the following way

#ifndef _HEAD_H_
#define _HEAD_H_

extern int my_var;

#endif

And for example add line in the module with main

#include "head.h"
int my_var = 100;
int main() { return 0; }

The problem is that as the header is included in two modules then each module contains a variable with external linkage with the same name as a variable in other module. And the linker does not know which variable to use.

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