在多个Cpp文件中使用变量 [英] Use variable across multiple Cpp files

问题描述

搜索了足够的答案，但没有一个解决方案适用于我。

场景：
我试图包括一个.h文件，

如果我将这个头文件包含在实际使用函数和变量的源文件中（精确到2），那么，最后一个编译有一个链接器错误，说明

未定义引用`abc :: myfun（char const *，char const *， char *）'

头文件中的所有函数和变量都声明为extern， >

我希望有一个cpp文件在.h文件中定义的变量中放置一个值，并且另一个cpp文件能够读取它。

如果它有帮助，我的每一段代码都在我定义的命名空间中（ namespace abc {//所有文件中的所有代码} ）

解决方案

.h 文件中的声明确切地说，让编译器知道 您计划在某处计划的对象。 某处将是一个编译单元（ .c 或 .cpp 文件）

code> foo.h ：

  extern int global_foo; 
  

foo.c ：

  #includefoo.h
 int global_foo; //可选你可以这样初始化：int global_foo = 123; 
  

main.c ：

  #includefoo.h
 void bar（）
 {
 global_foo = 0; //访问该文件外部的变量
  

不会意外嵌套命名空间，因为 abc :: something 不等于 abc :: abc :: something

Have searched enough answers but none of the solutions work for me.

scenario : I am trying to include a .h file that has some functions declared (not defined) and some variables declared.

If I include this header file in the source files (2 to be precise) that actually use the functions and variables, then the last one that compiles has a linker error that states

undefined reference to `abc::myfun(char const*, char const*, char*)'

All the functions and variables in the header files have been declared as extern and include guards are present.

I wish to have one cpp file put a value in the variable defined in the .h file and another cpp file to be able to read it.

Also if it helps, every piece of my code is in a namespace that I have defined ( namespace abc{ //all my code, in all the files } )

解决方案

Declarations in your .h file are doing exactly that - letting compiler know of the objects you are planning on declaring "somewhere". That "somewhere" would be a compilation unit (a .c or .cpp file) where that variable would be defined.

Here's an example (skipping the guards for simplicity):

foo.h:

extern int global_foo;

foo.c:

#include "foo.h"
int global_foo; // optionally you can initialize like this: int global_foo = 123;

main.c:

#include "foo.h"
void bar()
{
  global_foo = 0; // accessing that variable which is "outside" of this file

As paddy mentioned above - make sure you are not accidentally nesting namespaces, since abc::something is not the same as abc::abc::something

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