带外部链接的命名空间 [英] Namespaces with external linkage

问题描述

我所遇到的问题基本上与'greentype'提到的
http：// www。 cplusplus.com/forum/beginner/12458/

我通过命名空间共享变量，当我尝试将我的函数定义放入考虑下面的例子，其中我想传递变量'i'，在主代码中定义
到函数a（）：

<

---

* nn.h：* >

  #ifndef _NN_H_ 
 #define _NN_H_ 
 
命名空间nn {
 int i; 
} 
 #endif 
  

* main.cpp *

  #include< iostream> 
 #includenn.h
 using namespace std; 
 using namespace nn; 
 
 void a（）; 
 
 int main（）
 {
 i = 5; 
 a（）; 
} 
 
 void a（）
 {
 using namespace std; 
 using namespace nn; 
 
 i ++; 
 cout<< i =< i<< endl; 
} 
  

---

但现在，如果我把a（）的定义放到一个单独的文件...

---

* a.cpp *

  #include< iostream> 
 #includenn.h
 
 void a（）
 {
 using namespace std; 
 using namespace nn; 
 
 i ++; 
 cout<< i =< i<< endl; 
} 
  

---

...然后我得到'多重定义'错误时链接（g ++ main.cpp
a.cpp -o main）。如果我在头文件'extern'（如
在其他论坛中建议的）''''声明，我得到'未定义的引用'错误。我可以编译时，'i'在标题中声明为const，但这不是我想要的。

任何建议非常感谢。

解决方案

任何全局对象，例如 i

使用 extern 没有初始化器使声明只是一个声明。这适合你的头文件，但你必须仍然定义 i 。除了创建头声明 extern ，还需要添加一个定义（即，声明的副本不含 extern ）到一个而且只有一个源文件。

编辑：读取您的问题，你说要传递一个变量到功能。从样式和代码结构的角度来看，这通常不是使用共享（全局）变量的好理由。在没有任何重写的原因，你通常应该定义一个函数，它接受一个参数，并通过其参数传递一个值（可能从一个局部变量）从调用站点到该函数。

The problem I have is basically the same as 'greentype' mentions at http://www.cplusplus.com/forum/beginner/12458/

I'm sharing variables through namespaces and a problem arises when I try to put my function definitions into a separate file.

Consider the following example, where I want to pass variable 'i', defined in the main code, to the function a():

---

* nn.h: *

#ifndef _NN_H_
#define _NN_H_

namespace nn {
int i;
}
#endif

* main.cpp *

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

void a();

int main()
{
i=5;
a();
}

void a()
{
using namespace std;
using namespace nn;

i++;
cout << "i = " << i << endl;
}

---

But now if I put the definition of a() into a separate file ...

---

* a.cpp *

#include <iostream>
#include "nn.h"

void a()
{
using namespace std;
using namespace nn;

i++;
cout << "i = " << i << endl;
}

---

... then I get 'multiple definition' error when linking (g++ main.cpp a.cpp -o main). If I make 'i' declaration in the header file 'extern' (as suggested in other forums), I get 'undefined reference' error. I can compile when 'i' is declared as const in the header, but that's not what I want.

Any suggestions greatly appreciated.

解决方案

Any global object, like i, must have exactly one definition somewhere in the program, but it can be declared multiple times.

Using extern without an initializer makes a declaration just a declaration. This is appropriate for your header file, but you must still define i somewhere. As well as making the header declaration extern you also need to add a definition (i.e. a copy of the declaration without extern) to one and only one of your source files.

Edit: Reading your question, you say that you want to pass a variable to a function. From a style and code structure point of view, this isn't usually a good reason for using a shared (global) variable. In the absence of any overriding reasons you should normally define a function which takes a parameter and pass a value (possibly from a local variable) from the calling site to that function via its parameter.

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