具有外部链接的命名空间 [英] Namespaces with external linkage
问题描述
我遇到的问题与greentype"在http://www.cplusplus.com/forum/beginner/12458/
The problem I have is basically the same as 'greentype' mentions at http://www.cplusplus.com/forum/beginner/12458/
我通过命名空间共享变量，当我尝试将函数定义放入单独的文件时出现问题.
I'm sharing variables through namespaces and a problem arises when I try to put my function definitions into a separate file.
考虑下面的例子，我想传递变量'i'，定义在主代码中，到函数a():
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;
}
<小时>
但是现在如果我把 a() 的定义放到一个单独的文件中...
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;
}
<小时>
...然后在链接时出现多重定义"错误(g++ main.cppa.cpp -o 主要).如果我在头文件extern"中声明i"(如在其他论坛中建议)，我得到未定义的参考"错误.当 'i' 在标头中声明为 const 时，我可以编译，但这不是我想要的.
... 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.
非常感谢任何建议.
推荐答案
任何全局对象，例如 i，都必须在程序的某处有一个定义，但可以声明 多次.
Any global object, like i, must have exactly one definition somewhere in the program, but it can be declared multiple times.
使用没有初始化器的 extern 使声明只是一个声明.这适用于您的头文件，但您仍必须在某处定义 i.除了制作标头声明 extern 之外，您还需要将定义(即不带 extern 的声明副本)添加到您的一个且只有一个源文件中.
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.
阅读您的问题，您说您想将变量传递给函数.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.在没有任何重要原因的情况下，您通常应该定义一个函数，该函数接受一个参数并通过其参数将一个值(可能来自局部变量)从调用站点传递给该函数.
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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