头部保护难题 - 已经在.obj问题中定义 [英] header guard conundrum - already defined in .obj problem

问题描述

我有一个类（A），它必须包括两个文件，类X和类Y.不幸的是，类Y也需要在它的头文件中包含类X，因为构造函数接受一个类X类型的指针作为参数。

潜在的问题是会出现链接器错误？因为类A现在有两个类X的副本，它需要使用它，一个从类Y.这是头保护没有用的情况。我的问题是 - 这是纯粹的结构性问题还是有办法解决这个问题？

我真的不喜欢在类Y的头文件中包含任何东西，想要包括其他任何东西，但它是必要的，因为函数原型？

解决方案

如果您有以下信息：

Xh

  #ifndef X_H__ 
 #define X_H__ 
 
 class X 
 {
 public：
 int foo（）{return 1; } 
}; 
 
 #endif 
  

Yh >

  #ifndef Y_H__ 
 #define Y_H__ 
 
 #includeXh
 
 class Y 
 {
 public：
 Y（X * pX）{myval = pX-> foo（）; } 
 int myval; 
}; 
 
 #endif 
  

something.cpp / p>

  #includeXh
 
 ... 
  
 
 
 
  something_else.cpp  
  #includeYh
 
 ... 
  
 
 
 
 
但是，如果 Xh 改为：
  #ifndef X_H__ 
 #define X_H__ 
 
 class X 
 {
 public：
 int foo（）; 
}; 
 
 int X :: foo（）{return 1; } 
 
 #endif 
  
尝试链接 something.cpp 和 something_else.cpp 。  X :: foo 将被非内联定义为两个单独的翻译单元。
 
I have a class (A) which has to include two files, class X and class Y. unfortunately class Y also needs to include class X in it's header file because the constructor takes a pointer to a class X type as an argument. 

The potential problem is that there would be a linker error? as class A now has two copies of class X in it, one that it needs to use, and one from class Y. This is the situation where header guards are of no use. My question is - is this purely a structural problem or is there a way around this?

I really would prefer to not include anything in class Y's header file, in case I want to include THAT in anything else, but is it necessary because of the function prototype? 
解决方案
If you have the following:

X.h
#ifndef X_H__
#define X_H__

class X
{
public:
    int foo() { return 1; }
};

#endif
Y.h
#ifndef Y_H__
#define Y_H__

#include "X.h"

class Y
{
public:
    Y(X *pX) { myval = pX->foo(); }
    int myval;
};

#endif
something.cpp
#include "X.h"

...
something_else.cpp
#include "Y.h"

...
Then there should be no problem.

However, if X.h instead looks like this:
#ifndef X_H__
#define X_H__

class X
{
public:
    int foo();
};

int X::foo() { return 1; }

#endif
then you will indeed get a linker error when you try to link something.cpp and something_else.cpp.  X::foo will have been defined non-inline into two separate translation units.

                        
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