如何在C++中使用循环依赖类？

Question

此代码未编译.

我可以做些什么修改才能达到预期效果？

ClassOne.h

#ifndef _CLASS_ONE_
#define _CLASS_ONE_

#include <string>
#include "ClassTwo.h"

class ClassTwo;

class ClassOne
{
private:
    string message;     
    friend ClassTwo;
    ClassTwo m_ClassTwo;

public:
    ClassOne();
    void Display();
};

#endif

ClassTwo.h

#ifndef _CLASS_TWO_
#define _CLASS_TWO_

#include <string>
#include "ClassOne.h"

class ClassOne;

class ClassTwo
{
private:
    string message;
    friend ClassOne;
    ClassOne m_ClassOne;

public:
    ClassTwo();
    void Display();
};

#endif

ClassOne.cpp

#include "ClassOne.h"
#include "ClassTwo.h"
#include <iostream>

ClassOne :: ClassOne()
{
    std::cout<<"ClassOne()...called\n";
    this->m_ClassTwo.message = "ClassOne - Message\n";
}

void ClassOne :: Display()
{
    std::cout<<this->m_ClassTwo.message;
}

ClassTwo.cpp

#include "ClassTwo.h"
#include "ClassOne.h"
#include <iostream>

ClassTwo :: ClassTwo()
{
    std::cout<<"ClassTwo()...called\n";
    this->m_ClassOne.message = "ClassTwo - Message\n";
}

void ClassTwo :: Display()
{
    std::cout<<this->m_ClassOne.message;
}

main.cpp中

#include "ClassOne.h"
#include "ClassTwo.h"

int main()
{
    ClassOne one;
    one.Display();

    ClassTwo two;
    two.Display();
}

错误消息

1   error C2146: syntax error : missing ';' before identifier 'message'
2   error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
3   error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
4   error C2079: 'ClassTwo::m_ClassOne' uses undefined class 'ClassOne'
5   error C2146: syntax error : missing ';' before identifier 'message'
6   error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
7   error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
8   error C2039: 'message' : is not a member of 'ClassTwo'
9   error C2039: 'message' : is not a member of 'ClassTwo'
10  error C2146: syntax error : missing ';' before identifier 'message'
11  error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
12  error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
13  error C2079: 'ClassOne::m_ClassTwo' uses undefined class 'ClassTwo'
14  error C2146: syntax error : missing ';' before identifier 'message'
15  error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
16  error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
17  error C2039: 'message' : is not a member of 'ClassOne'
18  error C2039: 'message' : is not a member of 'ClassOne'
19  error C2146: syntax error : missing ';' before identifier 'message'
20  error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
21  error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
22  error C2079: 'ClassTwo::m_ClassOne' uses undefined class 'ClassOne'
23  error C2146: syntax error : missing ';' before identifier 'message'
24  error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
25  error C4430: missing type specifier - int assumed. Note: C++ does not support default-int

Answer 1

你不能编译那段代码.您已经描述了一个系统,其中类型A包含类型B,类型B包含类型A.因此,类型A和B都递归地且无限地包含它们自己,这是不可能的.您必须从根本上重新设计代码以消除此问题.

另外,你的friend语法错了.