c++ - 如何在C++中使用循环依赖类？

此代码未编译。

我可以做什么修改才能达到预期的效果？

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

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

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

    ClassTwo two;
    two.Display();
}

错误讯息

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

最佳答案

如前所述，您需要转发声明ClassOne或ClassTwo中的至少一个。

您的ClassOne.h可能看起来像这样:

#ifndef _CLASS_ONE_
#define _CLASS_ONE_

#include <string>

class ClassTwo;

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

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

#endif

如您所见，我们声明ClassTwo，但不包括它。我们基本上只告诉编译器是的，我们确实有一个ClassTwo，但是我们并不在乎它现在包含什么。

还要看看您的ClassTwo成员，这是一个指针。原因是成员需要编译器知道对象的大小，我们目前不知道这是什么。因此，您需要一个指针或一个引用。

接下来，在您的ClassOne.cpp中，将需要包含ClassTwo.h，以获取该类的功能和大小。

不过有几件事:
使用转发声明，您不能从ClassTwo继承，请在头文件中使用转发类的方法(编译器将如何知道存在哪些方法？)使用转发类定义函数或方法，即您必须通过引用传递或传递一个指针。

关于c++ - 如何在C++中使用循环依赖类？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/9461551/