类定义交叉引用时如何声明朋友方法? [英] How to declare friend method when class definitions cross reference?

问题描述

我想定义两个类，A和B.A有一个数据成员，该成员是B类对象，并且在类中进行了初始化. A也有一种方法来检索此B类型数据成员中的值，并且该方法将在B中声明为好友方法.这是我的代码:

I want to define two classes, A and B. A has a data member which is a Class B object and is in-class initialised. A also has a method to retrieve the value in this B type data member and this method would be declared as a friend method in B. Here is my code:

class A{
public:
    int getBValue();
private:
    B b=B(1);
};

class B{
public:
    friend int A::getBValue();
    B(int i):value(i){}
private:
    int value;
};

int A::getBValue(){
    return b.value;
}

毫不奇怪，由于类A定义中的类型B未知，编译失败.我试图在源代码中交换A和B的定义，结果甚至更糟.有没有可能解决A和B之间的交叉引用问题的方法?

And unsurprisingly the compilation had failed because of unknown type B in class A definition. I had tried to swap the definitions of A and B in the source and the result was even worse. Is there a possible way to resolve this cross reference issue between A and B?

推荐答案

如果这是您所拥有的完整代码，则问题在于编译器当时不知道B是什么编译类A.解决此问题的一种方法是创建指向B的指针，而不是拥有B本身:

If this is the complete code as you have it, then the problem is that the compiler doesn't know what a B is at the time it is compiling class A. One way to solve it is by creating a pointer to B instead of having a B itself:

A.h

#ifndef CLASS_A
#define CLASS_A

class B;

class A{
public:
    int getBValue();
private:
    B *b;
};

#endif

B.h

#ifndef CLASS_B
#define CLASS_B

#include "A.h"

class B{
public:
    friend int A::getBValue();
    B(int i):value(i){}
private:
    int value;
};

#endif

A.cpp

#include "A.h"
#include "B.h"

int A::getBValue(){
    return b->value;
}

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