在c++中，如何有效地在两个类之间共享数据？

class ClassB; // 前向声明ClassB

class ClassA {
public:
    friend void friendFunction(ClassA&, ClassB&); // 声明ClassB的友元函数

private:
    int privateData;
};

class ClassB {
public:
    void accessData(ClassA& obj) {
        cout << "ClassB accessing private data of ClassA: " << obj.privateData << endl;
    }
};

void friendFunction(ClassA& objA, ClassB& objB) {
    cout << "Friend function accessing private data of ClassA: " << objA.privateData << endl;
    objB.accessData(objA); // 可以调用ClassB的成员函数访问ClassA的私有数据
}

class ClassB; // 前向声明ClassB

class ClassA {
public:
    friend class ClassB; // 声明ClassB为友元类

private:
    int privateData;
};

class ClassB {
public:
    void accessData(ClassA& obj) {
        cout << "ClassB accessing private data of ClassA: " << obj.privateData << endl;
    }
};

class ClassB; // 前向声明ClassB

class ClassA {
public:
    void accessData(ClassB& obj); // 定义成员函数

private:
    int privateData;
};

class ClassB {
public:
    void accessData(ClassA& obj) {
        cout << "ClassB accessing private data of ClassA: " << obj.privateData << endl;
    }
};

void ClassA::accessData(ClassB& objB) {
    cout << "ClassA accessing private data of ClassB: ";
    objB.privateData; // 可以直接访问ClassB的私有数据
}