如何使用友元函数和继承拆分定义和声明

#include <iostream>

class Base {
private:
    int privateData;

public:
    Base(int data) : privateData(data) {}

    // 声明友元函数
    friend void accessPrivateData(Base& obj);

    // 声明虚函数
    virtual void display() {
        std::cout << "Base class display: " << privateData << std::endl;
    }
};

// 定义友元函数
void accessPrivateData(Base& obj) {
    std::cout << "Private data: " << obj.privateData << std::endl;
}

class Derived : public Base {
public:
    Derived(int data) : Base(data) {}

    // 重写虚函数
    void display() override {
        std::cout << "Derived class display: " << privateData << std::endl;
    }
};

int main() {
    Base base(10);
    Derived derived(20);

    base.display();
    derived.display();

    accessPrivateData(base);
    accessPrivateData(derived);

    return 0;
}

class Derived : public Base {
private:
    int derivedPrivateData;

public:
    Derived(int data) : Base(data), derivedPrivateData(data * 2) {}

    // 再次声明友元函数
    friend void accessPrivateData(Base& obj);
};

void accessPrivateData(Base& obj) {
    std::cout << "Base private data: " << obj.privateData << std::endl;
    Derived* derived = dynamic_cast<Derived*>(&obj);
    if (derived) {
        std::cout << "Derived private data: " << derived->derivedPrivateData << std::endl;
    }
}