• 友元函数
#include <iostream>
using namespace std;
//友元函数定义在类外部，有权访问类的所有私有和保护成员
// 友元函数不是成员函数
class Box{
public:
    double length;
    friend void printWidth(Box box);
    void setWidth(double width);

private:
    double width;
};
void Box::setWidth(double width) {

    this->width = width;
}

void printWidth(Box box){

    cout<<"width of box:"<<box.width<<endl;

}

int main(){
    Box box;
    box.setWidth(2);
    printWidth(box);
    return 0;
};
  • 友元函数需要访问
#include <iostream>

using namespace std;
//友元函数定义在类外部，有权访问类的所有私有和保护成员
// 友元函数不是成员函数
class Box{
public:
    double length=0;
    friend void printWidth(Box box);
    void setWidth(double width);

private:
    double width;
};
void Box::setWidth(double width) {

    this->width = width;
}

void printWidth(Box box){
    cout<<"test"<<endl;
    cout<<"height of box:"<<box.length<<endl;
    cout<<"width of box:"<<box.width<<endl;

}

int main(){
    Box box;
    box.setWidth(2);
    printWidth(box);
    return 0;
};
// 友元类
#include <iostream>

using namespace std;
//友元函数定义在类外部，有权访问类的所有私有和保护成员
// 友元函数不是成员函数
class Box{
public:
    double length=0;
    friend  class BigBox;
    void setWidth(double width);
    Box()
    {
        cout<<"普通构造函数"<<endl;
        this->setWidth(2);
    }
    Box(const Box &box){
        cout<<"复制构造函数"<<endl;

        this->width = box.width;

    }

private:
    double width;
};
void Box::setWidth(double width) {

    this->width = width;
}

class BigBox{
public:
    int length;
    double width;
    double height;
    void Print(Box box)
    {
        box.setWidth(2);
        cout<<"the box's width is "<<box.width<<endl;
    }

};
int main(){

    BigBox bigbox;
    Box box;
    box.setWidth(2);
    bigbox.Print(box);
    return 0;
};

C++从入门到精通——类与面向对象_封装