100.类型转换以及类类型转换

#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
using namespace std;

class myclass
{
public:
    int x;
    int y;

    myclass()
    {

    }
    //赋值初始化
    //加上explicit表示不能调用隐式的转换
    explicit myclass(int a) :x(a), y(a)
    {
        cout << "构造" << endl;
    }

    //赋值重载
    void operator =(int num)
    {
        cout << "赋值重载";
        this->x = 3;
    }

    //类类型转换为int 不能有参数
    //类类型转换只能是成员函数,必须有this指针
    operator int()
    {
        return (x + y);
    }

    operator double();
};

myclass::operator double()
{
    return x + y;
}

//CPP编译的是从上到下,会遇到不匹配问题
//类的声明可以拓展作用域,但是只能创建指针或者引用,不能创建对象
//完成类类型转换的时候,一般两个类的转换,两个转换接口都放在一个类

class mypoint2
{
public:
    int a;
    int b;

public:
    mypoint2(int x, int y) :a(x), b(y)
    {

    }

    mypoint2() :a(0), b(0)
    {

    }
};

//该类提供了两个接口,把mypoint转换为mypoint2 mypoint2转换为mypoint1
class mypoint
{
public:
    int x;
    int y;
    int z;
    mypoint() :x(10), y(20), z(20)
    {

    }

    //mypoint2类型转换为mypoint类型
    mypoint(const mypoint2 &my):x(my.a),y(my.b),z(0)
    {

    }

    //mypoint类型转化为mypoint2类型
    operator mypoint2()
    {
        mypoint2 temp(x, y);
        return temp;
    }

    //重载等于号
    void operator=(const mypoint2 &my)
    {
        x = my.a;
        y = my.b;
        z = 0;
    }
};

void main()
{
    //c++的转换
    ////自动转换
    //int a = 1.2;
    ////显式转换
    //int b = static_cast<int>(2.3);
    ////C风格
    //int c = (int)2.5;

    //int *p1 = nullptr;
    ////CPP风格的指针转换
    //char *p2 = reinterpret_cast<char *>(p1);

    //构造函数初始化,视作类型转换
    /*myclass my1 = static_cast<myclass>(5);
    myclass my2(2);
    myclass my3 = myclass(5);
    myclass my4 = (myclass)5;
    myclass my5;
    my5 = 3;*/

    //调用类类型转换为int类型
    //myclass my1;
    //my1.x = 4;
    //my1.y = 3;
    //int a = my1;
    //cout << a << endl;

    mypoint2 my1(1, 2);
    mypoint my2(my1);
    //mypoint my3 = my1;

    //调用operator mypoint2() 进行类型转换
    mypoint2 my4= my2;

    cin.get();
}

类类型转换在实际开发中的用途

#include "mainwindow.h"
#include <QApplication>
#include <QDebug>

class winsize
{
public:
    int x;
    int y;
};

class mywin
{
public:
    MainWindow *p;
    int cx;
    int cy;

public:
    mywin():cx(0),cy(0),p(nullptr)
    {

    }

    //mywin到winsize的类型转换
    mywin(const winsize &mysize):cx(mysize.x),cy(mysize.y)
    {
        p=new MainWindow;
        p->resize(cx,cy);
        p->show();;
    }

    //winsize到mywin的类型转换
    operator winsize()
    {
        winsize tmp{cx,cy};
        return tmp;
    }
    
    void operator = (const winsize &mysize)
    {
        if(p!=nullptr)
        {
            p->resize(cx,cy);
            p->show;
        }
        else
        {
            p = new MainWindow;
            p->resize(cx,cy);
            p->show();
        }
    }

};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    winsize win1{200,600};
    mywin my = win1;

    //winsize win2 = my;
    //上述语句的本质
    winsize win2 = my.operator winsize();
    
    qDebug()<<win2.x << win2.y;
    return a.exec();
}