c++沉思录 学习笔记 第六章 句柄(引用计数指针雏形？)

一个简单的point坐标类

 

class Point {
public:
Point():xval(0),yval(0){}
Point(int x,int y):xval(x),yval(y){}
int x()const { return xval; }
int y()const { return yval; }
Point& x(int xv) { xval = xv; return *this; }
Point& y(int yv) { yval = yv; return *this; }
private:
int xval, yval;
};

将handle 绑定到其上 进行一层封装

通过handle控制那些操作可以公开 那些操作私有 阻止用户对point的地址的操作

更重要的是提供 point的引用计数 将多个handle绑定到ppint上 当全部释放对point的使用权时

才进行释放point的操作

那么增加两个类 

一个引用计数类 class UPoint

class UPoint {
friend class Handle;
Point p;
int u;

UPoint() :u(1) {}
UPoint(int x, int y) :p(x, y), u(1) {}
UPoint(const Point& p0):p(p0),u(1){}
};

一个是handle句柄类  包含UPoint的指针  构造和析构的时候会将计数+1 

当计数到0 则释放指针内存

class Handle {
public:
Handle();
Handle(int, int);
Handle(const Point&);
Handle(const Handle&);
Handle& operator=(const Handle&);
~Handle();
int x()const;
Handle& x(int);
int y()const;
Handle& y(int);
private:
UPoint* up;
};

 

全部代码如下:

// sample0.cpp: 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include <iostream>

class Point {
public:
    Point():xval(0),yval(0){}
    Point(int x,int y):xval(x),yval(y){}
    int x()const { return xval; }
    int y()const { return yval; }
    Point& x(int xv) { xval = xv; return *this; }
    Point& y(int yv) { yval = yv; return *this; }
private:
    int xval, yval;
};

class UPoint {
    friend class Handle;
    Point p;
    int u;

    UPoint() :u(1) {}
    UPoint(int x, int y) :p(x, y), u(1) {}
    UPoint(const Point& p0):p(p0),u(1){}
};

class Handle {
public:
    Handle();
    Handle(int, int);
    Handle(const Point&);
    Handle(const Handle&);
    Handle& operator=(const Handle&);
    ~Handle();
    int x()const;
    Handle& x(int);
    int y()const;
    Handle& y(int);
private:
    UPoint* up;
};

Handle::Handle() :up(new UPoint) {}
Handle::Handle(int x, int y) : up(new UPoint(x, y)) {}
Handle::Handle(const Point& p) : up(new UPoint(p)) {}

Handle::~Handle() {
    if (up->u == 0)
        delete up;
}

Handle::Handle(const Handle& h) :up(h.up) { ++up->u; }
Handle& Handle::operator=(const Handle& h) {
    ++h.up->u;
    if (--up->u == 0) {
        delete up;
    }
    return *this;
}

int Handle::x()const { return up->p.x(); }
int Handle::y()const { return up->p.y(); }

Handle& Handle::x(int x0)
{
    up->p.x(x0);
    return *this;
}

Handle& Handle::y(int y0) {
    up->p.y(y0);
    return *this;
}


int main()
{
    Handle h;
    std::cout << h.x() << " " << h.y() << std::endl;
    return 0;
}