学习C++——类的自动转换和强制类型转换

类的自动转换和强制类型转换

/*本程序主要的用法是：1、类型转换函数的定义以及使用。（double类型赋值给对象类型，此时构造函数可以实现；  对象类型赋值给double类型，此时用转换函数来实现。）2、成员函数实现加法的重载。3、友元函数实现加法的重载。*/#ifndef STONEWT_H_#define STONEWT_H_class Stonewt{private:enum {Lbs_per_stn = 14};//每14磅等于1英石int stone;//英石double pds_left;//double pounds;//磅public:Stonewt(double lbs);//单位是磅Stonewt(int stn, double lbs);//单位是英石和磅Stonewt();~Stonewt();void show_lbs() const;void show_stn() const;/*转换函数：因为上面可以知道，存在Stonewt wolfe(25.6);也就是可以将一个double类型赋值给Stonewt对象，但是能否将一个Stonewt对象赋值给一个double类型，即double host =？ wolfe;这就需要用到转换函数了。转换函数的原型是：operator double();operator int();转换函数是类方法，需要通过类对象来调用，从而告知函数要转换的值。*///转换函数的声明operator double () const;operator int () const;//使用成员函数//Stonewt operator+(const Stonewt & st) const;//使用友元函数来重载加法friend Stonewt operator+(const Stonewt & st1, const Stonewt & st2);};#endif

#include <iostream>using std::cout;#include "16.h"Stonewt::Stonewt(double lbs)//构造函数{stone = int (lbs) / Lbs_per_stn;pds_left = int (lbs) % Lbs_per_stn + lbs - int (lbs);pounds = lbs;}Stonewt::Stonewt(int stn, double lbs)//构造函数{stone = stn;pds_left = lbs;pounds = stn * Lbs_per_stn + lbs;}Stonewt::Stonewt()//默认构造函数{stone = pounds = pds_left = 0;}Stonewt::~Stonewt()//析构函数{}void Stonewt::show_lbs() const//显示函数，显示磅{cout << pounds << " pounds\n";}void Stonewt::show_stn() const//显示函数，显示英石{cout << stone << " stone, " << pds_left << " pounds\n";}//转换函数的定义Stonewt::operator double () const{return pounds;}Stonewt::operator int() const{return int (pounds + 0.5);}//使用成员函数重载加法/*Stonewt Stonewt::operator+(const Stonewt & st) const{double pds = pounds + st.pounds;Stonewt sum(pds);return sum;}*///使用友元函数重载加法Stonewt operator+(const Stonewt & st1, const Stonewt & st2){double pds = st1.pounds + st2.pounds;Stonewt sum(pds);return sum;}

#include <iostream>#include "16.h"using std::cout;void display(const Stonewt & st, int n);int main(){Stonewt incognito = 275;Stonewt wolfe(287.7);Stonewt taft(21, 8);cout << "The incognito weighed ";incognito.show_stn();cout << "The wolfe weighed ";wolfe.show_stn();cout << "The taft weighed ";taft.show_lbs();incognito = 276.8;/*incognito = 276.8;使用接收double参数的构造函数，将276.8转换为一个Stonewt值，这将把incognito的pound成员设置为276.8。因为语句使用了构造函数，所以还将设置shone,pds_left成员。*/taft = 325;/*将一个int值转换为double类型，然后使用Stonewt(double)来设置全部3个成员*/cout << "\nincognito = 276.8;the incognito weighed ";incognito.show_stn();cout << "taft = 325;the taft weighed ";taft.show_lbs();cout << "\ndisplay(taft, 2);显示的结果：\n";display(taft, 2);cout << "\ndisplay(422, 2);显示的结果：\n";display(422, 2);/*先将422，转换为一个double数，然后使用Stonewt(double)将其转换为一个Stonewt对象。*/cout << "\n验证转换函数,定义了double变量poun:\n";double poun;poun = taft;cout << "poun = taft = " << poun << "\n";cout << "\n成员函数重载加法：\n";Stonewt st1 (12.3);Stonewt st2 (11.4);Stonewt sum_st;sum_st = st1 + st2;/*使用友元函数或者是使用成员函数实现加法重载，但是不能同时定义两个函数，只能定义其中一个。*/cout << "sum_st = " ;sum_st.show_lbs();std::cin.get();return 0;}void display(const Stonewt & st, int n){for(int i = 0; i<n; i++){cout << "wow! ";st.show_stn();}}