C++关于类的强制类型转换

在看《C++ Primer Plus》第六版的过程中，第十一章的后面部分讲到了关于类的强制类型转换。其中分为将内置类型转换为自定义类和将自定义类转换为内置类型。

1、将内置类型转换为自定义类

将内置类型转换为自定义类的方法很简单，即调用在类中定义的只有一个参数的构造函数。比如有一个Stonewt类声明如下（跟如何定义成员函数没有多大关系）：

// stonewt.h -- definition for the Stonewt class#ifndef STONEWT_H_#define STONEWT_H_class Stonewt{private:enum {lbs_per_stn = 14};// pounds per stoneint stone;// whole stonesdouble pds_left;// fractional poundsdouble pounds;// entire weight in poundspublic:Stonewt(double lbs);// constructor for double poundsStonewt(int stn, double lbs);// constructor for stone, lbsStonewt();// default constructor~Stonewt();void show_lbs() const;// show weight in pounds formatvoid show_stn() const;// show weigth in stone format// conversion functionsoperator int() const;operator double() const;};#endif

可以用如下语句将某种数值类型转换为Stonewt类型

Stonewt x = 255.5;Stonewt y = 12;

第一句将直接调用类的只有一个参数的构造函数（如果存在的话）；

第二句也会调用只有一个参数的构造函数，但是12会被先转换为double型。

2、将自定义类转换为内置类型

在C++中，使用转换函数来执行强制类型转换，函数声明格式如下：

operator TypeName();

注意几点：

i)转换函数必须是类成员函数

ii)转换函数不能有参数

iii)不能指定转换函数的返回类型

还是用上面的例子，转换函数已经声明在类中了，我们可以如下定义他们，让类的实例可以被强制转换为 int 类型 或 double 类型：

Stonewt::operator int() const{return int (pounds + 0.5);}Stonewt::operator double() const{return pounds;}

这样就可以如下对类使用强制类型转换了：

Stonewt x = 123.4;int y;y = int(x);

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另外，上面的类出自于《C++ Primer Plus》第六版

完整类定义如下：

// stonewt.cpp -- Stonewt methods#include <iostream>#include "stonewt.h"using std::cout;Stonewt::Stonewt(double lbs){stone = int(lbs) / lbs_per_stn; // integer divisionpds_left = int(lbs) % lbs_per_stn + lbs - int(lbs);pounds = lbs;}Stonewt::Stonewt(int stn, double lbs){stone = stn;pds_left = lbs;pounds = stn * lbs_per_stn + lbs;}Stonewt::Stonewt(){stone = pounds = pds_left = 0;}Stonewt::~Stonewt() {}// show weight in stonesvoid Stonewt::show_stn() const{cout << stone << " stones, " << pds_left << " pounds\n";}// show weight in poundsvoid Stonewt::show_lbs() const{cout << pounds << " pounds\n";}Stonewt::operator int() const{return int (pounds + 0.5);}Stonewt::operator double() const{return pounds;}

测试代码如下：

// stone.cpp -- user-defined conversions// compile with stonewt.cpp#include <iostream>#include <cstdlib>#include "stonewt.h"using std::cout;void display(const Stonewt & st, int n);int main(){Stonewt incognito = 275;Stonewt wolfe(285.7);Stonewt taft(21, 8);cout << "The celeberity weight ";incognito.show_stn();cout << "The detective weight ";wolfe.show_stn();cout << "The President weight ";taft.show_lbs();incognito = 276.8;taft = 325;cout << "After dinner, the celeberity weight ";incognito.show_stn();cout << "After dinner, the President weight ";taft.show_lbs();display(taft, 2);cout << "The wrestler weight even more.\n";display(422, 2);cout << "No stone left unearned\n";int x;Stonewt y = 235.5;cout << "before conversion: ";y.show_lbs();x = int(y);cout << "after conversion: " << x << "\n";system("pause");return 0;}void display(const Stonewt & st, int n){for(int i = 0; i < n; ++i){cout << "Wow! ";st.show_stn();}}

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