类类型与其他类型的转换

类型学转换成其他类型有两种方式：

    一、用构造函数，将基本类型转为构造类型

        1.用初始化的形式；

class X

{

public:

X(int n);

~X();

};

void f(X arg);

...

int main(){

    X(3);

    X=a;//a隐式调用构造函数X（int n）;

    f(5);//把5调用构造函数X(int n)转为X arg.然后调用构造函数。

    return 0;

}

    二、因为带参数的构造函数无法将类类型转为基本类型，所以设置类型转换函数

#include<iostream>

using namespace std;

class rational

{

public:

rational();

rational(int n,int d=1);

rational(double x);//将double类型转换成类类型。

operator double();//将类类型转变为double类型，且注意其没有返回值。

friend rational& operator+(const rational&, const rational&);

friend ostream & operator <<( ostream& os, const rational&);

//注意这里输出类ostream前面不可以加const因为在输出流经过os时会修改os

        //且类型一定是引用类型因为必须用cout对象本身

~rational();

private:

int numerator;

int denominator;

};

int gcd(int a, int b);

rational::rational()

{

numerator = 0;

denominator = 0;

}

rational::rational(int n,int d){

int g;

if (d == 1){

numerator = n;

denominator = d;

}

else{

g = gcd(n, d);//求分子分母的最大公约数

numerator = n / g;//把分母化成最简；

denominator = d / g;

}

}

rational::rational(double x){

int a, b, g;

a = int(x*1e5);//x乘10的5次方。把小数变成整数

b = int(1e5);

g = gcd(a, b);

numerator = a / g;

denominator = b / g;

}

rational::~rational()

{

}

rational::operator double(){//虽然没有返回值依然要返回double类型的数。

return double(numerator) / double(denominator);

}

rational& operator +(const rational& a, const rational& b){

rational c;

int d = a.denominator*b.denominator;

int n = a.numerator*b.denominator + a.denominator*b.numerator;

int g = gcd(n, d);

c.denominator = d / g;//将分子分母化为最简

c.numerator = n / g;

return c;//返回类型是引用还是类类型都可以。

}

ostream& operator <<(ostream& os, const rational& a){

os << a.numerator;

if (a.denominator != 1){

os << "/" << a.denominator;

}

return os;//这里返回os类型的引用目的是连续使用cout<<"ss"<<"sss";

}

int gcd(int n, int d){//求最大公约数的算法

if (d == 0) return n;

else{

return gcd(d, n%d);

}

}

int main(){

rational a(2, 4);

rational b = 0.3;

rational c = a + b;

cout << double(a) << "+" << double(b)<<"="<<double(c)<<endl;

//将类类型对象a,b,c转换成double类型。

cout << a << "+" << b << "=" << c << endl;

double x = b;

c = x + 1 + 0.6;

cout << x << "+" << 1 << "+" << 0.6 << "=" << c << endl;

cout << rational(x) << "+" << rational(1) << "+" << rational(0.6) << "=" << c << endl;

system("pause");

}