条款24：若所有参数皆需类型转换，请为此采用非成员函数

设计一个表示有理数的类时，允许从整数隐式转换为有理数是有用的：

class Rational {
public:
    Rational(int numerator = 0, // 该构造函数没有explicit限制;
        int denominator = 1); 

    int numerator() const;  
    int denominator() const;  

    const Rational operator*(const Rational& rhs) const;
private:
    int n,d;
};

Rational oneEighth(1, 8);
Rational oneHalf(1, 2);
Rational result = oneHalf * oneEighth; 	// 正确
result = result * oneEighth; 			// 正确
result = oneHalf * 2; // result = oneHalf.operator*(2); 正确
result = 2 * oneHalf; // result = 2.operator*(oneHalf); 错误!

?乘法应该是可交换的，但是由于参数的隐式类型转换只发生在参数列表中，所以2 * oneHalf会报错，由于2是int类型的参数。
?这时候我们可以将operator重载函数修改为非成员函数，编译器还会寻找非成员运算符(即命名空间或全局作用域中的运算符):

const Rational operator*(const Rational& lhs, const Rational& rhs) 
{
    return Rational(lhs.numerator() * rhs.numerator(),
        lhs.denominator() * rhs.denominator());
}
Rational oneFourth(1, 4);
Rational result;
result = oneFourth * 2; // 正确
result = 2 * oneFourth; // 欧耶，它起作用了!

#include <iostream>

class Rational {
public:
	Rational(int numerator = 0, // 该构造函数没有explicit限制;
		int denominator = 1):n{numerator},d{denominator}
	{ }

	int numerator() const {
		return n;
	}
	int denominator() const { 
		return d;
	}

	//const Rational operator*(const Rational& rhs) const {
	//	return Rational(this->n*rhs.n,this->d*rhs.d);
	//}
	void print() const {
		if(n%d==0)
			std::cout << n /d << std::endl;
		else if(d%n==0)
			std::cout << 1 << "/" << d/n << std::endl;
		else
			std::cout << n << "/" << d << std::endl;
	}
private:
	int n, d;
};
const Rational operator*(const Rational& lhs, const Rational& rhs)
{
	return Rational(lhs.numerator() * rhs.numerator(),
		lhs.denominator() * rhs.denominator());
}

int main()
{
	Rational oneEighth(1, 8);
	Rational oneHalf(1, 2);
	Rational result = oneHalf * oneEighth; 	// 正确
	result = result * oneEighth; 			// 正确
	result.print();
	result = oneHalf * 2; // result = oneHalf.operator*(2); 正确
	result.print();
	result = 2 * oneHalf; // result = 2.operator*(oneHalf); 
	result.print();
}