Effective C++ 若所有参数都需要类型转换，使用非成员函数

#include <iostream>

//有理数
class Rational {
public:
    Rational(double numerator = 0, double denominator = 1) :
        m_n(numerator), m_d(denominator) {}

    double numerator() const//分子
    {
        return m_n;
    }
    double denominator() const//分母
    {
        return m_d;
    }
    double value()
    {
        return (m_n / m_d);
    }

    //成员函数的发起者必须是类, 因为可能出现 int 类型在前的情况, 则无法执行乘法, 所以应使用非成员函数
    const Rational operator* (const Rational& lhs)
    {
        return Rational(lhs.numerator() * m_n,
            lhs.denominator() * m_d);
    }

private:
    double m_n;
    double m_d;
};

const Rational operator* (const Rational& lhs, const Rational& rhs)//非成员函数
{
    return Rational(lhs.numerator() * rhs.numerator(),
        lhs.denominator() * rhs.denominator());
}

int main()
{
    //有理数乘以有理数
    Rational oneHalf(1, 2);
    Rational result = oneHalf * oneHalf;

    //整数和有理数混乘
    result = oneHalf * 2;//non-explicit构造函数，编译器会根据2构造临时Rational
    std::cout << result.numerator() << "  " << result.denominator() << std::endl;
    result = 2 * oneHalf;
    std::cout << result.numerator() << "  " << result.denominator() << std::endl;

    std::cout << "Hello World!\n";
}