条款 24：若所有参数皆需类型转换，请为此采用 non-member 函数

条款 24：若所有参数皆需类型转换，请为此采用 non-member 函数

Declare non-member functions when type conversions should apply to all parameters.

class Rational
{
public:
    Rational(int numerator = 0, int denominator = 1);
    int numerator() const;
    int denominator() const;
}
class Rational
{
public:
    ...
    const Rational operator* (const Rational& rh) const;
}
Rational oneEighth(1, 8);
Rational oneHalf(1, 2);
result = oneHalf * 2;   // OK
result = 2 * oneHalf;   // ERR
const Rational operator*(const Rational& lhs, const Rational& rhs)
{
    return Rational(lhs.numerator() * rhs.numerator(),
                    lhs.denominator() * rhs.denominator())
}

第一种方式中由于不存在接受一个 int 和 Rational 作为参数的 non-member operator*, 因此失败。