必須返回對象時，別妄想返回其reference

class Rational
{
public:
    Rational(int numerator = 0, int denominator = 1) : n(numerator), d(denominator) {
        printf("Rational Constructor\n");
    }
    ~Rational() {
        printf("Rational Destructor\n");
    }
    Rational(const Rational& rhs) {
        this->d = rhs.d;
        this->n = rhs.n;
        printf("Rational Copy Constructor\n");
    }
private:
    int n, d;
    friend const Rational operator*(const Rational& lhs, const Rational& rhs);
};

Rational的*運算符能夠這樣重載：

const Rational operator*(const Rational& lhs, const Rational& rhs)
{
    Rational tmp(lhs.n * rhs.n, lhs.d * rhs.d);
    return tmp;
}

可是不能夠這樣重載：【區別在於一個&】

const Rational& operator*(const Rational& lhs, const Rational& rhs)
{
    Rational tmp(lhs.n * rhs.n, lhs.d * rhs.d);
    return tmp;
}

當這樣去使用：

Rational x(1, 2), y(2, 3);
Rational z = x * y;

第一種方法能夠獲得正確的結果，由於會調用Rational的拷貝構造函數將tmp賦給z，可是第二種方法返回的是tmp的引用，在函數退出前，tmp就被銷燬了，因此這樣作是不對的。

不過，第一種方法雖然功能上沒有問題，可是效率上有所欠缺，由於調用了三次構造函數，一次複製構造函數，一次析構函數

Rational Constructor
Rational Constructor
Rational Constructor
Rational Copy Constructor
Rational Destructor

能夠進行返回值優化以下：

const Rational operator*(const Rational& lhs, const Rational& rhs)
{
    return Rational(lhs.n * rhs.n, lhs.d * rhs.d);
}

Rational Constructor
Rational Constructor
Rational Constructor

優化以後少調用了一次複製構造函數和析構函數

完整代碼以下：

#include <stdio.h>
class Rational
{
public:
    Rational(int numerator = 0, int denominator = 1) : n(numerator), d(denominator) {
        printf("Rational Constructor\n");
    }
    ~Rational() {
        printf("Rational Destructor\n");
    }
    Rational(const Rational& rhs) {
        this->d = rhs.d;
        this->n = rhs.n;
        printf("Rational Copy Constructor\n");
    }
private:
    int n, d;
    friend const Rational operator*(const Rational& lhs, const Rational& rhs);
};

const Rational operator*(const Rational& lhs, const Rational& rhs)
{
    return Rational(lhs.n * rhs.n, lhs.d * rhs.d);
}

int main()
{
    Rational x(1, 2), y(2, 3);
    Rational z = x * y;
    return 0;
}