13.7 Overloading the comparison operators

https://www.learncpp.com/cpp-tutorial/overloading-the-comparison-operators/

이번 섹션에서는 비교 연산자에 대해서 살펴본다
==와 != 를 어떻게 구현하고 있는지 예시를 보자

#include <iostream>
#include <string>

class Car
{
private:
    std::string m_make;
    std::string m_model;

public:
    Car(const std::string& make, const std::string& model)
        : m_make{ make }, m_model{ model }
    {
    }

    friend bool operator== (const Car& c1, const Car& c2);
    friend bool operator!= (const Car& c1, const Car& c2);
};

bool operator== (const Car& c1, const Car& c2)
{
    return (c1.m_make == c2.m_make &&
            c1.m_model == c2.m_model);
}

bool operator!= (const Car& c1, const Car& c2)
{
    return (c1.m_make != c2.m_make ||
            c1.m_model != c2.m_model);
}

int main()
{
    Car corolla{ "Toyota", "Corolla" };
    Car camry{ "Toyota", "Camry" };

    if (corolla == camry)
        std::cout << "a Corolla and Camry are the same.\n";

    if (corolla != camry)
        std::cout << "a Corolla and Camry are not the same.\n";

    return 0;
}

위의 코드에서는 friend function으로 구현하고 있다

다음은 대소 연산자까지 추가한 example 코드이다

#include <iostream>

class Cents
{
private:
    int m_cents;

public:
    Cents(int cents)
	: m_cents{ cents }
	{}

    friend bool operator== (const Cents& c1, const Cents& c2);
    friend bool operator!= (const Cents& c1, const Cents& c2);

    friend bool operator< (const Cents& c1, const Cents& c2);
    friend bool operator> (const Cents& c1, const Cents& c2);

    friend bool operator<= (const Cents& c1, const Cents& c2);
    friend bool operator>= (const Cents& c1, const Cents& c2);
};

bool operator== (const Cents& c1, const Cents& c2)
{
    return c1.m_cents == c2.m_cents;
}

bool operator!= (const Cents& c1, const Cents& c2)
{
    return c1.m_cents != c2.m_cents;
}

bool operator< (const Cents& c1, const Cents& c2)
{
    return c1.m_cents < c2.m_cents;
}

bool operator> (const Cents& c1, const Cents& c2)
{
    return c1.m_cents > c2.m_cents;
}

bool operator<= (const Cents& c1, const Cents& c2)
{
    return c1.m_cents <= c2.m_cents;
}

bool operator>= (const Cents& c1, const Cents& c2)
{
    return c1.m_cents >= c2.m_cents;
}

int main()
{
    Cents dime{ 10 };
    Cents nickel{ 5 };

    if (nickel > dime)
        std::cout << "a nickel is greater than a dime.\n";
    if (nickel >= dime)
        std::cout << "a nickel is greater than or equal to a dime.\n";
    if (nickel < dime)
        std::cout << "a dime is greater than a nickel.\n";
    if (nickel <= dime)
        std::cout << "a dime is greater than or equal to a nickel.\n";
    if (nickel == dime)
        std::cout << "a dime is equal to a nickel.\n";
    if (nickel != dime)
        std::cout << "a dime is not equal to a nickel.\n";

    return 0;
}

Minimizing comparative redundancy

비교 연산자는 반대되는 연산자의 경우 유사한 구현을 가지고 있다
따라서 약간의 최적화를 통해 redundancy를 줄일 수 있다

#include <iostream>

class Cents
{
private:
    int m_cents;

public:
    Cents(int cents)
        : m_cents{ cents }
    {}

    friend bool operator== (const Cents& c1, const Cents& c2);
    friend bool operator!= (const Cents& c1, const Cents& c2);

    friend bool operator< (const Cents& c1, const Cents& c2);
    friend bool operator> (const Cents& c1, const Cents& c2);

    friend bool operator<= (const Cents& c1, const Cents& c2);
    friend bool operator>= (const Cents& c1, const Cents& c2);

};

bool operator== (const Cents& c1, const Cents& c2)
{
    return c1.m_cents == c2.m_cents;
}

bool operator!= (const Cents& c1, const Cents& c2)
{
    return !(operator==(c1, c2));
}

bool operator< (const Cents& c1, const Cents& c2)
{
    return c1.m_cents < c2.m_cents;
}

bool operator> (const Cents& c1, const Cents& c2)
{
    return operator<(c2, c1);
}

bool operator<= (const Cents& c1, const Cents& c2)
{
    return !(operator>(c1, c2));
}

bool operator>= (const Cents& c1, const Cents& c2)
{
    return !(operator<(c1, c2));
}

int main()
{
    Cents dime{ 10 };
    Cents nickel{ 5 };

    if (nickel > dime)
        std::cout << "a nickel is greater than a dime.\n";
    if (nickel >= dime)
        std::cout << "a nickel is greater than or equal to a dime.\n";
    if (nickel < dime)
        std::cout << "a dime is greater than a nickel.\n";
    if (nickel <= dime)
        std::cout << "a dime is greater than or equal to a nickel.\n";
    if (nickel == dime)
        std::cout << "a dime is equal to a nickel.\n";
    if (nickel != dime)
        std::cout << "a dime is not equal to a nickel.\n";

    return 0;
}

위의 코드를 보면 이미 구현된 operator를 재활용해 의도한 결과를 도출하고 있다