非メンバの演算子オーバーロード　解答ページ | Programming Place Plus　C++編【言語解説】　第３５章

#include <iostream>

class Size {
public:
    Size(int width, int height) :
        mWidth(width), mHeight(height)
    {}

    inline Size& operator+=(const Size& rhs)
    {
        mWidth += rhs.GetWidth();
        mHeight += rhs.GetHeight();
        return *this;
    }

    inline Size& operator-=(const Size& rhs)
    {
        mWidth -= rhs.GetWidth();
        mHeight -= rhs.GetHeight();
        return *this;
    }

    inline int GetWidth() const
    {
        return mWidth;
    }

    inline int GetHeight() const
    {
        return mHeight;
    }

    inline int GetArea() const
    {
        return mWidth * mHeight;
    }

private:
    int mWidth;
    int mHeight;
};

const Size operator+(const Size& lhs, const Size& rhs)
{
    return Size(lhs) += rhs;
}

const Size operator-(const Size& lhs, const Size& rhs)
{
    return Size(lhs) -= rhs;
}

bool operator==(const Size& lhs, const Size& rhs)
{
    return lhs.GetWidth() == rhs.GetWidth()
        && lhs.GetHeight() == rhs.GetHeight()
        ;
}

bool operator!=(const Size& lhs, const Size& rhs)
{
    return !(lhs == rhs);
}

bool operator<(const Size& lhs, const Size& rhs)
{
    return lhs.GetArea() < rhs.GetArea();
}

bool operator>(const Size& lhs, const Size& rhs)
{
    return lhs.GetArea() > rhs.GetArea();
}

bool operator<=(const Size& lhs, const Size& rhs)
{
    return !(lhs > rhs);
}

bool operator>=(const Size& lhs, const Size& rhs)
{
    return !(lhs < rhs);
}

std::ostream& operator<<(std::ostream& lhs, const Size& rhs)
{
    lhs << rhs.GetWidth() << ", " << rhs.GetHeight();
    return lhs;
}

int main()
{
    Size s1(5, 10);
    Size s2(10, 5);

    s1 = s1 + s2;
    std::cout << s1 << std::endl;

    s1 = s1 - s2;
    std::cout << s1 << std::endl;

    s1 += s2;
    std::cout << s1 << std::endl;

    s1 -= s2;
    std::cout << s1 << std::endl;

    s1 += Size(1, 1);
    std::cout << std::boolalpha 
              << (s1 == s2) << "\n"
              << (s1 != s2) << "\n"
              << (s1 < s2) << "\n"
              << (s1 > s2) << "\n"
              << (s1 <= s2) << "\n"
              << (s1 >= s2) << std::endl;
}