C++：算术运算符重载

Question

我有一个名为 HealthPoints 的 class，它有 hp 和 max_hp 成员。我想重载 + 运算符，所以它会像这样工作：

HealthPoints healthPoints1;
healthPoints1 -= 150; /* healthPoints1 now has 0 points out of 100 */
HealthPoints healthPoints2(150);
healthPoints2 -= 160; /* healthPoints2 now has 0 points out of 150 */
healthPoints2 = healthPoints1 + 160; /* healthPoints2 now has 100 points out of 100 */

我成功地让它工作了，但只是按照特定的顺序。如果我把数字放在对象之前，有没有办法让它识别出两者之间的数字并据此采取行动？目前它的工作方式，我得到 healthPoints2 现在有 160 分（满分 160）。

HealthPoints class:

class HealthPoints {
    public:
        static const int DEFAULT_MAXHP=100;
        static const int DEFAULT_HP=100;
        static const int MINIMUM_MAXHP=1;
        static const int MINIMUM_HP=0;
        explicit operator int() const;
        HealthPoints(int maxHP=HealthPoints::DEFAULT_MAXHP);
        ~HealthPoints() = default; // Destructor set to default
        HealthPoints(const HealthPoints&) = default; // Copy Constructor set to default
        HealthPoints& operator=(const HealthPoints&) = default; // Assignment operator set to default
        HealthPoints& operator+=(const HealthPoints& HP);
        HealthPoints& operator-=(const HealthPoints& HP);
        class InvalidArgument {};
    private:
        int m_maxHP;
        int m_HP;
        friend bool operator==(const HealthPoints& HP1, const HealthPoints& HP2);
        friend bool operator<(const HealthPoints& HP1, const HealthPoints& HP2);
        friend std::ostream& operator<<(std::ostream& os, const HealthPoints& HP);
};

HealthPoints::HealthPoints(int maxHP) {
    if(maxHP < HealthPoints::MINIMUM_MAXHP) {
        throw HealthPoints::InvalidArgument();
    }
    this->m_maxHP=maxHP;
    this->m_HP=m_maxHP;
}

HealthPoints operator+(const HealthPoints& HP1, const HealthPoints& HP2) {
    return HealthPoints(HP2)+=HP1;
}

HealthPoints operator-(const HealthPoints& HP1, const HealthPoints& HP2) {
    return HealthPoints(HP1)-=HP2;
}

HealthPoints& HealthPoints::operator+=(const HealthPoints& HP) {
    if(this->m_HP+HP.m_HP>this->m_maxHP) {
        this->m_HP=this->m_maxHP;
    } else {
        this->m_HP+=HP.m_HP;
    }
    return *this;
}

HealthPoints& HealthPoints::operator-=(const HealthPoints& HP) {
    if(this->m_HP-HP.m_HP>HealthPoints::MINIMUM_HP) {
        this->m_HP-=HP.m_HP;
    } else {
        this->m_HP=HealthPoints::MINIMUM_HP;
    }
    return *this;
}

Answer 1

我不完全确定您 HealthPoints class 的逻辑，因此请仔细检查下面的代码。不过，我的观点是实施：

    friend HealthPoints operator+(const HealthPoints& other, int value);
    friend HealthPoints operator+(int value, const HealthPoints& other);

实际上你只需要实现第一个，然后让第二个使用它：

HealthPoints operator+(const HealthPoints& hp, int value) {
    HealthPoints ret{hp};
    ret.m_HP = std::min(ret.m_maxHP, ret.m_HP + value);
    return ret;
}
HealthPoints operator+(int value, const HealthPoints& hp) {
    return hp + value;
}

您可以在此处查看实际效果：

[Demo]

int main() {
    HealthPoints hp1{100};
    hp1 -= 50;
    std::cout << "hp1: " << hp1 << "\n";
    auto hp2{hp1 + 10};
    std::cout << "hp2: " << hp2 << "\n";
    auto hp3{10 + hp2};
    std::cout << "hp3: " << hp3 << "\n";
}

// Outputs:
//   hp1: (max_HP = 100, HP = 50)
//   hp2: (max_HP = 100, HP = 60)
//   hp3: (max_HP = 100, HP = 70)

Answer 2

在此声明中：

healthPoints2 = healthPoints1 + 160;

您的 + 运算符正在创建一个新的 HealthPoints 对象，该对象是 healthPoints1 的副本，因此对象的 m_maxHP 设置为 100。然后将 160 添加到该对象，增加其 m_HP 但保留其 m_maxHP。然后将该对象分配给 healthPoints2，同时复制 m_HP 和 m_maxHP 值。这就是 healthPoints2 的 m_maxHP 从 150 变为 100 的原因。

在此声明中：

healthPoints2 = 160 + healthPoints1;

您的 + 运算符正在创建一个新的 HealthPoints 对象，其 m_maxHP 设置为 160。然后将 healthPoints1 添加到该对象，增加其 m_HP 但保留其 m_maxHP。然后将该对象分配给 healthPoints2，同时复制 m_HP 和 m_maxHP 值。这就是 healthPoints2 的 m_maxHP 从 150 变为 160 的原因。

如果您希望 operator 仅更新 m_HP 而不是更改 m_maxHP，您需要添加接受 int 值而不是 HealthPoints 个对象，例如：

HealthPoints& HealthPoints::operator+=(int value) {
    m_HP = std::min(m_HP + value, m_maxHP);
    return *this;
}

HealthPoints& HealthPoints::operator-=(int value) {
    m_HP = std::max(m_HP - value, HealthPoints::MINIMUM_HP);
    return *this;
}

HealthPoints operator+(const HealthPoints& HP, int value) {
    return HealthPoints(HP) += value;
}

HealthPoints operator+(int value, const HealthPoints& HP) {
    return HealthPoints(HP) += value;
}

HealthPoints operator-(const HealthPoints& HP, int value) {
    return HealthPoints(HP) -= value;
}

HealthPoints operator-(int value, const HealthPoints& HP) {
    return HealthPoints(HP) -= value;
}

然后，将 HealthPoints(int) 构造函数标记为 explicit 以避免在将 int 值传递给需要 HealthPoints 对象的参数时发生不需要的隐式转换。

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