為什么類是抽象的，盡管它不應該是？

我已經看了這個代碼幾個小時了，但我找不到為什么我不能實體化類。所以我有介面：

class ICloneable {
    public:
    virtual ICloneable* clone() const = 0;
    virtual ~ICloneable() = 0 {}
};


class IPrintable
{
    protected:
        virtual void print(std::ostream&) const = 0;
    public:
        virtual ~IPrintable() = 0;
        friend std::ostream& operator<<(std::ostream, const IPrintable&);
};
std::ostream& operator<<(std::ostream os, const IPrintable& other) {
    other.print(os);
    return os;
}


class IComparable {
    protected:
        virtual bool is_greater(const IComparable& other) const = 0;
        virtual bool is_equal(const IComparable& other) const = 0;
    public:
        virtual ~IComparable() = 0;
        virtual bool operator>(const IComparable& other) const {
            return is_greater(other);
        }
        virtual bool operator<(const IComparable& other) const {
            return !(is_greater(other) || is_equal(other));
        }
        virtual bool operator==(const IComparable& other) const {
            return is_equal(other);
        }
        virtual bool operator!=(const IComparable& other) const {
            return !(is_equal(other));
        }
};

我有兩個繼承這些介面的類：

class I2DShape : public IComparable, public IPrintable {
    public:
        virtual void print(std::ostream& os) const override final {
            os << "Circumference: " << this->circumference();
        }
        virtual bool is_greater(const I2DShape& other) const final {
            return this->circumference() > other.circumference();
        }
        virtual bool is_equal(const I2DShape& other) const final {
            return this->circumference() == other.circumference();
        }

        virtual double circumference() const = 0;
        virtual ~I2DShape();
};


class IPositionable : public IPrintable, public IComparable {
    public:
        virtual void print(std::ostream& os) const override final {
            
        }

        virtual bool is_greater(const IPositionable& other) const final {
            distance_from_origin() > other.distance_from_origin();
        }
        virtual bool is_equal(const IPositionable& other) const final {
            distance_from_origin() == other.distance_from_origin();
        }
        
        virtual double distance_from_origin() const {
            return sqrt(pow(center().get_x(), 2)   pow(center().get_y(), 2));
        }
        virtual Point center() const = 0;
        virtual ~IPositionable();
};

最后這兩個類被一個代表形狀的類繼承：

class Shape2D : public IPositionable, public I2DShape, public ICloneable {
    protected:
        int num_of_points;
        Point* points;
    public:
        Shape2D() : num_of_points(0), points(nullptr) {}
        Shape2D(int num) : num_of_points(num), points(new Point[num]) {}
        Shape2D(const Shape2D& other) : num_of_points(other.num_of_points) {
            points = new Point[num_of_points];
            for (int i = 0; i < num_of_points; i  ) {
                points[i] = other.points[i];
            }
        }
        Shape2D& operator=(const I2DShape& other) {
            
        }
        virtual Shape2D* clone() const override = 0;
        virtual ~Shape2D() {
            if(points)
                delete[] points;
        }
};

當我從 Shape2D 派生 Square 并創建用于克隆的函式時，我得到它是抽象類的錯誤：

class Square : public Shape2D {
    private:
        double side;
    public:
        Square() {}
        Square(double s, Point center) : side(s), Shape2D(1) { points[0] = center;}
        
        virtual Point center() const override{
            return points[0];
        }
        virtual double circumference() const override {
            return 4 * side;
        }
        virtual Square* clone() const override final {
            return new Square(*this); //error on this line
        }
};

錯誤：不允許抽象型別別“Square”的物件

uj5u.com熱心網友回復：

因為在介面中你宣告了解構式= 0是強制在可以實體化的子類中顯式實作它。

有兩種方法可以修復它。

• = 0使用{}或使用默認解構式（no ）以標準方式創建介面類= default。
• 添加顯式解構式 Square

請注意，帶有實作的純虛擬解構式被 gcc 和 clang 視為錯誤。

相關的問題：

• 為什么我們需要 C 中的純虛解構式？
• 帶實作的純虛函式

uj5u.com熱心網友回復：

該錯誤如@HolyBlackCat 在評論中所述，具有功能，is_greater并且is_equal因為它們在被覆寫時具有不同的引數。解決這個問題的簡單方法是= 0從這兩個函式中洗掉，IComparable這樣它們就不是純函式了。

標籤：C 遗产 抽象类 多重继承

上一篇：通過組合動態實作方法

下一篇：leetcode 8. 字串轉換整數 (atoi)