如何为抽象类重载运算符？

Question

我对 C++ 比较陌生，我的教授在上他的课时并没有像我想的那样详细介绍运算符重载。我正在尝试实现一种方法来比较所有继承抽象类的对象（使用 > 或

我尝试让它成为父类的成员，但我不知道如何从基类内部调用纯虚函数。然后我尝试使用模板，但这让我很头疼（我的教授也没有深入了解这些）。

我知道我在操作符函数中完全失败了（如果有任何有关正确语法的帮助，我们将不胜感激。

#include <iostream>

enum cType { point, maxima, inflection };

class CONSTRAINT {
public:
    //coordinates
    int x, y;
    //gets what type of constraint the object is
    virtual cType getType() = 0; //pure virtual
    //I'm sure this syntax is horrendous and completely wrong.
    //I was just trying to emulate what I found online :(
    bool operator > (const CONSTRAINT &rhs) { 
            //If the constraints have the same type, compare by their x-value
        if (getType() == rhs.getType())
            return (x > rhs.x);
            //Otherwise, it should be point > maxima > inflection
        else
            return (getType() > rhs.getType());
    }
};
class POINT : public CONSTRAINT {
public:
    virtual cType getType() { return point; }
};
class MAXIMA : public CONSTRAINT {
public:
    virtual cType getType() { return maxima; }
};
//I have another inflection class that follows the pattern

int main() {
    POINT point1, point2;
    point1.x = 3;
    point2.x = 5;
    MAXIMA maxima;
    maxima.x = 4;
    std::cout << (point1 > point2);
    std::cout << (point2 > point1);
    std::cout << (maxima > point2);
    std::cout << (point1 > maxima );
    return 0;
}

我希望：0110 如果程序可以编译。

相反，我收到以下错误：

“对象具有与成员函数“CONSTRAINT::getType”不兼容的类型限定符”

“'cType CONSTRAINT::getType(void)': 无法将'this'指针从'const CONSTRAINT'转换为'CONSTRAINT &'”

谢谢。

Answer 1

在

bool operator > (const CONSTRAINT &rhs)

rhs 是 const。它不能在此方法内更改。但是……

virtual cType getType() = 0; //pure virtual

不是const 方法。这意味着该方法可以更改rhs，因此编译器拒绝调用它。

解决方法：声明方法const

virtual cType getType() const = 0; //pure virtual

现在编译器承诺调用该函数将不允许更改rhs。如果getType 的实现试图更改调用它的对象，编译器也会强制执行此操作并拒绝编译程序。

旁注：

一旦方法被声明为virtual，所有覆盖也将是virtual。

override 关键字将在方法应覆盖但由于不匹配而未覆盖时捕获错误。将const 添加到基类方法，而不是派生类方法，是一个很好的例子，说明这很有帮助。

由于这段代码看起来利用了运行时多态性，因此您可能希望在基类中使用虚拟析构函数，以确保在您有一天希望通过指向基类的指针delete 派生类时销毁正确的类班级。

总结一下：

#include <iostream>

enum cType { point, maxima, inflection };

class CONSTRAINT {
public:
    //coordinates
    int x, y;

    virtual ~CONSTRAINT() = default;
//  ^ added

    //gets what type of constraint the object is
    virtual cType getType() const = 0; //pure virtual
//                          ^ added
    //I'm sure this syntax is horrendous and completely wrong.
    //I was just trying to emulate what I found online :(
    bool operator > (const CONSTRAINT &rhs) {
            //If the constraints have the same type, compare by their x-value
        if (getType() == rhs.getType())
            return (x > rhs.x);
            //Otherwise, it should be point > maxima > inflection
        else
            return (getType() > rhs.getType());
    }
};
class POINT : public CONSTRAINT {
public:
    cType getType() const     override { return point; }
//                  ^ added   ^ added
};
class MAXIMA : public CONSTRAINT {
public:
    cType getType() const     override { return maxima; }
//                  ^ added   ^ added
};
//I have another inflection class that follows the pattern

int main() {
    POINT point1, point2;
    point1.x = 3;
    point2.x = 5;
    MAXIMA maxima;
    maxima.x = 4;
    std::cout << std::boolalpha // < added. prints true and false instead of 1 and 0
              << (point1 > point2) << '\n'
              << (point2 > point1) << '\n'
              << (maxima > point2) << '\n'
              << (point1 > maxima);
    // took advantage of chaining and added newlines to the output for clarity
    return 0;
}

最后附注：一般建议将operator< 实现为Free Function。有关这方面的更多信息，以及关于运算符重载的许多其他智慧，请参阅What are the basic rules and idioms for operator overloading?