c++ - 多态性并在C++中定义不同对象的列表

Question

为什么我的代码返回错误？我定义了一个名为Shape的父类和两个派生类。我正在尝试存储由类定义的对象，并将它们存储在列表中。

# include <iostream>

using namespace std;

class Shape{
    public:
        virtual double area(const double &height, const double &weight) const = 0;
};

class Triangle: public Shape{
   public:
       double height, weight;
       Triangle(double height, double weight): height(height), weight(weight){}
   double area(){
       return (height*weight)/2;
   }
};

class Rectangle: public Shape{
    public:
        double height, weight;
        Rectangle(double height, double weight): height(height), weight(weight){}
    double area(){
        return height*weight;
    }
};

Shape *shapes[3];
shapes[0] = new Triangle(2, 1);
shapes[1] = new Rectangle(3, 2);
shapes[2] = new Rectangle(5, 2);

double * show(Shape *shapes){
    double arr[3];
    for (int i=0; i < 3; i++){
        arr[i] = shapes[i].area();
    }
    return arr;
}

int main(){
    double arr[3] = show(shapes);
    cout << arr[0] << endl;
    cout << arr[1] << endl;
    cout << arr[2] << endl;
}

但是我收到以下两个错误:

error: 'shapes' does not name a type Shapes[0] = new Trangle(2,1);

error: cannot convert 'shape**' to 'shape*' double arr[3] = show(shapes);

Answer 1

您的代码有几个问题:

您的抽象基类Shape具有接受参数的纯虚函数area()，但您的派生类则不接受。因此，它也使派生类成为abstract，因此您无法初始化派生类对象。

您不能声明指针Shapes* shapes[3];的数组，然后在全局范围中单独分配它们。这是您遇到的第一个错误。更完整的说明，请参见here

调用show(shapes)函数时，shapes是Shapes*数组的名称，它衰减为指向第一个元素的指针。结果类型应为Shapes**，这是您遇到的第二个错误。

shapes[i]函数中的show()是指向对象的指针，因此您应该使用->而不是.

您的double arr[3];是堆栈上的一个数组，因此函数完成并返回后，其内容将被删除。未将指针返回到已存储的元素是未定义的。

您没有对delete创建的所有对象进行new，导致内存泄漏。

为了使代码正常运行，对代码所做的最小更改是:

# include <iostream>

using namespace std;

class Shape {
public:
    virtual double area() const = 0;
};

class Triangle : public Shape {
public:
    double height, weight;
    Triangle(double height, double weight) : height(height), weight(weight) {}
    double area() const override {    //add override to let compiler check for correctness
        return (height * weight) / 2;
    }
};

class Rectangle : public Shape {
public:
    double height, weight;
    Rectangle(double height, double weight) : height(height), weight(weight) {}
    double area()const override {
        return height * weight;
    }
};

Shape* shapes[3] = { new Triangle(2, 1),  new Rectangle(3, 2) ,new Rectangle(5, 2) };

double* show(Shape** shapes) {
    double* arr = new double[3];
    for (int i = 0; i < 3; i++) {
        arr[i] = shapes[i]->area();
        delete shapes[i];    //free the object pointed in shapes[]
    }
    return arr;
}

int main() {
    double* arr = show(shapes);
    cout << arr[0] << endl;
    cout << arr[1] << endl;
    cout << arr[2] << endl;
    delete[] arr;
}

但是有一些建议可以改进，例如在不使用 new而不是堆的情况下在堆栈上创建对象，以及使用 std::array创建数组等。