C++ 破坏基类导致调试断言失败

Question

我正在尝试更多地了解多态性，并试图使其尽可能简单，所以我得到了基类 Figure 和 2 个派生类 Rectangle 和 Circle。

class Figure {
public:
    Figure() { cout << "Creating Figure\n"; }
    virtual ~Figure() { cout << "Destroying Figure\n"; }

    virtual double calculateField() {
            return 0;
    }
};

class Rectangle : public Figure
{
public:
    Rectangle(double m_a) : p_a(m_a) { cout << "Creating Rectangle\n"; }
    ~Rectangle() { cout << "Destroying Rectangle\n"; }

    virtual double calculateField() {
        return p_a*p_a;
    }

private:
    double p_a; 
};

class Circle : public Figure
{
public:
    Circle(double m_r) : p_r(m_r) { cout << "Creating Circle\n"; }
    ~Circle() { cout << "Destroying Circle\n"; }

    virtual double calculateField() {
        return p_r*3.14*3.14;
    }

private:
    double p_r; 
};

在主函数中，我创建了指向 Figure 类和 2 个对象的指针 - 矩形和圆形。然后我在其中 1 个上设置 Figure 指针，并在此指针上调用 calculateField()。有用。但最后我尝试在此指针上调用 delete，它使我的程序崩溃。

int main(){
    Figure *ptr = new Figure();
    Rectangle rec(5);
    Circle circ(5);

    cout << "Figure field: " << ptr->calculateField() << endl;

    ptr = &rec; 
    cout << "Rectangle field: " << ptr->calculateField() << endl;

    ptr = &circ;
    cout << "Circle field: " << ptr->calculateField() << endl;

    delete ptr;

    cin.get();
    return 0;
}

结果如下:

Creating Figure
Creating Figure
Creating Rectangle
Creating Figure
Creating Circle
Figure field: 0
Rectangle field: 25
Circle field: 49.298
Destroying Circle
Destroying Figure

But then I got Debug Assertion Failed

Expression: _BLOCK_TYPE_IS_VALID(pHead -> nBlockUse)

Answer 1

程序行为未定义。

您只能在从new 返回的指针上调用delete。

目前您正在尝试删除具有自动存储期限的内容。这永远不会有好结果。