基指针可以指向派生对象的数组吗？ [英] Can a pointer to base point to an array of derived objects?

问题描述

我今天去面试一个工作面试，并给了这个有趣的问题。

I went to a job interview today and was given this interesting question.

除了内存泄漏和事实，没有虚拟dtor，为什么这个代码崩溃？

Besides the memory leak and the fact there is no virtual dtor, why does this code crash?

#include <iostream>

//besides the obvious mem leak, why does this code crash?

class Shape
{
public:
    virtual void draw() const = 0;
};

class Circle : public Shape
{
public:
    virtual void draw() const { }

    int radius;
};

class Rectangle : public Shape
{
public:
    virtual void draw() const { }

    int height;
    int width;
};

int main()
{
    Shape * shapes = new Rectangle[10];
    for (int i = 0; i < 10; ++i)
        shapes[i].draw();
}

推荐答案

您已经分配了 Rectangles 的数组，并且存储了指向 shapes 中的第一个的指针。当您执行 shapes [1] 时，您要取消引用（shapes + 1）。这不会给你一个指向下一个 Rectangle 的指针，而是一个指向下一个 Shape 的指针推测数组 Shape 。当然，这是未定义的行为。

You cannot index like that. You have allocated an array of Rectangles and stored a pointer to the first in shapes. When you do shapes[1] you're dereferencing (shapes + 1). This will not give you a pointer to the next Rectangle, but a pointer to what would be the next Shape in a presumed array of Shape. Of course, this is undefined behaviour. In your case, you're being lucky and getting a crash.

使用指向 Rectangle 的指针可以建立索引

Using a pointer to Rectangle makes the indexing work correctly.

int main()
{
   Rectangle * shapes = new Rectangle[10];
   for (int i = 0; i < 10; ++i) shapes[i].draw();
}

如果你想有不同种类的 Shape s在数组中，并使用它们多态，你需要一个指针到Shape。

If you want to have different kinds of Shapes in the array and use them polymorphically you need an array of pointers to Shape.

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