用超类的实例覆盖子类的实例 [英] Overwriting an instance of a subclass with an instance of a superclass

问题描述

试图找出一些东西（在Ada，实际上）我想出了以下代码。为什么是合法的？

Trying to figure something out (in Ada, actually) I came up with the following code. Why is it legal?

class Superclass {
public:
    virtual void Announce() {
        printf("I am the superclass\n");
    }
};

class Subclass : public Superclass {
public:
    void Announce() {
        printf("I am the subclass\n");
    }
};

int main() {
    Superclass osuper;
    Subclass osub;

    Superclass* p = &osub;
    *p = osuper;
    osub.Announce();

    return 0;
}

在 main（），我创建子类的实例，然后使用超类的实例物理覆盖它。然后我成功地调用了覆盖（和这样损坏的）对象上的 Subclass 方法。

In main(), I create an instance of Subclass, then physically overwrite it with an instance of Superclass. I then successfully call a method of Subclass on the overwritten (and so corrupted) object.

直接分配 osub = osuper ，因为这没有意义，但通过一个指针，我似乎绕过了。上面的代码编译正常，没有警告，但是当我调用 osub.Announce（） osub 不再包含有效的子类对象。

I can't directly assign osub = osuper, because that makes no sense, but by going via a pointer I seem to bypass that. The above code compiles fine with no warnings, but by the time I call osub.Announce() the memory inside osub no longer contains a valid Subclass object.

这不能是类型安全的（甚至，通常安全），但编译器似乎完全幸福。为什么？

This can't possibly be type safe (or even, like, ordinarily safe), but the compiler seems perfectly happy. Why?

推荐答案

对某个对象使用=运算符无效，并且不覆盖任何内容，将会被使用。
如果你想真正覆盖内存中的对象，请尝试memcpy（p，& osub，sizeof（Superclass））。现在将被覆盖：）

Using the = operator on an object is not invalid, and it doesn't overwrite anything, a default copy constructor will be used. If you want to really overwrite the object in memory, try memcpy(p, &osub, sizeof(Superclass)). Now THAT would be an overwrite :)

请参阅 http：// www .cplusplus.com / articles / y8hv0pDG /

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