堆栈上的多态对象? [英] Polymorphic objects on the stack?

问题描述

在上一个问题

In a previous question I quoted Stroustrup on why a common Object class for all classes is problematic in c++. In that quote there is the statement:

使用通用基类意味着 成本:对象必须是堆分配的 是多态的；

Using a universal base class implies cost: Objects must be heap-allocated to be polymorphic;

我真的没看过两次，因为它的 Bjarnes主页我想很多眼睛已经扫描了该句子并报告了任何错误陈述.

I really didn't look twice at it, and since its on Bjarnes home page I would suppose a lot of eyes have scanned that sentence and reported any misstatements.

但是，有评论者指出，情况可能并非如此，回想起来，我找不到任何正确的理由.一个简短的测试用例产生了"VDerived :: f()"的预期结果.

A commenter however pointed out that this is probably not the case, and in retrospect I can't find any good reason why this should be true. A short test case yields the expected result of "VDerived::f()".

struct VBase
{
    virtual void f()    { cout<<"VBase::f()"<<endl; }
};

struct VDerived:VBase
{
    void f()  { cout<<"VDerived::f()"<<endl; }
};

void test(VBase& obj){
    obj.f();
}

int main(int argc, char** argv) {
    VDerived obj;
    test(obj);
    return 0;
}

当然，如果要测试的形式参数是test(VBase obj)，情况将完全不同，但这不是堆栈vs​​.堆参数，而是复制语义.

Of course if the formal argument to test was test(VBase obj) the case would be totally different, but that would not be a stack vs. heap argument but rather copy semantics.

Bjarne是不是完全错了，还是我在这里遗漏了一些东西?

Is Bjarne flat out wrong or am I missing something here?

推荐答案

在我看来，多态性.

具有间接的情况下，C ++中的多态性可以工作；即pointer-to-T或reference-to-T. T的存储位置完全不相关.

Polymorphism in C++ works when you have indirection; that is, either a pointer-to-T or a reference-to-T. Where T is stored is completely irrelevant.

Bjarne还犯了一个错误，说堆分配"在技术上是不准确的.

Bjarne also makes the mistake of saying "heap-allocated" which is technically inaccurate.

^{(注意:这并不意味着通用基类是好"！)}

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