为什么不能dynamic_cast“侧身”播放？在多重继承期间？ [英] Why can't I dynamic_cast "sideways" during multiple inheritence?

问题描述

以下代码抛出std :: bad_cast

The following code throws std::bad_cast

struct Foo {
    void foo () {}
};

struct Bar {
    Bar () {
        dynamic_cast <Foo &> (*this) .foo ();
    }
    virtual ~ Bar () {}
};

struct Baz : public Foo, public Bar {
};

int main ()
{
    Baz b;
}

我记得曾经读过dynamic_cast如何实现性能折衷，因为它遍历完整的继承格以正确评估。

I remember once reading how dynamic_cast has implementation performance trade-offs because "it traverses the full inheritence lattice" in order to evaluate correctly. What the compiler needs to do here is first cast up and then down again.

是否可以使以上工作正常进行，还是需要添加
<？ code>虚拟Foo * Bar :: as_foo（）= 0;
？

Is it possible to make the above work or do I need to add virtual Foo* Bar::as_foo()=0; ?

推荐答案

Foo中没有虚函数，因此dynamic_cast完全有可能失败。需要有一个虚函数。在施工期间这样做也是个坏主意，因为您会遇到施工订单问题。

There are no virtual functions in Foo, so dynamic_cast is perfectly obliged to fail. There needs to be a virtual function. It's also a bad idea to do so during construction, as you're going to run into construction order issues.

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