使用多重继承时，何时静态转换安全？ [英] When is static cast safe when you are using multiple inheritance?

问题描述

我发现自己处在某种类型的情况下。类型是三个（或更多）继承级别之一。我调用了返回 B * 的工厂，但是T是类型的最高级别（如果我的代码知道它是什么）或第二级别。

I found myself in a situation where I know what type something is. The Type is one of three (or more) levels of inheritance. I call factory which returns B* however T is either the highest level of a type (if my code knows what it is) or the 2nd level.

无论如何，我在模板中做了 static_cast ，这是错误的做法。我的问题是，何时可以安全地进行静态投射？有没有这样的时间？我之所以这样做，是因为当我不小心将T当作怪异的东西（发生了）而动态转换忽略（并返回null）时，我宁愿遇到编译错误。但是，当我知道正确的类型时，指针将无法调整，从而导致指针指向错误。我不确定为什么在这种情况下根本不允许使用静态投射。

Anyways, I did a static_cast in the template which is the wrong thing to do. My question is WHEN can I static cast safely? Is there ever such a time? I did it in this case because I'd rather get compile errors when I accidentally have T as something wacky which (has happened and) dynamic cast ignores (and returns null). However when I know the correct type the pointer is not adjusted causing me to have a bad pointer. I'm not sure why static cast is allowed in this case at all.

何时可以使用static_cast安全地向下投射？有没有情况？现在，使用 static_cast 似乎总是错误的（当目的是降低演员阵容时）

When can I use static_cast for down casting safely? Is there ever a situation? Now it seems like it always is wrong to use a static_cast (when the purpose is to down cast)

好我想出了如何重现它。

Ok I figured out how to reproduce it.

#include <iostream>
struct B { virtual void f1(){} };
struct D1 : B {int a;};
struct D2 : B {int a, b; };
struct DD : D1, D2 {};

int main(){
void* cptr = new DD(); //i pass it through a C interface :(
B*  a = (B*)cptr;
D2* b = static_cast<D2*>(a); //incorrect ptr
D2* c = dynamic_cast<D2*>(a); //correct ptr
std::cout << a << " " <<b << " " <<c;
}

推荐答案

A交叉广播：

struct Base1 { virtual void f1(); };
struct Base2 { virtual void f2(); };
struct Derived : Base1, Base2 {};

Base1* b1 = new Derived();
Base2* b2 = dynamic_cast<Base2*>(b1);

需要使用 dynamic_cast ， static_cast 无法完成（ static_cast 应该会导致编译时错误。） dynamic_cast 如果任一基类都不是多态的（虚拟函数的存在不是可选的），也会失败。

requires use of dynamic_cast, it cannot be done with static_cast (static_cast should have caused a compile-time error). dynamic_cast will also fail if either base class is not polymorphic (the presence of virtual functions is NOT optional).

请参见此解释MSDN上的

这篇关于使用多重继承时，何时静态转换安全？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！