如果static_cast< Derived *>（基本指针）给出编译时错误？ [英] Should static_cast<Derived *>(Base pointer) give compile time error?

问题描述

如果static_cast（基本指针）给出编译时错误？

Should static_cast(Base pointer) give compile time error?

class A
{
public:
    A()
    {

    }
};

class B : public  A
{
 public:
     B()
     {
     }
};

int main()
{
    A *a=new A();
    B * b=static_cast<B*>(a);   // Compile Error?
}

推荐答案

因为基于派生的关系可以在运行时存在，这取决于被转换的指针的地址。
static_cast 总是成功，但如果您未投放到正确的类型，则会产生未定义行为。 dynamic_cast 可能会失败或失败，实际上告诉您是否尝试投放到正确的类型。

It cannot give compile time error because a Base-Derived relationship can exist at runtime depending on the address of the pointers being casted. static_cast always succeeds, but will raise undefined-behavior if you don't cast to the right type. dynamic_cast may fail or not, actually telling you whether you tried to cast to the right type or not.

在我看来， static_cast 应该用于 downcast ，只有当设计可以确定存在这样的可能性。一个很好的例子是 CRTP 。所以在某些情况下是合乎逻辑的，但是尽量避免它，因为它是未定义的行为。

So in my opinion, static_cast should be used to downcast only if the design can establish that such a possibility exists. One good example of this is CRTP. So it is logical in some situations but try to avoid it as it is undefined-behavior.

static_cast 不需要RTTI，这可能会使理论上更快将随时对 static_cast 可能导致的未定义行为交易 dynamic_cast ！

RTTI is not needed for static_cast which might make it theoretically faster, but I will anytime trade-in a dynamic_cast against the undefined behavior that static_cast may cause!

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