static_cast<Derived *>(Base pointer) 是否应该给出编译时错误?

2021-12-31 00:00:00 casting c++

static_cast(Base pointer) 是否应该给出编译时错误?

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

它不能给出编译时错误，因为 Base-Derived 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，但如果你没有转换为正确的类型，将会引发 undefined-behavior.dynamic_cast 可能会失败，也可能不会，实际上是在告诉您是否尝试转换为正确的类型.

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.

所以在我看来，static_cast 应该用于向下转换，前提是设计可以确定存在这种可能性.一个很好的例子是 CRTP.所以在某些情况下这是合乎逻辑的，但尽量避免它，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，这可能使它理论上更快，但我会随时用 dynamic_cast 来抵消未定义的行为static_cast 可能导致！

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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