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? } 
2 Answers
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.
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.
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!
3 Comments
ibizaman
What do you mean by "dynamic_cast may fail or not" ? Do you mean that there is uncertainty about the fact that dynamic_cast will fail in the example above ? Or do you mean that it will always fail ?
Abhay
@ibizaman: There is no uncertanity here. Infact it will tell you when it failed and when it did not fail. I think if you read the complete sentence it should make sense.
ibizaman
indeed, it was just to be sure :)
It doesn't give a compile time error because the cast could very-well be valid, and you would often do it in practice, e.g.:
A* a = new B; B* b = static_cast<B*>(a); // OK In your code, as far as the compiler is concerned, you are doing the same thing. It cannot know that the cast would be invalid, so it allows it at compile time. At run time however, you're going to get some nasty errors as soon as you try to use a feature of B on an instance of A.
answered Mar 18, 2010 at 10:39
Peter Alexander
54.4k14 gold badges121 silver badges169 bronze badges
010button or indent by 4 spaces for code markup.cout<<statements in those constructors? As far as I see, they are unrelated to the question, and would be better removed.ais pointing to a B object, and you are lying to your compiler. Nasal demons are on their way.A *a=new A(); B * b=static_cast<B*>(a);is not a good practice. It will give you some nasty errors.