Can not use dynamic_cast to a const object

c++ constants dynamic-cast const-cast
Rakib picture Rakib · May 23, 2014 · Viewed 8.1k times · Source

I want to write a method where a Base object pointer will be passed as a parameter, and inside the method it will be casted to derived object pointer.

void func( const Base* const obj){
    Derived* der = dynamic_cast<Derived*>(obj);
}

But it shows error because dynamic cast cannot cast away const specifier. But I am not understanding why const specifier has to be removed here, all I am doing is creating a derived pointer which should point to some offset amount after the base pointer. I also tried const Derived* const der = dynamic_cast<Derived*>(obj);, but no result.

It is important to pass the parameter as const. How can I do this? Do I have to do it in the ugly way of first applying const_cast then dynamic_cast? is there any better way?

Answer

Donnie picture Donnie · May 23, 2014

You're casting away const because you didn't do this:

const Derived* der = dynamic_cast<const Derived*>(obj);

If you actually need a Derived* then you need to

Derived* der = dynamic_cast<Derived*>(const_cast<ObjType*>(obj));

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