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Home » Tip Bank » C++
Language: C++
Expertise: Intermediate
Feb 25, 1999

Performance of typeid vs. dynamic_cast<>

As far as design is concerned, dynamic_cast<> should be preferred to typeid because the former enables more flexibility and extensibility. Notwithstanding that, the runtime overhead of typeid can be less expensive than dynamic_cast<>, depending on the operands. An invocation of operator typeid is a constant time operation--it takes the same length of time to retrieve the runtime type information of every polymorphic object, regardless of the object's derivational complexity. On the other hand, dynamic_cast<> is not a constant time operation. It has to traverse the derivation tree of the operand until it has located the target object in it. The worst case scenario is when the operand is a deeply derived object and the target is a non-related class type. Then, dynamic_cast<> has to traverse the entire derivation tree before it can confidently decide that requested cast cannot be done.
Danny Kalev
 
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