RTTI Overhead in C++

Question

What are the memory/performance overheads of enabling RTTI in a C++ program?
Can anyone please throw some light between the internal implementation of RTTI mechanism and the relevant overheads?
I do understand how to use RTTI through typeid and dynamic_cast, what I am trying to know is the internal implementation details of how the run time keeps track of this information and how it is an overhead?

Answer 1

Enabling RTTI typically brings only a small overhead. The usual implementation carries a pointer to the type information structure in the vtable of an object. Since the vtable must be constructed anyway, the extra time is small - it's like adding another virtual function to the class.

typeid is therefore comparable to calling a virtual function. dynamic_cast is slower - it needs to traverse the inheritance hierarchy to do a cast. Calling dynamic_cast too frequently can be a performance bottleneck. By 'can' I mean that it usually won't …

There is a slight bloat in executable size since the typeinfo structures need to be stored somewhere. In most cases it won't be relevant.

Answer 2

Please read appropriate section in this document.

To sum up:

• typeid (5.3.7): find vtable, through that find most derived class object, then extract type_info from that object's vtable. It is still very slow comparing with function call;

• dynamic_cast (5.3.8): find type_info as described above, then determine whether conversion is possible, then adjust pointers. Run-time cost depends on the relative position in the class hierarchy of two classes involved. Down- and cross-casts are very slow these days (though here you can find the article about possible (but restricted) constant-time implementation of dynamic_cast).