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    <p>Virtual calls themselves are more time consuming than normal calls because it has to lookup the address of the actual function to call from the vtable </p> <p>Additionally compiler optimizations like inlining might be hard to perform due to the lookup requirement. Situations where inlining is not possible itself can lead to quite a high overhead due to stack pop and push and jump operations</p> <p>Here is a proper study which says the overhead can be as high as 50% <a href="http://www.cs.ucsb.edu/~urs/oocsb/papers/oopsla96.pdf" rel="nofollow noreferrer">http://www.cs.ucsb.edu/~urs/oocsb/papers/oopsla96.pdf</a></p> <p>Here is another resource that looks at a side effect of having a large library of virtual classes <a href="http://keycorner.org/pub/text/doc/kde-slow.txt" rel="nofollow noreferrer">http://keycorner.org/pub/text/doc/kde-slow.txt</a></p> <p>The dispatching of the virtual calls with multiple inheritances is compiler specific, so the implementation will also have an effect in this case. </p> <p>Regarding your specific question of having a large no of base classes, usually the memory layout of a class object would have the vtbl ptrs for all the other constituent classes within it. </p> <p>Check this page for a sample vtable layout - <a href="http://www.codesourcery.com/public/cxx-abi/cxx-vtable-ex.html" rel="nofollow noreferrer">http://www.codesourcery.com/public/cxx-abi/cxx-vtable-ex.html</a></p> <p>So a call to a method implemented by a class deeper into the heierarchy would still only be a single indirection and not multiple indirections as you seem to think. The call does not have to navigate from class to class to finally find the exact function to call. </p> <p>However if you are using composition instead of inheritance each pointer call would be a virtual call and that overhead would be present and if within that virtual call if that class uses more compositio,n more virtual calls would be made. That kindof a design would be slower depending on the amount of calls you made. </p>
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