Performance penalty for working with interfaces in C++?

Question

Is there a runtime performance penalty when using interfaces (abstract base classes) in C++?

Answer 1

Note that multiple inheritance bloats the object instance with multiple vtable pointers. With G++ on x86, if your class has a virtual method and no base class, you have one pointer to vtable. If you have one base class with virtual methods, you still have one pointer to vtable. If you have two base classes with virtual methods, you have two vtable pointers on each instance.

Thus, with multiple inheritance (which is what implementing interfaces in C++ is), you pay base classes times pointer size in the object instance size. The increase in memory footprint may have indirect performance implications.