> single inheritance allows C++ to implement fast polymorphism via virtual function Multiple inheritance can be implemented to be as fast as single inheritance

> single inheritance allows C++ to implement fast polymorphism via virtual function Multiple inheritance can be implemented to be as fast as single inheritance regarding virtual function calls, and typically is in C++ by keeping multiple virtual table pointers per object. The problem this brings is that we can have pointers/references to the different parts of the same same object. A Child* may point to the start of the object, but Parent* may point to somewhere in the middle (because that's where the parent's vtable pointer happens to be). This also means that casting a pointer can change it. This can introduce some "interesting" bugs, as you can imagine. OTOH, there are languages like C# that don't do that, but require 2 "hops" when calling an interface method, causing a slight performance penalty (a class can inherit from at most one other class, but can implement multiple interfaces). But a reference to an object always points to the object's start, which is very important for garbage collector.