Item 24: Understand the costs of virtual functions, multiple inheritance, virtual base classes, and RTTI.

When a virtual function is called, the code executed must correspond to the dynamic type of the object on which the function is invoked; the type of the pointer or reference to the object is immaterial. How can compilers provide this behavior efficiently? Most implementations use virtual tables and virtual table pointers. Virtual tables and virtual table pointers are commonly referred to as vtbls and vptrs, respectively.

A vtbl is usually an array of pointers to functions. (Some compilers use a form of linked list instead of an array, but the fundamental strategy is the same.) Each class in a program that declares or inherits virtual functions has its own vtbl, and the entries in a class's vtbl are pointers to the implementations of the virtual functions for that class. Each object whose class declares virtual functions carries with it a hidden data member that points to the virtual table for that class. This hidden data member — the vptr — is added by compilers at a location in the object known only to the compilers.