FAQ 21.04 How does C perform static typing while supporting dynamic binding?

FAQ 21.04 How does C++ perform static typing while supporting dynamic binding?

Static typing ensures that all declarations, definitions, and uses of a virtual function are consistent while dynamic binding provides the "plumbing" so that the right implementation is called at runtime.

Given a reference (or pointer) to an object, there are two distinct types in question: the static type of the reference and the dynamic type of the referent (that is, the object being referred to). In other words, the object may be an instance of a class that is derived from the class of the reference. Nonvirtual (statically bound) member functions are selected based on the (statically known) type of the reference. Virtual (dynamically bound) member functions are selected based on the (dynamically known) type of the referent.

The legality of the call is checked based on the (statically known) type of the reference or pointer. This is safe because the referent must be "at least as derived as" the type of the reference. This provides the following type safety guarantee: if the class of the reference has the indicated member function, then the class of the referent will as well.