17.5.3 Virtual methods

• 
  　　When an instance method declaration includes a virtual modifier, that method
  is said to be a virtual method.
  When no virtual modifier is present, the method is said to be a non-virtual
  method.
  The implementation of a non-virtual method is invariant: The implementation
  is the same whether the method is
  invoked on an instance of the class in which it is declared or an instance
  of a derived class. In contrast, the
  implementation of a virtual method can be superseded by derived classes.
  The process of superseding the
  implementation of an inherited virtual method is known as overriding that
  method (§17.5.4).
  In a virtual method invocation, the run-time type of the instance for which
  that invocation takes place determines
  the actual method implementation to invoke. In a non-virtual method
  invocation, the compile-time type of the
  instance is the determining factor. In precise terms, when a method named N
  is invoked with an argument list A
  on an instance with a compile-time type C and a run-time type R (where R is
  either C or a class derived from C),
  the invocation is processed as follows:
  ? First, overload resolution is applied to C, N, and A, to select a
  specific method M from the set of methods
  declared in and inherited by C. This is described in §14.5.5.1.
  ? Then, if M is a non-virtual method, M is invoked.
  ? Otherwise, M is a virtual method, and the most derived implementation of
  M with respect to R is invoked.
  For every virtual method declared in or inherited by a class, there exists
  a most derived implementation of the
  method with respect to that class. The most derived implementation of a
  virtual method M with respect to a class R
  is determined as follows:
  ?
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