C++ Definitions

A definition is a unique specification of an object or variable, function, class, or enumerator. Because definitions must be unique, a program can contain only one definition for a given program element.

There can be a many-to-one correspondence between declarations and definitions. There are two cases in which a program element can be declared and not defined:

• A function is declared but never referenced with a function call or with an expression that takes the function’s address.
  
  
• A class is used only in a way that does not require its definition be known. However, the class must be declared. The following code illustrates such a case:

  class WindowCounter;    // Forward reference; no definition
  
  class Window
  {
      static WindowCounter windowCounter;  // Definition of
                                           //  WindowCounter
                                           //  not required.
  };