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:

  // definitions.cpp
  class WindowCounter;   // Forward reference; no definition
  
  class Window
  {
     // Definition of WindowCounter not required
     static WindowCounter windowCounter;
  };
  
  int main()
  {
  }
  

See Also

Reference

C++ Declarations and Definitions