Declarations and Definitions

• A declaration introduces a name and information about the name into a scope or region of program text
• A definition is a special kind of declaration. Definitions cause the compiler to allocate memory
• In C++ all declarations are also definitions except the following:
• Declarations with the extern specifier and no initializer
• Function declarations without a body, with or without the extern specifier
• Declarations of static data members in a struct or class definition
• Forward declarations of struct and class names
• typedef declarations
• A declaration can appear any number of times in a program file
• There can be only one definition of any entity in a program. Even if the variable definitions are identical, it will cause a multiple definition linkage error
• Examples...
  

  extern int k;	// This is a declaration, indicating that k is
  defined
  		// externally in another file
  
  // The following are all definitions...
  int j;
  int m = 0;
  extern int n = 1;