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─────C/C++ Language─────────────────────────────────────────────────────────
 
     A declaration specifies certain attributes of an identifier
     but does not allocate storage. The extern keyword tells the
     compiler to defer allocation. An initializer is not allowed in
     a declaration. For example:
 
         extern int y;          // Names a variable. Defers storage
                                //    allocation.
         extern const start;    // Names a constant. Defers storage
                                //    allocation.
         struct person;         // Names a structure type. The members
                                //    are not defined and the size of
                                //    the struct is not known.
         int sum( int, int );   // Names a function and specifies return
                                //    type and number and type of
                                //    parameters. Parameter names are
                                //    recommended but not required.
 
     For a variable, a definition provides both a name and a type that
     allow the compiler to allocate a section of memory that is large
     enough to accommodate the type. An initializer is allowed in a
     definition. For example:
 
         int x;                 // Allocates an integer.
         int y = 0;             // Allocates and initializes an integer.
         const start = 1;       // Allocates and initializes a constant.
         struct person          // Defines a structure type and allocates
         {                      //    a variable of that type.
             long phone;
             char *name;
         } customer;
 
     For a function, a definition provides everything that a declaration
     provides, plus a function body. For example:
 
         int sum( int a, int b )
         {
             return a + b;
         }
                                    -♦-