C++ Default Arguments		    Bob Walton, 7/16/96


In C++, because function overloading is allowed, the
following is possible:

  int sum ( int x, int y )
      { return x + y; }
  int sum ( int x, int y, int z )
      { return x + y + z; }
  int sum ( int x, int y, int z, int w )
      { return x + y + z + w; }

This permits the calls:	    sum ( 1, 2 )
			    sum ( 1, 2, 3 )
			    sum ( 1, 2, 3, 4 )

The following is also allowed in C++:

    int sum ( int x, int y, int z = 0, int w = 0 )
	{ return x + y + z + w; }

The value given after the = sign for an argument is the
default value for the argument in case the argument is
omitted.  This makes the argument optional.  With this
last definition of sum:

  sum ( x, y )	   is equivalent to   sum ( x, y, 0, 0 )
  sum ( x, y, z )  is equivalent to   sum ( x, y, z, 0 )

Default values may only be given to arguments at the end
of the argument list.  Arguments may only be omitted
from the end of an actual argument list, and then only
if default values are given.

The declaration:

  int sum ( int x, int y, int z = 0, int w = 0 );

in effect declares three sum functions with 2, 3, and 4
arguments.  There must not be other declarations that
conflict with these.

Default argument values should be given only in .h
files, or when the function is first declared in a
translation unit (i.e. a .cc file and its inclusions).
Thus if the above declaration for sum is given in a .h
file included in a .cc file, a definition of sum in the
.cc file should have no default argument values, as in:

    int sum ( int x, int y, int z, int w )
	{ return x + y + z + w; }

Default argument values may be expressions that
reference global and static expressions and functions.
They are evaluated separately every time the function is
called.  Names used in these expressions are interpreted
in the scope they appear in; i.e., the name refers to
what it refers to where the default argument value
appears in the program code, and not what it might refer
to where the function is actually called.