Default parameter

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A default parameter also referred to as optional argument (or default argument) is a function or procedure parameter that has a default value provided to it. If the programmer does not supply a value for this parameter, the default value will be used. If the programmer does supply a value for the default parameter, the programmer-supplied value is used. The programmer can extend an existing function or procedure by adding some parameters that have default value instead of writing an identical method with different parameters.

Example deadline_day

{$mode objfpc}

uses SysUtils, DateUtils;

function deadline_day( day: integer; month: integer = 0 ) : tDateTime;
var
  n :TDateTime;
  y, m, d :word;
begin
  n := now;
  DecodeDate( n, y, m, d );
  if month = 0 then
     begin
       if d > day  then
         begin
           n := IncMonth( n );
           DecodeDate( n, y, m, d );
         end;
     end
   else
     begin
       if month < m then n := IncYear( n );
       DecodeDate( n, y, m, d );
       m := month;
     end;
    d := day;
   result := EncodeDate( y, m, d );
end;

Default parameter is used in place of the missing trailing parameter in a call

• in program call deadline_day( 5 ); //-> calls deadline_day( 5, 0 );
• in program call deadline_day( 5, 1 ); //-> calls deadline_day( 5, 1 );