Default

Default is

• a compiler intrinsic function returning a zeroed out value, or
• a keyword marking a class’s array property as implicitly accessible.

This article describes the compiler intrinsic. See other articles for the notion in OOP.

zero-idiom

Since FPC 3.0.0 default(dataType) returns a zero value for the specified dataType.

program defaults(input, output, stdErr);
var
	i: integer;
	s: string;
	r: tOpaqueData;
begin
	i := default(integer);     { assigns `0` }
	s := default(string);      { assigns `nil` or `''` (empty string) }
	r := default(tOpaqueData); { assigns `tOpaqueData[]` }
end.

advantages

• The most important “advantage” is that default can be used in generic definitions with template parameters.

  program defaultDemo(input, output, stdErr);
  {$mode objFPC}
  { --- generic thing -------------------------------------- }
  type
  	generic thing<storageType> = object
  			data: storageType;
  			constructor init;
  		end;
  constructor thing.init;
  begin
  	data := default(storageType);
  end;
  
  { === MAIN =============================================== }
  type
  	arr = array[1..10] of integer;
  var
  	x: specialize thing<tBoundArray>;
  	y: specialize thing<arr>;
  begin
  end.
  

  In the context of the definition of the generic data type thing it is not yet known what storageType will be. Therefore, we could not possibly write a literal value such as 0 or nil. However, we can use default to overcome this hurdle. Upon specialization the correct zero value will be inserted.
• Furthermore, the compiler can choose a faster implementation than, for instance, a respective fillChar(myVariable, sizeOf(myVariable), chr(0)).

caveats

• Despite its name, default is really just a synonym for zero. It can be used to assign values out of range:

  program faultyDefault(input, output, stdErr);
  {$rangeChecks on}
  type
  	typeWithoutZeroValue = -1337..-42;
  var
  	i: typeWithoutZeroValue;
  begin
  	i := -1024; { ✔ OK }
  	{i := 0; ✘ not OK }
  	i := default(typeWithoutZeroValue); { OK again }
  	writeLn(i);
  end.
  

  See FPC issue 34972.
• Default cannot be applied on file or text data types and structured data types containing such.

see also

• management operators