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.

{$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:

{$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

From the Free Pascal wiki, page "Default", as it stood on 2022-04-01 after a change by Kai Burghardt, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's Licensing page. If you have concerns about this material, message the admin.