Considerations for (simple) type helpers

10.4 Considerations for (simple) type helpers

For simple types, the rules are pretty much the same as for records, plus there are some extra requirements:

Support for type helpers needs to be activated using the modeswitch typehelpers:
{$modeswitch typehelpers}

This modeswitch is enabled by default only in mode Delphi and DelphiUnicode.
In Delphi (and DelphiUnicode) mode, for stricter Delphi compatibility, the record helpers must be used instead of a type helper.
The modes ObjFPC and MacPas use type helper, but the modeswitch TypeHelpers must be used.
The following types are not supported:
- All file types (Text, file of ...)
- Procedural variables
- Types like records, classes, Objective C classes, C++ classes, objects and interfaces are forbidden as well, the class helper must be used for classes. That means that for instance, the following will fail:
  TTestHelper = type helper for TObject
  end;
This of course means that all other simple types are supported.
Type helpers can implement constructors.
Inheritance of record helpers is only allowed in ObjFPC mode; In Delphi mode, it is not allowed.
Type helpers can only descend from other type helpers, not from class or record helpers.
A descendent type helper must extend the same type.

The following gives an idea of the possibilities:

{$mode objfpc}
{$modeswitch typehelpers}

type
TLongIntHelper = type helper for LongInt
   constructor create(AValue: LongInt);
   class procedure Test; static;
   procedure DoPrint;
end;

constructor TLongIntHelper.create(AValue: LongInt);

begin
  Self:=Avalue;
  DoPrint;
end;

class procedure TLongIntHelper.Test;

begin
   Writeln('Test');
end;

procedure TLongIntHelper.DoPrint;

begin
   Writeln('Value :',Self);
end;

var
  i: LongInt;
begin
  I:=123;
  i.Test;
  $12345678.Test;
  LongInt.Test;
  I:=123;
  i.DoPrint;
  $12345678.DoPrint;
end.

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