[fpc-pascal] Record Composition

[Thomas Kurz]

I'm trying to use this feature, but I'm having some trouble:

All of my records contain a parameterless `class function Create: TWhateverRecord; static;`.

Now, when I try to contain one within the other, I'm stumbling across this compiler error:

Error: Overloaded identifier "CREATE" isn't a function

I guess this is because both records now have a `Create` function with the same signature. But how can I resolve that conflict?

And one more question: Does `Default(TComposedRecord)` correctly intialize the complete structure?

Kind regards,
Thomas


----- Original Message ----- 
From: Mattias Gaertner via fpc-pascal <fpc-pascal at lists.freepascal.org>
To: fpc-pascal at lists.freepascal.org <fpc-pascal at lists.freepascal.org>
Sent: Monday, October 19, 2026, 11:40:23
Subject: [fpc-pascal] Record Composition

Hi all,

Thanks to Frederic Kehrein FPC main now supports Record Composition:

A record can now "contain" another record and have that record's members appear as its own.
Firstly this allows easy and automated conversion of C code, which uses anonymous structs and unions introduced in C11, into Pascal and to use C libraries frictionless and transparently through the same types and interfaces.
Secondly it allows for non trailing and multiple variant parts.
And last but not least it can avoid {$IFDef} structures within the record definition allowing to reuse existing types.

The feature is enabled with

{$modeswitch RecordComposition}

It is not part of any mode by default.


Example:

------------------------
program demo;
{$mode objfpc}{$H+}
{$modeswitch advancedrecords}
{$modeswitch recordcomposition}
type
   TPoint = record
     X, Y: Integer;
     procedure Offset(dx, dy: Integer);
   end;

   TSize = record
     Width, Height: Integer;
     function Area: Integer;
   end;

   TRectangle = record
     contains TopLeft: TPoint;  // named composition
     contains TSize;            // unnamed composition
     function Right: Integer;
   end;

procedure TPoint.Offset(dx, dy: Integer);
begin
   Inc(X, dx);
   Inc(Y, dy);
end;

function TSize.Area: Integer;
begin
   Result := Width * Height;
end;

function TRectangle.Right: Integer;
begin
   Result := X + Width;   // composed members, no qualifier needed
end;

var
   r: TRectangle;
begin
   r.X := 10;             // same as r.TopLeft.X
   r.Y := 20;
   r.Width := 300;
   r.Height := 200;
   r.Offset(5, 5);        // calls TPoint.Offset with Self = r.TopLeft
   WriteLn(r.TopLeft.X, ' ', r.Right, ' ', r.Area);  // 15 315 60000
end.
------------------------


See here for the more information:
https://wiki.freepascal.org/FPC_New_Features_Trunk#Record_Composition

Mattias


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