[fpc-devel] Re: Converting records back and forth. (Forward declarations for records needed ?!?)

[Skybuck Flying]

There appears to be a solution in Delphi for this:

It's called "record helpers":

Demonstration program:

// *** Begin of Test Program ***

program TestProgram;

{$APPTYPE CONSOLE}

{

Test circular records problem and solutions.

version 0.01 created on 19 september 2011 by Skybuck Flying

The solution below uses a "record helper" which seems to solve it nicely
and now I can write the code the way I want to...

Not sure if it compiles in free pascal, but it does compile in Delphi XE at 
least ;)

(Perhaps another solution could be to use type casting, but that would not 
solve
the circular problem, so not going to test that)

}

uses
SysUtils;

type
TrecordB = record
private
  mData : double;
public
  property Data : double read mData write mData;
end;

TrecordA = record
private
  mData : integer;
public
  property Data : integer read mData write mData;

  function ToRecordB : TrecordB;
end;

// solution: record helper
TrecordBhelper = record helper for TrecordB
private
//  mData : double;
public
//  property Data : double read mData write mData; // must be in TrecordB 
apperently ?!?

  function ToRecordA : TrecordA;
end;

function TrecordA.ToRecordB : TrecordB;
begin
result.mData := mData;
end;


function TrecordBhelper.ToRecordA : TrecordA;
begin
result.mData := Round(mData);
end;

procedure Main;
var
vRecordA : TrecordA;
vRecordB : TrecordB;
begin
writeln('program started');

vRecordA.mData := 555;
vRecordB.mData := 666.666;

writeln( 'vRecordA.mData: ', vRecordA.mData );
writeln( 'vRecordB.mData: ', vRecordB.mData );

writeln( 'vRecordA.ToRecordB.mData: ', vRecordA.ToRecordB.mData );
writeln( 'vRecordB.ToRecordA.mData: ', vRecordB.ToRecordA.mData );

writeln('program finished');
end;

begin
try
  Main;
except
  on E: Exception do
   Writeln(E.ClassName, ': ', E.Message);
end;
ReadLn;
end.


// *** End of Test Program ***

Bye,
  Skybuck.