Delphi record helper for record

Question

I have this code:

type
 TSolution = record
  x1, x2, i1, i2: double;
 end;

 TSupport = record helper for TSolution 
  function toFraction: string;
 end;

And I would like to be able to call something like this inside (for example) a button click event:

var k: TSolution;
begin

 //...
 Memo1.Lines.Add(k.x1.toFraction);
 //...

end;

Where toFraction is a function I have written that converts a double into a fraction. I think that a record helper is what I need but why doesn't this work?

{ TSupport }

function TSupport.toFraction: string;
begin
 Result := TFunction.DoubleToFraction(self);
end;

If you are wondering, TFunction is a class (in the same unit as the record/record helper) and it has the DoubleToFraction class function which returns a string of course.

E2010 Incompatible types: 'Double' and 'TSolution'

Ok this is the error but if I don't use the self then how can I take the parameter? The parameter would be x1, x2, i1 or i2 depending on what I need. In the example above, inside Memo1 I should convert x1 (parameter) to fraction. What to do here?

I have noticed that if you write for example

TSupport = record helper for double

Then the Self works! Basically I would like to know how to avoid the Self to refer to the TSolution record but to its double fields.

But why do you need helper for your own type? Why not add function/property into TSolution? — MBo
@MBo Because I already have a class function with the toFraction. I wanted to be able to add to my TSolution something "quick" like k.x1.toFraction instead of k.toFraction(k.x1) — Raffaele Rossi
That misses the point completely. Add a method to TSolution that calls your class function. Using helpers here is a complete abuse of the feature. — David Heffernan

asd-tm asd-tm · Accepted Answer · 2017-09-25T13:51:34

Here is the wrong context: you call

k.x1.toFraction

that means that you call a function toFraction that corresponds to x1, but it is a double, not TSolution.

Your helper function will be called form something like

k.toFraction

I assume that toFraction knows in advance which of the variables x1, x2, i1, i2: double; to take. Otherwise you can write a helper for double.

APENDED on @Remy Lebeau consideration

You can use a construction like this:

TMyDouble = record
  private
    class var FX: double;
  public
    class property X: double read FX write FX;
    class operator Implicit(a: TMyDouble): double;
    class operator Implicit(a: double): TMyDouble;
    function toFraction: string;
  end;

function TMyDouble.toFraction: string;
begin
  result := FloatToStr(self.FX { .Fraction } ); // or other FX.methods
end;

class operator TMyDouble.Implicit(a: TMyDouble): double;
begin
  result := FX
end;

class operator TMyDouble.Implicit(a: double): TMyDouble;
begin
  result.FX := a;
end;

It will emulate double functionality and seem a double being a record.

Delphi record helper for record

1 Answers