Advanced record constructors

[howardpc]

What is the essential difference between calling a record's constructor, and calling a 'standard' procedure of that record?
In other words, in what circumstances would you want to declare a constructor for an advanced record, rather than just declaring a plain procedure? And are such routines in records correctly termed 'methods', or is that term restricted to the routines of classes and interfaces?

[TRon]

Hello howardpc,

Perhaps you missed it because it is (officially) named extended records in Free Pascal, see https://www.freepascal.org/docs-html/ref/refse58.html#x118-1400009.1 and https://www.freepascal.org/docs-html/ref/refse59.html#x119-1410009.2

cheers

edit: and i just see that there is an error in that documentation. A constructor is allowed, as long as it is not parameterless.

[jamie]

What is the essential difference between calling a record's constructor, and calling a 'standard' procedure of that record?
In other words, in what circumstances would you want to declare a constructor for an advanced record, rather than just declaring a plain procedure? And are such routines in records correctly termed 'methods', or is that term restricted to the routines of classes and interfaces?

Maybe it behaves like OBJECTS do. the difference being one can have a Pointer referenced to it etc..

I don't Know, maybe I should experiment a bit 


EDIT:
  Must be something new, cause I can't do that in 3.0.4

[jamie]

Interesting,,,,

When dynamically creating a Record using the NEW(MYRecordPointer, Can't call a Method of this recorc);

 The NEW does not support calling of procedures/functions of said dynamic record …..

  Should this be reported ? Really thing NEW should be able to do this.

[TRon]

EDIT:
  Must be something new, cause I can't do that in 3.0.4

not sure what you are referring to exactly, but see https://wiki.freepascal.org/FPC_New_Features_2.6.0#Advanced_record_syntax

did you forgot the modeswitch perhaps ?

[howardpc]

The first page of documentation TRon referenced states:

Some of the restrictions when compared to classes or objects are obvious from the syntax diagram:

...

Constructors or destructors cannot be defined.

Yet this little program compiles and produces the expected output:

Code: Pascal  [Select][+][-]

1. program Project1;
2.  
3. {$mode objfpc}{$H+}
4. {$ModeSwitch advancedrecords}
5.  
6. type
7.   TPlainRec = record
8.   public
9.     procedure Init(const aText: String);
10.   end;
11.  
12.  
13.   TEnhancedRec = record
14.   public
15.     constructor Init(const aText: String);
16.   end;
17.  
18. var
19.   p: TPlainRec;
20.   e: TEnhancedRec;
21.  
22. { TEnhancedRec }
23.  
24. constructor TEnhancedRec.Init(const aText: String);
25. begin
26.   WriteLn(aText);
27. end;
28.  
29. procedure TPlainRec.Init(const aText: String);
30. begin
31.   WriteLn(aText);
32. end;
33.  
34. begin
35.   p.Init('plain');
36.   e.Init('enhanced');
37.   ReadLn;
38. end.

[jamie]

I wasn't reading the message close enough, it was "non parametered " create not allowed.

well well. so I can't use a plain ole  constructor, it needs a parameter..

[TRon]

@jamie
that it requires a parameter, make more or less sense to me.

You are indeed correct that you can't seem to use new and invoke the constructor at the same time ("Error: Pointer to object expected").

I wonder if that was done on purpose or that it was an oversight ?

[jamie]

I would say an oversight. The New isn't used much because people forget about it but I still use Objects because they work both ways, static or dynamic and for the most part I can do lots of things with objects and if I exit early I don't normally need to worry about cleanup in a block..

[jamie]

Well its confirmed , Fpc has a bug with the NEW and constructors in a Record.. at least the 3.0.4 does..

 NEW(PointerTORecord, ConstructorofSaidRecord(A_Parameter)) refuses to compile..

This works in Delphi without an issue but it does insist on using an Constructor, not just any method..

 Can anyone else confirm this with higher level of compilers ?

Code: Pascal  [Select][+][-]

1.   PMyType = ^TmyType;
2.   TMyType = Record
3.    Constructor init(A:Byte);
4.   end;  
5.   implementation
6.  
7. {$R *.lfm}
8. Constructor TmyType.Init(A:Byte);
9. Begin
10.  if A = 1 Then Beep;
11. End;
12. { TForm1 }
13. procedure TForm1.Button1Click(Sender: TObject);
14. Var
15.   P:PMyType;
16. begin
17.   New(P,init(1));  // This works in Delphi .Of course advancedRecords is on..
18. end;                                      
19.  

[TRon]

oh, sorry. i thought i mentioned it already 

3.2.0rc1 produced that error.

[egsuh]

AFAIK, constructors in records do nothing but setting initial values to the fields of the record.

Record variables p and e, in follwoing example, have their own local (or global? anyway not at memory heap) memory allocated at the definition.

Code: Pascal  [Select][+][-]

1. var
2.   p: TPlainRec;
3.   e: TEnhancedRec;

The issue seems that record variable allocated by "new" procedure does not support class-like methods. Not sure about Delphi.

[Thaddy]

This is only allowed with old school objects and Delphi behaves the same.
The whole KOL library is based on it, and simply works.

[PascalDragon]

What is the essential difference between calling a record's constructor, and calling a 'standard' procedure of that record?
In other words, in what circumstances would you want to declare a constructor for an advanced record, rather than just declaring a plain procedure? And are such routines in records correctly termed 'methods', or is that term restricted to the routines of classes and interfaces?

A constructor is essentially a class function. Though if jamie is right with his comment that in Delphi constructors can also be used for extended New then this would be a further difference to ordinary class methods and would additionally explain why the Delphi developers decided to use the constructor keyword.

And yes, the term method is also used here, essentially for any routine that has a hidden Self parameter (e.g. a static class function (compared to a class function) wouldn't be a method ).

This works in Delphi without an issue but it does insist on using an Constructor, not just any method..

It makes sense that it only works with constructors (the same is true for the TP style objects), because then the compiler can be sure about the function signature.

[ASerge]

Though if jamie is right with his comment that in Delphi constructors can also be used for extended New then

It's right. This simple project compiled and work in Delphi (at least 10.3), but not in FPC 3.0.4 or FPC 3.3.1:

Code: Pascal  [Select][+][-]

1. {$APPTYPE CONSOLE}
2. {$IFDEF FPC}
3.   {$MODE OBJFPC}
4.   {$MODESWITCH ADVANCEDRECORDS}
5. {$ENDIF}
6.  
7. type
8.   PMyType = ^TMyType;
9.   TMyType = record
10.     constructor Init(AValue: Integer);
11.   end;
12.  
13. constructor TMyType.Init(AValue: Integer);
14. begin
15.   Writeln('Init ', AValue);
16. end;
17.  
18. var
19.   P: PMyType;
20. begin
21.   New(P, Init(123)); // or P := New(PMyType, Init(123));
22.   Dispose(P);
23.   Readln;
24. end.