[Solved]Impossible to overload assignment of equal type: How to fix that?

[jamie]

I have a Record that maintains X,Y:Integer; and some other items .

I don't want to use the generic := because it will copy all of the record which I don't want.

I tried to implement the := operator so that I can simply assign the X,Y from another record but not the rest of the contents of the source record.

 Translated from a C/C++ app, where it obviously does work but FPC isn't allowing me to do the same.

 The rest of the operators work ok {+,-,= etc}

 So, if I am able to get the "=" to work, why can't I get the ":=" work, too?

 I understand to some degree this may not make sense to allow an operator like this since the compiler would simply assign one from the other, but in this case, its only some of it that gets assigned.

Is there a switch that I don't know about?

[MarkMLl]

Interesting. Does that also apply if the assignands are scalars, i.e. 32-bit or smaller, and specifically not composites such as a record?

MarkMLl

[Coldzer0]

I have a Record that maintains X,Y:Integer; and some other items .

I don't want to use the generic := because it will copy all of the record which I don't want.

I tried to implement the := operator so that I can simply assign the X,Y from another record but not the rest of the contents of the source record.

 Translated from a C/C++ app, where it obviously does work but FPC isn't allowing me to do the same.

 The rest of the operators work ok {+,-,= etc}

 So, if I am able to get the "=" to work, why can't I get the ":=" work, too?

 I understand to some degree this may not make sense to allow an operator like this since the compiler would simply assign one from the other, but in this case, its only some of it that gets assigned.

Is there a switch that I don't know about?

If I understand correctly, you want to copy x and y from one record to another.
Or Can you show an example of what you are trying to do; maybe I can help.

[jamie]

hand written here as best as I can explain it.

Code: Pascal  [Select][+][-]

1. Type
2.   TMyRecord = Record
3.     X,Y:Integer;
4.     SomeotherFields:Integer; //That I don't want to copy.
5.     ....
6.     class operator :=(A:TMyRecord; B:TMyRecord):TMyRecord;
7.   End;
8.  
9. Implementation
10. Class Operator TMyRecord.:=(A:TmyREcord; B:TMyRecord):TMyRecord
11. Begin
12.   A.X := B.X;
13.   A.Y := B.Y;
14.  Result := A;
15. End;
16.  
17.  

As it is, I can not use an := overload on this record because the compiler thinks I should use the normal := which will copy the whole record.

 I can implement a method to do this of course but then it is kind of a black sheep of the record because  I have all of the other common overloads already implemented.

[TRon]

Impossible to overload assignment of equal type: How to fix that?

By modifying the compilers source-code. End user is not allowed to override implicit assignment operator of the same type.

[Coldzer0]

You can do it using the operator Copy.

Code: Pascal  [Select][+][-]

1.   TMyRecord = Record
2.     X,Y:Integer;
3.     SomeotherFields:Integer; //That I don't want to copy.
4.     class operator Copy(constref src: TMyRecord; var dst: TMyRecord);
5.   End;
6.  
7.   class operator TMyRecord.Copy(constref src: TMyRecord; var dst: TMyRecord);
8.   begin
9.     dst.X := src.X;
10.     dst.Y := src.Y;
11.   end;
12.  
13. var
14.   a, b: TMyRecord;
15. Begin
16.   a.X := 1;
17.   a.Y := 2;
18.   a.SomeotherFields := 3;
19.   b.X := 4;
20.   b.Y := 5;
21.   b.SomeotherFields := 6;
22.  
23.   a := b;
24.  
25.   WriteLn(Format('a.X : %d | a.Y : %d | a.SomeotherFields : %d', [a.X, a.Y, a.SomeotherFields])); // will print "a.X : 4 | a.Y : 5 | a.SomeotherFields : 3"
26. end.
27.  

[TRon]

You can do it using operator Copy

Hmz nice find.

afaik only mentioned on the new features pages and the wiki as I was unable to locate it in the documentation.

[Coldzer0]

You can do it using operator Copy

Hmz nice find.

afaik only mentioned on the new features pages and the wiki as I was unable to locate it in the documentation.

I found it here https://wiki.freepascal.org/management_operators#Copy

[TRon]

I found it here https://wiki.freepascal.org/management_operators#Copy

Yeah I found it there as well but unfortunately only after you posted your example because I wasn't aware that management operators existed (and they seem to exist for quite some time) so before that example was posted I had no idea what to look for

I can blame that on myself for not looking close enough at the new features page (3.2.0) which links to that page but quite honestly I expected it to be mentioned somewhere in the official documentation (where I was unable to find anything on management operators but perhaps I overlooked).

Than you for bringing the example to my/our attention.

[Thaddy]

Unfortunaly the wiki entry is the only thing that comes close to official, although I suspect it is in the trunk/main documentation. But there are already many examples on this forum.

[jamie]

You can do it using operator Copy

Hmz nice find.

afaik only mentioned on the new features pages and the wiki as I was unable to locate it in the documentation.

I found it here https://wiki.freepascal.org/management_operators#Copy

Yes, that works. Thank you much!

While I was there at that page, I am also interested in the initializers and finalizers, those too, will also come in handy.
 

[PascalDragon]

Is there a switch that I don't know about?

No. Operators that are implemented by the compiler are not allowed to be overloaded to avoid potential confusion especially when global and scoped operators are mixed. As others wrote, use the Copy management operator for that as that correctly hooks into the copy mechanism of the RTL.

[jamie]

I was scoping around my Delphi and noticed they call it "Assign" not copy.

Trying to sync source codes with a few compiler processors to keep them happy on both ends.

Thanks.

[Thaddy]

Delphi did not have management operators until very recently. Freepascal has it for quite some time. That is the same as that Freepascal had generics way before Delphi.
It is thus quite common for us to see that they did not copy our syntax.
In due time, there probably will be a Delphi syntax compatible mode in {$mode delphi}. That is as usual.

[Thaddy]

Unless you mean the normal assign for classes.
The advantage of the management operator copy is that it allows the programmer to implement it as a deep copy, a shallow copy, or like in your case, a partial copy, while maintaining the use of the assignment operator to perform that automatically.