Is there a function/operator in FPC that returns the offset of a field ?

[440bx]

Hello,

given something like:

Code: Pascal  [Select][+][-]

1. type
2.   TSomeRecord = record
3.     FirstField       : word;
4.     SecondField      : byte;
5.     ThirdField       : qword;
6.   end;
7.  

is there an "offset" function/operator that would directly return the offset of any one field in the above record ?  for instance, offset(TSomeRecord.ThirdField) would return 8 (because of alignment.)

It is easy to calculate the offset as the difference of two pointers but, it would sure be nice if there is a direct way of getting it.  That's my question, is there such a nice way of getting a field's offset ?

Thank you for your help.

[Thaddy]

Afaik only like this, as you already observed:

Code: Pascal  [Select][+][-]

1. {$ifdef fpc}{$mode delphi}{$H-}{$endif}
2.  
3. type
4.   TSomeRecord1 = packed record
5.     FirstField       : word;
6.     SecondField      : byte;
7.     ThirdField       : qword;
8.   end;
9.  
10.   TSomeRecord2 = record
11.     FirstField       : word;
12.     SecondField      : byte;
13.     ThirdField       : qword;
14.   end;
15.  
16. var R1: TSomeRecord1;
17.     R2: TSomeRecord2;
18. begin
19.  writeln(Addr(R1.ThirdField) - Addr(R1)); //3  
20.  writeln(Addr(R2.ThirdField) - Addr(R2)); //8 , depending on alignment settings)
21. end.

[/code]

[440bx]

Afaik only like this, as you already observed:

Code: Pascal  [Select][+][-]

1. {$ifdef fpc}{$mode delphi}{$H-}{$endif}
2. type
3.   TSomeRecord = record
4.     FirstField       : word;
5.     SecondField      : byte;
6.     ThirdField       : qword;
7.   end;
8.  
9. var R: TSomeRecord;
10. begin
11.  writeln(Addr(R.ThirdField) - Addr(R));  
12. end.

Thanks Thaddy.  I was afraid that would be the answer but, I figured I'd ask just in case. 

[Thaddy]

I expect the compiler to use an intrinsic, but it is not public.
In assembler it is no problem.

[PascalDragon]

The following expression might look strange, but we consider it one of the official ways to retrieve the offset to a field

Code: Pascal  [Select][+][-]

1. PrnUInt(@TSomeRecord(Nil^).ThirdField)

[440bx]

The following expression might look strange, but we consider it one of the official ways to retrieve the offset to a field

Code: Pascal  [Select][+][-]

1. PrnUInt(@TSomeRecord(Nil^).ThirdField)

Thank you.  I see what it does.

Definitely an improvement over using pointer difference.

It still cannot be used to define a constant equal to the offset of a field in some record.  If it could be used for that, then it would probably be a full replacement for an offset function.

[Thaddy]

That would require an extension of the macro support (the const), since it is align dependent.
Rather nifty reply from Sven...
On second thought: test that!
This works:

Code: Pascal  [Select][+][-]

1. program untitled;
2. {$mode objfpc}
3. type
4.  TSomerecord = record
5.  a:integer;
6.  b:byte;
7.  c:word;
8.  end;
9.  
10. const
11.   OffsetC:PtrUint = NativeUint(@TSomeRecord(Nil^).C);
12. begin
13.   writeln(OffsetC);
14. end.

<grin>

[440bx]

@Thaddy,

The compiler is not happy with your example.  I get an "illegal expression" on the definition of OffsetC.

[Thaddy]

@Thaddy,

The compiler is not happy with your example.  I get an "illegal expression" on the definition of OffsetC.

Why does it work for me?

[440bx]

Why does it work for me?

I'm using FPC 3.0.4.  What FPC version are you using ?

[Thaddy]

3.2.0 and trunk 3.3.1 (daily rolling compile).

[440bx]

3.2.0 and trunk 3.3.1 (daily rolling compile).

That's likely why it works for you and not for me. 

[guest58172]

suggestions: add a `.OffsetOf` property to aggregate members. Too bad if the member is itself an aggregate type with a member called OffsetOf...
but still doable with a deprecation. 'deprecation: OffsetOf is a reserved identifier inside aggregates'.

[jamie]

This works for me using 1.8.4/3.0.4

Code: Pascal  [Select][+][-]

1. procedure TForm1.Button1Click(Sender: TObject);
2. begin
3.   caption := PtrUint(@TRect(nil^).top).ToSTring;
4. end;                                              
5.  

if you are looking for a built in function to give you an offset I am not sure how much value it would have since we can do it
this way?

[440bx]

This works for me using 1.8.4/3.0.4

Code: Pascal  [Select][+][-]

1. procedure TForm1.Button1Click(Sender: TObject);
2. begin
3.   caption := PtrUint(@TRect(nil^).top).ToSTring;
4. end;                                              
5.  

Yes, that works because caption is being assigned at run time instead of compile time.  if you try this:

Code: Pascal  [Select][+][-]

1. const
2.   caption : ptruint = PtrUint(@TRect(nil^).top).ToSTring;
3.  

it won't work in FPC 3.04  (unfortunately)

But, thank you, I appreciate the help.