Delphi: Offset of record field

Question

I\'m looking for ways to obtain the offset of a field in a Delphi record. These 2 following methods work but i was hoping for a cleaner way. Basically i would have liked the

Answer 1

I always use this approach:

Offset := Integer(@rec_a(nil^).c);

Don't let the use of nil^ put you off, it's perfectly safe. And don't worry about 64 bit pointer truncation. If you have a record whose size is >4GB then you have bigger problems!

Answer 2

You could also use a generic approach:

uses
  System.SysUtils,TypInfo,RTTI;

function GetFieldOffset( ARecordTypeInfo : PTypeInfo;
                         const ARecordFieldName : String) : Integer;
var
  MyContext: TRttiContext;
  MyField: TRttiField;
begin
  if (ARecordTypeInfo.Kind <> tkRecord) then
    raise Exception.Create('Not a record type');
  for MyField in MyContext.GetType(ARecordTypeInfo).GetFields do
    if MyField.Name = ARecordFieldName then
    begin
      Exit(MyField.Offset);
    end;
  raise Exception.Create('No such field name:'+ARecordFieldName);
end;

And call it like this:

ShowMessage( IntToString( GetFieldOffset( TypeInfo(rec_a),'c')));

---

Not as fast as your other alternatives, but gives a unified generic solution.

---

Looking at your options here for a clean solution, it seems the best is to declare a generic function:

function GetFieldOffset( const P : Pointer) : Integer; Inline;
// Example calls :
//   GetFieldOffset( @PMyStruct(nil).MyParameter);
//   GetFieldOffset( @TMyStruct(nil^).MyParameter);
begin
  Result := Integer( P);
end;

So even if the call looks awkward, the function name tells you what's going on. Inlining the call removes the function call overhead, so it will work as a code beautifier.

It is possible to get constant values for a record base and field address:

const
  cStruct : MyStruct = ();
  cMyInteger3Offs : Pointer = @cStruct.MyInteger3;
  cMyStructBase   : Pointer = @cStruct;

But this will not make code look cleaner.