Locations vs Values: using RTTI to work with value types

[pyscripter 689]

I was looking at an old blog post by Barry Kelly.  

 

In particular the function:

function TLocation.FieldRef(const name: string): TLocation;
var
  f: TRttiField;
begin
  if FType is TRttiRecordType then
  begin
    f := FType.GetField(name);
    Result.FLocation := PByte(FLocation) + f.Offset;
    Result.FType := f.FieldType;
  end
  else if FType is TRttiInstanceType then
  begin
    f := FType.GetField(name);
    Result.FLocation := PPByte(FLocation)^ + f.Offset;
    Result.FType := f.FieldType;
  end
  else
    raise Exception.CreateFmt('Field reference applied to type %s, which is not a record or class',
      [FType.Name]);
end;

I am puzzled by the line:

Result.FLocation := PPByte(FLocation)^ + f.Offset;

If Flocation is an object (FType is TRttiInstance type) and I am having a record field inside the object, the Result.FLocation should be PByte(FLocation) + f.offset, i.e. the same as for FType is TRttiRecord.

 

Barry Kelly is probably the guy that wrote the Rtti stuff, so he knows what he is talking about.   What I am missing?

[Attila Kovacs 629]

it's in the comments

 

Quote

The code is different between instances and records because instances need a dereference before adding the offset. It could be written differently, of course.

 

[pyscripter 689]

@Attila Kovacs Thanks I missed that.  Now I get it.  The key is in 

 

class function TLocation.FromValue(C: TRttiContext; const AValue: TValue): TLocation;
begin
  Result.FType := C.GetType(AValue.TypeInfo);
  Result.FLocation := AValue.GetReferenceToRawData;
end;

If AValue contains an object, Flocation would a pointer to a pointer.  That was not the case in my testing.  He could do the dereferencing in this function of course.

Edited May 3, 2020 by pyscripter