Get the offset of a field in a delphi record at runtime

Question

Given a record type:

TItem = record
   UPC : string[20];
   Price : Currency;
   Cost : Currency;
   ...
end; 

And the name of a field as a string, how can I get the offset of that field within the record? I need to do this at runtime - the name of the field to access is decided at runtime.

Example:

var
   pc : Integer;
   fieldName : string;
   value : Currency;
begin
   pc := Integer(@item);                    // item is defined and filled elsewhere
   fieldName := Text1.Text;                 // user might type 'Cost' or 'Price' etc
   Inc(pc, GetItemFieldOffset(fieldName));  // how do I implement GetItemFieldOffset?
   value := PCurrency(pc)^;
   ..

I'm using Delphi 7.

Answer 1

You can't. Delphi 7 does not emit RTTI for records. There are other options (as seen the previous answers) but those require manual mapping of "Field Name" -> "Offset".

Answer 2

As alex said, Delphi 7 doesn't emit RTTI for records, so you can't retrieve the required info at runtime. However, in later versions (Delphi 2010+) it does, and the following code:

TItem = record
   UPC : string[20];
   Price : Currency;
   Cost : Currency;
//...
end;
    var
       rttiContext: TRttiContext;
       rttiType: TRttiType;
       fields: TArray<TRttiField>;
       item: TItem;
    begin
        rttiType := rttiContext.GetType(TypeInfo(TItem));
        caption := rttiType.Name + ' {';
        fields := rttiType.GetFields;
        for i := low(fields) to high(fields) do
        begin
          caption := caption +'{name='+fields[i].Name+',';
          caption := caption +'offset='+IntToStr(fields[i].Offset)+'}';
        end;
        caption := caption + '}';

will produce 'TItem {{name=UPC,offset=0}{name=Price,offset=24}{name=Cost,offset=32}}'

You can also set the field value in a particular instance (although you should really also verify the type) using:

if fields[i].Name = 'Price' then
  fields[i].SetValue(@item, 10);

Answer 3

Following would work for your simplified scenario but I doubt it will be possible to make a generic function for this kind of thing.

The best I can think if is to add some kind of registration object but it still would require you to register all the records you need an offset from.

function GetItemFieldOffset(const Value: string): Integer;
var
  item: TItem;
begin
  if Value = 'UPC' then Result := 0
  else if Value = 'Price' then Result := Integer(@item.Price) - Integer(@item)
  else if Value = 'Cost' then Result :=  Integer(@item.Cost) - Integer(@item)
  else raise Exception.CreateFmt('Unhandled condition (%0:s)', [Value]);
end;