[fpc-pascal] How to avoid Copy

[Michael Van Canneyt]

On Fri, 29 Dec 2023, Amir via fpc-pascal wrote:

> 
> 
> On Dec 29, 2023 9:50 PM, Adriaan van Os <adriaan at adriaan.biz> wrote:
>
>       Amir--- via fpc-pascal wrote:
>       > Hi all,
>       >
>       >  I have a List of record, where the record has a WideString field.
>       >   I have some code like the following:
>       >
>       > function check(constref v: TMyRecord; data: TListOfMyRecord): Boolean;
>       > var
>       >   r: TMyRecord;
>       >
>       > begin
>       >   Result := False;
>       >   for r in data do
>       >     if r.State = v.State then
>       >       Exit(True);
>       > end;
>       >
>       > I call this method a lot and the CPU profiling shows a lot of cpu time
>       > spent on "fpc_copy_proc" (which I assume is doing the deep copy on
>       > records) from "TCustomListEnumerator.GetCurrent".
>       > I considered other alternatives like using enumerators but they all need
>       > a to return a record (and hence copying the widestring field).
>       > I can think of two solutions to get rid of the wasting(!) so much time
>       > on "fpc_copy_proc":
>       > 1) Changing the TMyRecord to TMyClass. But then I need to Create and
>       > Free a "lot" of objects.
>       > 2) Update TListOfMyRecord to TListOfPointerToMyRecord. This requires a
>       > "lot" of memory allocation/fragmentation.
>       >
>       > Is there a better solution?
>
>       Pass the data parameter by reference.
> 
> This means I need to have a ListOfMyRecord and a ListOfConstRefMyRecord, right?

It means you use 
constref  data: TListOfMyRecord
or 
var data: TListOfMyRecord

And try to avoid "for r in data" because it will create a copy for each element
in the list.

I don't know how you defined TListOfMyRecord, but if it is a
generic TList<TMyRecord>, better change it to a TArray<TMyRecord> or Array
of TMyRecord, in which case you can use a pointer to the consecutive records
in the array. Generics are convenient, but slow.

Michael.