[fpc-devel] Copy/move operator

[Ben Grasset]

On Sun, Jun 16, 2019 at 9:30 AM Ryan Joseph <genericptr at gmail.com> wrote:

> Here’s a little example of the problem. What I propose is that we either
> add a 3rd boolean parameter to the Copy operator or add a new Move operator
> which is preferred over the Copy operator if it exists (this is much more
> work obviously but maybe has some benefit).
>

I can't actually check the assembly at the moment, but I'm pretty sure if
your example was written like the following, it would not call `Copy` for
the `TList<Integer>.Create(10)` assignment:

{$mode objfpc}
{$modeswitch advancedrecords}

program test;

type
  generic TList<T> = record
      data: array of T;
      // static class function instead of actual constructor
      class function Create(num: integer): TList; static; inline;
      class operator Copy(constref src: TList; var dest: TList);
  end;

class function TList.Create(num: integer): TList;
begin
  SetLength(result.data, num);
end;

class operator TList.Copy(constref src: TList; var dest: TList);
begin
  // no reason not to use `move` here, I wouldn't say
  move(src.data, dest.data, sizeof(T) * Length(src.data));
end;

var
  a, b: specialize TList<integer>;
  i: integer;
begin
  // TList.Create allocates a new array but we don’t really need to copy
  // on assignment because the source is a temporary object that doesn’t
  // actually exist at any static address.
  a := specialize TList<integer>.Create(10);
  for i := 0 to 9 do a.data[i] := i;
  b := a;
  for i := 0 to 9 do writeln(b.data[i]);
end.
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