Re: Reserve_Capacity for Unbounded_String?

[Maciej Sobczak <see.my.homepage@gmail.com>]

On 22 Lip, 23:32, Robert A Duff <bobd...@shell01.TheWorld.com> wrote:

> > Why there is no Reserve_Capacity for Unbounded_String?
>
> Probably for historical reasons.

Would introducing it break any existing code?

> > declare
> >    S : Unbounded_String;
> >    My_Capacity : constant := 1000;
> > begin
> >    S := To_Unbounded_String (My_Capacity);
>
> I would write
>
>   To_Unbounded_String (Length => My_Capacity);
>
> to make it clear which To_Unbounded_String you're calling.

Yes. I try to follow the "rule" that names associations should be used
for any subprogram call with more than one parameter. I've never
considered hardening this rule for overloaded subprograms.

> > "Seems to work fine" means that after the above two operations the
> > string is logically empty, but the subsequent appends run faster
> > (which is actually the original motivation).
>
> How much faster (I'm curious)?

This of course depends on what is the ration between allocations and
reservations in a given test.

Here:

with Ada.Strings.Unbounded;
procedure String_Test is
begin
   for I in 1 .. 10000 loop
      declare
         S : Ada.Strings.Unbounded.Unbounded_String;
         use Ada.Strings.Unbounded;
      begin
--       S := Ada.Strings.Unbounded.To_Unbounded_String (10000);
--       Ada.Strings.Unbounded.Delete (S, 1, 10000);
         for J in 1 .. 10000 loop
            Ada.Strings.Unbounded.Append (S, 'A');
         end loop;
      end;
   end loop;
end String_Test;

On my machine and with option -O2 uncommenting the two lines makes a
difference between 1.86s and 1.43s, which is about 20% gain. Depending
on a situation it might or might not be worth the hassle (the hassle
is in determining the correct reservation size - and if we can know it
fully in advance, then maybe there is no point in using unbounded at
all).

--
Maciej Sobczak
http://www.msobczak.com/