Re: Containers - nontrivial element access

[Matthew Heaney <mheaney@on2.com>]

On Oct 1, 5:55 pm, Maciej Sobczak <see.my.homep...@gmail.com> wrote:
>
> There is a record type (for example Person) with a couple of fields
> (for example Salary) that together make the whole a bit heavy, so that
> unnecessary copying of the whole is to be avoided. Objects of this
> type are stored in the container.

Right, so you probably want to avoid operations Element or
Replace_Element, and use Query_Element or Update_Element instead.


> I would like to swap salaries of two guys which I can refer with the
> index/iterator/cursor/etc. The reference method is not really
> important - what is important is the problem of modifying more than
> one element in the container.
>
> C++ example is easy:
>
> vector<Person> people;
> // ...
> swap(people[x].salary, people[y].salary);


V : Person_Vectors.Vector;

declare
   procedure Process_X (PX : in out Person) is
      procedure Process_Y (PY : in out Person) is
         SX : constant Salary_Type := PX.Salary;
      begin
         PX.Salary := PY.Salary;
         PY.Salary := SX;
      end;
   begin
      V.Update_Element (Y, Process_Y'Access);
   end;
begin
   V.Update_Element (X, Process_X'Access);
end;



> I hope it is obvious what it does (suppose x and y are some indices
> into the vector). Just in case it isn't - all the components of the
> two records stay intact except the salary, which is swapped between
> the two. C++ makes it possible by explicitly returning a reference
> from the method that accesses the element.

Right, but Ada doesn't have explicit reference types, so you have to
use Update_Element, which allows in-place editing.


> What would you suggest as the Ada solution for this problem?

See above.


> The Update_Element procedure with its access to the user-provided
> modifying procedure requires to pass data "under the table" (like with
> a separate variable declared aside the modifying procedure) - and
> seems to be just clunky. Is this the only possibility?

I don't understand what you think the problem is.  There is no need
for a separate variable or anything else.