Re: OO Style with Ada Containers

On Nov 19, 8:44 am, Maciej Sobczak <see.my.homep...@xxxxxxxxx> wrote:
On 19 Lis, 03:53, Matthew Heaney <matthewjhea...@xxxxxxxxxxxxx> wrote:

Can you convince me that Ada.Containers is closer to STL than to Java
containers (for example)?

But the burden of proof is on you, to prove that the container library
is *not* closer to the C++ than it is to the Java containers! (And
since I know C++ but not Java, such a proof would indeed be
interesting!)


If not, then why it is repeated so often
that Ada.Containers were influenced by STL?

You can read my original library proposal here:

http://home.earthlink.net/~matthewjheaney/charles/ai302.txt

As I stated then:

"This library API is modeled on the STL. It has the same containers
as
the STL does, using the same container names and semantics. If you
know
the STL already then you will instantly understand how this library
works. It does not have any of the STL algorithms, but that's only
because I wanted to keep the API small (and not because it's not
possible to support STL-style algorithms -- it is). The API described
here has exactly the minimum set of containers I believe all Ada
developers need, in exactly the form they need it."


Do you by any chance find
something in C++ that is worth imitating? :-)

Yes, of course: the STL.


I don't claim that the designers of Ada.Containers didn't have STL in
mind when taking some decisions, but the essence [*] of STL just
didn't get there.

Well, the library is modeled on the STL. There are necessarily
differences, because the languages are different, and my design
philosophy was (and is) to use Ada idioms where appropriate.


Ada has other mechanisms, such as nested procedures and
downward closures, and the container library was designed to use the Ada
mechanisms.

How can I use these mechanisms to make iterator adaptors?

A generic algorithm in Ada would look something like:

generic
type Cursor is private;
with function Next (C : Cursor) return Cursor is <>;
with function Has_Element (C : Cursor) return Boolean is <>;
...
procedure Generic_Algorithm (C : Cursor);

You can instantiate this on the container types directly, e.g.

package Integer_Vectors is new Vectors (Integer);
use Integer_Vectors;

procedure Algorithm is new Generic_Algorithm (Cursor);

This instantiation will deliver every integer element in the vector.
An iterator adapter just means replacing the cursor operations for the
container with something else, e.g.

function Return_Odd_Only (C : Cursor) return Cursor is
CC : Cursor := C;

begin
while Has_Element (CC)
and then Element (CC) rem 2 = 0
loop
Next (CC);
end loop;
return CC;
end Return_Odd_Only;

procedure Algorithm is
new Generic_Algorithm
(Cursor,
Next => Return_Odd_Only,
others => <>); -- verify syntax

procedure Op (V : Vector) is
begin
Algorithm (V.First);
end;


Consider a
filtering iterator that automatically skips unwanted (for the given
predicate) elements.

See above for an example.


Consider a virtual iterator that does not have
*any* container behind, but can serve as a sequence generator. And so
on.

But the generic algorithms depend only on a cursor as a generic formal
-- the algorithms aren't tied to containers directly (which was
Stepanov's great insight). For example, let's use the algorithm above
with an array:

type Integer_Array is (Positive range <>) of Integer;
-- the "container" type

procedure Op (A : Integer_Array) is
function Has_Element (J : Integer) return Boolean is
begin
return J <= A'Last;
end;

procedure Algorithm is
new Generic_Algorithm
(Cursor => Integer,
Next => Integer'Succ);

begin
Algorithm (A'First);
end;

.

Relevant Pages

  • Re: OO Style with Ada Containers
    ... modified without naming the container in addition to providing the ... cursor/iterator and this is different from both STL and Java (STL ... type Cursor is private; ... You can see here that the algorithm does not name a container. ...
    (comp.lang.ada)
  • Re: Unified Ada library
    ... I think a cursor should be a modeled explicitly as a pointer, ... it's how an algorithm to reverse a sequence would be ... an element in the container. ...
    (comp.lang.ada)
  • Re: Ada.Containers.Indefinite_Ordered_Maps of gcc 4.0.1 has bug ?
    ... Empty_Map: constant Map; ... No_Element: constant Cursor; ... function Length (Container: Map) return Count_Type; ... pragma Inline; ...
    (comp.lang.ada)
  • Re: Run-time accessibility checks
    ... No I made it having same interface, ... container, *because* it is of a different type. ... The cursor is the index in the model you have here. ... to be used to achieve the goal, and secondly return by reference is marked ...
    (comp.lang.ada)
  • Re: GCC 4.0 Ada.Containers Cursor danger.
    ... To impose an order on them standard needs to put a restriction on implementarion algorithms. ... There is no need for cursors during iterations. ... has exactly the same semantics for _any_ container type. ... This could be done using a generic algorithm which is similar ...
    (comp.lang.ada)
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