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Subject: Question[s] about aliased extended return.
From: Shark8 <onewingedshark@gmail.com>
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Date: 2012-11-02T08:55:49-07:00
List-Id: <comp.lang.ada>

The syntax for the extended return given in the RM [ http://www.ada-auth.or=
g/standards/12rm/html/RM-6-5.html ] allows for the usage of the keyword ALI=
ASED, with the restriction of "if the keyword aliased is present in an exte=
nded_return_object_declaration, the type of the extended return object shal=
l be immutably limited."

The 2012 rationale entry regarding the extended return [ http://www.ada-aut=
h.org/standards/12rat/html/Rat12-4-6.html ] says of immutably limited objec=
ts:
* it is an explicitly limited record type,
* it is a task type, protected type or synchronized interface,
* it is a non-formal limited private type that is tagged or has an access d=
iscriminant with a default expression, [or]
* it is derived from an immutably limited type.

(1) Doesn't this reduce the usefulness of the extended return?

There are a couple of times building the OpenGL binding where I would have =
liked to do something like:
Function some_vector( [params] ) Return Vector_of_points is
begin
  Return Result : access Vector_of_points( 1..size_from_params ) do
    glFunctionPopulatingVector( glEnum_from_params, Result'access )
  end return;
end some_vector;
in order to populate the vector with the requisite data, but this is not al=
lowed.
(2) Is there a reason this is not allowed? (Thick/thin 'pointer' difference=
s?)

Lastly, the GNAT compiler seems to reject the following, even though the ra=
tionale says a "explicitly limited record type" qualifies as immutably limi=
ted:
    Type LR is limited record
	Data : Character:=3D 'X';
    End record;
   =20
    Function Get_LR Return LR is
    begin
	Return Result : aliased LR do
	    null;
	end return;
    end Get_LR;
(3) Is this a bug, or am I just misreading everything?