Re: new type'class'() legal?

[stt@houdini.camb.inmet.com (Tucker Taft)]

Simon Wright (simon@pogner.demon.co.uk) wrote:

: The code below compiles fine on Gnat 3.07, various platforms, but
: fails on Academic AdaMagic and ObjectAda (the demo version) at the
: point indicated with "e has no possible interpretation as an
: expression of the expected type base'class".

: I hope Gnat is right?

Nope.  If you check RM95 4.7(3) on qualified expressions, in T'(X),
X must resolve to be of type T (or a universal type that covers it).
In your example, "T" is base'class, and e is of type ext'class --
definitely not the same type.

The simple solution is to write:

   p := new ext'class'(e);

The implicit conversion to base'class happens automatically
as part of an allocator -- per RM95 4.8(3), the designated
type (base'class) of the access type ("base_p") only needs to "cover"
the type of the qualified expression.

Note that a qualified expression is about the only context where
you don't get implicit conversion to a class-wide type.  The reason
for this special case is that one of the points of a qualified
expression is to resolve overloading.  If the qualified expression
allowed implicit conversion to a class-wide type, it would be less
useful in resolving overloading.  

The fact that a qualified expression is also the way to create
an initialized allocator does muddy things up a bit ;-), but
that is why the name resolution rules for allocators allows
for the implicit conversion (as implied by the "covers" terminology).

: (in case you're wondering why I wrote it this way, the original code
: has derived types ext1, ext2 which are each the root of a tree of
: derived types, and which have sufficiently different properties that I
: wanted to make sure that the appropriate proc() was called).

: --
: package classp is
:   type base is tagged null record;
:   type base_p is access base'class;
:   type ext is new base with null record;
:   procedure proc (e : ext'class);
: end classp;
: package body classp is
:   p : base_p;
:   procedure proc (e : ext'class) is
:   begin
:     p := new base'class'(e);
: -------------------------^
:   end proc;
: end classp;

-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Cambridge, MA  USA