Re: Dispatching Example

[Tucker Taft <stt@avercom.net>]

Pat Rogers wrote:
> 
> Yet AARM 3.9.2{19a} sez:
> 
> "Ramification: This includes the cases of a tag-indeterminate procedure
> call, and a tag-indeterminate function_call that is used to initialize a
> class-wide formal parameter or class-wide object. "
> 
> Hence I would have thought (and indeed, have taught) that:
> 
>     C    : T1'Class := Is_Item;
> 
> is a call to the function returning a value of type T1.

This is still ambiguous, because either interpretation of
Is_Item would be allowed by the overload resolution rules.
However, presuming you only have one of them visible, then
you are correct that 3.9.2(19) applies, and it would be
a statically bound call rather than dynamically bound.

However, I believe the original question related to an
assignment statement rather than an initialization.
With an assignment statement, the LHS already has a value,
and hence a tag, and the implementation uses that tag
to control the dispatch.  In an initialization, the LHS
doesn't have a tag yet, so the tag is statically determined
by the "compile-time" return type of Is_Item.

-Tucker Taft  stt@avercom.net

> 
> "Tucker Taft" <stt@avercom.net> wrote in message
> news:3B9547D9.BE2CC282@avercom.net...
> > Ed Colbert wrote:
> > >
> > > Hi All,
> > >
> > > Below is a simplified example from Richard Riehle's & my course on Ada.
> > > GNAT v3.13p says that the call Is_Item is an mbiguous expression, and
> > > ObjectAda v 7.2 says the statement is illegal based on LRM 5.2(6).  Both
> > > Richard and I thought that the expression is tag indeterminate, LRM
> > > 3.9.2(6), and would static resolved, LRM 3.9.2(19).  Are Richard and I
> > > missing something or are both compilers wrong?
> >
> > The call on Is_Item is ambiguous, even though one of the
> > possible interpretations (the one returning type T1) violates
> > 5.2(6).  This is one of those cases where the overload
> > resolution rules are less picky than the legality rules,
> > which can be a bit confusing.
> >
> > ObjectAda is not being very helpful by telling you that one
> > of the interpretations is illegal, without first telling
> > you that the expression is considered ambiguous by the
> > overloading rules.
> >
> > In any case, if only the Is_Item that returned type T1 were
> > directly visible, the expression would not be ambiguous, and
> > the call would be legal.  It would involve a "dispatching
> > on result" where the tag of "C" would determine which body
> > of Is_Item were called.
> >
> > In general the overload resolution and legality rules relating
> > to classwide assignment are confusing at best.  We tried other
> > rules, but they had worse problems, so I suppose the current
> > rules are the best of a set of unpleasant alternatives.
> > ObjectAda's error message was unfortunately only adding to
> > the confusion.
> >
> > -Tucker Taft  stt@avercom.net
> > >
> > > package Dispatching_Examples
> > > is
> > >   type T1 is tagged private;
> > >   function Is_Item return T1;
> > >
> > >   type T2 is new T1 with private;
> > >   function Is_Item return T2;
> > >
> > > private
> > >    -- full definition of T1 and T2
> > >   type T1 is tagged null record;
> > >   type T2 is new T1 with null record;
> > > end Dispatching_Examples;
> > >
> > > with Dispatching_Examples;
> > > use Dispatching_Examples;
> > > procedure Dispatching_Examples_Driver is
> > >     A, B : T1;                    -- simple variables
> > >     Q, R : T2;                    -- simple variables
> > >     C    : T1'Class := A;     -- class-wide variable
> > >     D    : T2'Class := R ;    -- class-wide variable
> > > begin
> > >     C := Is_Item ;              -- static resolution?
> > > end Dispatching_Examples_Driver;
> >
> > --
> > -Tucker Taft   stt@avercom.net   http://www.avercom.net
> > Chief Technology Officer, AverCom Corporation (A Titan Company)
> > Bedford, MA  USA (AverCom was formerly the Commercial Division of
> AverStar:
> > http://www.averstar.com/~stt)

-- 
-Tucker Taft   stt@avercom.net   http://www.avercom.net
Chief Technology Officer, AverCom Corporation (A Titan Company) 
Bedford, MA  USA (AverCom was formerly the Commercial Division of AverStar:
http://www.averstar.com/~stt)