Re: Ada.strings.bounded problems?

[kst@alsys.com (Keith Thompson)]

In <Mats.Weber-1701951908250001@mlma11.matrix.ch> Mats.Weber@matrix.ch (Mats Weber) writes:
> In article <3fduto$ta7@watnews1.watson.ibm.com>, ncohen@watson.ibm.com wrote:
> > Ada 95 allows predefined equality to be overridden, even for nonlimited
> > types, so the declaration of a type Bounded_String with a discriminant
> > specifying the maximum length should be accompanied by the declaration
> > for the following function: 
> > 
> >    function "=" (Left, Right: Bounded_String) return Boolean is ...
> 
> This helps some, but still has severe problems. For instance if I write
> 
>    type Pair is
>       record
>          First,
>          Second : Bounded_String;
>       end record;
> 
> then "=" is broken for Pair. This situation is very bad because overriding
> "=" on Bounded_String gives the programmer the feeling that the problem is
> solved, but it reappears whenever a Bounded_String is used as a component
> of a composite type.

Interesting.  This is a potential problem that didn't occur (as seriously)
in Ada 83.  Since Ada 83 only allowed "=" to be overloaded for limited
types, and a type with a limited component is also limited, a type with
a subcomponent that has a user-defined "=" operator generally wouldn't
have its own "=" operator unless the user explicitly defined one (unless
you used the generic trick).

In Ada 95, it's very easy to have a case like the above where the
user-defined "=" operator for a subcomponent is bypassed.  Section II-6.3
of the Rationale, which discusses overloading of "=" and "/=", doesn't
mention this problem.  Quite possibly it was simply overlooked.

It's obviously too late to fix this in the language by requiring
predefined "=" to invoke user-defined "=" for subcomponents.

I suggest that Ada 95 compilers should issue a warning for a type
whose predefined "=" operator bypasses a user-defined "=" operator
for a subcomponent.  Determining exactly when to issue such a warning
may be difficult ("=" can be declared to return a type other than
Standard.Boolean, the left and right operands can be different types,
"=" and "/=" can be overloaded separately in some circumstances, the
declaration for "=" needn't be in the same scope as the operand type(s),
etc.), but catching the most common cases shouldn't be difficult.

-- 
Keith Thompson (The_Other_Keith)  kst@thomsoft.com (kst@alsys.com still works)
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