In article <m3ivavlhdh.fsf@gabi-soft.fr>, kanze@gabi-soft.fr (J. Kanze) wrote:
>tottinge@csci.csc.com (Tim Ottinger) writes:
>
>>         struct CustomerKey{
>>                 // ...
>>                 typedef Customer MyObject;
>>                 // ...
>>         };
>>
>>         template<class IDClass>
>>         IDClass::myObject&* getObject(IDClass x)
>                           ^^
>
>Is this a typo?  If not, what is it supposed to mean.  The function
>should return either a pointer, or a reference, or a reference to a
>pointer.  On the other hand, there's no such thing as a pointer to a
>reference.

Yes, this is a typo. It was not supposed to have an & in it.
I must have landed a finger squarely between the 7 and 8 to
do that.

Of course, it should be one or the other, or a return by value.
Either case is okay with me.  For now, consider it * as I'd
intended.

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tottinge@csci.csc.com (Tim Ottinger) writes:

>         struct CustomerKey{
>                 // ...
>                 typedef Customer MyObject;
>                 // ...
>         };
>
>         template<class IDClass>
>         IDClass::myObject&* getObject(IDClass x)
                           ^^

Is this a typo?  If not, what is it supposed to mean.  The function
should return either a pointer, or a reference, or a reference to a
pointer.  On the other hand, there's no such thing as a pointer to a
reference.

>         {
>                 // ...
>         }
>
> This way, to get an object from a key is a matter of:
>
>         CustomerKey x;
>         // ...
>         MyCustomer* cptr  = getObject(x);

Other than the above typo, I believe that this should work.  There's
plenty of similar stuff in the standard library, and I've used similar
stuff a lot myself.

Such things do tend to stress the template instantiation mechanism, so
be prepared to run up against compiler bugs.  (My examples won't
compile/link with g++, for example, nor with anything but the very
latest version of Sun CC.)

--
James Kanze           (+33) 88 14 49 00          email: kanze@gabi-soft.fr
GABI Software, Sarl., 8 rue des Francs Bourgeois, 67000 Strasbourg, France
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This is a 'relational database extended into c++' thing, but it grew into
a question I'd like to have a good answer to.  I'll give background a bit
first, then the problem.  Then I'll sit back and await your judgement:

What I'd really love to be able to do is have more-or-less opaque objects
which serve as identifiers for other objects and nothing else. These are
essentially "keys" and "foreign keys".

        struct CustomerKey{
                // ...
                long customerNumber;
        };

This is easy enough, of course.  These can be embedded in lots-o-classes:

        class CableDataSubscription {
                // ...
                CustomerKey myCust;
                EquipmentKey myEquip;
                // ...
        };


Then I thought it would be "cool" to have a very simple function which pulled
data out of a type-specific cache (i.e. all Customers are cached together, all
Equipment cached together, etc).  This way, my caching is simple, and I still
don't have to have all types descending from a common base (which would
not be good in this world).

I conceived of a cute ploy using typedefs and template functions, but I can't
find a compiler which likes the idea.  I actually crash the compilers with
this, so I will do it another way, but I thought maybe you would like tell me
why this can't be done:

        struct CustomerKey{
                // ...
                typedef Customer MyObject;
                // ...
        };

        template<class IDClass>
        IDClass::myObject&* getObject(IDClass x)
        {
                // ...
        }

This way, to get an object from a key is a matter of:

        CustomerKey x;
        // ...
        MyCustomer* cptr  = getObject(x);



It's not that I can't work around it, but it seemed like such a good idea, I'm
not sure why it's not possible.  Maybe you can tell me.  Should it be?


+----------------------------------------------------------------------------+
| Tim Ottinger, Sr Tech           tottinge@csci.csc.com |
| CSC Communications Industry Services             217-351-8508x2420 |
| TRIS Division -- Cellular Billing and Support             Fax 217-351-2640 |
+----------------------------------------------------------------------------+
| Requirements have always been the least stable part of any project I have  |
| worked on.                         -- Robert Martin via Usenet 12/02/95 |
+----------------------------------------------------------------------------+
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