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From: px@fctunl.rccn.pt (Joaquim Baptista [pxQuim])
Newsgroups: comp.compilers
Subject: Re: Defining polymorphism vs. overloading
Keywords: polymorphism
Message-ID: <PX.90Sep20145320@hal.fctunl.rccn.pt>
Date: 20 Sep 90 18:53:19 GMT
References: <19400003@inmet>
Sender: compilers-sender@esegue.segue.boston.ma.us
Reply-To: px@fctunl.rccn.pt (Joaquim Baptista [pxQuim])
Organization: Universidade Nova de Lisboa -- Lisbon, Portugal
Lines: 42
Approved: compilers@esegue.segue.boston.ma.us
In-Reply-To: stt@inmet.inmet.com's message of 15 Sep 90 14:32:00 GMT

In article <19400003@inmet> stt@inmet.inmet.com writes:

   Here is my favorite so far:

   > - Overloading is the ability to have the same name (symbol) attached
   > to different entities (functions, procedures, variables or whatsoever).
   > 
   > - Polymorphism is the ability to have the compiler decide *at runtime*
   > which entity to use depending on the runtime type (class or whatever)
   > of the contents of variables.
   > 
   > - Generic code is a chunk of code that has the ability of performing
   > the same algorithm with more than one type. The actual types may be
   > known at runtime (as in Smalltalk) or at compile time (as in Ada).
   > . . .
   > Joaquim Manuel Soares Baptista, aka px@fctunl.rccn.pt, px@unl.uucp

   However, I would revise the definition of polymorphism as follows:

   - Polymorphism is the ability to have a single entity (usually a parameter or
   a pointer) refer to objects/values of more than one type.  Run-time
   polymorphism (like C++) allows the dispatch to type-specific code to be made
   at run-time.  Compile-time polymorphism (like Ada's generics) performs the
   "dispatch" to type-specific code at compile-time.

It's always nice to be appreciated, but still I must disagree with you.

I disagree with your difference of run-time vs. compile-time polymorphism. I
call the first polymorphism and the second one overloading.

Although they look similar, they have a different expressive power.  Run-time
dispatching is more powerful than compile-time "dispatching", and I think
that they deserve different names, ie, polymorphism and overloading.

I can elaborate on this "expressive power" business if that proves to
be necessary.
--
Joaquim Manuel Soares Baptista, aka px@fctunl.rccn.pt, px@unl.uucp
Snail: CRIA, UNINOVA, FCT/UNL, 2825 Mt Caparica, Portugal
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