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From: chip@tct.uucp (Chip Salzenberg)
Newsgroups: comp.lang.c++
Subject: Re: asking an object for its type
Message-ID: <27C95350.1558@tct.uucp>
Date: 25 Feb 91 18:11:28 GMT
Article-I.D.: tct.27C95350.1558
References: <1991Feb22.195826.14008@gpu.utcs.utoronto.ca> <27C6DBF3.28A3@tct.uucp> <27114@dime.cs.umass.edu>
Organization: Teltronics/TCT, Sarasota, FL
Lines: 42

According to connolly@livy.cs.umass.edu (Christopher Connolly):
>In article <27C6DBF3.28A3@tct.uucp> chip@tct.uucp (Chip Salzenberg) writes:
>>If the "same" operation is actually several operations that are
>>distinct in semantic terms (as opposed to implemention), then folding
>>them together into one virtual function is a design error.
>					      ^^^^^^^^^^^^ 
>
>Consider the operation "+": I can use simple integer addition if both
>my coefficients are integers.  The "+" operation results in an integer
>in this case.  On the other hand, adding an integer to an algebraic
>number usually results in another algebraic number.  Integer addition
>and algebraic number addition are quite different algorithms,
>producing quite different results.

But both of those algorithms implement "addition" in the sense that a
mathemetician may use the word.  So I probably would not consider your
example to be an implementation of "several different operations [that
are] distinct in semantic terms."

>I see no way around defining a generic "+" operation which takes
>different data types, and returns different data types as a result
>(all derived from the "coefficient" class, of course).

Sure.  Make coefficient an abstract base class.  Derive int_coeff
(integer) and alg_coeff (algebraic).  Have all "+" operations return a
coefficient, thus allowing you to use the virtual function mechanism.
Make a virtual member function "operator +=", and implement a
non-member "operator +" using "+=".

Then, the key: a virtual member function "int integer_value(int &n)",
which sets "n" to the integer value of the given coefficient and
returns true for success, or false for failure.  Only int_coeff will
define this function to return true; all others will return false.

Then use it:

      coefficient c = coefficient(4) + coefficient(7);
      int n;
      if (c.integer_value(n))
          cout << "c is " << n << "\n";
      else
          cout << "c isn't an integer!\n";