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From: cimshop!davidm@uunet.UU.NET (David S. Masterson)
Newsgroups: comp.lang.c++
Subject: Re: Seeking neat way to do binary "virtual" functions.
Message-ID: <CIMSHOP!DAVIDM.91Mar24133334@uunet.UU.NET>
Date: 24 Mar 91 21:33:34 GMT
References: <NGO.91Mar19114016@tammy.harvard.edu> <13683@darkstar.ucsc.edu>
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In-reply-to: ericg@ucschu.ucsc.edu's message of 21 Mar 91 21:14:22 GMT

>>>>> On 21 Mar 91 21:14:22 GMT, ericg@ucschu.ucsc.edu (Eric Goodman) said:

Eric> What I want:

Eric> class B {
Eric>      virtual B& operator+(B&);
Eric> };
Eric> class D: public B{
Eric>      B& operator+(D&);
Eric> };

Eric> the D::operator+() function is not overloaded.  I'd like a mechanism
Eric> that would check the actual type of the second arguent, and call the
Eric> correct function at run time:

Eric> B& f(B& one, B& two) {
Eric>      return (one+two);       // if (either or) both are B's, use
Eric>                                     // B::operator+(B&)
Eric>                                     //  if both are D's, use 
Eric>                                     //  D::operator+(D&)
Eric> };

Something that might be of interest is the article on "Generalized Arithmetic
in C++" by Tim Budd in the February issue of the Journal of Object-Oriented
Programming.  It describes a couple of techniques (coercive generality and
double polymorphism) that might show good ways of doing this and the pitfalls.
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