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From: bgbg@cbnewsd.att.com (brian.g.beuning)
Newsgroups: comp.lang.c++
Subject: Re: Seeking neat way to do binary "virtual" functions.
Message-ID: <1991Mar24.045917.29264@cbnewsd.att.com>
Date: 24 Mar 91 04:59:17 GMT
References: <13683@darkstar.ucsc.edu>
Distribution: comp
Organization: AT&T Bell Laboratories
Lines: 45

From article <13683@darkstar.ucsc.edu>, by ericg@ucschu.ucsc.edu (Eric Goodman):
> ...  With binary operators of 
> unknown actual type only the first object gets run time determined.
> To test type compatibility for two arbitrary base class references
> there is no way to determine whether or not they are "the same" other
> than that they are derived from the same class.

There was an article in a recent Journal of OOP (JOOP) about
supporting arithmetic in C++ that got into this topic.
One suggestion was to use:

class B {
public:
	virtual B& operator+( B& );
	virtual B& add_to_D( class D& );
	virtual B& add_to_E( class E& );
};
class D: public B {
public:
	B& operator+( B& );
	B& add_to_D( D& );	// D + D
	B& add_to_E( E& );	// E + D
};
class E: public B {
public:
	B& operator+( B& );
	B& add_to_D( D& );	// D + E
	B& add_to_E( E& );	// E + E
};

B&
D::operator+( B& arg )
{
	return( arg.add_to_D( *this ) );	// second run-time lookup
}
B&
E::operator+( B& arg )
{
	return( arg.add_to_E( *this ) );
}

If you have N derived classes, you end up with about N^2 methods.
But you can do it.  It is also extendable to more than 2 arguments.

		Brian Beuning