Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!wuarchive!uunet!tdatirv!sarima
From: sarima@tdatirv.UUCP (Stanley Friesen)
Newsgroups: comp.lang.c++
Subject: Re: virtual operators
Message-ID: <16@tdatirv.UUCP>
Date: 3 Jun 91 19:48:48 GMT
References: <1991Jun1.175904.20181@ux1.cso.uiuc.edu>
Reply-To: sarima@tdatirv.UUCP (Stanley Friesen)
Organization: Teradata Corp., Irvine
Lines: 57

I think this is getting to FAQ level.

In article <1991Jun1.175904.20181@ux1.cso.uiuc.edu> bernied@yoyodyne.ncsa.uiuc.edu (Bernhard Damberger) writes:
>
>	I 'am having some problems with virtual operators...
 
>class Base {
>	...
>public:
>	virtual int operator< (Base &) const;	// less than operator for Base
>	virtual int operator> (Base &) const;	// greater than operator
>	...					// etc...
>};
>
>class TestInt : public virtual Base {
>	...
>public:
>	virtual int operator< (const TestInt &) const;	// overide Base
>	virtual int operator> (const TestInt &) const;	// overide Base
>	...		// overide the rest of Base's relational operators
>};

OK, here's your problem.   These do *not*, I repeat, *not*, override the
"corresponding" virtual functions in Base!   And why not?

Because they do not have *exactly* *matching* signatures.   By making the
parameters 'const TestInt &' instead of 'Base &' you are *replacing*, not
overriding, the functions in Base.

Now, since the functions in Base have never been overridden, then a call
through a Base pointer or reference will, naturally, use the pre-defined
base functions.

<<Irrelevant detail deleted>>

>	The problem occurs when the "less than" operator is applied.  I 
>want CFront to call the most specific operator< as possible.  In this 
>case that would mean if I called BinaryTree::insert(foo), where foo is 
>a TestInt pointer, I would want CFront to call TestInt::operator<(TestInt &) 
>But, instead it calls the more basic Base::operator< (instead of
>TestInt::operator<).

As indeed it should, given the above definitions.

>	So how do I get CFront to use the virtual operators?  When are 
>the types bound (it looks like at compile time instead of run time)?  
>How do I get CFront to call the most specific operator method?

This is quite simple, make the TestInt relational functions take parameters
of type (Base &).

Oh, you need 'const' parameters?  Then you need to add the const qualified
forms as virtual functions in the definition of class Base.
-- 
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uunet!tdatirv!sarima				(Stanley Friesen)