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From: steve@lia (Stephen Williams)
Newsgroups: comp.lang.c++
Subject: Re: Bug in multiple inheritence
Message-ID: <1991Jan17.190452.5204@lia>
Date: 17 Jan 91 19:04:52 GMT
Reply-To: steve@lia.com (Stephen Williams)
Organization: Litton/Integrated Automation, Alameda, California
Lines: 51


Let me remind you of my problem. ...

>The following program demonstrates an apparent bug in C++ 2.0 and 2.1.
>Am I seeing things?
>
>
>class X { int x, y; };
>class Y { int a, b; };
>class Mess  : public X, public Y { int m; };
>
>
>main()
>{
>	Mess m;
>	Mess &n = m;
>
>	X x = n;
>	Y y = n;	// <---- C++ Error!!!!  In this case y should get the
>		// Y part of n, but is instead getting the X part.
>		// An offset is missing.
>	X &xp = n;
>	Y &yp = n;	// <---- Works.  In this case the correct code
>			// is generated.
>}


ken@tucana.csis.dit.csiro.au respondes: 

>Isn't it unsafe anyway to assign a derived class instance to a base class
>instance?


I decided to check this out.  I searched the Ellis-Stroustrup C++ book
and found no mention either way about casting a derived INSTANCE to
a base class instance, but by experiment it seems that the AT&T compiler
does the assignment for single inheritence correctly, even to the point
of getting virtual functions right.  However, using the example above
and assuming that class X had a virtual function and Mess redefined
it, x.vfunc() would call X::vfunc(), whereas xp.vfunc() would call Mess::vfunc().
The problem in my test was that the compiler was inlining the virtual
functions for x.vfunc(), but not for xp.vfunc().

The moral:  although assignment of derived class instance to base
class instance is allowed by the compiler, it is probably illegal in the
language, and again I complain about the compiler.

Comments?

--Steve
steve@lia.com