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From: jamesh@cs.umr.edu (James Hartley)
Newsgroups: comp.lang.c++
Subject: How smart is operator resolution?
Message-ID: <1747@umriscc.isc.umr.edu>
Date: 27 Nov 90 20:15:43 GMT
Sender: news@umriscc.isc.umr.edu
Organization: University of Missouri - Rolla
Lines: 47
Originator: jamesh@mcs213f.cs.umr.edu

I recently tried taking advantage of virtual functions by creating a generic
routine which would take a base class pointer and output the appropriate 
derived class where the insertor was properly overloaded.  The following 
program attempts to illustrate the idea.  Turbo C++ balks at the output
statement in main() because the insertor is not defined with respect to
the base class.  Is this operator resolution beyond the restraints of the
language, or is this a bug in the compiler?

-----------------------  BEGIN CODE -------------------------------------

#include <iostream.h>

const float pi = 3.14159;

class figure {			// abstract base class definition
protected:
    float r, h;
public:
    virtual float volume(void) = 0;
};

class sphere : public figure {	// derived class definition
public:
    sphere(float radius)  { r = radius; }
    virtual float volume(void)  { return 4.0 / 3.0 * pi * r * r * r; }
    friend ostream &operator<<(ostream&, sphere&);
};

ostream &operator<<(ostream &strm, sphere &sph) {
    strm << "VOLUME OF SPHERE:\n";
    strm << "radius = " << sph.r << "\n";
    strm << "volume = " << sph.volume() << "\n";
    return strm;
}

main() {
    figure *fig = new sphere(2.0);
    cout << *fig << "\n";
    return 0;
}

-------------------------  END CODE -------------------------------------

-- 
James J. Hartley                      Internet: jamesh@cs.umr.edu
Department of Computer Science          Bitnet: jamesh@cs.umr.edu@umrvmb.bitnet
University of Missouri - Rolla            UUCP: ...!uunet!cs.umr.edu!jamesh