Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!usc!apple!netcom!teda!ditka!mcdchg!tellab5!balr!clrcom!rmartin
From: rmartin@clear.com (Bob Martin)
Newsgroups: comp.lang.c++
Subject: Re: How smart is operator resolution?
Message-ID: <1990Nov30.042621.18213@clear.com>
Date: 30 Nov 90 04:26:21 GMT
References: <1747@umriscc.isc.umr.edu>
Organization: Clear Communications, Inc.
Lines: 64

In article <1747@umriscc.isc.umr.edu> jamesh@cs.umr.edu (James Hartley) writes:
>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;
>}
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This function defines the << operator for a sphere but not for a figure.
>
>main() {
>    figure *fig = new sphere(2.0);
>    cout << *fig << "\n";
^^^^^^^^^^^^^^^^^^^^^^^^^^
The compiler cannot resolve this statement because there is not << operator
defined for a figure.
>    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

What you need to do is declare a virtual function (say printOn) for sphere.
Then in operator<<(ostream&, figure&) call the virtual function.



-- 
+-Robert C. Martin-----+:RRR:::CCC:M:::::M:| Nobody is responsible for |
| rmartin@clear.com    |:R::R:C::::M:M:M:M:| my words but me.  I want  |
| uunet!clrcom!rmartin |:RRR::C::::M::M::M:| all the credit, and all   |
+----------------------+:R::R::CCC:M:::::M:| the blame.  So there.     |


Brought to you by Super Global Mega Corp .com