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From: schweitz@lexvmc.iinus1.ibm.com ("Eric Schweitz")
Newsgroups: comp.lang.c++
Subject: re: C++ typing not so strong
Message-ID: <9102182022.AA05904@ucbvax.Berkeley.EDU>
Date: 18 Feb 91 20:23:57 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 70

| David R. Cok
| Eastman Kodak Company
| cok@Kodak.COM
| 716-477-7086

writes:

| We had a base class Image from which some derived classes DoubleImage,
| FloatImage, IntImage etc. were derived.  We defined lots of relevant
| functions including (for example) operator- .
|
| ... we defined member functions of the form
|
|                 const DoubleImage& DoubleImage::operator-() const;
|                 const FloatImage&  FloatImage::operator-() const;
|
| But we also want some "virtualness" here.

Perhaps you could tell us why you `needed' these member functions to be
virtual.

| Example A:
|
| class Image {
|     public:
|         virtual const Image& operator-() const = 0;
| };
|
| class DoubleImage: public Image {
|     public:
|         const DoubleImage& operator-() const;
| };
|
| BUT THIS IS ILLEGAL because the two operator- declarations do not return the
| same type.

You're right here. However, if ``operator-'' had been declared as a
non-`pure virtual' member function, you would have simple overloading of
the unary ``-'' operator.

| Question 1: Does anyone have a better suggestion?

Yeah, don't make the operator- a pure virtual function if you don't have to.

| Question 2: What is the rationale for requiring that the implementations of
|         virtual functions in derived classes return exactly the same type as is
|         declared in the base class?

The rationale behind this is that it would make the following possible :

   //....
   Image i, *ip;
   DoubleImage *dp;

   ip = &i;
   dp = func ((DoubleImage*) ip);
   //....

where, func () is:

   //....
   DoubleImage* func (DoubleImage* p)
   {
      return p->operator-();
   }
   //....

hence, func() would return a Image* to dp which is expecting a DoubleImage*.

Schweitz.   schweitz@lexvmc.iinus1.ibm.com