minor annoyance: derived class' operator=

From: Skluzacek, Christian (C.Skluzacek++at++fokkerspace.nl)
Date: 02/15/2002 05:14:31

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Hi all,

This is probably more of a C++ question but it came up when I was deriving
my classes from pfMemory. Two small things really:

1. pfMemory declares operator= a virtual function which gives me warnings
during compilation because the derived class' signature does not match
pfMemory's signature.

2. From what I've seen, operator= usually uses references instead of
pointers. pfMemory uses pointers.

Ok, these aren't really errors but it is kind of annoying to see all of
these warning messages come up when I am compiling (under Irix) with
-fullwarn.

Following is a test program which reproduces these warnings. Compile (under
Irix) with:

CC -LANG:std -o opequals opequals.C

add -DUSEPTR to use the pointer version
add -DVIRTUAL=virtual to use the virtual version.

opequals.C:
-----------------
#include <iostream>

#ifndef VIRTUAL
#define VIRTUAL
#endif

////////////////////////////////////////////////////////////////////////////
//
struct Base
{
  Base( int val = 0 ) : baseData(val) {}
#ifdef USEPTR
  VIRTUAL Base * operator=( const Base * rhs )
    {
      if ( this != rhs ) {
        baseData = rhs->baseData;
      }
      return this;
    } // operator=
#else
  VIRTUAL Base & operator=( const Base & rhs )
    {
      if ( this != &rhs ) {
        baseData = rhs.baseData;
      }
      return *this;
    } // operator=

#endif

  bool operator==( const Base & rhs ) const
    {
      return baseData == rhs.baseData;
    } // operator==

  friend std::ostream& operator<<( std::ostream & os, const Base & b )
    {
      os << "Base: " << b.baseData; return os;
    } // operator<<

  int baseData;
}; // class Base
////////////////////////////////////////////////////////////////////////////
//
struct Derived : public Base
{
  Derived( int dval = 0, int bval = 0) : Base(bval), derivedData(dval) {}

#ifdef USEPTR
  VIRTUAL Derived * operator=( const Derived * rhs )
    {
      if ( this != rhs ) {
        Base::operator=(rhs);
        derivedData = rhs->derivedData;
      }
      return this;
    } // operator=
#else
  VIRTUAL Derived & operator=( const Derived & rhs )
    {
      if ( this != &rhs ) {
        Base::operator=(rhs);
        derivedData = rhs.derivedData;
      }
      return *this;
    } // operator=
#endif

  bool operator==( const Derived & rhs ) const
    {
      return ( Base::operator==(rhs) &&
               derivedData == rhs.derivedData );
    }

  friend std::ostream& operator<<( std::ostream & os, const Derived & d )
    {
      os << static_cast<const Base &>(d) << std::endl
         << "Derived: " << d.derivedData; return os;
    } // operato<<

  int derivedData;
}; // class Derived

////////////////////////////////////////////////////////////////////////////
//
int main( int, char ** )
{
  Base b1(1), b2(2);
  Derived d1(11,21), d2(12,22);

  std::cout<<b1<<std::endl;
  std::cout<<d1<<std::endl;
  b1 = d1;
// d2 = b1; <--- illegal
  std::cout<<b1<<std::endl;

} // main

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