Re: Compound += operator overriding with derived classes

From:
Dragan Milenkovic <dragan@plusplus.rs>
Newsgroups:
comp.lang.c++.moderated
Date:
Tue, 13 Jul 2010 16:43:52 CST
Message-ID:
<i1iff4$ts7$1@speranza.aioe.org>
On 07/13/2010 12:27 AM, cpp4ever wrote:

On 07/12/2010 02:15 PM, Goran wrote:

[snip]

P.S. BTW, in raw C, these things work with primitive types (e.g. int,
float) through implicit conversions, not through operator
overloading ;-) Perhaps you should try that instead, but then again,
I'd be wary.


Without careful design, I'd agree, but in this case I would define the
operator += () in a standard form as follows:

class A
{
   public:
    virtual const A&operator += (const A&);
}

class B : public A
{
   public:
    virtual const B&operator += (const A&);
    const B&operator += (const B&)
}

B operator + (const A&inp1, const B&inp2)
{
   B tmp = inp2;
   tmp += inp1;
   return tmp;
}

So for any class operator +=() will always returns a const reference to
the class object that handled it. Whereas for operator +() the simplest
solution is to return by value the most appropriate class type.


Allow me to build up on your design...

class C : public A
{
  public:
     virtual const C & operator += (const A&);
     const C& operator += (const C&)
};

Now, given:
C c;
B b;

.... what would this do and how should it be implemented?
b += c;

Will it call B::operator+=(const A&) ? What will it do?
Does B know about C, too? And when we add a new type D,
will the knowledge of D need to be add everywhere? This
is an obvious abuse of inheritance and virtual functions.
The original advice regarding conversion vs inheritance
was sound.

--
Dragan

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