Re: Pointers to member functions are NOT useless

From:

"Francesco S. Carta" <entuland@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Sun, 25 Jul 2010 02:29:15 +0200

Message-ID:

<4c4b855a$0$6831$5fc30a8@news.tiscali.it>

Ian Collins <ian-news@hotmail.com>, on 25/07/2010 12:18:04, wrote:

On 07/25/10 11:57 AM, Francesco S. Carta wrote:

Jonathan Lee <jonathan.lee.975@gmail.com>, on 24/07/2010 08:42:08, wrote:

The problem is there seem to be more suitable idioms in most cases.
Take your example. I'd rather use polymorphism and write something
like

void enter(Field f) { current_field = f; }

void move(Movement m) {
switch(current_field) {
case water: return moveWater(m);
case air: return moveAir(m);
case ground: return moveGround(m);
default: throw std::runtime_error("WTField");
}
}

and let the derived classes implement moveXXX(). And
it's not like it's just the example you came up with
that this could be done with. I think you could
refactor this whole category of uses.

....useful in particular to avoid the kind of code you just posted - as
I previously said, in the OP.

You just moved the decision about which function to call from the rarely
used function (the one that sets the context) to the often used function
(the one that should just "do it").

You wasted cycles, and potentially, you heavily slowed down the program:
imagine if there are hundreds or thousands of such objects in a
simulation, would you then try to save as much as possible? I know I
would.

There's probably a template solution as well if opening up the code to
optimisation is important. That's the thing about C++, there are
invariably many ways to skin a particular cat. Of all the possible
options, function pointers are the least friendly to the optimiser.

That's sure, putting the optimiser in condition to squeeze out the most
is an important thing for me, and I didn't consider the last point you
mention above - I simply wasn't aware of it.

I'm really interested in this subject, because I have implemented
heavily populated simulations in the past and I'm willing to deepen my
grasp on the subject, so any further information about how to replace
pointers to member functions in cases like this would be really welcome.

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