Re: [Dev-C++] Optimizing using the STL

[David M. <ci...@ya...>]

Whew, long e-mail.

Some interesting ideas there. You are right I am not too
familar with the
STL. I have a question though. What is the difference
between passing a
refrence and passing a const refrence? Is that to ensure
that the original
can never be changed(as would happen of I passed by value)?

My other comment is that current_players is a member of
pc_char. It is
static and thus shared by all instances of pc_char. So I am
guessing the
function signature stays the same.

From the looks of it using for_each may not be worth it
especially since the
function is not finished. What has to be added to this
function is a check
to make sure the write_to_player is not called for the
object invoking
write_to_all_pc (We don't want the message being sent to
the person who is
sending it). Would something like this work? (or rather is
it reccomended?)

void pc_char::write_to_all_pc(const string& to_send, bool
add_eoln)
{
  for(map<string, pc_char *>::iterator i =
current_players.begin();
    i != current_players.end(); ++i)
    if((*i).second != this)
       (*i).second->write_to_player(to_send, add_eoln);
}

With respect to the books. I will look into getting my
hands on a copy.

Thanks ALOT!
David Mcken


----- Original Message ----- 
From: "Hossein Haeri" <ult...@em...>
To: <ci...@ya...>
Sent: Sunday, July 27, 2003 4:07 PM
Subject: Re: [Dev-C++] Optimizing using the STL


> David,
>
> > Some interesting comments. (I know I need to increment
the
> > iterator, I
> > forgot that when I sent this e-mail). So far the only
> > optimization that
> > would be applicable is the passing by refrence. This
> > program is
> > multi-threaded so even though I could use mutexes, etc.
to
> > prevent the
> > string from changing between calls to write_to_player I
> > would rather not
> > have that headache. If you are wondering I plan to put
the
> > map in a
> > Read-write mutex (the only time that map changes is
when a
> > player logs in or
> > logs out).
>
> Consider the following if you want to adhere to your own
handcraft
> loop:
>
> void pc_char::write_to_all_pc(const string& to_send, bool
add_eoln)
> {
>   for(map<string, pc_char *>::iterator i =
current_players.begin();
>     i != current_players.end(); ++i)
>     (*i).second->write_to_player(to_send, add_eoln);
> }
>
>
> It is a truly mentioned tip that you must prefere using
references
> to constants instead of passing arguemnts by value. But,
I presume that
> you know that it "may" not have any effect to the
underlying code
> generated by the compiler, as in most of the modern C++
> implementations, std::string is implemented using
reference-counted
> smart pointers. But it is not wise, anyway, to rely on
this fact. Keep
> on using that cute refernce to const!
>
> Another point is that I have placed "i" to lie inside the
loop body to
> optimise the lifetime issues. It is not needed outside!
>
> And the last point is that I have chosen preincrementing
the loop index
> to the postincrementing it. I hope you know how much this
can save you!
>
> Furthermore, it seems to me that the "current_players"
map is not a
> data member of pc_char. If I'm right, it means that,
according to the
> Const-Correctness issues you must change the function
signature to the
> following:
>
> void pc_char::write_to_all_pc(/*The same parameters*/)
const;
>
> As this member function is not mutating the internal
status, so it must
> be qualified const.
>
> > > I was hoping to use for_each but kept coming across
the
> > > problem of "How the hell am I going to pass the
arguments
> > > to write_to_player?"
>
> It is not that hard! It is enough to write a unary
function object that
> serves the action you need for a given object of the
desired type
> (using a member function invokation).
>
> E.g., consider the following:
>
> class write_to_all_pc_fun
>   : public std::unary_function<pc_char, void>
> {
> public:
>   write_to_all_pc_fun(const string& to_send, bool
add_eoln)
>     : ts(to_send), aeoln(add_eoln) {}
>   void operator () (pc_char& p) {p.write_to_all_pc(ts,
aeoln);}
> private:
>   string ts;
>   bool aeoln;
> };
>
> Now you could use std::for_each like this:
>
> void pc_char::write_to_all_pc(const string& to_send, bool
add_eoln)
> {
>   std::for_each(
>     current_players.begin(),
>     current_players.end(),
>     write_to_all_pc_fun(to_send, add_eoln));
> }
>
> To be honest, this is not the neatest solution! In fact,
the best
> solution is to either combine the gadgets like
std::mem_fun1_ref and
> the binders, or use Boost stuff in Lambda Library
> (http://www.boost.org). But I haven't considered those
solutions, as
> even STL-experinced programmers may fail to digest the
code truly.
> Especially, because that way, you have to either write
your own
> handcrafted mem_fun2_ref (that's whithin the ability of
> STL-professionals that are heavily experts in Generic
Programming as
> well), or search whitin the Function Library of Boost to
find an
> appropriate tool. (It seems to me that you are not so
familiar with STL
> and Generic Programming. I hartappologise if that's not
the case! But
> if I'm right, consider the following Bibles of STL and
Generic
> Programming:
>
> Matthew H. Austern, Generic Programming and the STL:
Using and
> Extending the C++ Standard Template Library,
Addison-Wesley, 1999,
> ISBN: 0-201-30956-4.
>
> Scott Meyers, Effective STL: 50 Specific Ways to Improve
Your Use of
> the Standard Template Library, Addison-Wesley, 2001,
> ISBN: 0-201-74962-9.
>
> Don't think that the first one is old. It is still the
best manual for
> learning STL and especially Generic Programming. The
second one,
> indeed, assumes that you already have used STL for an
enough long time.
>
> I recommend those books to any C++ programmer, especially
my friends,
> including you! Furthermore, if you need better reviews of
the books,
> consider the following best C++ books reviewer:
http://www.accu.org)
>
> Regards,
> --Hossein
> -- 
>
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