Re: using map, list etc. in const methods
From: Donovan Rebbechi (abuse_at_aol.com)
Date: 01/23/04
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Date: Fri, 23 Jan 2004 19:26:30 +0000 (UTC)
In article <burrlk$j5g$07$1@news.t-online.com>, Christof Krueger wrote:
> Hello,
>
> I'm quite new to C++ so maybe there's something I miss.
> I write a simple board game. It has a board class. This class has a
> method that returns the count of pieces a player has on the board. Since
> this function does not change anything in the class I declared it as
> const. To count all pieces of a given color the functions iterates
> through a "map" of CNode-pointers. "CNode" is another class that is
> irrelevant to the problem.
>
> Here comes the function: (EColor is an enum of colors, not important here)
>
> int
> CBoard::countPieces(EColor color) const
> {
> // "nodes" is a private class member
> // with type map<string,CNode*>
> int count = 0;
> for (map<string,CNode*>::iterator i=nodes.begin();
> i != nodes.end();
> i++)
> {
> // do something with (*i)
> // ...
> }
> return count;
> }
>
> The compiler refuses to compile the code, because I violate the
> constness of the function in the line where I declare the iterator.
> Is there a proper way (or workaround) to leave the function const?
map<string,CNode*>::const_iterator
btw, you can do this using
count_if(themap.begin(),themap.end(),isWhite); // isWhite returns true if the piece is white
> Another point: I've heard, that it is not good to store pointers to
> objects in stl-containers like map, list, vector etc. because of
> ownership problems (?).
It's not necessarily a bad thing. But you need to make sure that you understand
who "owns" a given pointer, and you need to make sure that the map doesn't hold
on to "dead pointers".
Here's my question: how are your pointers allocated and deleted ? Who owns them ?
> Do you know a good webpage or tutorial about
> this topic? I would like to learn how to do it the right way.
This topic is too complicated to be discussed exhaustively in "a webpage". You
need to read a couple of good books that deal with memory management issues.
> There are a lot of classes in the STL and I would like to know how to
> use their whole power. At the moment I'm just using lists, vectors and
> maps in lack of a good book or online tutorial discussing the STL in detail.
Two good books about the C++ standard library:
"Accelerated C++" Koenig and Moo
"The Standard C++ Library A Tutorial and a Reference" Josuttis
Cheers,
-- Donovan Rebbechi http://pegasus.rutgers.edu/~elflord/
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