Re: using map, list etc. in const methods

From: Christof Krueger (donotspamnews_at_pop2wap.net)
Date: 01/23/04 

Date: Fri, 23 Jan 2004 20:51:14 +0100

Donovan Rebbechi wrote:

> 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
thanks, that makes sense :)

> btw, you can do this using
>
> count_if(themap.begin(),themap.end(),isWhite); // isWhite returns true if the piece is white
very handy. I'll use it.

>
>>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 ?
In this case the board creates the nodes, stores them in the map and
deletes them in it's destructor. I see that ownership is a question of
design. Thanks!

>>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
I'll take a look on Josuttis book.

Another question:
If I had a public method getNodeMap() that should return my nodes-map to
the caller, is there a possibility to make this method const and to
return a const version of the map, so that the caller may not change the
returned map? I tried that once, but failed because of a lot of compiler
errors. 

-- 
Regards,
   Christof Krueger
Remove "donotspam" if you want to email me.

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