Re: confusing delete problem
From: Thomas Matthews (Thomas_MatthewsSpitsOnSpamBots_at_sbcglobal.net)
Date: 02/24/04
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Date: Tue, 24 Feb 2004 18:26:39 GMT
Joerg Toellner wrote:
> Hi John and Clark,
>
> thx for your replies.
>
> I haven't my code right by hand now. So i only wrote the situation out of
> the head. Of course x is defined in the real code destructor.
>
> I'm glad that my suggestion that i have to delete it, is right.
>
> To John:
> Can't understand your comments about copy constructor. There will be
> definitely ONE instance/object of type class map. map is a map (or say a
> playfield) for a game that will hold the parts of the playfield (the single
> tiles of the map). So there is only one map consisting of maaaaany tiles.
>
> Do i need the copy constructor in this case too? And why do i need it when i
> never want to copy the map-object? Even if it is a good fashion or
> recommended by default.
>
> Of course i'm thankful for your hint. I will remember this for the objects
> where i need more than one instance.
>
> I'll check my code further to find who is deleting my tiles before i get
> hand on it. Your comments are important for me that it is worth searching
> for the bug further and not hunting C++ ghosts which every experienced
> C++-Guru already knows.
>
> Thx. again to you both and to John for maybe another little comment about
> the copy constructor.
>
> CU
> Joerg Toellner
>
> PS:
> I already own Stroustroup C++ Language and Effective C++ and More effective
> C++ as books. But you will understand, nobody can remember all the details
> and s.t. it is like chasing an elephant with an microscope. You overseas the
> obvious.
>
> "John Harrison" <john_andronicus@hotmail.com> schrieb im Newsbeitrag
> news:c1fqa1$1he3vq$1@ID-196037.news.uni-berlin.de...
>
>>"Joerg Toellner" <toellner@oss-gmbh.de> wrote in message
>>news:c1fpmi$pce$02$1@news.t-online.com...
>>
>>>Hi Group,
>>>
>>>i stumbled over a delete problem which confuses me. I'm not very
>>
>>experienced
>>
>>>with C++ programming and so i am not able to understand what's going on
>>>here.
>>>
>>>I have the following situation:
>>>
>>>-------------SNIP---------------------
>>>// File: map.cpp
>>>
>>>class tile;
>>>
>>>class map
>>>{
>>> map();
>>> ~map();
>>> // ... some other unrelated stuff here
>>> tile *m_MyTiles[144];
>>>}
>>>
>>>void map::map()
>>>{
>>> int x;
>>> for(x = 0; x < 144; x++)
>>> m_MyTiles[x] = new tile;
>>>}
>>>
>>>void map::~map()
>>>{
>>> for(x = 0; x < 144; x++)
>>> delete m_MyTiles[x];
>>>}
>>>-------------SNIP---------------------
>>>
>>>When i construct a new instance of class map and later delete it again
>
> my
>
>>>program crash at the delete command in the destructor. It seems that the
>>
>>(in
>>
>>>the constructor of class map) created 144 instances of class Tile will
>
> be
>
>>>deleted somehow automagically.
>>
>>That's not true, you new them so you delete them.
>>
>>
>>>When i comment out the code in the destructor
>>>of class map, all works fine. But i assume that then i will leave some
>>>memory leaks behind, right?
>>>
>>
>>Right.
>>
>>
>>>Can s.o. please direct me in the right direction what i'm doing wrong?
>>
>>Must
>>
>>>i delete my with new created instances of the tiles in the destructor of
>>
>>map
>>
>>>(where the pointers are member variables)? Or who will do this for me
>>
>>behind
>>
>>>the scene? Or when this will leave memory leaks, how do i delete my
>>>tiles-objects correctly without a program crash?
>>>
>>
>>What you've forgotten to do is write a copy constructor and assignment
>>operator for your class.
>>
>>If you have two copies of the same object, then at present you will have
>
> two
>
>>copies of the same pointers. When the first object gets destructed
>>everything is OK, but when the second gets destructed you are deleteing
>>pointers that have been deleted already, so your program crashes.
>>
>>
>>>Sorry for the dumb question, but i can't figure it out myself.
>>>
>>
>>This is *the* classic newbie C++ gotcha. You need a good C++ book, any C++
>>book that doesn't explain copy constructors is not worth reading.
>>
>>
>>
>>>TIA very much
>>>Joerg Toellner
>>>
>>
>>john
If you only want one instance of the board / map,
I suggest you search the web and newsgroups for
the "Singleton Design Pattern". The Singleton pattern
is when you only want one instance of an object.
--
Thomas Matthews
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- Next message: Scott Danzig: "About memory management with references"
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