Question

pointers and dynamic classes

Asked by: gem56

hey guys,

im building a small console app and i seem to be getting some funky behavior with my pointers. im still relatively new at c++ and im a little weak on pointers.

my app is a small state manager consisting of a main 'game' class, a menu 'type' abstract class, and half a dozen other menu classes (inheriting from menu). my game class has a pointer which creates new menu classes as the user changes their menu.

class Game
{
   .....
   class Menu *currentMenu;
}

class Menu
{
   virtual .... = 0
}

class MainMenu : public Menu
{ ..... }

class AboutMenu : public Menu
{ ..... }


now this is the part im not sure if im doing correct. when i 'currentMenu = new AboutMenu()' im supposed to delete the old object previously pointed to by my pointer, i do this by 'delete this' (from my MainMenu class) when i assign the pointer a new object. i take it this is correct.

the other thing is im getting confused between deleting the object that my pointer is pointing too....and deleting the pointer itself (if that makes sense). as far as i can remember, if i 'delete currentMenu' it'll delete the actual object being pointed too, however, it will still point to the objects memory location unless i 'currentMenu = null'. is this the correct way of going about it?

lastly when i 'delete this' from MainMenu or AboutMenu, the rest of the code in the function is still executed. i'd assume if the object no longer existed, there would be nothing to execute?

apologies for the beginner quiestions...cant seem to find anything directly related on google :S

thanks for any help :)

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Asked On
2009-08-29 at 05:13:39ID24691838
Topics

C++ Programming Language

,

Microsoft Visual C++

Participating Experts
4
Points
500
Comments
8

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Answers

 

by: itsmeandnobodyelsePosted on 2009-08-29 at 05:42:01ID: 25213847

In C++ you rarely need pointers. You better try with out pointers first:

// forward declarations
class Menu;
class AboutMenu;
class MainMenu;

class Game
{
    ...
    public:
        void showAboutMenu();
};

enum MenuItemMethod { MIM_HELP, MIM_NEW, MIM_STORE, ...  };

class MenuItem
{
    std::string text;
    int shortcut_pos;
    bool check;
    int   method;
public:
    MenuItem(const char* pszText, int shortcut, bool chk, MenuItemMethod mim)
     : text(pszText), shortcut_pos(shortcut), check(chk), method(mim) {}
};

class Menu
{
    std::vector<MenuItem> items;
public:
   void show();
   void addItem(const MenuItem& mi);

   
};

class MainMenu : public Menu
{
};

class AboutMenu : public Menu
{
};

void Game::showAboutMenu()
{
     AboutMenu menu;
     menu.addItem("Help", 0, false, MIM_HELP);
     menu.addItem(...);
     ...
     menu.show();
     
};




 

by: pgnatyukPosted on 2009-08-29 at 07:45:42ID: 25214221

If you have a real working application and you see that everything you are asking works fine, so where is the problem to solve? :)

I didn't understand this paragraph:
lastly when i 'delete this' from MainMenu or AboutMenu, the rest of the code in the function is still executed. i'd assume if the object no longer existed, there would be nothing to execute?

If I do understand, put "delete this" as the last line in the function. The function "exists" seperately from the class variables and you don't delete the function when you delete the class object.

Previous two paragraphs will work as you described - delete will delete the correct object if your didn't forget to make the destructor virtual.

Scott Meyers likes these theretical questions: http://www.aristeia.com/publications.html

@itsmeandnobodyelse: C++ without pointer? :) Please do not discuss it here. It just sounds funny for me.

 

by: gem56Posted on 2009-08-29 at 10:44:14ID: 25214715

hey guys thanks for the replies.

pgnatyuk, thanks for clarifying that up. my abstract and derived classes dont have any dynamically created objects/variables, therefore no (meaningful) destructor is needed, however yes i understand it should be virtual so each classes destructor is called in turn all the way up the 'family tree' :)

if you could please just clarify one last query... when deleting the pointers object (either via 'delete currentMenu' or 'delete this'), what is generally considered good programming practice to do with the pointer itself? set it to point to null until we create another object for it to point to? or something else?

cheers

 

by: Infinity08Posted on 2009-08-29 at 14:05:56ID: 25215359

>>  i do this by 'delete this' (from my MainMenu class) when i assign the pointer a new object. i take it this is correct.

I recommend against using 'delete this'. Not only doesn't it make sense (why would an object destroy itself - especially since it doesn't know whether it has been allocated dynamically), it's also potentially dangerous.

For as much as possible, keep the code that allocates memory in the same module as the code that de-allocates the same memory. ie. write the corresponding 'new' and 'delete' statements at the same location (level). In this case, both will probably be in the Game class.


>> if i 'delete currentMenu' it'll delete the actual object being pointed too, however, it will still point to the objects memory location unless i 'currentMenu = null'. is this the correct way of going about it?

After deleting, the pointer will indeed still point to the same location in memory, but dereferencing the pointer now has undefined behavior (potentially even crashing your code).
To avoid that, programmers will often set the pointer to NULL right after deleting it (to show that the pointer is not pointing anywhere meaningful and should not be dereferenced). Although it is recommended, it is not necessary.

 

by: Infinity08Posted on 2009-08-29 at 14:37:42ID: 25215452

Regarding 'delete this', I just remembered that the C++ FAQ Lite also talks about it :

        http://www.parashift.com/c++-faq-lite/freestore-mgmt.html#faq-16.15

 

by: itsmeandnobodyelsePosted on 2009-08-29 at 15:04:14ID: 25215551

>>>> C++ without pointer? :) Please do not discuss it here
I won't discuss but you may look at my code which doesn't use one single pointer.

>>>> delete this
I know only one case where the 'delete this' was senseful. That was to delete a modeless dialog when the ultimative last message to the corresponding window was handled. The reason to use 'delete this' in that case is cause there was no more trigger for deleting the pointer after the last message was sent. And before it couldn't be deleted cause the class object still had to handle messages.

You will surely see that your case is much less complex. So, I strongly support the advise Infinity gave you to not using 'delete this'.  The delete operation always should be made on same level as the new operation. In your case you created the menu object in some Game member function, so you should delete it in some Game member function.

If you would follow my advise and not use pointers you wouldn't need to care for deletion at all.

 

by: efnPosted on 2009-08-29 at 15:17:05ID: 25215608

> when deleting the pointers object (either via 'delete currentMenu' or 'delete this'), what is generally considered good programming practice to do with the pointer itself?

I don't think there is consensus among experts about this, except perhaps that you should avoid having pointers that may or may not point to something.

I have known people who reflexively always set a pointer to null after deleting what the pointer addresses, and they recommend this practice, because it's easier than actually thinking about whether nullifying the pointer is necessary.  Similarly, they always check whether a pointer is null before dereferencing it, whether it could possibly be null or not.  I guess they have been burned by null pointers so many times that they just don't trust any pointer not to be null.

I personally aspire to tighter coding than this.  My guidelines are:

First, avoid using pointers if you can, as my esteemed colleague itsmeandnobodyelse has already recommended here.  Pointers are not evil, they are just dangerous.  Using a smart pointer class, as has been extensively recommended on this site, is preferable.

Second, if you use pointers, avoid keeping pointers that may or may not point to anything.

Third, if you use a pointer that may or may not point to anything, be sure you always set it to null when it doesn't point to anything, and be sure you always check whether it is null before dereferencing it.

--efn

 

by: gem56Posted on 2009-08-29 at 23:23:49ID: 25216526

hey guys,

many thanks for all the replies, definitely learnt a few things :)

just for reassurance, the only place i have 'delete this' statements are in the same spots i have '= new Menu()', so we're not completely failing at programming :P

anyways i'll make the changes to my program now, thanks for all the help.. points for everyone :-D

20120131-EE-VQP-002

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