Re: Hashtables--More Info on Original Question - C / C++ / MFC Discussion Boards

Re: Hashtables

Prakash Nadar31-Dec-05 19:39

Prakash Nadar

31-Dec-05 19:39

Michael Dunn wrote:
Are you using normal or managed C++?

I would like to see the abnormal ones Laugh | :laugh:

-Prakash

Re: Hashtables

Michael Dunn1-Jan-06 8:00

Michael Dunn

1-Jan-06 8:00

Some people considered the "Managed Extensions for C++" to be abnormal. Wink | ;)

--Mike--
Visual C++ MVP Cool | :cool:

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Re: Hashtables

John R. Shaw31-Dec-05 20:43

John R. Shaw

31-Dec-05 20:43

It is true that for every NEW you need to call DELETE. The only reason that you should receive an error, is that it was already deleted. That implies an error in your coding, becuase if the the pointer to the hash-table is a member of your class, then the memory-block should exist until the class object is destoryed.

If there is the slitest chance that the memory pointed-to by the pointer might be be dealocated by another function, then you should set the pointer to NULL after you deallocate it (delete does nothing if the pointer is NULL). The only problem with this, is that it hides the fact that you are trying to delete the same memory-block two or more times.

My recommendation is that you put a trace (or out-put it to a file) statement after every NEW and DELETE associated with the the hash-table. That way you will know when it is allocated and when it is destoryed. If the NEWs and DELETEs do not match up, then you will know that some thing else is going on.

INTP
Every thing is relative...

Re: Hashtables

Prakash Nadar31-Dec-05 20:56

Prakash Nadar

31-Dec-05 20:56

I hope u didnt say delete Hashtable
it should be delete hashTable

its kinda not rite to name the variable name same as the datatype with difference in the case, Sometimes it creates hard to understand compiler errors, like this one.

-Prakash

Re: Hashtables--More Info on Original Question

jblau31-Dec-05 21:23

jblau

31-Dec-05 21:23

wow, I am surprised at how many people responded, thanks everyone!

Here is some more information which might allow you guys to set me straight.

I am using Visual Studio.net 2003
The type of project that I opened is a "Windows Form Application (.Net)"
(so I don't know if that means that I am using "Normal" or "Managed" C++ ?)

My code is something like this:

void LoadLists()
{
Hashtable *hashSelectedItems = new Hashtable;

......(code working with the hashtable)

delete hashSelectedItems;
hashSelectedItems = NULL;
}

When I try to build the project, I get this error message: error C3841: illegal delete expression: managed type 'System::Collections::Hashtable' does not have a destructor defined

I'm thinking that perhaps it is, as one of you mentioned, being automatically managed, but I'm not certain.
It was my impression that if I wanted to use a Managed object I would specifiy so in the declaration, for example, in declaring an array, if I wanted it to be a managed array I would declare it like so: int c __gc[] = new int __gc[12]; but if I didn't want it Managed, I would leave out the __gc part.

************So given all of this, what do I need to know about declaring Hashtables without leaving any memory leaks?
Thanks,
Jody Blau

Re: Hashtables--More Info on Original Question

John R. Shaw31-Dec-05 22:22

John R. Shaw

31-Dec-05 22:22

I do not work with managed any thing, but I can tell you where you made a mistake. Do not allocate an object and delete an object in the same function, unless you have to. Instead you should just declare it as a stack object and let it clean up after its self.

void LoadLists()
{
    Hashtable hashSelectedItems;
    ......(code working with the hashtable)
}

The above code does not require a delete, becuase no new was called. The 'hashSelectedItems' variable is on the stack and will clean up after its self when the function returns.

INTP
Every thing is relative...

Re: Hashtables--More Info on Original Question

Gary R. Wheeler1-Jan-06 1:51

Gary R. Wheeler

1-Jan-06 1:51

John R. Shaw wrote:
Do not allocate an object and delete an object in the same function, unless you have to

Why? That's a perfectly normal thing to do, especially when the object in question has some runtime-determined characteristic (e.g. a non-trivial constructor). It's appropriate if you don't need the object for the entire life of the function. Also, if the object is large, you tend to get better working set behavior if you allocate it from the heap rather than the stack.

Software Zen: delete this;

Fold With Us![^]

Re: Hashtables--More Info on Original Question

John R. Shaw1-Jan-06 9:02

John R. Shaw

1-Jan-06 9:02

Gary R. Wheeler wrote:
Why?

It is like doing this:
char* buffer = new char[32];
when you should be doing this:
char buffer[32];

The allocation in this case is not a normal thing to do.

Another example:

class MyClass
{
   int n1,n2;
public:
   // member operations
   ......
};
// function using class
void MyFunc1()
{
   // this makes no since (sizeof(MyClass) is only 8 bytes)
   MyClass* mc = new MyClass;
   ......
   delete mc;
}
// function using class
void MyFunc2()
{
   // this makes more since
   MyClass mc;
   ......
}

Gary R. Wheeler wrote:
It's appropriate if you don't need the object for the entire life of the function.

Insted of allocating it, you can use scoping. The pratice of using scoping to limit an objects life time is common and is used in the MFC library its self.

void MyFunc()
{
   ......
   {
      MyClass mc;
      ......
   } // class is destoyed automatically here
   ......
}

When you allocate unnecessarily, you introduce one more factor that can go wrong.

INTP
Every thing is relative...

Re: Hashtables--More Info on Original Question

Prakash Nadar1-Jan-06 4:25

Prakash Nadar

1-Jan-06 4:25

check http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/frlrfsystemcollectionshashtableclasstopic.asp[^] how the hashtable is used.

-Prakash

Re: Hashtables--More Info on Original Question

Michael Dunn1-Jan-06 8:07

Michael Dunn

1-Jan-06 8:07

jblau wrote:
illegal delete expression: managed type 'System::Collections::Hashtable' does not have a destructor defined

OK, that shows that you're creating a managed object. Don't delete it, the CLR garbage collector will take care of that for you. The reason you don't need __gc in the declaration is the compiler infers it from the context - Hashtable is a managed object so __gc is the only possibility.
Yes, this is confusing and it's the main reason why the MC++ syntax was replaced with C++/CLI in VC8.

--Mike--
Visual C++ MVP Cool | :cool:

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Re: Hashtables--One More Follow Up

jblau1-Jan-06 9:49

jblau

1-Jan-06 9:49

Thanks for all of the helpful information.

As I understand it now, the way my project is setup, the hashtable is a managed object, so I can't delete it mannualy, (as I get errors when I try).

So my follow up question is this: If I store an object that was declared with a NEW, and store it in the hashtable, do I need to go back through the hashtable and delete those objects, or does the managed code do that as well?

Example:
MyObject *tempObject = new MyObject;
Hashtable *hashObjects = new Hashtable;

hashObjects->Add(key, tempObject);

So if I store several tempObjects in my hashtable, do I need to traverse through the hashtable and DELETE the tempObjects, or does the garbage collector handle this as well as the hashtable itself?

If I do need to delete them, will a call to hashObjects->clear(); delete the objects?

Thanks,

Jody Blau

-- modified at 15:53 Sunday 1st January, 2006

Re: Hashtables--One More Follow Up

Prakash Nadar1-Jan-06 17:44

Prakash Nadar

1-Jan-06 17:44

Try posting this as a new question in the forum so that others can also look at it.

-Prakash

CButton displayes dark edge after first click

spiritofklanawa31-Dec-05 14:23

spiritofklanawa

31-Dec-05 14:23

I create a button at runtime using CButton.Create() function. The button looks fine until it is clicked the first time. From then on the black rectangle around the button becomes slightly wider and does not go away with releasing the button.

It's cosmetic problem only, subtle but annoying nevertheless.

I suspect it's something to do with window-style and extended-window-style but I had no success so far.

Thanks in advance for your suggestions. Sigh | :sigh:

spiritofklanawa

Re: CButton displayes dark edge after first click

Prakash Nadar31-Dec-05 16:55

Prakash Nadar

31-Dec-05 16:55

Could it be focus rectangle drawn to the Button, Did you try taking the focus off the button, like pressing the Tab button.

-Prakash

Re: CButton displayes dark edge after first click

dipuks31-Dec-05 16:58

dipuks

31-Dec-05 16:58

If you are trying to use a push button, then make sure you put style as BS_PUSHBUTTON.

You can also try, SetState(FALSE);, which will remove any highlighting

Re: CButton displayes dark edge after first click

Michael Dunn31-Dec-05 19:06

Michael Dunn

31-Dec-05 19:06

The extra-dark border marks the default button. If more than one button has the BS_DEFPUSHBUTTON style, that will muck up the dialog manager and buttons will be drawn incorrectly.

--Mike--
Visual C++ MVP Cool | :cool:

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Laugh it up, fuzzball.

Re: CButton displayes dark edge after first click

spiritofklanawa10-Jan-06 6:29

spiritofklanawa

10-Jan-06 6:29

that was it ! I just added a line of code in the BN_CLICKED message handler to remove the "BS_DEFPUSHBUTTON" style. The trouble was that multiple buttons showed this dark outline which looked goofy.

Thanks

How to add new thread to CDialog

dipuks31-Dec-05 11:09

dipuks

31-Dec-05 11:09

I have a MFC MDI App, to which i have added new CDialog. Now when i pop-up the new dialog from MDI and use the controls in it, it will be using the main thread of the MDI App.
I wanted to make this new dialog run on a new thread, so that all controls in it will run on new thread and the MDI will have its main thread always free from dialog. Can anyone tell me how to do this?
Thanks

Re: How to add new thread to CDialog

Prakash Nadar31-Dec-05 16:52

Prakash Nadar

31-Dec-05 16:52

You need to create a UI Thread in MFC, check CWinThread for more info. You can then start your dialog in this thread.

-Prakash

getting filename

RabiaAslam31-Dec-05 9:15

RabiaAslam

31-Dec-05 9:15

do u have an idea of getting the filename from the handle returned by the FirstFindChangeNotification.

RabiaAslam

Re: getting filename

Michael Dunn31-Dec-05 10:42

Michael Dunn

31-Dec-05 10:42

FindFirst/NextChangeNotification() don't work on files, they watch entire directories. You have to figure out on your own what changed.
You can use ReadDirectoryChangesW() to get more details change details.

--Mike--
Visual C++ MVP Cool | :cool:

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Application in a system tray

RabiaAslam31-Dec-05 9:13

RabiaAslam

31-Dec-05 9:13

how to load an application developed in MFC in system tray of the windows taskbar.

RabiaAslam

Re: Application in a system tray

Jack Puppy31-Dec-05 14:13

Jack Puppy

31-Dec-05 14:13

http://www.codeproject.com/shell/trayicons.asp

"My dog worries about the economy. Alpo is up to 99 cents a can. That's almost seven dollars in dog money" - Wacky humour found in a business magazine

Re: Application in a system tray

ThatsAlok1-Jan-06 19:01

ThatsAlok

1-Jan-06 19:01

RabiaAslam wrote:
how to load an application developed in MFC in system tray of the windows taskbar.

http://www.codeproject.com/shell/CTrayIcon.asp[^]

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

cheers,
Alok Gupta
VC Forum Q&A :- I/ IV

Custom Caption Bar

Dave Regan31-Dec-05 7:33

Dave Regan

31-Dec-05 7:33

I would like to implement a custom caption bar with CLose, Min and Help buttons. The framework does not allow this. Any suggestions

Dave Regan

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.