C / C++ / MFC

Re: Can I create another CMutex object when a named CMutex object being locked?

7-Jul-11 21:26

Yes you may contruct the MFC object anyway, but you HAVE to call GetLastError (as Albert Holguin already noted) to see what really happened.

Jun Du wrote:
I don't know how CMutex behaves exactly, as we don't have its source.

Is by chance your Visual Studio installation broken? Do you know MFC comes with source code, don't you?

As you are using MFC synchronization objects the following Newcomer's article is a worthy reading: "Avoid CMutex, CEvent, CSemaphore and CCrticalSection"[^].

ThatsAlok7-Jul-11 21:33

ThatsAlok

7-Jul-11 21:33

Jun Du wrote:
I suspect that you can create multiple CMutex objects that use the same named kernel object. I don't know how CMutex behaves exactly, as we don't have its source

Nope, if you try to create same named mutex it going to give your exception, simple technique is one would create it and other would open it!

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow
Never mind - my own stupidity is the source of every "problem" - Mixture

cheers,
Alok Gupta
VC Forum Q&A :- I/IV
Support CRY- Child Relief and You

CComboBox in a Modeless Dialog

softwaremonkey7-Jul-11 8:08

softwaremonkey

7-Jul-11 8:08

OK, I'm going mad here. I have a dialog class which I am creating from a dialog resource and displaying as modeless. I have data members assigned to the various controls and most of them work fine, except the CComboBox data member. I am trying to add some string data to the control in the CMyDialog::OnCreate(LPCREATESTRUCT lpCreateStruct) but it asserts because the control's hwnd is null. Do I need to do anything special with a CComboBox? My code is like this...

int CFlushDialog::OnCreate(LPCREATESTRUCT lpCreateStruct)
{
	if (CDialog::OnCreate(lpCreateStruct) == -1)
		return -1;

	SetWindowPos(NULL, m_nXPos, m_nYPos, 0, 0, SWP_NOSIZE | SWP_NOZORDER);

	m_comboDataMember.AddString("Hello");   // BOOM!
        // Other data members CEdit, CButton work OK
	return 0;
}

I display the dialog like this...

m_flushDialog.Create(IDD_FLUSH_DLG, this);
m_flushDialog.ShowWindow(SW_SHOW);

Anyone have any ideas? Confused | :confused:

Tony

Maximilien7-Jul-11 8:20

Maximilien

7-Jul-11 8:20

Why don't you add the data to the combobox in the OnInitDialog method ?

Watched code never compiles.

Richard MacCutchan7-Jul-11 9:22

Richard MacCutchan

7-Jul-11 9:22

I think you will find that your ComboBox does not get created until after the dialog creation has completed. Use the OnInitDialog() function to handle intialisation of dialog controls.

The best things in life are not things.

softwaremonkey7-Jul-11 10:59

softwaremonkey

7-Jul-11 10:59

Thanks Richard (and also Maximilien), your suggestion works great. I wasnt aware that OnInitDialog() still got called with modeless dialogs.

Big Grin | :-D

Tony

Pure virtual base class question

Albert Holguin7-Jul-11 11:00

Albert Holguin

7-Jul-11 11:00

As others have suggested, all GUI initialization should occur in OnInitDialog(). The fact that some controls work is probably more of a coincidence in how they're implemented but should not be an indication that this is the proper place for this.

Erudite_Eric6-Jul-11 22:56

6-Jul-11 22:56

So class _base is a pure virtual class.

Class _child derives from it and implements the functions.

The code creates an instance of _base and uses it.

Question. It seems that when an instance of a pure virtual base class is created, an instance of its child is in fact created. Is this so?

An observation. VS finds it impossible to debug step into any functions of the _child instance.

Further question. What if you have two descendents from _base. How does the compiler know which one to instantiate?

While I know C++ well, I have never come across this before, and I would never code this way, it looks ugly, is unclear, and makes debugging impossible. (I work mostly in the kernel in C)

==============================

Nothing to say.

వేంకటనారాయణ(venkatmakam)6-Jul-11 23:23

వేంకటనారాయణ(venkatmakam)

6-Jul-11 23:23

Eric__V wrote:
instance of _base

I doubt on the term instance in case of pure virtual class.You can't create an instance of a class if it have a pure virtual function.May be the code is declaring the pointer of base class and assigning the instance of derived class.Like,

_base *pbase = new basederive();

Eric__V wrote:
Further question. What if you have two descendents from _base. How does the compiler know which one to instantiate?

This will be done with help of virtual pointer table.There are lot of articles in code project on this.Start from this link,
ATL Under the Hood - Part 2

http://www.mono-project.com/Main_Page

Erudite_Eric7-Jul-11 0:31

7-Jul-11 0:31

Yeah, I just dug a bit dfeeper and indeed, it is newed off the derived class.

I still want to know why VS cant step into the damn code though. Its really pissing me off.

==============================

Nothing to say.

Richard MacCutchan7-Jul-11 1:36

Richard MacCutchan

7-Jul-11 1:36

Eric__V wrote:
I still want to know why VS cant step into the damn code though.

Have you compiled and built it in Debug mode?

The best things in life are not things.

Erudite_Eric7-Jul-11 22:28

7-Jul-11 22:28

Yeah, its running on a remote machine, so I am doing an 'attach to process' in VS. pdbs are on the remote machine with the exes and dlls (best way to handle symbols for VS IMO). OS symbols are stored on the dev machie locally and off Microsofts symsrv.

But weird, just cant step into the code. Its not even complaining about lack of source code and offering the view assembler approach.

Cant really devote time to finding out why its not doing it, too much real work to do else where. Smile | :)

==============================

Nothing to say.

Richard MacCutchan7-Jul-11 23:02

Richard MacCutchan

7-Jul-11 23:02

Sorry, no more ideas, I've never tried debugging a remote process.

The best things in life are not things.

Niklas L7-Jul-11 8:54

Niklas L

7-Jul-11 8:54

Eric__V wrote:
How does the compiler know which one to instantiate?

venkatmakam wrote:
This will be done with help of virtual pointer table.

No, instantiation is always explicitly written in code, and the proper virtual table is selected on instantiation.

CPallini7-Jul-11 2:52

7-Jul-11 2:52

Eric__V wrote:
It seems that when an instance of a pure virtual base class is created, an instance of its child is in fact created.

When an instance of the derived class is created then you have also an instance of the base class (because 'derived' is 'base'): you cannot instantiate an abstract class.

Eric__V wrote:
An observation. VS finds it impossible to debug step into any functions of the _child instance.

Visual Studio debugger should be able to do that (the debugger is one of the amazing features of Visual Studio, in my opinion).

Eric__V wrote:
Further question. What if you have two descendents from _base. How does the compiler know which one to instantiate?

Choosing the appropriate derived class to instantiate (to get base class functionality) is up to you.

Erudite_Eric7-Jul-11 22:31

7-Jul-11 22:31

I dug a bit deeper and in fact the _base pointer instance is new'ed as the sub classes.

And now I saw that, I remember from about 12 years ago, that I have seen this before. Spent the last 10 years pretty much 100% in the kernel so I guess I got a bit rusty with the old C++ inheritance game. Smile | :)

==============================

Nothing to say.

Maximilien7-Jul-11 3:10

Maximilien

7-Jul-11 3:10

Eric__V wrote:
The code creates an instance of _base and uses it.

Maybe I do not understand the question correctly. Confused | :confused:

You cannot create an instance of the base class if it contains pure virtual member functions.

class Onion
{
public:
	virtual void f( ) = 0;
};

class Tata : public Onion
{
	virtual void f();
};

...
Onion onion; // will generate a compiler error.
...

Watched code never compiles.

CPallini7-Jul-11 3:14

7-Jul-11 3:14

Even

Tata tata;

would generate a compiler error...
Poke tongue | ;-P

Maximilien7-Jul-11 3:32

Maximilien

7-Jul-11 3:32

CPallini wrote:
would generate a compiler error...

No, that does not generate a compiler error.

But maybe you're laughing at something else ? Confused | :confused:

Watched code never compiles.

CPallini7-Jul-11 10:27

7-Jul-11 10:27

Did you try it?
In your code the function implementation is missing.
Smile | :)

Niklas L7-Jul-11 20:46

Niklas L

7-Jul-11 20:46

I think he meant that it won‘t be a compiler problem, but rather something the linker will stumble upon.

CPallini7-Jul-11 21:14