Re: Deep copy and Shallow copy w.r.t Copy Constructor and Assignment Operator - C / C++ / MFC Discussion Boards

KTTransfer6-Dec-09 21:01

6-Dec-09 21:01

Then instead of #import what else we can use to include?

I tried making it visible and then used quit but no use.

It's not getting killed.

Deep copy and Shallow copy w.r.t Copy Constructor and Assignment Operator

hrishiS3-Dec-09 22:27

hrishiS

3-Dec-09 22:27

Hi to Everyone.

Its been a long time, I have been searching for an clear cut idea of
....shallow copy and deep copy with regards to Copy constructor and assignment operator.

I search the web, but many places has been confused me...I know that the problem comes only when we do use of pointers...But, dont know exact situation and solution for it. when to use which one..ie in which situation to used which one....

Can anyone please help me understanding the concept better.

A good example problem with relating to all these topic would be appreciated.
May be could you please relate the example with the following situations

1. Default copy constructor
2. Default assignment operator
3. overloaded copy constructor
4. overloaded assignment operator

Thanks in advance

-----------------------------
I am a beginner

Re: Deep copy and Shallow copy w.r.t Copy Constructor and Assignment Operator

Cedric Moonen3-Dec-09 22:55

Cedric Moonen

3-Dec-09 22:55

There's no difference concerning this behavior between a copy constructor and an assignement operator, so my explanation is valid for both.

The problem with a default constructor is that it simply copies byte by byte all the content of your class. This is ok for most of the cases when your class contains only plain data type for instance.

Now, if you have a class which contains a pointer and then if you copy your class using a shalow copy, both class instances will contain a pointer pointing at the same memory location. If you have a destructor like this:

MyClass::~MyClass() 
{
  if (myPointer) 
  {
    delete myPointer;
    myPointer = NULL;
  }
}

Then you will end up in big troubles once one of the class instances gets destroyed because its destructor will be called and you will end up with a pointer pointing to memory which has been release in your other instance.

So, what you have to do instead is allocate a new pointer and copy the content when you copy the class:

MyClass::MyClass(const MyClass& copy)
{
  myPointer = new ....;  // Allocate a new pointer
  // Copy the content of the pointer based on what is in "copy"
  ......
}

Cédric Moonen
Software developer

Charting control [v2.0]
OpenGL game tutorial in C++

Re: Deep copy and Shallow copy w.r.t Copy Constructor and Assignment Operator

hrishiS3-Dec-09 23:02

hrishiS

3-Dec-09 23:02

Thanks a lot for your valuable reply
Got some more doubts again.

Cedric Moonen wrote:
The problem with a default constructor is that it simply copies byte by byte all the content of your class.

What does the code of default constrictor is...what I mean is what is the code present in the default constructor?

Cedric Moonen wrote:
Now, if you have a class which contains a pointer and then if you copy your class using a shalow copy

Could you please elaborate this, what exactly is meant by copying using shallow copy? Does this mean copying with the default copy constructor what we have?

-----------------------------
I am a beginner

Re: Deep copy and Shallow copy w.r.t Copy Constructor and Assignment Operator

Rajesh R Subramanian3-Dec-09 23:12

Rajesh R Subramanian

3-Dec-09 23:12

See Copy constructor and default constructor[^]

“Follow your bliss.” – Joseph Campbell

Re: Deep copy and Shallow copy w.r.t Copy Constructor and Assignment Operator

Cedric Moonen3-Dec-09 23:13

Cedric Moonen

3-Dec-09 23:13

hrishiS wrote:
What does the code of default constrictor is...what I mean is what is the code present in the default constructor?

If you don't provide any constructor yourself, the default constructor will simply assign all variables, similarly as you would do something like this:

MyClass::MyClass(const MyClass& copy)
{
   member1 = copy.member1;
   member2 = copy.member2;
   ....
}

So, it means that if your class contains a pointer, this will result in something like this:

MyClass::MyClass(const MyClass& copy)
{
   myPointer = copy.myPointer;
}

And of course this is not nice because both of your instances will have a pointer pointing at the same location and there's no way anymore to know which of the two has ownership of the pointer (so, which one is responsible for deleting it). And if you destroy that pointer in the class destructor, then you end up with the problem I described earlier: one instance will end up with a pointer pointing to memory which has been released (and thus which can contain corrupted data).

Furthermore, you can also encounter another problem than with the one you have when manipulating pointers. If one (or more) member of your class is an object that cannot be copied (because its copy constructor has been made private for instance), then your code won't even compile because there's no way for the default constructor of your class to copy the object (as the copy constructor is private). Thus, the only way to be able to copy your class is to provide a copy constructor that instead of calling the copy constuctor of your member, it will create an new instance of this member and initialize it properly.

hrishiS wrote:
Could you please elaborate this, what exactly is meant by copying using shallow copy? Does this mean copying with the default copy constructor what we have?

It is assigning all members from one class to another as I described at the begining of this message.

Cédric Moonen
Software developer

Charting control [v2.0]
OpenGL game tutorial in C++

Re: Deep copy and Shallow copy w.r.t Copy Constructor and Assignment Operator

hrishiS3-Dec-09 23:59

hrishiS

3-Dec-09 23:59

Cedric Moonen wrote:
If you don't provide any constructor yourself, the default constructor will simply assign all variables,

But the default constructor doesn't have any parameter with it...So in that case to which value the member variable will get initialized to?...

Cedric Moonen wrote:
It is assigning all members from one class to another as I described at the begining of this message.

Does it meant that shallow copy is member to member copying of each member variable one by one...?
is it,...Copy Constructor is doing shallow copy?
In that case, if we dont provide an copy constructor does the default copy constructor provides deep copy?

-----------------------------
I am a beginner

Re: Deep copy and Shallow copy w.r.t Copy Constructor and Assignment Operator

Cedric Moonen4-Dec-09 0:12

Cedric Moonen

4-Dec-09 0:12

hrishiS wrote:
But the default constructor doesn't have any parameter with it...So in that case to which value the member variable will get initialized to?...

We are talking about the default copy constructor here. The default copy constructor receives the object from which to copy.

hrishiS wrote:
Does it meant that shallow copy is member to member copying of each member variable one by one...?
is it,...Copy Constructor is doing shallow copy?

Yes, that's what I showed you in my previous reply: a default copy constructor simply assigns all members one by one (in fact, it is using the copy constructor of each member to create them and not the assignement operator, but this is not important in the discussion here). So, in that sense a default copy constructor is doing shallow copy.

hrishiS wrote:
In that case, if we dont provide an copy constructor does the default copy constructor provides deep copy?

No, I just said the opposite: a default copy constructor does not make a deep copy. How could it do it ? You are the one how wrote the clas, so you know you have to copy some specific members (like pointers). The compiler doesn't know anything of that, he can simply assign all members from one object to the other. How can he know that for a pointer he has to allocate a new pointer and copy the content of the pointer ?

Cédric Moonen
Software developer

Charting control [v2.0]
OpenGL game tutorial in C++

How can I close an CChildFrame within CHtmlView ? [modified]

mesajflaviu3-Dec-09 22:14

mesajflaviu

3-Dec-09 22:14

I have an MDI application with CHtmlView. In CInvoiceHtmlView::OnBeforeNavigate2(...)I see if a special button is pressed. When is pressed , I would like to close child frame , but I don't know where I do that ...
Here is my trials :

<br />
void CInvoiceHtmlView::OnBeforeNavigate2(LPCTSTR lpszURL, DWORD nFlags, LPCTSTR lpszTargetFrameName, CByteArray& baPostedData, LPCTSTR lpszHeaders, BOOL* pbCancel) <br />
{<br />
	// TODO: Add your specialized code here and/or call the base class<br />
<br />
	if(m_bProcessNavigate)		// set not very first time<br />
	{<br />
		CString sBytes;<br />
<br />
		if(baPostedData.GetSize())<br />
		{<br />
			for(int i = 0;i < baPostedData.GetSize();i++)sBytes += (char)baPostedData[i];<br />
		}<br />
<br />
		sBytes = sBytes.Left(7);<br />
		if(sBytes == "button2")m_bClose = TRUE;<br />
	}<br />
<br />
	CHtmlView::OnBeforeNavigate2(lpszURL, nFlags,	lpszTargetFrameName, baPostedData, lpszHeaders, pbCancel);<br />
}<br />

now : m_bClose show me if special button was fired , but I don't know how can I use this to close child window ...

modified on Friday, December 4, 2009 4:51 AM

multi-processes's memory sharing problem?

nenfa3-Dec-09 19:45

nenfa

3-Dec-09 19:45

as we know, we can share the same memory in multi-processes through dll-library. if i use stl container as the public accessing memory in multi-processes, does it could do effect, thanks.

Re: multi-processes's memory sharing problem?

norish3-Dec-09 22:12

norish

3-Dec-09 22:12

What is a point? How do you want to write down as codes?

Re: multi-processes's memory sharing problem?

Michael Schubert4-Dec-09 5:25

Michael Schubert

4-Dec-09 5:25

norish wrote:
What is a point?

A point is an entity that has a location in space or on a plane, but has no extent.

Re: multi-processes's memory sharing problem?

nenfa7-Dec-09 20:08

nenfa

7-Dec-09 20:08

thank you, i had tried and it can't do.

Accessing Dialog member Variable problem

kamalilam3-Dec-09 19:30

kamalilam

3-Dec-09 19:30

Hi Friends,

How can I Access CDialog Class Member Variables in CDocument Class For Some Process...i Tried a lot..but it give error..plz gve me a solution .. Smile | :)

Re: Accessing Dialog member Variable problem

«_Superman_»3-Dec-09 19:40

«_Superman_»

3-Dec-09 19:40

What is the relationship between the CDocument and CDialog classes in your project?

«_Superman_»
I love work. It gives me something to do between weekends.

Microsoft MVP (Visual C++)

Re: Accessing Dialog member Variable problem

Madhu Nair3-Dec-09 19:44

Madhu Nair

3-Dec-09 19:44

kamalilam wrote:
Access CDialog Class Member Variables in CDocument Class

If the member variables are declared as Public, use it directly from the object of the dialog class.

For e.g ,

CDocument* pDoc;
pDoc->m_oDialog.m_nMember1 = 10;

Re: Accessing Dialog member Variable problem

Cedric Moonen3-Dec-09 20:10

Cedric Moonen

3-Dec-09 20:10

The same way you would do for any other classes. Your CDialog and CDocument classes are still classes and they follow the same rules as normal classes. So, to be able to access a variable from a CDialog object, you first need to have an instance of that class (e.g. CDialog myDialog, in which case myDialog is your instance). Then your CDocument needs to be able to access that variable (if he create the instance, then he already owns it and can use it). Finally, the members you want to access in your dialog class have to be accessible, which means either they are public or you provide a getter method for them.

But this is really basic C++ knowledge, I suggest you take some time to read and understand the concept of classes, because that's exactly what it is here. It's not because those classes are document and dialog classes that they do not follow the standard rules of C++.

Cédric Moonen
Software developer

Charting control [v2.0]
OpenGL game tutorial in C++

Re: Accessing Dialog member Variable problem

Rajesh R Subramanian3-Dec-09 21:14

Rajesh R Subramanian

3-Dec-09 21:14

Will you please include some relevant information in your post?

Are the CDialog members you're talking about public? How is this CDialog object related to the CDocument object in your application?

kamalilam wrote:
Tried a lot..but it give error.

*What* did you try and *what* were the errors for Chri' sake?

“Follow your bliss.” – Joseph Campbell

Which window has the focus [modified]

SujayG3-Dec-09 18:17

SujayG

3-Dec-09 18:17

I have to return the path of the selected file , EITHER in the Desktop or in the Windows Explorer.

I can get the path of the selected file both on the Windows Explorer and the desktop

My problem is that I am unable to find which window has the current focus

1) The user has selected a file on the desktop
2) The user has selected a file in the Windows explorer. ( So this has the focus now )

How do I programatically find that out

I am using the following code

HWND hWndDesktop = GetDesktopHandle(); // Gets the handle of the SyslistView32 window

DWORD dwThread = ::GetWindowThreadProcessId(hWndDesktop, NULL);

GUITHREADINFO gtf ;

gtf.cbSize = sizeof(GUITHREADINFO);

BOOL b = GetGUIThreadInfo(dwThread, >f);

if(hWndDesktop == gtf.hwndFocus)
{
// this window currently has the focus
}

But I am getting different handles for hWndDesktop and gtf.hwndFocus .

Please let me know if I am missing something

EDIT

I am trying to get the Window handle across processes .

Thanks
Sujay

modified on Friday, December 4, 2009 12:40 AM

Re: Which window has the focus

«_Superman_»3-Dec-09 18:21

«_Superman_»

3-Dec-09 18:21

Use GetFocus to get the handle to the window that has the focus.

«_Superman_»
I love work. It gives me something to do between weekends.

Microsoft MVP (Visual C++)

Re: Which window has the focus

SujayG3-Dec-09 18:31

SujayG

3-Dec-09 18:31

HI ,

GetFocus does return me the correct window handle also.

I had tried with GetFoucs,GetActiveWindow, none of them are working

Re: Which window has the focus [modified]

Madhu Nair3-Dec-09 19:28

Madhu Nair

3-Dec-09 19:28

Superman, Does the GetTopWindow help in this regard ?

Or, What about using File Change Notification[^] to get the changes user made in file system ? Not sure these are relevant for the OP.

modified on Friday, December 4, 2009 1:37 AM

Re: Which window has the focus

vasu_sri3-Dec-09 20:18

vasu_sri

3-Dec-09 20:18

i have tried what u have done .
that GetFocus function is retirning is NULL when iam calling in thread.

so, that i tried one more function GetForegroundWindow();
this also returns HWND.

CWnd *wnd=NULL;
HWND hWnd = NULL;
CString strwndName;

while (1)
{
    hWnd=::GetFocus();
    if(hWnd==NULL)
        hWnd=GetForegroundWindow();
    GetWindowText(hWnd,strwndName.GetBuffer(),20);
    //displaying in edit box.
    dlg->m_ed1.SetWindowText(strwndName);
}

just try it.. it will be work...

Regards,
Srinivas

Is there a convenient way to change a System environment variable in MFC?

Erik3-Dec-09 18:16

Erik

3-Dec-09 18:16

Hi,

I'm using VS 2005 and MFC, and I want to modify a system environment variable. So I looked up in the MSDN help and found that it is easy to set a user environment variable. However, for changing a system variable, the help says:

"Calling SetEnvironmentVariable has no effect on the system environment variables. The user can add or modify system environment variables using the Control Panel. To programmatically add or modify system environment variables, add them to the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\Environment registry key, then broadcast a WM_SETTINGCHANGE message. This allows applications, such as the shell, to pick up your updates. Note that environment variables listed in this key are limited to 1024 characters."

I understand that, but find this way very error-prone and complicated. Is there really no easy, straightforward way to do such a simple thing as setting a system variable? No other methods availbale in MFC?

Thanks alot

Re: Is there a convenient way to change a System environment variable in MFC?

T21023-Dec-09 19:27

T2102

3-Dec-09 19:27

That is a better method than I am aware of. I typically modify the registry via code and reboot the computer when necessary.

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