Re: const keyword - C / C++ / MFC Discussion Boards

Re: const keyword

Dominik Reichl2-Mar-07 1:09

Dominik Reichl

2-Mar-07 1:09

SyamlalS wrote:
Will it cause any kind of errors if I take the return value to a CString variable??

No. The const declaration of the function has nothing to do with its return value.

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Re: const keyword

Stephen Hewitt4-Mar-07 13:12

Stephen Hewitt

4-Mar-07 13:12

Nibu babu thomas wrote:
This means GetItemText function does not modify any of the member variables of the class to which this function belongs niether does it call any member function that modifies any member variables of the said class.

EXCPETION: mutable data members can be changed by const member functions.

Steve

Re: const keyword

prasad_som2-Mar-07 0:58

prasad_som

2-Mar-07 0:58

SyamlalS wrote:
CString GetItemText(int nRow, int nCol) const;

This function does not change state/value of member variables of class, it is function of.

Prasad
Notifier using ATL | Operator new[],delete[][^]

Re: const keyword

Roger Stoltz2-Mar-07 1:55

Roger Stoltz

2-Mar-07 1:55

In addition to what Prasad and Nibu babu thomas already said...

Consider the const keyword as a promise to the compiler that you won't change the value of the variable. If you don't honour this promise the compiler will remind you of it. This applies in al situations for the use of the const keyword.

SyamlalS wrote:
CString GetItemText(int nRow, int nCol) const;

If you try to alter member values inside a member function declared as const, you'll get a compiler error.

So what is the benefit of the const keyword in this situation?
The answer is that you can call such functions on a const declared object.
Explanation in code:

class CMyClass
{
public:
   int GetValue() { return m_nValue; }
   int GetValueConst() const { return m_nValue; }
};

// Try to use the class declared above...
void UseClass( const CMyClass theClass ) // You promise not to change the object
{
    int nTheValue;
    nTheValue = theClass.GetValue();       // Compiler error
    nTheValue = theClass.GetValueConst();  // OK
}

It's considered good practice to declare member functions that don't alter any member variables as const.

"It's supposed to be hard, otherwise anybody could do it!" - selfquote
"High speed never compensates for wrong direction!" - unknown

Re: const keyword

David Crow2-Mar-07 2:51

David Crow

2-Mar-07 2:51

Roger Stoltz wrote:
nTheValue = theClass.GetValue(); // Compiler error

Exactly how is this modifying a member variable?

"Approved Workmen Are Not Ashamed" - 2 Timothy 2:15

"Judge not by the eye but by the heart." - Native American Proverb

Re: const keyword

Roger Stoltz2-Mar-07 3:11

Roger Stoltz

2-Mar-07 3:11

Roger Stoltz wrote:
nTheValue = theClass.GetValue(); // Compiler error

DavidCrow wrote:
Exactly how is this modifying a member variable?

I never said it would and the code also shows that it doesn't.
The compiler error is due to the instance of CMyClass being a constant object and the CMyClass::GetValue() member function is not declared as constant.
I posted it as an explanation for the use of this syntax.

"It's supposed to be hard, otherwise anybody could do it!" - selfquote
"High speed never compensates for wrong direction!" - unknown

Re: const keyword

toxcct2-Mar-07 1:55

toxcct

2-Mar-07 1:55

in addition, such a constness concerning a member function is allowed to modify a data member if and only if it was declared mutable.

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Re: const keyword

vibindia2-Mar-07 4:07

vibindia

2-Mar-07 4:07

class sample
{
CString s1;
mutable CString s2;
public:
CString GetItemText(int nRow, int nCol) const;
};

CString sample::GetItemTextint nRow, int nCol) const;
{
s1 = _T("abc");//error because an const function can't change the data memter
//They are only for accesing
s2 = _T("abc");//works the keyword mutable is given
return s1;
}

//U have to read more on const object too Smile | :)

VIBIN

"Fool's run away,where angle's fear to tread"

Re: const keyword

John R. Shaw2-Mar-07 20:25

John R. Shaw

2-Mar-07 20:25

Glad you mentioned that, because it is important and I have found that aspect very useful. Although that ability needs to be used with great care and forethought, as it can easily be misused.

INTP
"Program testing can be used to show the presence of bugs, but never to show their absence."Edsger Dijkstra

Info about CListCtrl ?

sakthimuthu2-Mar-07 0:16

sakthimuthu

2-Mar-07 0:16

Hi,

I need to have a radio button inside the list control. The radio button should have the background color as white.

How can i do this ?

Thanks.

Re: Info about CListCtrl ?

Hamid_RT2-Mar-07 0:20

Hamid_RT

2-Mar-07 0:20

See CQuickList[^]

WhiteSky

Any error in my code! [modified]

ShaluMalu2-Mar-07 0:16

ShaluMalu

2-Mar-07 0:16

Hi everybody!

I wrote a code to display a messagebox when WM_QUERYENDSESSION message is posted.This is the code i wrote!
bool CMainWnd::OnQueryEndSession()
{
MessageBox(NULL,"QueryEndSession message is posted",MB_OK);
}

Is this correct! I expected to get a messagebox when i wanted to logoff the system. But it didnt work!Is anything wrong with my code!If so,I humbly request you to respond me!

Thanks in advance!

Regards
Shalini

-- modified at 6:55 Friday 2nd March, 2007

Re: Any error in my code!

prasad_som2-Mar-07 1:18

prasad_som

2-Mar-07 1:18

ShaluMalu wrote:
Is anything wrong with my code

No.

Are yo sure ON_WM_QUERYENDSESSION() is there is message map.

Prasad
Notifier using ATL | Operator new[],delete[][^]

Thread proble.

david bagaturia2-Mar-07 0:14

david bagaturia

2-Mar-07 0:14

Hi All.

I have problem, in my program I use 4 thread and thay all use:

<br />
<br />
//  in ---> "D:\\Documentations ( Russian )\\C++\\For_Windows\\msc_6\\*.exe"<br />
//  out ---> "D:\\Documentations ( Russian )\\C++\\For_Windows\\msc_6\\"<br />
//<br />
wchar_t* MyFolder(wchar_t* pSearch)<br />
{<br />
	wchar_t Folder[2048]= {L'\0'};<br />
	size_t n=0; <br />
<br />
	wcscpy(Folder,pSearch);<br />
<br />
	for(n=wcslen(Folder); n >= 0; n--)<br />
		if(Folder[n] == L'\\')<br />
		{<br />
			Folder[n+1]=L'\0';<br />
			break;<br />
		}<br />
	<br />
	return Folder;<br />
}

this function, and wen I debug it, I olvais have problem to this function.
I think thare is one of two:

1) I have not much experience
2) I do some mistake [in threads or in this function].

can help?

Re: Thread proble.

_anil_2-Mar-07 0:23

_anil_

2-Mar-07 0:23

Read about CCriticalSection on MSDN

Regards
Anil

Re: Thread proble.

User 5838522-Mar-07 0:36

User 583852

2-Mar-07 0:36

david bagaturia wrote:
wchar_t Folder[2048]= {L'\0'};

david bagaturia wrote:
return Folder;

Folder goes out of scope when the function exits so you should not return a pointer to that memory. Allocate the memory using new() or allocate the buffer in the calling function and pass it to this function

System.IO.Path.IsPathRooted() does not behave as I would expect

Re: Thread proble. [modified]

prasad_som2-Mar-07 1:34

prasad_som

2-Mar-07 1:34

As indicated by Josh, the problem is because, you are returning address of local object. Create it on heap, and return it.

david bagaturia wrote:
for(n=wcslen(Folder); n >= 0; n--)

This is in fact very bad coding practice, ~~It causes unnecessary multiple calls of a functions~~.
Modify it to,

size_t n =wcslen(Folder);
for(n; n >= 0; n--)

-- modified at 9:07 Friday 2nd March, 2007

Prasad
Notifier using ATL | Operator new[],delete[][^]

Re: Thread proble.

David Crow2-Mar-07 3:02

David Crow

2-Mar-07 3:02

prasad_som wrote:
It causes unnecessary multiple calls of a functions.

Actually it doesn't. wcslen() is only called once.

"Approved Workmen Are Not Ashamed" - 2 Timothy 2:15

"Judge not by the eye but by the heart." - Native American Proverb

Re: Thread proble.

prasad_som2-Mar-07 3:06

prasad_som

2-Mar-07 3:06

My Bad !

It was my oversight.
I thought, its comparing each time.

Prasad
Notifier using ATL | Operator new[],delete[][^]

MDIChild not getting WM_CLOSE

_anil_2-Mar-07 0:11

_anil_

2-Mar-07 0:11

I want to give user some messages before closing the MDIchild window.
But I am not getting the WM_CLOSE message. I receive WM_DESTROY messsage.
How to get the WM_CLOSE message in MDIChild window.

Regards
Anil

Re: MDIChild not getting WM_CLOSE

prasad_som2-Mar-07 1:28

prasad_som

2-Mar-07 1:28

_anil_ wrote:
How to get the WM_CLOSE message in MDIChild window

Add handler for it.
Are you sure ON_WM_CLOSE() is there in message map ?

Prasad
Notifier using ATL | Operator new[],delete[][^]

Re: MDIChild not getting WM_CLOSE

_anil_2-Mar-07 2:22

_anil_

2-Mar-07 2:22

Yes ofcourse I have added.

Regards
Anil

Re: MDIChild not getting WM_CLOSE

prasad_som2-Mar-07 2:30

prasad_som

2-Mar-07 2:30

Thenm, there should not be any problem.

Prasad
Notifier using ATL | Operator new[],delete[][^]

Re: MDIChild not getting WM_CLOSE

Mark Salsbery2-Mar-07 8:54

Mark Salsbery

2-Mar-07 8:54

How is the window being closed?

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Handle left-click on popup menu

Nicolas Bonamy1-Mar-07 23:51

Nicolas Bonamy

1-Mar-07 23:51

Hello,

I have been searching for this but can't find any answer. I would like my app to be able to handle left-click on popup menu (not menu items). Suppose my menu is:

Item 1
Popup > PopupItem1
PopupItem2
Item2

Everybody known how to handle left-click on Item1, Item2, PopupIem1 and PopupItem2. But how can one detect a left-click on the "Popup" item? Is this feasible with a regular CMenu instance?

Thanks!

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