C / C++ / MFC

Re: size of an empty class

30-Jul-03 5:33

Yeah, I'm sure Dr. Stroustrup missed that action item. Wink | ;)

Anyway, don't confuse the size of an item with the size of its address. The size of the class is 1 byte, but it's address is still 4 bytes.

Joaquín M López Muñoz30-Jul-03 6:35

Joaquín M López Muñoz

30-Jul-03 6:35

As others pointed out, the size of empty class objects is not zero so that pointers pointing to different objects do not have the same address value. Your confusion arises from asuming that the object somehow stores its own address: it doesn't, it is the pointers that store the address to the piece of memory occupied by the object (typically one byte). If you don't see it yet, consider the case of chars and pointers to char.

On a sidenote, there's an optional feature called empty base optimization by which an empty class object can effectively occupy zero bytes when it acts as a base of another class. Consider:

struct A{}; // size=1, cannot be 0
struct B{char m;}; // size=1
struct C:public A{char m;}; // size=1 instead of 1+1!

Class C can be applied this optimization so that, so to say, its A base takes no space. This can be done as the space occupied by C itself (not taking into account A) serves as the "token" byte where pointers to A can be set to.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo

Re: size of an empty class

melwyn30-Jul-03 20:57

melwyn

30-Jul-03 20:57

Thank you Joaquín for taking the time to explain it to me. It's now become clear to me. Essentially the address of every object of a class (whether empty or containing members) should be unique.

Regards,
Melwyn

CListCtrl scroll behaviour

Brian van der Beek30-Jul-03 4:27

Brian van der Beek

30-Jul-03 4:27

I have a CListCtrl that I use for logging. For every item that is inserted (always at the back/bottom) I call EnsureVisible(). The annoying thing is, when I try to scroll to an older entry, the list control jumps to the new entry being inserted (ofcourse).
What I would like to do is implement the behaviour of the visual studio output window, i.e. only jump to the latest entry when the vertical scrollbar is set to it's max. (bottom) position. However, when I try something like this:

if (GetScrollPos(SB_VERT) == GetScrollLimit(SB_VERT) -1)<br />
        {<br />
            // make sure the last item is visible<br />
            EnsureVisible(GetItemCount() - 1, TRUE);<br />
            SetScrollPos(SB_VERT, GetScrollLimit(SB_VERT) -1);<br />
        }

the cursor of the scrollbar keeps jumping up and down.

Anyone know how to do this the right way?

gdi+

wb30-Jul-03 4:14

30-Jul-03 4:14

Hi!

If I use some "normal" MFC class, VisualStudio offers
such a nice name completition (inteli sense?!)
but with the gdi+ classes, the IDE dont show me the
hints, which methods a class has. Frown | :(

anybody an idea, how to enable it?

Re: gdi+

Kannan Kalyanaraman30-Jul-03 5:47

Kannan Kalyanaraman

30-Jul-03 5:47

Intellisense in VisualStudio(6) is more or less broken, particularly when you are using third party libraries, you can get away with tools like Visual Assist.
Check out http://www.wholetomato.com
I think they have a 30 day trial.

Cheers,
Kannan

Restarting a loop from another cpp file?

keegan30-Jul-03 4:13

30-Jul-03 4:13

Ok, this is going to be confusing to explain, but simple to fix (me thinks).

I have a dialog box that pops up asking a user for a password. if they enter the icorrect password, another box pops up saying "Retry or Cancel". when the user hits Retry, i want it to start over (ie pop the password dialog box back up.)

Here's the code:

///*in PropsView.cpp*///

<br />
void CPropsView::OnWindowTransducers()<br />
{<br />
        m_password.DoModal();<br />
	bool bleh = g_password;<br />
	if(bleh == TRUE)<br />
	{<br />
		m_chsheet.DoModal(); //if pass is the right one, fo this<br />
	}<br />
	else<br />
	{<br />
		m_fail_pass.DoModal();//otherwise, propmt for a retry or to cancel<br />
		m_password.DestroyWindow();<br />
	}<br />
        g_password = FALSE;<br />
}<br />

in another cpp file, I have an OnOk() statement if the user hits "retry". But, since that onok is in another cpp file, i can't do m_fail_pass.DestroyWindow() to close the retry/cancel window, and i dont know how to make the loop start at the top again with the Password Dialog.

Does anyone get what i mean?

*.*
cin >> knowledge;

David Crow30-Jul-03 4:23

30-Jul-03 4:23

Do not dismiss the password box until the correct password has been entered, or until a certain number of failures has been met.

For a modal dialog, there is no need to call DestroyWindow(). When DoModal() returns, the window is gone.

void CPropsView::OnWindowTransducers()
{
    CMyPasswordDlg dlg;

    if (dlg.DoModal() == IDOK)
    {
        AfxMessageBox("Correct password was entered.");
        m_bPassword = true;
    }
    else
    {
        AfxMessageBox("Correct password was not entered.");
        m_bPassword = false;
    }
}

void CMyPasswordDlg::OnOK()
{
    CString str;

    m_pwd.GetWindowText(str);
    if (str == "password")
        CDialog::OnOK();
    else
    {
        AfxMessageBox("Incorrect.  Try again.");
        m_pwd.SetFocus();
        m_tries++
    }
}

keegan30-Jul-03 4:46

30-Jul-03 4:46

i didn't know what to define m_pwd as, so i defined it as CMyPasswordDlg m_pwd in the okok() function, and that gives a debug error when i hit ok.

Frown | :(

*.*
cin >> knowledge;

David Crow30-Jul-03 4:52

30-Jul-03 4:52

m_pwd

is an edit control. Note the use of GetWindowText()!! What is the "okok" function? Define "debug error."

Re: [edit]

keegan30-Jul-03 4:59

30-Jul-03 4:59

oops. okok = OnOk() but i mistyped it.

I changed m_pwd with my edit control variable, and now when i compile, i get these errors:

error C2039: 'GetWindowTextA' : is not a member of 'ATL::CStringT<basetype,stringtraits>'
with
[
BaseType=char,
StringTraits=StrTraitMFC_DLL<char>
]

hmm, was m_pwd supposed to me a "Control" variable or a "value" variable when i created it witht he add variable wizzard? i made it a value, so its a CString, not a CEdit.

[Edit]
~it works! that was it. thanks a whole whole whole lot!

*.*
cin >> knowledge;

David Crow30-Jul-03 5:08

30-Jul-03 5:08

keegan wrote:
hmm, was m_pwd supposed to me a "Control" variable or a "value" variable when i created it witht he add variable wizzard? i made it a value, so its a CString, not a CEdit.

It is a "control" variable. You would not be able to use GetWindowText() on a "value" variable.

keegan30-Jul-03 5:10

30-Jul-03 5:10

just figured that out, once again you saved me from the horrors of MFC. Big Grin | :-D

thanks.

*.*
cin >> knowledge;

John M. Drescher30-Jul-03 4:25

John M. Drescher

30-Jul-03 4:25

It should not make a difference that something is in another cpp file if your variable vaild in the scope that you need to use it.

The way I do this is in your OnOK dialog for the password entry you do the required password varification and create the second dialog if necissary. As a result of the second dialog you either exit the first dialog by calling the base class CDialog::OnOK() or continue by not calling it.

John

keegan30-Jul-03 4:42

how can I warn to make a choice for numeric edit box?

30-Jul-03 4:42

well, i would make the dialog i need top be created if the password is correct (m_sh), but the only place i can call

m_sh.DoModal()

is in PropsView.cpp, and the onok() is in property.cpp. If, in Property.cpp, i try to include PropsView.h, where m_sh is defined, i get a whole lotta errors. so, as far as i can figure out i have to create the new dialog in PropsView.cpp, and do the authentication of the password there.

grr.

*.*
cin >> knowledge;

Emre Zorlu30-Jul-03 4:02

Emre Zorlu

30-Jul-03 4:02

when I use string ,I can write

m_StringeditBox.GetWindowText (str);
if(str.IsEmpty()){
MessageBox("please,choose one of them ! ");
return;
}
like tihs !

if I use edit box for numeric value how can I warn ?

emroz

Re: how can I warn to make a choice for numeric edit box?

David Crow30-Jul-03 4:24

Re: how can I warn to make a choice for numeric edit box?

30-Jul-03 4:24

In the same way.

Emre Zorlu30-Jul-03 4:41

Emre Zorlu

30-Jul-03 4:41

it didn't work Frown | :(

emroz

Re: how can I warn to make a choice for numeric edit box?

David Crow30-Jul-03 4:45

Re: how can I warn to make a choice for numeric edit box?

30-Jul-03 4:45

How about a code snippet?

Emre Zorlu30-Jul-03 20:36

Emre Zorlu

30-Jul-03 20:36

When I add information to access table ,I use this code !

set.m_access_attribute = GetDlgItemInt (IDC_EDIT1);

How can I warn to fill edit box ?

emroz

Re: how can I warn to make a choice for numeric edit box?

David Crow31-Jul-03 2:45

Re: how can I warn to make a choice for numeric edit box?

31-Jul-03 2:45

I don't use GetDlgItemInt() (see below), but if that is what you are limited to, you'll want to use its second parameter, lpTrans. If it is 0 upon return, an error was encountered in translating the edit control text. This includes being empty.

Whether I'm dealing with letters, numbers, or a combination of the two, I extract edit box data using GetWindowText(). It's easy to check for emptiness, and if the end result needs to be stored in a number, atoi() is only a function call away.

keegan30-Jul-03 4:54