Re: Virtual function design pattern [modified] - C / C++ / MFC Discussion Boards

George_George19-Mar-08 3:15

19-Mar-08 3:15

Thanks CPallini,

So, your answer and pattern to this question is, if code are common in both base and derived class, and derived class has no need to override it, do not make it virtual?

regards,
George

Re: Virtual function design pattern

CPallini19-Mar-08 3:29

CPallini

19-Mar-08 3:29

My answer to the question is slighty different: if small pieces of code are common in the hierarchy of classes (it applies mainly to complex class hierachies) then make those methods available to the whole class hierarchy (hence the base class is a good place for them) but don't make them virtual (if you don't need them, don't use them), leaving polymorphism be done in the right places, i.e. bigger methods (of course small and big tradeoff is subjective).
The above is, IMHO, the substance of the original text.
Smile | :)

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-- Iain Clarke

Re: Virtual function design pattern

George_George19-Mar-08 17:20

George_George

19-Mar-08 17:20

Thanks CPallini,

A small confusion. I think whether or not making a method virtual is dependent on whether derived class needs to override it, and has nothing to do with whether the method is big or small.

Why do you making decision whether a method is virtual or not also based on whether the method is big or small? what is your trade-off?

regards,
George

Re: Virtual function design pattern

CPallini19-Mar-08 22:26

CPallini

19-Mar-08 22:26

George_George wrote:
A small confusion. I think whether or not making a method virtual is dependent on whether derived class needs to override it, and has nothing to do with whether the method is big or small.

Well this is true. Neverthless whenever we're talking about little pieces of common code, IMHO we have two points to consider:
(1) such methods have little functionality, i.e. if you don't need it you can replace the call with few lines of code.
(2) such methods are shared by a relevant number of other methods, i.e. you can find calls to such methods here and there.

From this viewpoint, IMHO, the design is cleaner if such pieces of code remains unchanged.

But, I know, at the end, it is matter of taste.

George_George wrote:
Why do you making decision whether a method is virtual or not also based on whether the method is big or small? what is your trade-off?

No. I make my decision based on the overall structure of the project and provided the points (1) and (2) hold for such 'helper' methods.
Smile | :)

Re: Virtual function design pattern

George_George19-Mar-08 22:38

George_George

19-Mar-08 22:38

Thanks CPallini,

Looks cool! You have made me clear about your points.

regards,
George

Re: Virtual function design pattern

followait19-Mar-08 1:42

followait

19-Mar-08 1:42

For mechanism, it's more flexible with an efficency tradeoff.

For semantics, the design will not be so clear.

Using virtual only when necessary will be a good choice considering efficiency and flexibility.

Re: Virtual function design pattern

George_George19-Mar-08 2:21

George_George

19-Mar-08 2:21

Thanks followait,

What is your trade-off? I think for the "little piece" common part, derived class can utilize it and overrides it, so why not make it virtual and protected?

regards,
George

Re: Virtual function design pattern [modified]

followait19-Mar-08 2:49

followait

19-Mar-08 2:49

In my opinion, a design would be better if it can guide the using of it, for example, it should prevent coders from overriding the functions that should not be overridden.

modified on Wednesday, March 19, 2008 8:57 AM

Re: Virtual function design pattern

George_George19-Mar-08 3:16

George_George

19-Mar-08 3:16

Thanks followait,

I think your points and answers to this question are, if code are common in both base and derived class, and derived class has no need to override it, do not make it virtual?

regards,
George

Re: Virtual function design pattern

followait19-Mar-08 3:37

followait

19-Mar-08 3:37

Simply, virtual function is invented for polymorphism.

In essence, a virtual function is an indirect call to a non-virtual function, so it can always replace a non-virtual function, with more flexibility and less efficiency.

Re: Virtual function design pattern

George_George19-Mar-08 17:22

George_George

19-Mar-08 17:22

Thanks followait,

1.

followait wrote:
a virtual function is an indirect call to a non-virtual function

You mean through vtable?

2.

followait wrote:
so it can always replace a non-virtual function

You mean virtual function has all the functions of a non-virtual function?

regards,
George

Re: Virtual function design pattern

followait19-Mar-08 18:00

followait

19-Mar-08 18:00

1. Yes.
2.
Yes, but more.

Re: Virtual function design pattern

George_George19-Mar-08 18:30

George_George

19-Mar-08 18:30

Thanks followait,

Question answered.

regards,
George

Is it not safe to use RectF::Equals? (float comparison problem) [modified]

followait18-Mar-08 19:10

followait

18-Mar-08 19:10

Here is its implementation. Simply compared with ==. It seems not very useful in practical, doesn't it?

BOOL Equals(IN const RectF & rect) const
{
    return X == rect.X &&
           Y == rect.Y &&
           Width == rect.Width &&
           Height == rect.Height;
}

modified on Wednesday, March 19, 2008 2:00 AM

Re: Is it not safe to use RectF::Equals? (float comparison problem)

Mark Salsbery18-Mar-08 20:05

Mark Salsbery

18-Mar-08 20:05

Ouch!

It may be fine if you use integers/whole numbers Roll eyes | :rolleyes:

Mark Salsbery
Microsoft MVP - Visual C++
Java | [Coffee]

not installaing visual studio .Net 2003 in vista?

rowdy_vc++18-Mar-08 18:55

rowdy_vc++

18-Mar-08 18:55

Hi 2 All!!!

When I install visual studio .Net 2003 in Windows Vista it does not installed why?

*****THANKS N ADVANCE****
Mathen.K
(I WILL TRY MY LEVEL BEST )

Re: not installaing visual studio .Net 2003 in vista?

Naveen18-Mar-08 19:18

Naveen

18-Mar-08 19:18

Google[^]

nave

[OpenedFileFinder]

Re: not installaing visual studio .Net 2003 in vista?

Rajesh R Subramanian18-Mar-08 22:05

Rajesh R Subramanian

18-Mar-08 22:05

Probably because you suck at asking questions. It's been a long time, I am seeing you ask two different questions over and over. One is that your program will work in XP, but not on Vista. And the other is that you fail to install VS 2003 on Vista. And you won't ever give us a remotest clue to solve the problem.

Nobody can give you wiser advice than yourself. - Cicero
.·´¯`·->Rajesh<-·´¯`·.
Codeproject.com: Visual C++ MVP

Re: not installaing visual studio .Net 2003 in vista?

David Crow19-Mar-08 4:23

David Crow

19-Mar-08 4:23

rowdy_vc++ wrote:
why?

Maybe because it failed. Other than that, I'm unsure.

"Normal is getting dressed in clothes that you buy for work and driving through traffic in a car that you are still paying for, in order to get to the job you need to pay for the clothes and the car and the house you leave vacant all day so you can afford to live in it." - Ellen Goodman

"To have a respect for ourselves guides our morals; to have deference for others governs our manners." - Laurence Sterne

Structure array initialization error

LiYS18-Mar-08 18:53

LiYS

18-Mar-08 18:53

Hi all,

I have something like this in one of the header file:

VAL Val1[1];

where VAL is defined as:

struct VAL
{
    LPWSTR      pszVal;
    LPWSTR      pszEmailAdrVal;
    ULONG       ulVal;
    SYSTEMTIME  stVal;
    BOOL        fVal;
};

when I doing something like this in CPP file:

Val1[0] = {L"Jack",      L"Jack@somewhere.com",      256,     {2006, 3, 4, 9, 0, 0, 0, 0},   TRUE};

I got "missing ';' before '{'" compilation errors. What's the reason behind this?

Thanks,

Re: Structure array initialization error

Naveen18-Mar-08 19:08

Naveen

18-Mar-08 19:08

LiYS wrote:
Val1[0] = {L"Jack", L"Jack@somewhere.com", 256, {2006, 3, 4, 9, 0, 0, 0, 0}, TRUE};

You cannot do like this after the object has been constructed. The object gets contructed at the statement

VAL Val1[1];

So the only option is to do as follows

VAL Val1[1] = {L"Jack",      L"Jack@somewhere.com",      256,     {2006, 3, 4, 9, 0, 0, 0, 0},   TRUE};

nave

[OpenedFileFinder]

Re: Structure array initialization error

ThatsAlok18-Mar-08 20:00

ThatsAlok

18-Mar-08 20:00

LiYS wrote:
VAL Val1[1];

HI, if you need only one object, why not you declare VAL Val1; instead of VAL Val1[1];

"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
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How to show a bitmap in a CStatic control?

fantasy121518-Mar-08 17:55

fantasy1215

18-Mar-08 17:55

In the default CAboutDlg, I just erase all controls. And I add a CStatic control. I just want to show a bitmap in the CStatic control.
And I also import a bitmap file to the resource.
my codes:

The header file

class CAboutDlg : public CDialog
{
  CBitmap m_bitmap;
  CStatic m_ctlPic;
}

The cpp file:

CAboutDlg::CAboutDlg()
{
  m_bitmap.LoadBitmap(IDB_BITMAP1);
}
CAboutDlg::~CAboutDlg()
{
  if (m_bitmap.GetSafeHandle())
    m_bitmap.DeleteObject();
}

BEGIN_MESSAGE_MAP()
  ON_WM_PAINT()
END_MESSAGE_MAP()

void CAboutDlg::OnPaint()
{
CPaintDC dc(this);
CRect rcWnd;
m_ctlPic.GetWindowRect(&rcWnd);
this->ScreenToClient(&rcWnd);
BITMAP bm = {0};
m_bitmap.SelectObject(&bm);
CDC dcMem;
dcMem.CreateCompatibleDC(&dc);
dcMem.SelectObject(&m_bitmap);
dc.StretchBlt(rcWnd.left, rcWnd.top, rcWnd.Width(), rcWnd.Height(), &dcMem, 0, 0, bm.bmWidth, bm.bmHeight, SRCCOPY);
}

But when I open the CAboutDlg, the bitmap shows flash, and vanish immediately, Please tell me what's wrong in my mind?

Re: How to show a bitmap in a CStatic control?

Naveen18-Mar-08 18:31

Naveen

18-Mar-08 18:31

fantasy1215 wrote:
How to show a bitmap in a CStatic control?

Why dont you use a Picture control?

nave

[OpenedFileFinder]

Re: How to show a bitmap in a CStatic control?

MANISH RASTOGI18-Mar-08 18:35

MANISH RASTOGI

18-Mar-08 18:35

Use SetBitmap() method of static control.

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