Re: The type of *this - C / C++ / MFC Discussion Boards

George_George15-Dec-07 21:57

15-Dec-07 21:57

Thanks SANTHOSH,

My question is not what is this pointer, but when we return *this, should we treat return type as reference to object type, object type, or both are ok?

regards,
George

Re: The type of *this

Rajasekharan Vengalil15-Dec-07 20:35

Rajasekharan Vengalil

15-Dec-07 20:35

foo1 returns a reference to the object pointed at by this. foo2 returns a copy of the object pointed at by this.

--
gleat
http://blogorama.nerdworks.in[^]
--

Re: The type of *this

George_George15-Dec-07 21:55

George_George

15-Dec-07 21:55

Thanks gleat,

You mean if we invoke,

1. fun1, like A& a = fun1(), then return a reference;
2. fun2, like A a = fun2(), return a new copy of instance (invokes assignment operator on A)

Both are good code, right?

regards,
George

Re: The type of *this

Rajasekharan Vengalil15-Dec-07 22:03

Rajasekharan Vengalil

15-Dec-07 22:03

Yep. That's the idea. As for whether its good code or not depends on what you want to do. Some simple thumb-rules are:

Don't pass around data heavy structures by value as the copying overhead can prove to be a performance drain. Be especially wary of passing them via STL (or any other) collection types. Imagine returning a vector<BigHeavyClass> containing 500 items by value!
When passing around objects by reference remember to use const if you do not intend to alter the object's state via that reference.

--
gleat
http://blogorama.nerdworks.in[^]
--

Re: The type of *this

George_George15-Dec-07 22:11

George_George

15-Dec-07 22:11

Thanks for your advice, gleat!

My question is answered.

regards,
George

Re: The type of *this

Gary R. Wheeler16-Dec-07 4:20

Gary R. Wheeler

16-Dec-07 4:20

George_George wrote:
(invokes assignment operator on A)

Not exactly true. It invokes the copy constructor, which is not the same thing.

Software Zen: delete this;

Fold With Us![^]

Re: The type of *this

George_George16-Dec-07 20:27

George_George

16-Dec-07 20:27

Thanks Gary, you are correct. I want to confirm with you that,

1.

A a = fun2() will invoke copy constructor on a;

2.

A a;
a = fun2();

will invoke assignment operator on a.

Both are correct?

regards,
George

Re: The type of *this

Gary R. Wheeler18-Dec-07 14:36

Gary R. Wheeler

18-Dec-07 14:36

Since foo1 returns a reference (which is really just a pointer), it can just return the address of the 'this' object.

The other function, foo2 actually returns an object of type A. In order to do that, it invokes the copy constructor for A, copying the contents of this to the destination.

Software Zen: delete this;

Fold With Us![^]

Re: The type of *this

George_George18-Dec-07 18:49

George_George

18-Dec-07 18:49

Thanks Gary,

How about your comments on my two questions before?

--------------------
1.

A a = fun2() will invoke copy constructor on a;

2.

A a;
a = fun2();

will invoke assignment operator on a.

Both are correct?
--------------------

regards,
George

Re: The type of *this

Gary R. Wheeler19-Dec-07 12:29

Gary R. Wheeler

19-Dec-07 12:29

I believe your statements are both true. The compiler is supposed to turn #1 into this:

A a(fun2());

automatically, which is a call to the copy constructor.

#2 obviously calls the assignment operator. Note that the default constructor for A is called first, when the variable is declared.

Software Zen: delete this;

Fold With Us![^]

Re: The type of *this

George_George19-Dec-07 18:52

George_George

19-Dec-07 18:52

Thanks for your clarification, Gary!

regards,
George

Re: The type of *this

CPallini15-Dec-07 22:14

CPallini

15-Dec-07 22:14

George_George wrote:

class A{

A& foo1()
{
return *this;
}

A foo2()
{
return *this;
}
};

Well, there's nothing special related to this in the above snippet:
you can do as well:

class B
{
  int i;
  int * p;
...
  int getiValue(){ retirn i;}
  int & getiRef(){ return i;}
  int getpValue(){ return *p;}
  int & getpRef(){ return *p;}
...
}

it is simply C++ syntax for dealing with references.

Smile | :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.

Re: The type of *this

George_George15-Dec-07 22:40

George_George

15-Dec-07 22:40

Cool, CPallini! Thanks, and my question is answered.

regards,
George

return reference to local variable, good code?

George_George15-Dec-07 18:46

George_George

15-Dec-07 18:46

Hello everyone,

I am reading some code from other people, there are some code like this,

class Foo {

};

Foo& func()
{
	Foo foo;
	return foo;
}

int main()
{
	Foo& foo = func();

	return 0;
}

I want to confirm with you that it is not good code, since we return a reference to local object instance, right? Even if the code has expected function currently.

thanks in advance,
George

Re: return reference to local variable, good code?

Rajasekharan Vengalil15-Dec-07 20:38

Rajasekharan Vengalil

15-Dec-07 20:38

Yep! Whatever else it is, it most certainly is NOT good code! The local instance of foo inside func will get deallocated when control returns from func. So the reference foo in main is undefined.

--
gleat
http://blogorama.nerdworks.in[^]
--

Re: return reference to local variable, good code?

George_George15-Dec-07 21:31

George_George

15-Dec-07 21:31

Thanks gleat,

My question is answered.

regards,
George

How to get Instance of Main Window from MFC Activex

ashishbhatt15-Dec-07 18:45

ashishbhatt

15-Dec-07 18:45

Hi All,,

I have created MFC Activex Control in VS 2005.In that I want to get the instance of the main window.
As like in Win32 Application we can get this instance from Winmain() function like below...

<br />
HINSTANCE	hAppInstance;<br />
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR szCmdLine, int iCmdShow)<br />
{<br />
   hAppInstance = hInstance;<br />
   <br />
   // Any Code....<br />
}<br />

.. and this Instance I can use in futher application.(for example to create window CreateWindow() function)

Same way how can I get this Instance form MFC Activex Control Application??

Plz suggest me if you know anything about this.

Thanks in Advance.

Re: How to get Instance of Main Window from MFC Activex

Mike Dimmick16-Dec-07 1:37

Mike Dimmick

16-Dec-07 1:37

I suggest you take a step back and tell us what problem you are actually trying to solve.

The HINSTANCE passed to CreateWindow is used to allow multiple independently-developed libraries to register window classes using the same name. The HINSTANCE differentiates between the different classes. Generally when registering a window class you pass your DLL or EXE's HINSTANCE value, then use the same value when creating a window of that class.

For more see http://blogs.msdn.com/oldnewthing/archive/2005/04/18/409205.aspx[^].

DoEvents: Generating unexpected recursion since 1991

Re: How to get Instance of Main Window from MFC Activex

ashishbhatt16-Dec-07 2:00

ashishbhatt

16-Dec-07 2:00

Thank you very much for reply.

I was waiting fro reply for very long time.

Now ,I want to get instance of the main window which I want to give to SetWindowsHookEx(...)
function.

I got the instance by using AfxGetInstanceHandle() function,but during hooking it gives the messages for the window in which I am running this Activex(e.g. Explorer and Activex control Container).But I want to get hooking messages for all desktop windows.

Any suggetion?Plz reply me.

GDI+ Alpha changed, the image is still opaque why? [modified]

followait15-Dec-07 18:01

followait

15-Dec-07 18:01

Bitmap *pbm=::new Bitmap(rt.Width,rt.Height,bm.GetPixelFormat());
Graphics *pg=::new Graphics(pbm);
pg->SetInterpolationMode(InterpolationModeHighQuality);
pg->DrawImage(&bm,0,0,rt.Width,rt.Height);


//locked this way
pbm->LockBits(&Rect(0,0,rt.Width,rt.Height),ImageLockModeRead|ImageLockModeWrite,PixelFormat32bppARGB,&bmd);
...
//r,g,b can be changed normally. a can't. why?
pClr->SetValue(Color::MakeARGB(100,0xff,pClr->GetGreen(),pClr->GetBlue()));
//trace here, a is 100
a=pClr->GetAlpha();

//UnlockBits is also called

I used ImageAttributes to roate and adjust intensity when drawing.
Here is the code:

Matrix NM;
PointF fpt((float)pCB->m_Center.X,(float)pCB->m_Center.Y);
NM.RotateAt(pCB->m_Angle,fpt);
Matrix  OM;
m_pMemGraphics->GetTransform(&OM);
m_pMemGraphics->SetTransform(&NM);

ColorMatrix cm = {
    1.0f,0.0f,0.0f,0.0f,0.0f,
    0.0f,1.0f,0.0f,0.0f,0.0f,
    0.0f,0.0f,1.0f,0.0f,0.0f,
    0.0f,0.0f,0.0f,1.0f,0.0f,
    pCB->m_IntensityAdjust,pCB->m_IntensityAdjust,pCB->m_IntensityAdjust,0.f,1.0f
};
ImageAttributes ImAtt;
ImAtt.SetColorMatrix( &cm,ColorMatrixFlagsDefault,ColorAdjustTypeBitmap);
m_pMemGraphics->DrawImage(pbm,rt,0,0,rt.Width,rt.Height,UnitPixel,&ImAtt);

modified on Sunday, December 16, 2007 12:08:59 PM

Re: GDI+ Alpha changed, the image is still opaque why?

Mark Salsbery16-Dec-07 6:25

Mark Salsbery

16-Dec-07 6:25

followait wrote:
GDI+ Alpha changed, the image is still opaque why?

You should create your destination Bitmap (pbm) with format
PixelFormat32bppARGB, not bm.GetPixelFormat().

You obtain a copy of the pixel bits in PixelFormat32bppARGB format
with LockBits(), but when you call UnlockBits(), the pixel bits are
converted back to the original Bitmap format.

Mark

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

Re: GDI+ Alpha changed, the image is still opaque why?

followait16-Dec-07 6:29

followait

16-Dec-07 6:29

Thanks so much.

The task has been blocked until now.

GDI+ how to do bitmap mask efficiently

followait15-Dec-07 16:07

followait

15-Dec-07 16:07

Because size of the mask bitmap is different from of the to-mask bitmap,
how to do stretch efficiently.

Bitmp bm("myimage.jpg");
Graphics gp(&bm);
//Can I stretch bm here?
//Can I use the bm which is stretched here?

Re: GDI+ how to do bitmap mask efficiently

Christian Graus15-Dec-07 16:15

Christian Graus

15-Dec-07 16:15

Well, any stretch is going to create fuzzy edges on your masks, but you can just create a new bitmap using the old bitmap and the new size as parameters to the constructor, I thought.

Christian Graus - Microsoft MVP - C++

"also I don't think "TranslateOneToTwoBillion OneHundredAndFortySevenMillion FourHundredAndEightyThreeThousand SixHundredAndFortySeven()" is a very good choice for a function name" - SpacixOne ( offering help to someone who really needed it ) ( spaces added for the benefit of people running at < 1280x1024 )

Re: GDI+ how to do bitmap mask efficiently

Mark Salsbery16-Dec-07 6:33

Mark Salsbery

16-Dec-07 6:33

As Christian mentioned, you'll have problems with
the mask pixel values at edges if you stretch it.

You could set the interpolation mode of the Graphics
object you're stretching to InterpolationModeNearestNeighbor
so you don't get new mask colors introduced, but that may
not give you satisfactory results.

How do you have a mask that doesn't match the size of its
associated bitmap?

Mark

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

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