C / C++ / MFC

22-Jun-07 2:45

hey folks,
I wrote a little wrapper for the gsl-library and had to face a problem I never had expected (and never noticed before).
It's a simple class-question... So first the code:

<br />
// Class Header<br />
class myClass<br />
{<br />
public:<br />
	myClass();<br />
	~myClass();<br />
	myClass Testfunction();<br />
	int val;<br />
};<br />
<br />
// Class Implementation<br />
myClass::myClass()<br />
{<br />
	val=0;<br />
}<br />
<br />
myClass::~myClass()<br />
{<br />
	int t = 0;<br />
	int h = t;<br />
	this->val = val;<br />
	cout << "Destructor called\n";<br />
}<br />
<br />
myClass myClass::Testfunction()<br />
{<br />
	myClass newClass;<br />
	newClass.val = 7;<br />
	return newClass;<br />
}<br />
<br />
// Main<br />
int main()<br />
{<br />
	myClass A;<br />
	myClass B;<br />
	cout << "Calling test function\n";<br />
	B = A.Testfunction();<br />
<br />
	cout << "Print out value:\n";<br />
	cout << B.val << "\n";<br />
<br />
<br />
	cout << "Waiting for user...\n";<br />
	cin.get();<br />
}<br />

Allright... actually there shouldn't be any destructor called BEFORE the cin.get().
The output is:

<br />
Calling test function<br />
Destructor called                  // WHY???<br />
Destructor called                  // WHY???<br />
Print out value:<br />
7<br />
Waiting for user...<br />

The Destructor is somehow called for the return value and I don't know why...
And even if I get an explanation... is there a way of not calling the destructor here? (Compiler setting???)
because I actually free some memory in my prog in the destructor and when the memory is freed I never get it back... Frown | :(

any idea??

thx in advance
zqueezy

Iain Clarke, Warrior Programmer

Iain Clarke, Warrior Programmer22-Jun-07 3:01

22-Jun-07 3:01

The Testfunction returns myClass also.

newclass doesn't exist past the end of testfunction, so the function returns a copy of it... Temporary copy needs constructing, and (drumroll) destructing.

Try adding another member to myClass, and a constructor which assigns a random value to it. Then also spit it out in the destructor. See how its actually different myClass instances getting killed.

As to the avoidance? That all depends on what you were really trying to do. What you have posted is a very artificial example to show the "double" destructing, it doesn't show what you were trying to do when you ran into this issue.

And just in case this is homework, as least I've only done 50% of your work.

Iain.

zqueezy22-Jun-07 3:12

22-Jun-07 3:12

well, no it's no homework. I'm well aware of the fact that the copy-call is run for the object that went out of scope...

To make my example less artificial:
In gsl a matrix need allocation of memory (alloc) and freeing the memory, of course.
I tried to write a matrix-class which does the allocation in the constructor and which does the freeing in the destructor.
Now I wrote an operator function for the multiplication:
CMatrix CMatrix::operator *(const CMatrix& factorMatrix);
when I try to return the resulting matrix the destructor is called and the memory freed thus the matrix deleted and cannot be assigned with any operation like the operator=...

better?
zqueezy

Iain Clarke, Warrior Programmer

Iain Clarke, Warrior Programmer22-Jun-07 4:05

22-Jun-07 4:05

The "extra" destructor you're getting is because you're being nice and elegant and writing a * operator, which (quite correctly) returns an instance (albeit a temporary one) on CMatrix.

If the constructor / destructor is "expensive", then you're going to have to do it a different way.

class CMatrix
{
public:
    ....
    BOOL Multiply (CMatrix const &lhs, CMatrix const &rhs)
    {
        if (!ValidToDoThisMaths (lhs, rhs))
            return FALSE;
        // Do gnarly maths here to multiply lhs & rhs and keep the results in this
        ....
        return TRUE;
    }
    ....
};

void main ()
{
    CMatrix A,B,C;
    
    A.Create (blah);   // <-- however you make them have something interesting in them
    B.Set (blah);

    // we'd like   C = A * B;   but that would mean expensive operations with temp matrix
    C.Multiply (A,B);

    C.DoStuff ();
}

Its less elegant, but it would solve your problem.

It would also lose the ability to do:

D = A * B * C

but frankly, matrices are enough of a pain to work with [ (n x m) * (m x n) etc ] that that's probably not a huge loss.

I hope that helped,

Iain.

Nemanja Trifunovic22-Jun-07 3:03

Nemanja Trifunovic

22-Jun-07 3:03

zqueezy wrote:
Allright... actually there shouldn't be any destructor called BEFORE the cin.get().

Of course it should.

zqueezy wrote:
The Destructor is somehow called for the return value and I don't know why...

Because newClass went out of scope, and the destructor must be called in that case.

zqueezy wrote:
is there a way of not calling the destructor here?

Rewrite the function like this:

void myClass::Testfunction(myClass& obj)
{
obj.val = 7;
}

Create the object before calling it, and pass it as the argument.

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zqueezy22-Jun-07 3:24

22-Jun-07 3:24

when I do the whole thing as reference... why is the destructor called anyways... isn't the reference just an address?

Nemanja Trifunovic22-Jun-07 3:28

Nemanja Trifunovic

22-Jun-07 3:28

zqueezy wrote:
when I do the whole thing as reference... why is the destructor called anyways

No, it is not.

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Iain Clarke, Warrior Programmer

Iain Clarke, Warrior Programmer22-Jun-07 3:03

22-Jun-07 3:03

Ach, I'm weak.

Below is a copy of a =operator function, where you can see the return type is a reference to object, rather than the object. This allows you to dasiy chain loads of =s together.

RgbQuad &operator=(RgbQuad &r)
{
    rgbRed  = r.rgbRed;
    rgbGreen= r.rgbGreen;
    rgbBlue = r.rgbBlue;
    return  *this;
};

I hope that steers you in the right direction.

Iain.

Taka Muraoka22-Jun-07 3:06

Taka Muraoka

22-Jun-07 3:06

zqueezy wrote:
The Destructor is somehow called for the return value and I don't know why...

The newClass object in your test function is created on the stack and so cannot be used by anyone once the function exits. So the compiler has to copy it into your B variable, then destroy the newClass object. I think you're getting two calls to the destructor because the compiler is copying the newClass object into a temporary "return value" object and then copying this one into the B variable. Google for something called "return value optimization" for info on how to get rid of this.

zqueezy wrote:
is there a way of not calling the destructor here?

Nope. The newClass object *must* be destroyed (since it's on the stack).

zqueezy wrote:
because I actually free some memory in my prog in the destructor

This is usually not the best way of designing things but if you absolutely have to do it like this, allocate the newClass object from the heap (using "new") and return a pointer to that.

I enjoy occasionally wandering around randomly, and often find that when I do so, I get to where I wanted to be [^].

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zqueezy22-Jun-07 3:14

22-Jun-07 3:14

I didn't want to do a heap allocation... for the reason of the easy-user interface (see my first reply) but perhaps... that's the only possibility.
thanx for all your help

zqueezy25-Jun-07 11:27

25-Jun-07 11:27

To give my end-solution:
I didn't know I had to create a Copy-Constructor for my class,
you need a copy-constructor for classes that allocate memory dynamically...

Rotating Bitmaps !!!!

Adno22-Jun-07 2:43

Adno

22-Jun-07 2:43

Im using this method needless to say is not working and need some help.

void rotateb(Bitmap* image, HDC dc){<br />
	Graphics graphics(dc);<br />
	Bitmap* SrcBitmap1 = image;<br />
	Color mc;<br />
<br />
	int angle=45;        //45° for example <br />
	//Convert degrees to radians <br />
	float radians=(2*3.1416*angle)/360; <br />
<br />
	float cosine=(float)cos(radians); <br />
	float sine=(float)sin(radians); <br />
<br />
	float Point1x=(-SrcBitmap1->GetHeight()*sine); <br />
	float Point1y=(SrcBitmap1->GetHeight()*cosine); <br />
	float Point2x=(SrcBitmap1->GetWidth()*cosine-SrcBitmap1->GetHeight()*sine); <br />
	float Point2y=(SrcBitmap1->GetHeight()*cosine+SrcBitmap1->GetWidth()*sine); <br />
	float Point3x=(SrcBitmap1->GetWidth()*cosine); <br />
	float Point3y=(SrcBitmap1->GetWidth()*sine); <br />
<br />
<br />
	float minx=min(0,min(Point1x,min(Point2x,Point3x))); <br />
	float miny=min(0,min(Point1y,min(Point2y,Point3y))); <br />
	float maxx=max(Point1x,max(Point2x,Point3x)); <br />
	float maxy=max(Point1y,max(Point2y,Point3y)); <br />
<br />
	int DestBitmapWidth=(int)ceil(fabs(maxx)-minx); <br />
	int DestBitmapHeight=(int)ceil(fabs(maxy)-miny);<br />
<br />
	Bitmap* DestBitmap = new Bitmap(DestBitmapWidth, DestBitmapHeight);<br />
<br />
	//----------<br />
	for(int x=0;x<DestBitmapWidth;x++) <br />
	{ <br />
	  for(int y=0;y<DestBitmapHeight;y++) <br />
	  { <br />
		int SrcBitmapx=(int)((x+minx)*cosine+(y+miny)*sine); <br />
		int SrcBitmapy=(int)((y+miny)*cosine-(x+minx)*sine); <br />
		if(SrcBitmapx>=0&&SrcBitmapx<SrcBitmap1->GetWidth() && SrcBitmapy>= 0 && <br />
			 SrcBitmapy<SrcBitmap1->GetHeight()) <br />
		{ <br />
			SrcBitmap1->GetPixel(SrcBitmapx,SrcBitmapy,&mc);<br />
			DestBitmap->SetPixel(x,y,mc);<br />
		} <br />
	  } <br />
	} <br />
<br />
	graphics.DrawImage(DestBitmap,0, 0); <-- this does not <br />
	//graphics.DrawImage(SrcBitmap1,0,0); <-- this displays <br />
}

Thank you

rrrado22-Jun-07 3:05

rrrado

22-Jun-07 3:05

what does it display? Try to set some pixels in destination bmp with some constatnt color to check if there is problem in rotation alghoritm or displaying

rrrado

Mark Salsbery22-Jun-07 5:55

Mark Salsbery

22-Jun-07 5:55

Also, if you aren't required to write the rotation code yourself, GDI+ will do it for you Smile | :)

Mark

"Posting a VB.NET question in the C++ forum will end in tears." Chris Maunder

Adno22-Jun-07 7:34

Adno

22-Jun-07 7:34

hi the GDI+ i find is not very precise Mark i mean the image center shifts, and you have to draw the image then rotate the DC then draw it again you end up with two images and a rotated DC lol

the method i posted is skipping the second for loop for some reason WTF | :WTF:

thanks for your replies... back to digging Smile | :)

Mark Salsbery22-Jun-07 7:58

Mark Salsbery

22-Jun-07 7:58

hmm I've found it to be precise. Controlling the image center is easy handled.
I'm not sure what you mean by rotating a DC and ending up with two images - It uses pretty much
the same technique you are using.

Check this out (if you want to Smile | :)

). This spins an image (in a window, so you can watch) around
its center point (like a propellor LOL) at 5 degree increments...

Gdiplus::Bitmap SrcBitmap(L"C:\\test.bmp", FALSE);
 
Graphics DstGraphics(*this); // <code>use an appropriate HWND here</code>
 
REAL angle = 0.0f;
for (int i = 0; i < 200; ++i)
{
   DstGraphics.ResetTransform();
   DstGraphics.RotateTransform(angle);
   DstGraphics.TranslateTransform(SrcBitmap.GetWidth() / 2.0f, SrcBitmap.GetWidth() / 2.0f, MatrixOrderAppend);
   DstGraphics.DrawImage(&SrcBitmap, -((INT)SrcBitmap.GetWidth() / 2), -((INT)SrcBitmap.GetHeight() / 2), 
                 SrcBitmap.GetWidth(), SrcBitmap.GetHeight());
 
   angle += 5.0f;
   if (angle >= 360.0f)
      angle -= 360.0f;
 
   //::Sleep(70);
}

"Posting a VB.NET question in the C++ forum will end in tears." Chris Maunder

Removing Redundant Functions

Adno22-Jun-07 8:20

Adno

22-Jun-07 8:20

[d3m0n]22-Jun-07 2:42

[d3m0n]

22-Jun-07 2:42

I am working on a large codebase on VC++ 6.0, which contains many unused functions. i.e. functions that are never called from anywhere else in the code. I would like to delete these functions.

Without having to run every possible scenario with some sort of code coverage tool, is there any possible way to find these functions? Currently all I can do is do a Find In Files on the function name and work it out.

Thanks for your help...

Cheers
d3m0n

Re: Removing Redundant Functions

zqueezy22-Jun-07 3:25