C / C++ / MFC Discussion Boards

Re: Does anyone know this c plus plus knowledge?

CPallini15-Jan-13 11:30

15-Jan-13 11:30

That code creates (and destroys) a temporary CTempLock object. The net effect is the call of the class constructor and destructor. Try the following code:

C++

#include <iostream>
using std::cout;
using std::endl;

class A
{
public:
    A(){cout << "ctor" << endl;}
    ~A(){cout << "dtor" << endl;}
};

int main()
{
    A();
}

Veni, vidi, vici.

Re: Does anyone know this c plus plus knowledge?

yu-jian16-Jan-13 3:17

yu-jian

16-Jan-13 3:17

It's the same with my code. But i am silly why the c plus plus allow defining a variable without name?

Re: Does anyone know this c plus plus knowledge?

CPallini16-Jan-13 4:04

CPallini

16-Jan-13 4:04

For instance, this allows you writing:

C++

vector <A> v;
v.push_back(A());

Veni, vidi, vici.

Re: Does anyone know this c plus plus knowledge?

yu-jian16-Jan-13 17:44

yu-jian

16-Jan-13 17:44

Do you think that it's right? ~A() will be called after A() immediately.

Re: Does anyone know this c plus plus knowledge?

jschell16-Jan-13 9:10

jschell

16-Jan-13 9:10

yu-jian wrote:
why the c plus plus allow defining a variable without name?

It isn't doing that.

You are confusing 'variable' with 'expession'.

In the following the left 'x' is a varible while the right side is an expression. The execution of the expression is not dependent upon the variable. The variable is just where the result of the expression is stored.

x = doSomething();

Re: Does anyone know this c plus plus knowledge?

JackDingler16-Jan-13 11:19

JackDingler

16-Jan-13 11:19

Your line with "CTempLock (&(this->m_Lock));" will create an unnamed variable of type CTempLock, call the destructor specified, and then call the destructor and destroy the object.

As another poster pointed out, using a constructor reference in this way, is supported as a means to pass temporary objects to a function.

The temporary object only exists on the stack, long enough to make a function call, that is intended to use the temporary object as a reference.

modified 16-Jan-13 17:28pm.

Re: Does anyone know this c plus plus knowledge?

yu-jian16-Jan-13 17:50

yu-jian

16-Jan-13 17:50

Right. Perhaps, it can use this way to do something in the constructor function of CTempLock.

Re: Does anyone know this c plus plus knowledge?

JackDingler17-Jan-13 5:15

JackDingler

17-Jan-13 5:15

Yes.

1. The object is created on the stack.
2. The constructor is called.
3. The destructor is called.
4. The object is then forgotten.

The example you provided is a good demo of how you can introduce locking bugs. Because the object goes out of scope right away, it doesn't really do anything useful.

The proper way to do it is:

C++

CTempLock Lock(&(this->m_Lock));

If you do something like this...

C++

void ProcessLock(const CTempLock & TempLock)
{
  // do stuff to TempLock here....
}

ProcessLock(CTempLock(&(this->m_Lock)));

Then the temporary object will exist for the lifetime of the call.

deque problem. Does this code has any problem? [solved]

yu-jian14-Jan-13 3:47

yu-jian

14-Jan-13 3:47

{
    CTempLock tempLock(&m_FrameListLock);
    if (m_FrameQueue.size() == 0)  return 0L;

    pFrameData = m_FrameQueue.front();
    m_FrameQueue.pop_front();
}

if (WaitForSingleObject(this->m_hCloseEvent, 0) == WAIT_OBJECT_0)
{
    return ZMD_OK;
}

try
{
    if (pFrameData != NULL)
    {
        if (pFrameData->lpData != NULL)
        {
            try
            {
                this->DistributeFrame(pFrameData->lpData, pFrameData->iSize);
                byte*  pBuf = reinterpret_cast<byte*>(pFrameData->lpData);
                delete[] pBuf;
            }
            catch (CException* e)
            {
                e->Delete();
            }
        }
        delete pFrameData;
        pFrameData = NULL;
    }
}
catch (CException* e)
{
    e->Delete();
}

}

XML

There is a error that happens in the following code.
byte*  pBuf = reinterpret_cast<byte*>(pFrameData->lpData);
<b>delete[] pBuf;</b>

Just as the image, here.
https://www.dropbox.com/s/ckqkz88qnifus4g/Error.png
I find that the pFrameData->lpData is a wild buffer point.  I do not know when the buffer is released?  Or this deque has some problem?

The class is here.

XML

class CStreamProcess
{
    HRESULT ProcessFrames();

    void DistributeFrame(const LPVOID const lpBuf, int  dwSize);

    HANDLE  m_hCloseEvent;
    HANDLE  m_hFrameProcessEvent;
    std::deque<SZMDFrameData>   m_FrameQueue;
    CCriticalSection    m_FrameListLock;
};

--------------------
I have resolved this problem. Its a mistake of using CCriticalSection.

modified 15-Jan-13 3:35am.

Re: deque problem. Does this code has any problem?

CPallini14-Jan-13 6:59

CPallini

14-Jan-13 6:59

Quote:
m_FrameQueue.pop_front();

The removed element is destroyed, see, for instance std::deque::pop_front at C++ reference[^].

Veni, vidi, vici.

Re: deque problem. Does this code has any problem?

yu-jian14-Jan-13 13:50

yu-jian

14-Jan-13 13:50

Hi CPallini,

Thank you very much. You help me greatly.

Re: deque problem. Does this code has any problem?

yu-jian14-Jan-13 16:02

yu-jian

14-Jan-13 16:02

Not this problem, I made a test that has a temporary class. CMyClass. When pop_front() the desconstructor function of class CMyClass will not be called.

CDC Problem with coordinates

Argonia14-Jan-13 1:50

Argonia

14-Jan-13 1:50

Hello,
I need to add a string on top / on bottom of a few drown GDI objects on printing. So far i override the OnPrint function.The problem i am facing is to decide where the text should be added in the CDC (i cant find the coordinates needed for TextOut)
I tried with CDC::GetBoundsRect(&dcRect, 0); but in dcRect i get nothing but zeros and the function returns 1. I think its returning DCB_RESET but i am not sure.
I tried with GetCurrentBitmap and GetBitmapDimension but still i get CSize variable with nothing but zeros.
All help is welcome Smile | :)

Re: CDC Problem with coordinates

Argonia14-Jan-13 21:39

Argonia

14-Jan-13 21:39

The best thing i found is in the Microsoft Superpad example on the next link
Superpad
You can see how to put Header / Footer in it.

Re: CDC Problem with coordinates

Vaclav_15-Jan-13 9:53

Vaclav_

15-Jan-13 9:53

Using API In MFC OnDraw

TextOut(pDC->GetSafeHdc(),1, 20, "HELLO HELLO", 11);

I am not sure if that will print alligned properly.

Re: CDC Problem with coordinates

Argonia15-Jan-13 21:23

Argonia

15-Jan-13 21:23

My problem is finding the right coordinates for the TextOut function but after a talk with my Team manager we decided its better to be done as a header / footer.
Before that i just wanted to get the BoundRectangle with the hopes that it will be describing only the part in the page where the elements will be drown not the whole page itself.
I was wondering if there is a way to get out of the CDC member information about the position of the elements or something like that but i couldn't find any.

DhcpRequestParams()

Man in the mirror13-Jan-13 19:46

Man in the mirror

13-Jan-13 19:46

I want to use the function DhcpRequestParams() to get some information from the DHCP Server,but failes.

Any can help me?
Examples are best.

Re: DhcpRequestParams()

Richard MacCutchan13-Jan-13 22:11

Richard MacCutchan

13-Jan-13 22:11

Try http://msdn.microsoft.com/en-us/library/windows/desktop/aa363345(v=vs.85).aspx[^].

One of these days I'm going to think of a really clever signature.

SOLVED From HANDLE to BITMAPINFO - another basic question for gurus

Vaclav_13-Jan-13 5:30

Vaclav_

13-Jan-13 5:30

<pre lang="text">Found this very useful piece of code to get bitmap info from handle and would like to know HOW it works.
I think if I get how the LPBITAMPINFO gets filled I probably will also understand why using global handle is necessary. Or maybe not.</pre>

// a DIB is in the clipboard, draw it out
GLOBALHANDLE hGMem ;
LPBITMAPINFO lpBI ;
void* pDIBBits;
OpenClipboard() ;
hGMem = GetClipboardData(CF_DIB) ;
ASSERT(hGMem);
TRACE("\nfills LPBITMAPINFO");
lpBI = (LPBITMAPINFO)GlobalLock(hGMem) ;

Appreciate any help.
CHeers Vaclav

-- modified 15-Jan-13 15:54pm.

Re: From HANDLE to BITMAPINFO - another basic question for gurus

Richard MacCutchan13-Jan-13 5:44

Richard MacCutchan

13-Jan-13 5:44

C++

// a DIB is in the clipboard, draw it out
GLOBALHANDLE      hGMem ;    // create a global handle
LPBITMAPINFO      lpBI ;     // a pointer to the resultant bitmap
void*             pDIBBits;  // a pointer to the contents of the bitmap

OpenClipboard() ;                   // open the clipboard
hGMem = GetClipboardData(CF_DIB) ;  // get a handle (pointer) to the data in the clipboard
ASSERT(hGMem);                      // make sure it is valid
TRACE("\nfills LPBITMAPINFO"); 
lpBI = (LPBITMAPINFO)GlobalLock(hGMem) ; // lock that memory block and return a pointer to the DIB

At this point you can copy the memory block pointed to by lpBI into your program's address space and process it as required. You should then unlock and release hGMem (which is a system resource), and release the clipboard so other applications can use it.

One of these days I'm going to think of a really clever signature.

Re: From HANDLE to BITMAPINFO - another basic question for gurus

Vaclav_13-Jan-13 6:08

Vaclav_

13-Jan-13 6:08

Richard,
thanks for adding the comments.
I am still unclear how you get from HANDLE (pointer) - returned by GetClipboardData to BITMAPINFO (pointer).

I guess I still do not get the casting.

This may be to stupid , but why this would not work?

lpBI = (LPBITMAPINFO) GetCLipboardData(CF_DIB)

Maybe the key is really in usage of GLOBALHANDLE.

Re: From HANDLE to BITMAPINFO - another basic question for gurus

Richard MacCutchan13-Jan-13 6:32

Richard MacCutchan

13-Jan-13 6:32

Vaclav_Sal wrote:
Maybe the key is really in usage of GLOBALHANDLE.

Exactly so. The GetCLipboardData() function returns a global handle, and you then need to use the GlobalLock() function to get a pointer to addressable memory. Remember that HANDLEs are 'opaque' types which you cannot use directly, even though they may at times point to some real memory. This is because they are owned by the Windows system and their contents at any one time is not guaranteed to be useful in user space.

One of these days I'm going to think of a really clever signature.

How to get detail error info from E_FAIL?

Falconapollo11-Jan-13 21:23

Falconapollo

11-Jan-13 21:23

When I use ATL in MFC Application(Don't ask me why, just because I like), sometimes I got the error code: E_FAIL. However, it's almost useless for me to locate the specific reason.

I have googled so many times, but found nothing related. I thought there should be something like try{} catch{} in ATL.

Here's some sample code:

C++

CAxWindow  m_wndView;  // ActiveX host window class.
CComPtr<IWMPPlayer>  m_spWMPPlayer;  // Smart pointer to IWMPPlayer interface.

AtlAxWinInit();
CComPtr<IAxWinHostWindow>  spHost;
HRESULT  hr;
CRect rcClient;
GetClientRect(&rcClient);
m_wndView.Create(m_hWnd, rcClient, NULL, WS_CHILD | WS_VISIBLE | WS_CLIPCHILDREN, WS_EX_CLIENTEDGE);
//when I switch to Static Link to ATL from Dynamic Link to ATL, I always get E_FAIL
hr = m_wndView.QueryHost(&spHost);

Re: How to get detail error info from E_FAIL?

«_Superman_»11-Jan-13 23:43

«_Superman_»

11-Jan-13 23:43

Use the _com_error class.
Construct an object of this class by passing in the HRESULT value and then use its WCode or ErrorMessage methods to get the error code mapped to the HRESULT value.

«_Superman_»
I love work. It gives me something to do between weekends.

Microsoft MVP (Visual C++) (October 2009 - September 2013)

Polymorphism in C

Re: How to get detail error info from E_FAIL?

Falconapollo12-Jan-13 0:00

Falconapollo

12-Jan-13 0:00

Thank you for your reply.

I have did as what you said, but I only get the error message:
Unspecified error.

Here is my code:

C++

_com_error err(hr);
auto  d = err.WCode();
auto s = err.Description();
auto msg = err.ErrorMessage();//Unspecified error

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