C / C++ / MFC

getting pixel(s) directly from the screen

30-Mar-08 3:38

Hello everyone,

Two basic questions about function invocation from STA to MTA.

1.

In my understaning, the owning thread of STA should not touch MTA component, and also in MTA there is no message queue for MTA owning thread to pumping messages.

My question is, if the owning thread of STA calls MTA component (by interface pointer), which thead is responsible for executing in MTA? From my above analysis, seems both STA owner thread (can not touch MTA component) and MTA owner thread (can not receive calls from STA through message queue, snice no message queue in MTA.)

2.

Before function call returns from MTA, will STA thread waiting or it could execute in asynchronous way?

thanks in advance,
George

shja8830-Mar-08 3:26

shja88

30-Mar-08 3:26

I need to get the color from a specific pixel on the screen.
this pixel is not a part of my console application.

my basic method is:

keybd_event(VK_SNAPSHOT,0x9e, 0, 0); //pressing print screen
keybd_event(VK_SNAPSHOT,0x9e, KEYEVENTF_KEYUP, 0); //releasing print screen

if(OpenClipboard(NULL))
{
    HBITMAP hClipboardBmp = (HBITMAP) GetClipboardData(CF_BITMAP);
}

but i wonder if theres a way to get that pixel(s) directly from the screen.

thank you very much for your time and effort Smile | :)

Re: getting pixel(s) directly from the screen

CPallini30-Mar-08 3:53

CPallini

30-Mar-08 3:53

HDC hdc = GetDC(NULL);
COLORREF cr = GetPixel(hdc, x,y);
ReleaseDC(hdc);

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.

This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke

Re: getting pixel(s) directly from the screen

shja8830-Mar-08 4:13

shja88

30-Mar-08 4:13

it's perfect, thank you very much! Big Grin | :-D

Re: getting pixel(s) directly from the screen

Hamid_RT7-Apr-08 2:31

Hamid_RT

7-Apr-08 2:31

It seems you need toget pixel of a bmp file that you read of clipboard,right?

auto_ptr assignment compile error

George_George30-Mar-08 0:51

30-Mar-08 0:51

Hello everyone,

I posted the code and related compile error below. My analysis below,

1. I think the reason of compile error is, the copy constructor of Goo accepts const reference as input parameter, so the pf member of input parameter is also treated const?
2. And since auto_ptr does not have a copy constructor which accepts const reference auto_ptr as input parameter, the compile error occurs.

My analysis (1) and (2) are both correct?

What makes me confused is it seems the compile error message does not reflect my analysis (1) and (2) above. Any ideas?

#include <memory>

using namespace std;

// Foo is Pimpl class for Goo, suppose.
class Foo {

};

class Goo {

public:

	auto_ptr<Foo> pf;

	Goo (const Goo& input)
	{
		this -> pf = input.pf;
	}
};

1>d:\visual studio 2008\projects\test0330\test0330\main.cpp(18) : error C2679: binary '=' : no operator found which takes a right-hand operand of type 'const std::auto_ptr<_Ty>' (or there is no acceptable conversion)
1>        with
1>        [
1>            _Ty=Foo
1>        ]
1>        d:\program files\microsoft visual studio 9.0\vc\include\memory(689): could be 'std::auto_ptr<_Ty> &std::auto_ptr<_Ty>::operator =<Foo>(std::auto_ptr<_Ty> &) throw()'
1>        with
1>        [
1>            _Ty=Foo
1>        ]
1>        d:\program files\microsoft visual studio 9.0\vc\include\memory(701): or       'std::auto_ptr<_Ty> &std::auto_ptr<_Ty>::operator =(std::auto_ptr<_Ty> &) throw()'
1>        with
1>        [
1>            _Ty=Foo
1>        ]
1>        d:\program files\microsoft visual studio 9.0\vc\include\memory(707): or       'std::auto_ptr<_Ty> &std::auto_ptr<_Ty>::operator =(std::auto_ptr_ref<_Ty>) throw()'
1>        with
1>        [
1>            _Ty=Foo
1>        ]
1>        while trying to match the argument list '(std::auto_ptr<_Ty>, const std::auto_ptr<_Ty>)'
1>        with
1>        [
1>            _Ty=Foo
1>        ]

thanks in advance,
George

CPallini30-Mar-08 1:33

CPallini

30-Mar-08 1:33

IMHO your analysis is correct (both point 1 and 2. However, with regard to point 1, my interpretation is: since Goo copy constructor declares its argument const, then its content cannot be modified, but assigment operator of auto_ptr modify its argument, hence the problem).

George_George wrote:
What makes me confused is it seems the compile error message does not reflect my analysis (1) and (2) above. Any ideas?

I think that (it's just an hypothesis), since no auto_ptr copy constructor can match the this -> pf = input.pf;, the compiler tries to search for an assignment operator match, but it fails again and gives such error.

Smile | :)

George_George30-Mar-08 2:38

30-Mar-08 2:38

Cool, CPallini!

1.

CPallini wrote:
I think that (it's just an hypothesis), since no auto_ptr copy constructor can match the this -> pf = input.pf;, the compiler tries to search for an assignment operator match, but it fails again and gives such error.

You mean compiler search for all forms of assignment operator of auto_ptr, then no one matches so the compile errors come?

2.

The source of error is because the basic C++ ruls is violated -- can not assign const reference (input.pf) to non-const reference (input parameter of assignment operator of class auto_ptr)?

regards,
George

CPallini30-Mar-08 2:43

CPallini

30-Mar-08 2:43

George_George wrote:
1.

CPallini wrote:
I think that (it's just an hypothesis), since no auto_ptr copy constructor can match the this -> pf = input.pf;, the compiler tries to search for an assignment operator match, but it fails again and gives such error.

You mean compiler search for all forms of assignment operator of auto_ptr, then no one matches so the compile errors come?

Yes, I think so.

George_George wrote:
2.

The source of error is because the basic C++ ruls is violated -- can not assign const reference (input.pf) to non-const reference (input parameter of assignment operator of class auto_ptr)?

Yes.

George_George30-Mar-08 2:49

the debug problem in VC++

30-Mar-08 2:49

Thanks CPallini,

Question answered! Cool!!

regards,
George

wendyyue29-Mar-08 23:19

wendyyue

29-Mar-08 23:19

say i have a function int fun(int *a),

and other function int fa(); int fb(), int fc(),....int fn();

all the them will call the fun(int *a),

but the problem is, when I run the program, the vc++ tells me some of them pass the NULL pointer to the fun(int *a) which make mistake, but could not tell me which function calls the fun(int *a) and then pass the NULL pointer to it,

so dear guys, how could I know which one call the fun(int *a) that pass the NULL pointer?

Thanks.

Cedric Moonen29-Mar-08 23:51

Cedric Moonen

29-Mar-08 23:51

I didn't really understand your problem. Do you have a crash and would like to find the problem ? If yes, did you use your debugger to track the problem ? Using the callstack you'll be able to find which function passed a NULL pointer.

Cédric Moonen
Software developer

Charting control [v1.3]

rp_suman30-Mar-08 1:06

rp_suman

30-Mar-08 1:06

Hi,

If your program stops with runtime error, as Cedric said, use the "call stack" window and see which one of fa(), fb().. fn() is called just before fun().

Or try using only one function from fa(), fb() ... fn() at a time
and find out.

If you have problem still post the relevant code snippet.

Best Regards,
Suman

--
"Programming is an art that fights back!"

Saurabh.Garg30-Mar-08 1:59

Saurabh.Garg

30-Mar-08 1:59

Well one way like others said, if you are programming in Visual Studio, you can set break point and check the call stack when program crashes.

Another way is to do it like this:

int fa()<br />
{<br />
    int* a;<br />
    .<br />
    .<br />
    .<br />
    if(a == NULL)<br />
    {<br />
        cout << "fa is the culprit";<br />
    }<br />
    fun(a);<br />
}

-Saurabh

David Crow30-Mar-08 2:54

David Crow

30-Mar-08 2:54

Use the debugger to set a breakpoint in fun(). When a equals NULL, check the call stack.

"Love people and use things, not love things and use people." - Unknown

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