Drawing from Object

I'm trying to create a separate object from my program ('trackerObj.h') which I can use in my main window.

I want to be able to draw a rectangle for each new trackerObj.

For example,

Code:

---

void CVisualAuidoView::OnLButtonDown(UINT nFlags, CPoint point)
{
      trackObj *mytrack = new trackObj();
}

---

my trackObj class is:

Code:

---

#pragma once

#include "stdafx.h"

class trackObj : public CObject
{
public:
        //Construction
        trackObj();

public:
        //Attributes
        CRectTracker m_tracker;
protected:
        CWnd* m_pWnd;

public:
        //Operations
        void drawMyTracker(int t);

public:
        //Implementation
        virtual ~trackObj();
};

trackObj::trackObj()
{
        int t = 60;
        drawMyTracker(t);
}

trackObj::~trackObj()
{
}

void trackObj::drawMyTracker(int t)
{
        CClientDC dc(m_pWnd);
        CClientDC* pDC=&dc;
        pDC->MoveTo(t, 20);
        pDC->LineTo(60, 122);
        pDC->LineTo(264, 122);
}

---

The program starts to run, but then gives me a dialog which says:

Code:

---

Unhandled exception at 0x009a1c39 in VisualAuido.exe: 0xC0000005: Access violation reading location 0xcdcdcded.

---

Could someone give me a hand with drawing from other classes - I can't make an OnDraw method so no idea how to go about this!

Cheers
Gareth

What does the debugger tell/show you during the crash ?

Quote:

---

Originally Posted by Skizmo

What does the debugger tell/show you during the crash ?

---

Well it looks as if its going to run, then I get a dialog with the message.

wingdi.cpp opens (which I've never seen before)

and I have a little yellow aarow on the left pointing at the line starting "ASSERT" in the code below:

Code:

---

CClientDC::CClientDC(CWnd* pWnd)
{
        ASSERT(pWnd == NULL || ::IsWindow(pWnd->m_hWnd));

        if (!Attach(::GetDC(m_hWnd = pWnd->GetSafeHwnd())))
                AfxThrowResourceException();
}

---

The "Output" window (which is set to "Show output from: Debug") shows:

Code:

---

'VisualAuido.exe': Loaded 'C:\Users\Gareth\Documents\Visual Studio 2008\Projects\VisualAuido\Debug\VisualAuido.exe', Symbols loaded.
'VisualAuido.exe': Loaded 'C:\Windows\System32\ntdll.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\kernel32.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\KernelBase.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\user32.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\gdi32.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\lpk.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\usp10.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\msvcrt.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\comdlg32.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\shlwapi.dll'
'VisualAuido.exe': Loaded 'C:\Windows\winsxs\x86_microsoft.windows.common-controls_6595b64144ccf1df_6.0.7000.0_none_ee2c028959bec753\comctl32.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\shell32.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\winspool.drv'
'VisualAuido.exe': Loaded 'C:\Windows\System32\advapi32.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\rpcrt4.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\oledlg.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\ole32.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\oleaut32.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\imm32.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\msctf.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\avgrsstx.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\dui70.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\uxtheme.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\cryptbase.dll'
'VisualAuido.exe': Loaded 'C:\Windows\System32\dwmapi.dll'
First-chance exception at 0x009a1c39 in VisualAuido.exe: 0xC0000005: Access violation reading location 0xcdcdcded.
Unhandled exception at 0x009a1c39 in VisualAuido.exe: 0xC0000005: Access violation reading location 0xcdcdcded.

---

If you need any more info please ask - I'll be watching this thread all night! :D

Cheers
Gareth

Quote:

---

Originally Posted by gareth.h.rees

If you need any more info please ask - I'll be watching this thread all night! :D

Cheers
Gareth

---

Nobody needs any more information, including you. The answer is directly in your post:

Code:

---

ASSERT(pWnd == NULL || ::IsWindow(pWnd->m_hWnd));

---

One of the two conditions is NOT true.

Quote:

---

Originally Posted by TheCPUWizard

Nobody needs any more information, including you. The answer is directly in your post:

Code:

---

ASSERT(pWnd == NULL || ::IsWindow(pWnd->m_hWnd));

---

One of the two conditions is NOT true.

---

Sorry,

I'm pretty new to C++, and VERY new to MFC/Visual Studio...

I've tried using all sorts in the CClient dc( ____ ); code...

In most examples I've seen, 'this' has been in the brackets.

For example, when I do:

Code:

---

void trackObj::drawMyTracker(int t)
{
        CClientDC dc(this);
        //CClientDC* pDC=&dc;
        dc.MoveTo(t, 20);
        dc.LineTo(60, 122);
        dc.LineTo(264, 122);       
}

---

I get the error:

trackobj.h(38) : error C2664: 'CClientDC::CClientDC(CWnd *)' : cannot convert parameter 1 from 'trackObj *const ' to 'CWnd *'


I have no idea how to get around this :(

Cheers
Gareth

Quote:

---

Originally Posted by gareth.h.rees

trackobj.h(38) : error C2664: 'CClientDC::CClientDC(CWnd *)' : cannot convert parameter 1 from 'trackObj *const ' to 'CWnd *'

---

Why do you think you should be able to convert a CONSTANT tackObj into a modifiable CWnd???

Quote:

---

Originally Posted by TheCPUWizard

Why do you think you should be able to convert a CONSTANT tackObj into a modifiable CWnd???

---

To be fair, I don't really have that much of an idea what that means :S

I understand that C++ has drawing areas, but unlike Java, it appears that its harder to draw from any method?

I'm just trying to figure out how to create a new object that includes drawing code, but alas.. not quite sure how to go about it??

Cheers
Gareth

You are missing some VERY VERY basic C++ knowledge. You are not nearly to the point of writing this type of application if you do not understand casting and const-correctness.

Simply put:

If you tell the compiler that you are not allowed to modify something, then attempt to say that you can, the compiler stops you.

So, DONT try to lie to the compiler, DONT try to outsmart the compiler.

DO, get a good C++ text book, and start at page #1, reading every word, and typing in EVERY line of code, and stepping through EVERY line of code with your debugger.

When you finish the book in a few weeks (assuming several hours per day dedicated to the task), you can look at this project, and immediately see the problem.

Quote:

---

I will teach any man to fish, but nobody is going to eat the fish I caught.

---

Quote:

---

Originally Posted by TheCPUWizard

You are missing some VERY VERY basic C++ knowledge. You are not nearly to the point of writing this type of application if you do not understand casting and const-correctness.

Simply put:

If you tell the compiler that you are not allowed to modify something, then attempt to say that you can, the compiler stops you.

So, DONT try to lie to the compiler, DONT try to outsmart the compiler.

DO, get a good C++ text book, and start at page #1, reading every word, and typing in EVERY line of code, and stepping through EVERY line of code with your debugger.

When you finish the book in a few weeks (assuming several hours per day dedicated to the task), you can look at this project, and immediately see the problem.

---

I appreciate that I don't have a good knowledge of C++. It's part of my final year university project, and since it was a 'set' project I assumed the project supervisor would be able to help me out when I got stuck. When I started coding I found he didn't know much about Visual C++ either, so I was pretty much thrown right in the deep end!!

I've done most of the Sams Teach Yourself Visual C++ in 21 days but a lot of the examples were difficult to get to work - possibly because I'm using VS2008, rather than 2005?

Either way, I've had to try to push on as best I can with the project rather than spending forever reading books - I *had* to try to make progress.

I know if I had plenty of time that going through books would be the best way to learn, but I'm a tight deadline - I have 7 weeks to do the project AND a project report.

I don't want to sound impatient - I'm just convinced this is an easy thing to do so don't want to spend hours and hours on it since I've got much more to do after this.

Thanks
Gareth

Quote:

---

Originally Posted by gareth.h.rees

I've done most of the Sams Teach Yourself Visual C++ in 21 days but a lot of the examples were difficult to get to work - possibly because I'm using VS2008, rather than 2005?

---

There should not be any issues VS-2008/2005 related. However, if you just "skimmed" the book, I am not suprised if you had difficulty.

That book requires following the advice I gave earlier (read everything, type in everything, debug everything). It is very easy in this (or any other book) to mis a single paragraph or even sentance that is critical to understanding the following information.

For the people I know who have followed the book successfully, they usually spend 6-8 hours per chapter.

ps: Public Disclaimer: I was the technical editor of that book for some editions, and the Author is also here on CG...

Quote:

---

Originally Posted by TheCPUWizard

There should not be any issues VS-2008/2005 related. However, if you just "skimmed" the book, I am not suprised if you had difficulty.

That book requires following the advice I gave earlier (read everything, type in everything, debug everything). It is very easy in this (or any other book) to mis a single paragraph or even sentance that is critical to understanding the following information.

For the people I know who have followed the book successfully, they usually spend 6-8 hours per chapter.

ps: Public Disclaimer: I was the technical editor of that book for some editions, and the Author is also here on CG...

---

I think its more the layout of the IDE that makes it confusing - VS2008 seems to have a lot of differences.

I got through all of chapter one, and most of chapter two - the later pages didn't seem to be much about graphics.

Is there an easy fix to my problem? As mentioned, I really need to get things done. I'd like to try to understand what I'm doing a lot more, and I'm sure it would help in the future. However, I have other modules all with coursework/tests, as well as two part time jobs, so I just physically don't have the time to really read through books when they may or may not help.

Cheers
Gareth

Your problem has NOTHING to do with Graphics, or even with with Windows programming. It is simple basic flaws in your logic.

For example, given the posted code (btw: you should ALWAYS post a minimal yet complete - compilable, runnable - that duplicates the error...)

Your class has an m_pWnd which you are using, but I do not see any place where it is initialized.

Then later you try to treat your object as if it WAS a CWnd, but it clearly is not.

Quote:

---

Originally Posted by TheCPUWizard

Your problem has NOTHING to do with Graphics, or even with with Windows programming. It is simple basic flaws in your logic.

---

Sorry, as mentioned - I know I don't know an awful lot about C++. I'm not trying to be wrong, theres just a lot to learn

Quote:

---

Originally Posted by TheCPUWizard

For example, given the posted code (btw: you should ALWAYS post a minimal yet complete - compilable, runnable - that duplicates the error...)

---

My code is several files long - I can post it all if required, but figured the bits I've posted are the main parts? If you need any more please do ask - I'm guessing that experienced programmers will know more what they're looking for than me..

Quote:

---

Originally Posted by TheCPUWizard

Your class has an m_pWnd which you are using, but I do not see any place where it is initialized.

---

I put it near the top of the trackObj.h as per an example I found on the net

Code:

---

public:
        //Attributes
        CRectTracker m_tracker;
protected:
        CWnd* m_pWnd;

---

Quote:

---

Originally Posted by TheCPUWizard

Then later you try to treat your object as if it WAS a CWnd, but it clearly is not.

---

Not intentional - just not that sure on how to use these drawing/window methods?

How do I pass the CWnd to the new object class in order to draw?

Cheers
Gareth

1) If you work with minimal code, it will never be long. Your entire problem can be reproduced in under 20 lines.

2) Go back and re-read what I said about assigbning a value to m_pWnd!!!!!

3) To pass a CWnd - You create a new CWnd or gain access to an existing one.

I posted my entire trackObj class - however, the visual studio project wizard created various things (ie, the Doc, View and MainFrame)...

Okay - I can see I haven't actually assigned anything to m_pWnd. I thought it was just a built in variable.

I noticed I can do CVisualAuidoView::GetDC and ..::GetActiveWindow.

Is that what I'm looking for?

I'd imagine the GetDC would work?

Quote:

---

Originally Posted by gareth.h.rees

Okay - I can see I haven't actually assigned anything to m_pWnd. I thought it was just a built in variable.?

---

Where in the book did you EVER see the term "built in variable"???

Quote:

---

Originally Posted by TheCPUWizard

Where in the book did you EVER see the term "built in variable"???

---

"Member Variable" was probably what I meant to say - ie, one that is pre-defined for you?

Quote:

---

Originally Posted by gareth.h.rees

"Member Variable" was probably what I meant to say - ie, one that is pre-defined for you?

---

Nothing is "PRE-DEFINED", you can easily see that it is EXPLICITLY declared in the heater file as a completely uninitialized pointer!!!

Where does the INITIALIZATION come from??????

Quote:

---

Originally Posted by TheCPUWizard

Nothing is "PRE-DEFINED", you can easily see that it is EXPLICITLY declared in the heater file as a completely uninitialized pointer!!!

Where does the INITIALIZATION come from??????

---

I tried to use this to base my object on :

http://www.codeguru.com/forum/archiv.../t-262641.html

I thought that I was declaring a pointer to the Window of where the object was called from (ie, my View class).

Quote:

---

Originally Posted by TheCPUWizard

Where does the INITIALIZATION come from??????

---

Do you mean where was m_pWnd first defined? That, I do not know.

Would it be better for me to use ::GetDC as mentioned before?

Quote:

---

Originally Posted by gareth.h.rees

Do you mean where was m_pWnd first defined? That, I do not know.

---

No, I mean show me the line of code where a value is WRITTEN into m_pWnd to initialize it.

Quote:

---

Originally Posted by TheCPUWizard

No, I mean show me the line of code where a value is WRITTEN into m_pWnd to initialize it.

---

Okay - I've got that bit wrong.

There isnt.

Quote:

---

Originally Posted by gareth.h.rees

I tried to use this to base my object on :

http://www.codeguru.com/forum/archiv.../t-262641.html

I thought that I was declaring a pointer to the Window of where the object was called from (ie, my View class).

---

I hate to break this to you, but you can't get a C++ project done with this type of attempt of learning both the C++ language and Windows programming. Regardless of what it seems, the type of program you're attempting to write assumes you know C++ already to the point of understanding fully or (close to fully) classes, pointers, objects, etc.

As TheCPUWizard pointed out, a lack of understand of C++ (forget about graphics) is the issue. All member variables, whether they are pointers to CWnd, pointers to doubles, or pointers to anything you want, must be initialized before using them (i.e. dereferencing them).

Quote:

---

Do you mean where was m_pWnd first defined? That, I do not know.

---

Not knowing what your own program is doing doesn't bode too well for your assignment...

Quote:

---

Would it be better for me to use ::GetDC as mentioned before?

---

And what is the window handle (HWND) you will be using as the argument to this function?

Regards,

Paul McKenzie

Yeah - tell me about it hah! When I started this project I assumed my supervisor would be able to guide me in learning C++ while I do this project. I've done a little in a module, but we've mostly been taught Java.

I appreciate that my whole approach is wrong, but theres not much I can do about it now really - I *have* to make some attempt at getting this done.

The main part of my project is moving shapes around - I've done this with the help of the "tracker" example, but I want to be able to create more than one shape to move around so I thought the best way would be to use objects.

Once I figure out the drawing stuff I should be able to progress a little - though no doubt find a whole new bunch of problems.

Like I said, I know its a rubbish way of going about it, but not even my project supervisor.. who is a lecturer in the University and set the project!!!! knows how to do this..

Gareth

hmm anyone? :(

You might want to check out the DRAWCLI sample from MSDN. It uses drawing objects with their own drawing routines and has tracker rects for selecting and moving, bu it is a pretty complicated project.

http://msdn.microsoft.com/en-us/library/9t94971d.aspx

Quote:

---

Originally Posted by hoxsiew

You might want to check out the DRAWCLI sample from MSDN. It uses drawing objects with their own drawing routines and has tracker rects for selecting and moving, bu it is a pretty complicated project.

http://msdn.microsoft.com/en-us/library/9t94971d.aspx

---

Thanks for that. I have had a look at it previously but couldn't make much sense of it, but will look again now I understand the tracker better.

Perhaps if someone could explain in English what I need to do, I'll be able to code it myself..?

Cheers
Gareth

Quote:

---

Originally Posted by gareth.h.rees

Perhaps if someone could explain in English what I need to do, I'll be able to code it myself..?

---

To draw on a window, you need the windows device context, which means you must have a valid HWND that represents that window to get the device context. A CWnd is nothing more than an MFC class that wraps an HWND.

Your original code created a class that has no relationship at all with anything to do with a window or an HWND, but you tried to make it a CWnd by casting. In effect, you were trying to turn an concrete block into gold. Given this, the obvious explanation that is brief enough is to make sure you have a window handle so that you can get its device context.

Other than that, no message board is going to give you an in-depth explanation of MFC, Windows, HWND's or CWnd's. That is in the realm of books and sample programs.

Regards,

Paul McKenzie

Quote:

---

Originally Posted by gareth.h.rees

Perhaps if someone could explain in English what I need to do, I'll be able to code it myself..?

Cheers
Gareth

---

A choice....

1) Spend a few weeks/months properly learning what you need to know.
2) Pay someone who has invested the requisite time.

Since this is (I believe) a school assignment, option #2 would be considered cheating.

A final alternative would be to ask the instructor (in writing) to provide (again in writing) references to the material that was covered in the class [or is documented as a pre-requisite for the class] that would provide you with sufficient information to know how to properly program with MFC classes (or even Win32 API calls) ....

Quote:

---

Originally Posted by Paul McKenzie

Other than that, no message board is going to give you an in-depth explanation of MFC, Windows, HWND's or CWnd's. That is in the realm of books and sample programs.

---

Thanks for the help so far Paul - even though I'm not really getting anywhere I do feel I'm beginning to understand a little more.

I appreciate that its probably not "the done thing" to code things for people, but all I was after was a quick and rough example of how to draw from a separate object - not an in depth tutorial.

I'd love to actually understand what I'm doing a lot more... but alas - that won't actually go that far to help me pass my degree. Counter-intuitive, I know!

Cheers
Gareth