Managed C++/CLI

2-Jun-05 8:34

Now I get the error;
Just-In-Time Debugging
System.NullReferenceException

This is how I want it:

lblQ->Text = myQuestions[CurrentQuestion]->getQuestion()

myQuestions is a pointer class (declared: Question myQuestions __nogc[CurrentQuestion]; ) and getQuestion returns the string theQuestion. lblQ is btw just an Label (Label *lblQ)

and also I just tried two ways:
1:
lblQ->Text = new System::String(myQuestions[CurrentQuestion]->getQuestion();.c_str());

2:
myQuestions[CurrentQuestion]->setQuestion("hi");
string temp = myQuestions[CurrentQuestion]->getQuestion();
lblQ->Text = new System::String(temp.c_str());

Both gave the Just-In-Time debugger error.

Best Regards,
Hmmkk

Christian Graus2-Jun-05 10:02

2-Jun-05 10:02

myQuestions[CurrentQuestion] obviously == NULL ??? Is it a pointer and it needs to be allocated with new ?

Christian Graus - Microsoft MVP - C++

Hmmkk2-Jun-05 10:25

2-Jun-05 10:25

Yes, I guess your quite right there.
I bet there is some major error in the code that could be much better done=s But after all this is just a test program that I'm making.
To give you some insider/background info about the program:
I'm trying to create a Quiz program. I create by a class MainForm that inherits Form (the system one). The MainForm consists pretty much only by a previous and next button for CurrentQuestion++ or -- then call UpdateScreen, where it's suppose to update the Label lblQs text to the "theQuestion" string in a Question class, which shall store questions and answers.

Is there anyway you could watch through my short code or do you have any idea of a sollution?

Best Regards,
Hmmkk

Christian Graus2-Jun-05 10:30

2-Jun-05 10:30

Hmmkk wrote:
But after all this is just a test program that I'm making.

Then it's a good chance to learn how to do things well Smile | :)

There's never a reason to write sloppy code.

Having said that, this code doesn't look that sloppy to me, it's just a simple mistake, you're trying to access something and you need to check that it exists first, and create it if it doesn't and it needs to.

Hmmkk wrote:
Is there anyway you could watch through my short code or do you have any idea of a sollution?

At this stage, I don't even know what your list contains. HOwever, the solution is that you need to create an object to sit at the position you're trying to access, before you try to access it, as I said initially.

Christian Graus - Microsoft MVP - C++

Hmmkk2-Jun-05 10:38

2-Jun-05 10:38

//Object creation
static const int MAX_QUESTIONS = 10;
int CurrentQuestion;
Question *myQuestions[MAX_QUESTIONS];

//Some of the Question class
class Question
{
public:
Question();
~Question();
void setQuestion(string Question) {theQuestion = Question;}
string getQuestion() { theQuestion = "hi"; return theQuestion.c_str();}
string getClue() { return theClue;}

private:
string theQuestion;
};

//The update method
void MainForm::UpdateScreen()
{
lblStatus->Text = S"Updating";
lblQ->Text = new System::String(myQuestions[CurrentQuestion]->getQuestion().c_str());
}

I bet there's something wrong about the declaration of object or?

Best Regards,
Hmmkk

Christian Graus2-Jun-05 10:46

2-Jun-05 10:46

Hmmkk wrote:
I bet there's something wrong about the declaration of object or? 

No, it's exactly what I said it was

Hmmkk wrote:
Question *myQuestions[MAX_QUESTIONS];

You have an array of pointers. That means you have MAX_QUESTIONS ( currently 10 ) memory addresses in a row, and the compiler knows that you expect these memory addresses to point to instances of the Question class. However, they will point to random areas of memory until you reassign them. Given that your array is fixed, I'd be inclined to drop the *, and make it an array of Questions, not pointers. If you must use pointers, then you need an initialisation loop that does either of these:

for(int i =0;i<MAX_QUESTIONS;++i)
{
myQuestions[i] = new Question();
}

which will fix your problem, but gives you no advantage over not using pointers, or

for(int i =0;i<MAX_QUESTIONS;++i)
{
myQuestions[i]= NULL;
}

Now you can create questions as you need them, by checking first if the value is NULL ( but if you want to create a dynamically sized list, you should use a vector instead ).

Either way ( even if you use a vector ) your program will leak memory unless at the end you step through the list and call delete on each object.

Christian Graus - Microsoft MVP - C++

Hmmkk2-Jun-05 10:48

2-Jun-05 10:48

Yay!=) I actually made it work now, at least I think so. Instead of initiating a pointer to the class, I use normal creation.
Question myQuestions[MAX_QUESTIONS];
and also static is gone.
So I guess I was right about being "gone with the wind" with the Object part, but how would it look like if I used it as a pointer? How to make it work?
Thanks alot btw!

Best Regards,
Hmmkk

Hmmkk2-Jun-05 10:57

2-Jun-05 10:57

Oki, you answered the question even before I could post=)
Thanks alot, now I know how to initialize pointer objects too=)
You are a really kind man, sir. Thanks!

Best Regards,
Hmmkk

Christian Graus2-Jun-05 12:18

Passing data between forms

2-Jun-05 12:18

No worries - glad to help.

Christian Graus - Microsoft MVP - C++

richiemac1-Jun-05 8:58

1-Jun-05 8:58

I have two forms (Form1 and Form2). Form1 will create an instance of Form2. After some processing certain elements from Form2 will be passed back to Form1. I think I know what I need todo (roughly) but I have a problem including each Form header in the other. I don't think this is possible (is it?) so how do I go about creating instances of each form if I can't include their headers?

I know there was a similar post on this but it didn't answer my specific question.

Thanks in advance.

Saksida Bojan1-Jun-05 19:40

Saksida Bojan

1-Jun-05 19:40

If you are including the same header to the forms, instead using #pragma once
use Old Defenition style Example:

#ifndef _CORE_H_
#define _CORE_H_

public __gc class Core
{
//To Do:
};

#endif //_CORE_H_

With this you can use it from anywhere and you can include header into any file. Or multiple file. you will not get redefenition error

Lagwagon561-Jun-05 10:12

Lagwagon56

1-Jun-05 10:12

I had a similar problem. I just created a class that I pointed both forms to and used it to pass information between the forms.
So creat a new __gc class. From form1 create an object of that class. then point it to form2's object.
Pass is the class i'm using to pass information.

<br />
//Form1.h<br />
Pass *pass = new Pass();<br />
Form2 *input = new Form2(); <br />
input->pass1 = pass;<br />
input->ShowDialog();<br />
<br />
label_InputSomthing->Text = pass->somthing;<br />
Invalidate();<br />
<br />
//Form2.h<br />
Pass *pass1;<br />
pass1->something = textBox_something->Text;<br />
<br />
Close();<br />

richiemac1-Jun-05 21:32

1-Jun-05 21:32

Thaks guys I'll give it a go.

Saksida - will the old style definitions go inside the namespace tag or outside. e.g

#ifndef _CORE_H_
#define _CORE_H_
namespace test
{ ... }
#endif

or

namespace test
{
#ifndef _CORE_H_
#define _CORE_H_
...
#endif
}

richiemac1-Jun-05 22:37

1-Jun-05 22:37

Do I need to use the #indef definition in both forms or jsut the main one?? If I use it in both I get errors.

error C2143: syntax error : missing ';' before '*'
error C2501: 'test::Form1::form2' : missing storage-class or type specifiers
error C2501: 'test::Form1::f2' : missing storage-class or type specifiers

When I try to declare an instance of form2.

If I use it in the main form only I get only one error.

error C2011: 'test::form2' : 'class' type redefinition

I could do with some help here. I'm not very experienced with c++ let alone managed c++. Been on this for a couple of days and nothing I try wants to work.

Lagwagon562-Jun-05 4:27

Lagwagon56

2-Jun-05 4:27

Make sure to #include both your passing class and form2 in your form1.

Saksida Bojan2-Jun-05 20:10

Saksida Bojan

2-Jun-05 20:10

Yes you need to use but not #indef but #ifndef

And of course you need outside namespace. This means it must be in the beginning of file #ifndef and #define

And on the end it must be #endif

If you realy want to have an example then download this:
http://users.volja.net/bsaksida/CDDVD Organizer.zip[^]

Look under Core Folder, you will see CDDVDcore.h and you will see how has been defined. this program verzion is 0.1 there are a lot of bugs. Don't wory this program will be released as opensource.

richiemac5-Jun-05 5:34

5-Jun-05 5:34

Thanks for the help guys. I've written a little test prog that I've managed to get going with your help.

Thanks again. Smile | :)

panel component problems

richiemac31-May-05 6:44

Re: panel component problems

31-May-05 6:44

Hi all,

I'm trying to create a few panels in a VC++.net application. I need the panels to be visible on a number of screens and I don't want to reproduce the same components many times over. When I run my app the panel is only visible in one screen only no matter what I do.

I'm doing something along the lines of

<br />
private: System::Void application_menuItem_Click(System::Object *  sender, System::EventArgs *  e)<br />
		 {<br />
			 this->application_panel->Visible = true;<br />
			 this->low_level_panel->Visible = false;<br />
		 }<br />

which works for one screen but not if I select any other screen.

Any ideas gratefully recieved.

Christian Graus31-May-05 13:49

Re: panel component problems

31-May-05 13:49

What do you mean by a screen ? A form ? How does this panel get to be on these forms ?

Christian Graus - Microsoft MVP - C++

richiemac31-May-05 21:47