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Visual C++ 2008 : problems of using public accessors created within a Form1 Class

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TheMerovingian

Thu Jan 29 2009, 11:21AM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

To allow access to properties of Form1 objects i created a public function within the form1 class

public ref class Form1 : public System::Windows::Forms::Form
	{
	public:
		Form1(void)
		{
			InitializeComponent();
			//
			//TODO: aggiungere qui il codice del costruttore.
			//
		}

		String^ GetTextBox1Text()
		{

        return this->textBox1->Text;
		}
... continues

when i'm trying to access to the function

void display ()
{
	String ^st = Form1::GetTextBox1Text();

from another form or a function the compiler flags me an error (C2352) class::function' : illegal call of non-static member function

Why i cannot access the data from outside the Form if a created a public accessor with this purpose?

many thanx in advance

Steve Conner

Thu Jan 29 2009, 03:02PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

This is a classic C++ misunderstanding. You need to instantiate an object before you can call its member functions. The class is just a kind of template for making an object, not the object itself. For example, in the beginning of your program put:

Form1 MyForm; // this creates an object of type Form1, called MyForm

Then you can call

String Mystring = MyForm.GetTextBox1Text(); // this gets the text from MyForm

provided of course that the "MyForm" object is in your scope.

If you declare the member functions static, then you can actually call them without an instance of the object, as in your example code. But this defeats the whole point of OO, since they don't have any object to operate on. I only mention this because it explains the error message you're getting.

If you're trying to call the member functions of one Form1 object from inside another Form1 object, it starts to get messy. You can do it by passing "this" pointers around, but you probably want to avoid it by redesigning your program.

Better still, forget all this OO nonsense, real men use FORTRAN with lots of inline assembler ;)

Zenador

Thu Jan 29 2009, 03:14PM

Registered Member #1733 Joined: Thu Oct 02 2008, 03:17PM
Location: Hamilton, ON, Canada
Posts: 100

Have you "stepped" through the functions and the calls of the functions? Visual Studio has nifty debugging tools.

My first guess (without seeing all the code) would be the text box object on the form cannot be made public. I ran into this with VB all the time. For objects which I needed to access from multiple forms, I have always used variables in the (General) - Declarations. On a focus change within the containing form, the current value would be written to the general variables. This works well if all your forms are loaded on startup and just hide the ones you don't need.

If you would like, I can go through the code... It's a slow day at work...

Steve Conner

Thu Jan 29 2009, 04:04PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

It's C, so you can't step through it if you can't even get it to compile.

I'll restate what I said another way: You are using the "this" pointer in your GetTextBox1Text() member function. But "this" has no meaning until you've created an instance of the object. Just like the spoon in The Matrix.

TheMerovingian

Fri Jan 30 2009, 04:03PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

[quote]
This is a classic C++ misunderstanding. You need to instantiate an object before you can call its member functions. The class is just a kind of template for making an object, not the object itself. For example, in the beginning of your program put:

Form1 MyForm; // this creates an object of type Form1, called MyForm

Then you can call

String Mystring = MyForm.GetTextBox1Text(); // this gets the text from MyForm

provided of course that the "MyForm" object is in your scope.
[/quote1233331447]

It is a different function

Steve McConner wrote ...

If you declare the member functions static, then you can actually call them without an instance of the object, as in your example code. But this defeats the whole point of OO, since they don't have any object to operate on. I only mention this because it explains the error message you're getting.

I know, in fact the "this" pointer isn't available, because there's no object itself

Steve McConner wrote ...

If you're trying to call the member functions of one Form1 object from inside another Form1 object, it starts to get messy. You can do it by passing "this" pointers around, but you probably want to avoid it by redesigning your program.

I tried this route, creating a public function that returns the this pointer but this doesn't work for the same reason , cannot access the function

Steve McConner wrote ...

Better still, forget all this OO nonsense, real men use FORTRAN with lots of inline assembler ;)

I used fortran but c++ is more flexyble and windows related (using .NET 2.0 framework functions)

I dind't explain myself well. The class is created and the object too, in fact Form1 esist , and it is allocated in memory using the (gc)new statement in the "main" function, but the compiler doesn't allow me to access to public member functions of Form1 neither from different Form (Form2) member functions or pure functions (void calculate(void) {blabla} )

Zenador wrote ...

Have you "stepped" through the functions and the calls of the functions? Visual Studio has nifty debugging tools.

My first guess (without seeing all the code) would be the text box object on the form cannot be made public. I ran into this with VB all the time. For objects which I needed to access from multiple forms, I have always used variables in the (General) - Declarations. On a focus change within the containing form, the current value would be written to the general variables. This works well if all your forms are loaded on startup and just hide the ones you don't need.

If you would like, I can go through the code... It's a slow day at work...

Using global pointers or variables defeats all the purpose of Object Oriended Programming, public accessors are cleaner and easier to maintain

I will write a small program stripped from all that doesn't is realated to the error and publish

Edit:
Yeah I see, i'm calling the Member function from the class and not from the object itself...

This is a problem without solution...

Solved this way:

int main(array<System::String ^> ^args)
{

// Attivare gli effetti visivi di Windows XP prima di creare i controlli
Application::EnableVisualStyles();
Application:: SetCompatibleTextRenderingDefault(false);

Form1 ^myform = gcnew Form1();

myform->textBox1->Text = "Vaffanculo";

// Creare la finestra principale ed eseguirla
Application::Run(myform);

return 0;
}

Of course it returns me an illegal access error because TextBox1 is private, but making it public or creating a member function will reset the problem

For course this doesn't solve the problem of accessing one form from another , unless i make the pointers global...

an ugly solution but should work...

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