update statusbar's text property in the form by another form's button's click event

Question

Hi I am unable to update the status bar's text property in the form by another form's button click. It is successfully compiled and able to run until I clicked The error I got is "Access violation at address: XXXXXXXX....."

C++ Builder XE7 in Win 7 is used.

Form1:

#include "Main.h"
#include "minor.h"
....
TForm1 *Form1;
TForm2 *Form2;
....

Form2:

#include "minor.h"
#include "Main.h"
....
TForm2 *Form2;
TForm1 *Form1;
....
__fastcall TForm2::TForm2(TComponent* Owner)
: TForm(Owner)
{
TButton* btnTest2 = new TButton(this);
btnTest2->Height = 50;
btnTest2->Width = 200;
btnTest2->Left = 220;
btnTest2->Top = 50;
btnTest2->Caption = "Updated Statusbar Button";
btnTest2->Visible = true;
btnTest2->Enabled = true;
btnTest2->Parent = this;
btnTest2->OnClick = &ButtonClicked2; // Create your own event here manually!
}
//---------------------------------------------------------------------------

void __fastcall TForm2::ButtonClicked2(TObject *Sender)
{
Form1->statusbarMain->Panels->Items[0]->Text = "Hello2";   // PROBLEM!!!
}

Any idea why it got problems? Please advise.. Thanks

Answer 1

Your Form2 unit is declaring its own Form1 variable that is separate from the Form1 variable in the Form1 unit, and that second variable is not being initialized to anything meaningful. That is why your code is crashing - you are accessing the wrong Form1 variable.

You need to get rid of the Form1 variable in the Form2 unit and #include Form1's header file instead (it should already have an extern declaration for Form1's Form1 variable). This is also necessary because Form2 needs the full declaration of the TForm1 class in order to access its members, such as statusbarMain.

Same thing with the Form2 variable that you are declaring in the Form1 unit. You need to remove it and use #include "Form2.hpp" instead (which should have a suitable extern declaration).

Form1.h:

...
class TForm1 : public TForm
{
__published:
    TStatusBar *statusbarMain;
    ...
};
extern TForm1 *Form1;
...

Form1.cpp:

#include "Main.h"
#include "minor.h"
#include "Form2.hpp" // <-- add this
....
TForm1 *Form1;
//TForm2 *Form2; // <-- get rid of this
....

Form2.h:

...
class TForm2 : public TForm
{
    ...
};
extern TForm2 *Form2;
...

Form2.cpp:

#include "minor.h"
#include "Main.h"
#include "Form1.hpp" // <-- add this
....
TForm2 *Form2;
//TForm1 *Form1; // <-- get rid of this
...
void __fastcall TForm2::ButtonClicked2(TObject *Sender)
{
    Form1->statusbarMain->Panels->Items[0]->Text = "Hello2";
}