Board index » cppbuilder » "Control '' has no parent window" error when deriving from TWinControl

2005-10-31 08:17:08 PM
cppbuilder76
Hi,
I'm deriving my class from TWinControl just to enable it to receive
windows messages.
class PACKAGE TMyClass : public TWinControl
{
public:
__fastcall virtual TMyClass (TComponent* AOwner) :
TWinControl(AOwner) { };
__fastcall TMyClass(HWND ParentWindow) : TWinControl(ParentWindow) {};
}
and in my main form i create it:
void __fastcall TForm1::btConnectClick(TObject *Sender)
{
myobj = new TMyClass(this);
// an exception 'Control '' has not parent window' is thrown at the
// line below. This is just a demonstration.
ShowMessage( IntToStr( myobj->Handle ) );
}
Did i forget to initialize something in the constructor?
Thanks in advance
Muzaffar
 

Muzaffar Mahkamov wrote:
Hans.
 

Hans Galema wrote:
Project E:\dev\test12\test.exe faulted with message: 'access violation
at 0x7c90eddc: write of address 0x00030ee0'. Process Stopped. Use Step
or Run to continue.
 

{smallsort}

Muzaffar Mahkamov wrote:
Please show minimal code that demonstrates the AV problem.
Hans.
 

Hans Galema wrote:
{
myobj = new TMyClass(this);
myobj->Parent = this;
// an AV occurs in the line below
ShowMessage( IntToStr( myobj->Handle ) );
}
Did the same thing in Delphi and it works fine even without assigning
Parent manually.
 

Muzaffar Mahkamov wrote:
With the one you supplied in your first post all goes well here (bcb5).
Hans.
 

Hans Galema wrote:
constructor/destructor are inlined. No other code is involved.
 

Muzaffar Mahkamov wrote:
Please show complete code that causes the AV.
Hans.
 

Hans Galema wrote:
ShowMessage( IntToStr( myobj->Handle ) );
causes the AV. Particularly, the call to myobj->Handle, which is not my
code. Even if i do:
int i = myob->Handle;
the application crashes with AV.
My configuration is: Borland C++ Builder 6 Enterprise, Update 4
 

Hans Galema wrote:
never thought that an incorrect declaration would cause an AV or Stack
overflow.
class PACKAGE TMyClass: public TWinControl
{
BEGIN_MESSAGE_MAP
VCL_MESSAGE_HANDLER(WM_DEVICE_CONNECTED, TMessage,
WMDEVICECONNECTED)
VCL_MESSAGE_HANDLER(WM_DEVICE_DISCONNECTED, TMessage,
WMDEVICEDISCONNECTED)
END_MESSAGE_MAP(TMyClass)
}
changed it to:
class PACKAGE TMyClass: public TWinControl
{
BEGIN_MESSAGE_MAP
VCL_MESSAGE_HANDLER(WM_DEVICE_CONNECTED, TMessage,
WMDEVICECONNECTED)
VCL_MESSAGE_HANDLER(WM_DEVICE_DISCONNECTED, TMessage,
WMDEVICEDISCONNECTED)
END_MESSAGE_MAP(TWinControl)
}
and it works fine.
Thanks for you help :)
 

Muzaffar Mahkamov wrote:
you were asked to post all the code. And twice you stated that
class PACKAGE TMyClass : public TWinControl
{
public:
__fastcall virtual TMyClass (TComponent* AOwner) : TWinControl(AOwner) { };
__fastcall TMyClass(HWND ParentWindow) : TWinControl(ParentWindow) {};
}
was all there was.
If you need help then please be more exact.
Hans.