How to check if a pointer is "valid" ?

[Manuel]

I think I can't find this on google/books because is soooooooo basic...

This function assign a listner pointer to an image widget:
------------------------------------------------------
void image::setListener(AbstractListener *listener)
{
this->listener = listener;
}
------------------------------------------------------
this function use the listner:

------------------------------------------------------
bool image::isMouseOver(GLint mouseX, GLint mouseY)
{
bool isOver = false;
if (mouseX > positionX
&& mouseX < positionX + width
&& mouseY > positionY
&& mouseY < positionY + height)
{
listener->mouseOver(mouseX, mouseY, userValue);
isOver = true;
}
return isOver;
};
------------------------------------------------------

Of course, If I call the function above, without call setListner before,
the application crash.

So I must check listner is OK.
I'm trying somewhat like:

------------------------------------------------------
bool image::isMouseOver(GLint mouseX, GLint mouseY)
{
bool isOver = false;
if (mouseX > positionX
&& mouseX < positionX + width
&& mouseY > positionY
&& mouseY < positionY + height)
{
if (listener)
{
listener->mouseOver(mouseX, mouseY, userValue);
}
isOver = true;
}
return isOver;
};
------------------------------------------------------

But it crash anyway...

Please, help...

Regards,

Manuel

[Victor Bazarov]

Manuel wrote:

I think I can't find this on google/books because is soooooooo basic...

This function assign a listner pointer to an image widget:
------------------------------------------------------
void image::setListener(AbstractListener *listener)
{
this->listener = listener;
}
------------------------------------------------------
this function use the listner:

------------------------------------------------------
bool image::isMouseOver(GLint mouseX, GLint mouseY)
{ [..]
listener->mouseOver(mouseX, mouseY, userValue);
[..]
};
------------------------------------------------------

Of course, If I call the function above, without call setListner before,
the application crash.

So I must check listner is OK.
Simple. Initialise 'listener' with 0 in the constructor. Check for its
value before using it:

if (listener)
listener->mouseOver(...
I'm trying somewhat like:

------------------------------------------------------
bool image::isMouseOver(GLint mouseX, GLint mouseY)
{
bool isOver = false;
if (mouseX > positionX
&& mouseX < positionX + width
&& mouseY > positionY
&& mouseY < positionY + height)
{
if (listener)
{
listener->mouseOver(mouseX, mouseY, userValue);
}
isOver = true;
}
return isOver;
};
------------------------------------------------------

But it crash anyway...

Not if you actually initialise 'listener' to 0. Otherwise, it's left
uninitialised, and using an uninitialised pointer causes undefined
behaviour.

V

[Dan Cernat]

"Manuel" <ma**********************@tin.it> wrote in message
news:43**********************@reader4.news.tin.it. ..

I think I can't find this on google/books because is soooooooo basic...

This function assign a listner pointer to an image widget:
------------------------------------------------------
void image::setListener(AbstractListener *listener)
{
this->listener = listener;
}
------------------------------------------------------
this function use the listner:

------------------------------------------------------
bool image::isMouseOver(GLint mouseX, GLint mouseY)
{
bool isOver = false;
if (mouseX > positionX
&& mouseX < positionX + width
&& mouseY > positionY
&& mouseY < positionY + height)
{
listener->mouseOver(mouseX, mouseY, userValue);
isOver = true;
}
return isOver;
};
------------------------------------------------------

Of course, If I call the function above, without call setListner before,
the application crash.

So I must check listner is OK.
I'm trying somewhat like:

------------------------------------------------------
bool image::isMouseOver(GLint mouseX, GLint mouseY)
{
bool isOver = false;
if (mouseX > positionX
&& mouseX < positionX + width
&& mouseY > positionY
&& mouseY < positionY + height)
{
if (listener)
{
listener->mouseOver(mouseX, mouseY, userValue);
}
isOver = true;
}
return isOver;
};
------------------------------------------------------

But it crash anyway... one cannot check to see if a pointer is valid or not. Not in standard C++.

Please, help... in the constructor of image object, set listener pointer to zero.


Regards,

Manuel

[Artie Gold]

Manuel wrote:

I think I can't find this on google/books because is soooooooo basic...
No, you can't find this on Google or in books because it's impossible[1]!

This function assign a listner pointer to an image widget:
------------------------------------------------------
void image::setListener(AbstractListener *listener)
{
this->listener = listener;
}
------------------------------------------------------
this function use the listner:

------------------------------------------------------
bool image::isMouseOver(GLint mouseX, GLint mouseY)
{
bool isOver = false;
if (mouseX > positionX
&& mouseX < positionX + width
&& mouseY > positionY
&& mouseY < positionY + height)
{
listener->mouseOver(mouseX, mouseY, userValue);
isOver = true;
}
return isOver;
};
------------------------------------------------------

Of course, If I call the function above, without call setListner before,
the application crash.

So I must check listner is OK.
I'm trying somewhat like:

------------------------------------------------------
bool image::isMouseOver(GLint mouseX, GLint mouseY)
{
bool isOver = false;
if (mouseX > positionX
&& mouseX < positionX + width
&& mouseY > positionY
&& mouseY < positionY + height)
{
if (listener)
{
listener->mouseOver(mouseX, mouseY, userValue);
}
isOver = true;
}
return isOver;
};
------------------------------------------------------

But it crash anyway...

The only suggestion I would have would be to initialize the listener to
NULL (0) -- otherwise there's no way to distinguish between
uninitialized garbage and a valid value.

HTH,
--ag

[1] There may be a way to perform a `sanity check' in a platform
specific way -- but even that would not be reliable (and nothing *I*
would use for production code).
--
Artie Gold -- Austin, Texas
http://goldsays.blogspot.com
http://www.cafepress.com/goldsays
"If you have nothing to hide, you're not trying!"

[Manuel]

> in the constructor of image object, set listener pointer to zero.
It works, THANKS GUYS!