Re: goocanvasmm: problem on accessing cairo matrix

[David Davies <David questans co uk>]

Many thanks Murray.

As you suspected pMatrix is null until translate() is invoked, when it 
is becomes initialized.
Presumably, the other transform methods would initialize it similarly 
though I haven't tested this yet.
I find this strange but no longer a problem.

DavidGSD

Murray Cumming wrote:
> On Mon, 2009-07-13 at 18:28 +0100, David Davies wrote:
>   
> > void ExampleWindow::update_label()
> > {
> >     cairo_matrix_t * pMatrix = m_text->property_transform(); // line A
> >     double x0 = pMatrix->x0; // line B
> >
> >     std::stringstream str;
> >    ...
> >    etc
> >
> > My aim is to output the components of the transform matrix so that I
> > can 
> > understand what is happening to it.
> > The program compiles and links in CDT with no reported error.
> >
> > If  I comment out line B, it runs OK and, as expected, gives the same 
> > output as the original example.
> > With line B however, the program terminates and outputs a
> > <terminated> 
> > message without opening a window.
> > It does not report an error.
> >     
>
> You really need to use a debugger. I suspect that pMatrix is null. You
> should check pointers for null before dereferencing them anyway.
>
> I don't know _why_ it is null. Maybe that's a normal value, if there is
> no "transformation matrix" set for the item, though I don't quite know
> when that would be set.
>
>