Why does ishghandle(0) alwys return true?

Why does

ishghandle(0)

return true, even if there is no existent graphic object (actually, I executed this line of code first after Matlab startup).

Answer 1

[Stephen23]

Because the graphics root always exists:

https://www.mathworks.com/help/matlab/creating_plots/graphics-objects.html

Using zero is discouraged now, but in versions prior to R2014b was equivalent to groot:

https://www.mathworks.com/help/matlab/ref/groot.html

[Georg Nestlinger]

Thanks!

Answer 2

[Georg Nestlinger]

I'd like to extend my question. The source of the problem was the following:

I initilized a graphics object using

gobjects(1)

Then, checking with ishghandle() worked as expected in R2014b and above, but failed in R2013a.

What would be the recommended way for pre R2014b (I guess that was the release the new graphis system was introduced), ideally giving same results in R2013a and R2014b?

[Stephen23]

"I initilized a graphics object using gobjects(1) Then, checking with ishghandle() worked as expected in R2014b and above, but failed in R2013a."

What do you mean by "failed"? Did MATLAB crash, or did your computer stop responding, or did you just get an unexpected output? What output do you expect to get?

obj = gobjects(1);
ishghandle(obj)

ans = logical
   0

[Georg Nestlinger]

Sorry for beeing unclear:

In R2014b i got the expected output: false

In R2013a i got the contrary: true

[Stephen23]

AFAIK, versions before R2014b used numeric (double) scalars as a kind of graphics object "handle" (really more like an ID number). My guess is that the functions that "check" the existence simply have a different behavior when applied to doubles (IDs) vs graphics object handles.

In the back of my head I recalled an earlier discussion on this topic, but I can't find it now.

Is it really required to test for the existence of the graphics object? Surely if you are the author of your GUI and you keep track of the objects (by storing them in arrays, etc.), then there is little need to "guess" what graphics objects might be hanging around.

[Georg Nestlinger]

I'm using this for a visualization in Simulink and i want the figure to be created when the simulation starts or if I closed it unintentionally.

It seems that using the undocumented second argument like

ishghandle(obj, 'figure')

does what I want. So I will go with this approach for now.