why i couldn't creat hgtransform graphics object before the objects belong to it

this code is ok

>> ph=fill([0 -0.05 0.1 -0.05],[0 0.05 0 -0.05],'r');
>> hgh=hgtransform;
>> set(ph,'parent',hgh)

but these are wrong:

>> hgh=hgtransform;
>> ph=fill([0 -0.05 0.1 -0.05],[0 0.05 0 -0.05],'r');
>> set(ph,'parent',hgh)
Error using set
Invalid object handle 

>> hgh=hgtransform;
>> fill([0 -0.05 0.1 -0.05],[0 0.05 0 -0.05],'r','parent',hgh)
Error using fill
Not enough input arguments.

i coudn't understand why it can't work

[213]

Note   Many plotting functions clear the axes (i.e., remove axes children) before drawing the graph. Clearing the axes also deletes any hgtransform objects in the axes.

[Mike Garrity]

Yes, that's exactly why the one with the set failed.

You can see the hgtransform get deleted by the clear of the axes if you do the following:

hgh=hgtransform('DeleteFcn','disp bye');
ph=fill([0 -0.05 0.1 -0.05],[0 0.05 0 -0.05],'r');
ishghandle(hgh)

You'll see the hgtransform object print bye from within the fill command. Then when we get to the ishghandle command, hgh is no longer a valid handle.

Your last example is actually a bug. The fill command is deleting the hgtransform object as you noted, but it shouldn't have because you passed it in as the parent.

You can work around either of these by using "hold on".