How to check if handle is to a deleted axes?

I am running an old plotting routine in R2015b for the first time. It is crashing on a legend command because apparently the axes handle points to a deleted axes, and I am stumped on how to check for this. Can you please replace the line (if legh == 'handle to deleted Axes') to correctly check whether the axis is deleted?

figure; % Make demo figure
peaks;
hAx = gca; % Get demo axis
delete(hAx); % Delete axis, changing state of axis handle
% Replace following line so it correctly checks if axis is deleted
if hAx == 'handle to deleted Axes'
  isDeleted = true;
else
  isDeleted = false;
end

if ~isDeleted
  disp('Axis exists. OK to add legend')
end

Answer 1

[Mike Garrity]

A couple of choices.

h = plot(1:10);
delete(h)
isvalid(h)
isgraphics(h)

Either of those will return false for a deleted handle. They differ on what they return true for. The first will return true for any valid handle. The second one will return true only for graphics objects. It also supports an argument to ask if it is a particular type of graphics object.

[K E]

Confirming that both work in my example. Thanks!

[Adam]

I usually use

ishghandle( h );

There seem to be a plethora of similar functions in this area!

I wasn't aware of

isgraphics( h )

though I do like the specific version:

isgraphics( h, type )

that would shorten some of the multi-line validation I do with ishghandle.

[Mike Garrity]

It's relatively new. I think it was added in R2014b. If you need your code to run in older versions, you might still need your multi-line validation.

[Adam Danz]

The problem with these approaches is that non-graphics objects also return false and some integers return true such as 0 or an integer that is also represented by an existing figure.

isgraphics(pi)

ans = logical
   0

isgraphics('a','axes')

ans = logical
   0

isgraphics(0)

ans = logical
   1

Imagine testing if the first input 'h' to a function is an axis handle or not and define 3 different behaviors if...

• h is a valid axis isgraphics returns true
• h is a deleted axis isgraphics returns false
• h is not an axis isgraphics returns false
• h is 0 or a fig number isgraphics returns true

There's an undocumented method to test if a variable is a deleted axis but this could change in future releases.

figure()
ax = gca();
delete(ax)

isDeletedAx = ~isgraphics(ax, 'axes') && isa(ax,'matlab.graphics.axis.AbstractAxes')

isDeletedAx =

logical

1

Or use the class of the expected axes.

isDeletedAx = ~isgraphics(ax, 'axes') && isa(ax,'matlab.graphics.axis.Axes')

[Walter Roberson]

For a number of purposes, 0 and positive integers that match figure numbers are still accepted in graphics operations. Interestingly, if you double() a handle to a non-figure graphic object, then most functions will not treat it as a graphic handle any more (but set() will. Also I think I noticed some aspects of Simulink still using the doubles)

[Adam Danz]

@Walter Roberson your comment just led me to find another way to test for deleted axes although it fails when testing any numeric value less than 0.

isDeletedAxes = @(h)double(h)<0;

ax = axes(); 
delete(ax)

isDeletedAxes(ax)

ans = logical
   1

[Walter Roberson]

Nice hack ;-)

Answer 2

[Adam Danz]

How to check if handle is to a deleted object?

Anonymous function to detect if a variable is a deleted object.

isDeletedObj = @(h)isobject(h) & ~isgraphics(h);

Note: Tests below show that this function will also identify graphics placeholders. To eliminate detection of graphics place holders,

isDeletedObj_2 = @(h)isobject(h) & ~isgraphics(h) & ~isa(h,'matlab.graphics.GraphicsPlaceholder');

To test for a specific type of deleted object, add a condition that tests the class of the input variable.

isDeletedAxes = @(h)isobject(h) & ~isgraphics(h) & isa(ax,'matlab.graphics.axis.Axes');

Example

fig = figure(); 
ax = axes(fig); 
h = plot(ax,rand(1,5)); 

delete(ax) 

idx = isDeletedObj(ax)              % example 1

idx = logical
   1

idx = isDeletedObj([fig, ax, h])    % example 2 

idx = 1×3 logical array
   0   1   1

Comparison with other methods

Both versions of isDeletedObj() return true for deleted object handles; other functions return false.

tf = [ishandle(ax), ...
    ishghandle(ax), ...
    isvalid(ax), ...
    isgraphics(ax), ...
    isDeletedObj(ax), ...
    isDeletedObj_2(ax)]

tf = 1×6 logical array
   0   0   0   0   1   1

All functions correctly return false for (most) non-graphics objects. Non-graphics inputs can occur when validating inputs to functions that contain optional parent handles as inputs. Since functions other than isDeletedObj and isDeletedObj_2 return false in this example and the example above, a false output does not mean that the input is a deleted object!

% isvalid() removed; will cause error
tf = [ishandle(pi), ...
    ishghandle(pi), ...
    isgraphics(pi), ...
    isDeletedObj(pi), ...
    isDeletedObj_2(pi)]

tf = 1×5 logical array
   0   0   0   0   0

Both versions of isDeletedObj() correctly return false for existing object handles; other functions correctly return true.

tf = [ishandle(fig), ...
    ishghandle(fig), ...
    isvalid(fig), ...
    isgraphics(fig), ...
    isDeletedObj(fig), ...
    isDeletedObj_2(fig)]

tf = 1×6 logical array
   1   1   1   1   0   0

Both versions of isDeletedObj() correctly return false for an integer that coincides with an existing figure number or matlab root number (0). All other tests fail to distinguish figure handles from their integer representation! This is because the Matlab functions support backward compatibility prior to the graphics changes in r2014b.

% isvalid() removed, will cause error
tf = [ishandle(1), ...
    ishghandle(1), ... 
    isgraphics(1), ...
    isDeletedObj(1), ...
    isDeletedObj_2(1)]

tf = 1×5 logical array
   1   1   1   0   0

When testing graphics placeholders (gobjects), isDeletedObj() and isvalid() return true while isDeletedObj_2() returns false. This examples shows the only difference between the two versions isDeletedObj.

obj = gobjects(1); 
tf = [ishandle(obj), ...
    ishghandle(obj), ... 
    isvalid(obj), ...
    isgraphics(obj), ...
    isDeletedObj(obj), ...
    isDeletedObj_2(obj)]

tf = 1×6 logical array
   0   0   1   0   1   0

Summary

• A false output to Matlab's object validity tests (ishandle, ishghandle, isvalid, isgraphics) does not mean that the input is a deleted object.
• Matlab's object validity tests return true for an input 0 or any integer that corresponds to a an existing figure number.
• isDeletedObj only returns true when the input is a deleted object or object place holder (gobjects).
• isDeletedObj_2 only returns true when the input is a deleted object and returns false for obj place holders.