MATLAB function run in 2021a producing graphics handles as unique doubles (pre-2014 behavior) rather than handle objects

I *just* discovered that MATLAB even had a different way of handling graphics in previous releases, as i began using MATLAB in 2020. I discovered it now because i am running a great user-contributed function, superbar by Scott Lowe, where various aspects of a custom barplot are output as separate handles for further control. My question is: despite this being an older function (released in 2016), why am i observing MATLAB 2021a (v i'm currently running) produce graphics handles as unique doubles rather than handle objects? And is there a way to make MATLAB 2021a treat assigned handles using the updated behavior i am used to post-2014? (i have literally never observed this unique double method)... i even tried to make a new version of this function to save locally, hoping that it would somehow be viewed as an 'updated' (?) function and ignore any versions possibly defaulted into the build.

Answer 1

[Steven Lord]

Without seeing what you're doing I'm only guessing, but I suspect you're preallocating an array as a double array and assigning a graphics handle into it using subscripted assignment. When you do this, MATLAB tries to convert the thing you're assigning into the array into the type of the thing into which you're assigning.

So if I have a double value:

x = pi

x = 3.1416

that I'm trying to assign into an int32 array, that double will be converted into an int32 value.

y = zeros(1, 3, 'int32')

y = 1×3

0 0 0

class(y)

ans = 'int32'

y(2) = x % double pi gets converted into int32 3

y = 1×3

0 3 0

For backwards compatibility, MATLAB knows how to convert a graphics handle into a double value (which is how graphics handles were stored in the original Handle Graphics system.)

z = zeros(1, 3)

z = 1×3

0 0 0

class(z)

ans = 'double'

h = figure

h = 
  Figure (1) with properties:

      Number: 1
        Name: ''
       Color: [1 1 1]
    Position: [671 661 577 433]
       Units: 'pixels'

  Show all properties

class(h)

ans = 'matlab.ui.Figure'

z(2) = h % the Figure object h is converted to double when stored in z

z = 1×3

0 1 0

get(z(2), 'Position') % Can use that double handle with get to retrieve properties

ans = 1×4

671 661 577 433

h.Position

ans = 1×4

671 661 577 433

If you're updating that code to use graphics objects you could preallocate the arrays into which you want to store graphics handles using gobjects instead of zeros.

g = gobjects(1, 3)

g = 
  1×3 GraphicsPlaceholder array:

    GraphicsPlaceholder    GraphicsPlaceholder    GraphicsPlaceholder

g(2) = h

g = 
  1×3 graphics array:

    GraphicsPlaceholder    Figure                 GraphicsPlaceholder

[Connor Gallimore]

Outstanding. I am impressed with your intuition; you are exactly correct. Under the hood of the function i was using, graphics objects were being pre-allocated with

nan(size(data))

pre-allocating with:

gobjects(size(data))

produced exactly the result you suggest, and the one i was expecting. Accepted and upvoted, case closed! Hope folks who encounter this previous (and somewhat difficult to track) behavior find this page. Thanks Steven!

[Steven Lord]

You're welcome!