Checking if two handle objects are identical using 'isequal' and '=='

I am studying object and class in MATLAB and I found something I can't understand.

From this link (https://www.mathworks.com/help/matlab/matlab_oop/comparing-handle-and-value-classes.html)

I see that "a == b" for handle object "a" and "b" determines if "a" and "b" refere to the same object

and "isequal(a,b)" determines if "a" and "b" have the same values for their attributes.

So for the histogram objects as below, I expected that the results of "a==b" and "isequal(a,b)" should be "0" and "1" respectively,

because they do not refer to the same objects but they have the same values for all attributes.

But my thought was wrong and I don't know why. please help

x = randn(1000,1);
h = histogram(x);

g = histogram(x);

isequal(g,h)

ans = logical
   0

g==h

ans = logical
   0

Answer 1

[DGM]

I don't know if there's a generalized canonical way to compare the similarity of graphics objects, but they're going to differ.

% fake data
x = randn(1000,1);

% two objects of the same type, using the same data
h = histogram(x); hold on
g = histogram(x);

% they're not equal
isequal(h,g)

ans = logical
   0

% find where props differ
% this includes properties which aren't public
% though i'm only checking one layer deep
hprops = struct2cell(struct(h));
gprops = struct2cell(struct(g));
equalprops = false(size(hprops));
for k = 1:numel(hprops)
    equalprops(k) = isequal(hprops{k},gprops{k});
end
fnames = fieldnames(struct(h));

% a list of the fieldnames and values where they differ
d = [fnames(~equalprops) hprops(~equalprops) gprops(~equalprops)]

d = 12×3 cell array
    {'AutoColor'           }    {[                          0 0.4470 0.7410]}    {[                     0.8500 0.3250 0.0980]}
    {'Face'                }    {1×1 Quadrilateral                          }    {1×1 Quadrilateral                          }
    {'Edge'                }    {1×1 LineStrip                              }    {1×1 LineStrip                              }
    {'BrushFace'           }    {1×1 Quadrilateral                          }    {1×1 Quadrilateral                          }
    {'BrushEdge'           }    {1×1 LineStrip                              }    {1×1 LineStrip                              }
    {'NodeChildren'        }    {4×1 Graphics                               }    {4×1 Graphics                               }
    {'DataTipTemplate'     }    {1×1 matlab.graphics.datatip.DataTipTemplate}    {1×1 matlab.graphics.datatip.DataTipTemplate}
    {'Behavior'            }    {1×1 struct                                 }    {1×1 struct                                 }
    {'Behavior_I'          }    {1×1 struct                                 }    {1×1 struct                                 }
    {'SerializableBehavior'}    {1×1 struct                                 }    {1×1 struct                                 }
    {'SeriesIndex'         }    {[                                        1]}    {[                                        2]}
    {'SeriesIndex_I'       }    {[                                        1]}    {[                                        2]}

Things like handles to their hidden descendant objects will differ. The colors of objects (e.g. line objects in a plot) will differ, and their level in the ui stack will differ.

I'm assuming that a test for equivalence will have to consider which properties actually matter and compare those properties alone.

[Jiyeol Oh]

Very interesting. Thank you for your great answer, DGM!

Things like levels in the UI stack are something I've never been considered before..

Testing the equivalence of two handle objects with the predetermined properties looks clear

Answer 2

[Dyuman Joshi]

When you call histogram() again, the handle to the previous histogram is deleted, thus comparing handles results in 0.

The overwriting takes place with most funtions used to plot - plot(), scatter(), surf() etc.

x = randn(1000,1);
f = histogram(x);

g = histogram(x);

whos

  Name           Size            Bytes  Class                                        Attributes

  cmdout         1x33               66  char                                                   
  f              1x1                 8  matlab.graphics.chart.primitive.Histogram              
  g              1x1                 8  matlab.graphics.chart.primitive.Histogram              
  x           1000x1              8000  double                                                 

f

f = 
  handle to deleted Histogram

get(f)

Error using matlab.graphics.chart.primitive.Histogram/get
Invalid or deleted object.

Error in hggetdisp (line 8)
v = get(h,sp);

[Jiyeol Oh]

Thank you!

Great answer to stupid question... lol

[Dyuman Joshi]

One such object that does not over-write the previous one is Constant-line objects - https://in.mathworks.com/help/matlab/ref/matlab.graphics.chart.decoration.constantline-properties.html

You can test their equality, and the results are as expected -

y1 = yline(1);
y2 = yline(1);

y1

y1 = 
  ConstantLine with properties:

    InterceptAxis: 'y'
            Value: 1
            Color: [0.1500 0.1500 0.1500]
        LineStyle: '-'
        LineWidth: 0.5000
            Label: ''
      DisplayName: ''

  Use GET to show all properties

y2

y2 = 
  ConstantLine with properties:

    InterceptAxis: 'y'
            Value: 1
            Color: [0.1500 0.1500 0.1500]
        LineStyle: '-'
        LineWidth: 0.5000
            Label: ''
      DisplayName: ''

  Use GET to show all properties

isequal(y1, y2)

ans = logical
   1

y1==y2

ans = logical
   0