MATLAB: Testing private functions in classes

object-oriented progammingoopunit tests

I'm wondering what the best way is to write unit tests for private functions and properties in classes. Say for example, I have a class that represents a ball launched as a projectile:
class ball
  %


  properties (Access = private)
     initialSpeed = 25;  %m/s
     acceleration = -10;  %m/s^2
  end
  %
  methods 
    function distTravelled = getDistanceTravelled(obj, timeElapsed)
        % Estimate total distance traveled by the ball 
        % (I know this is a bad approximation)
        currSpeed = obj.getCurrSpeed(timeElapsed);
        distTravelled = 0.5 * (obj.initialSpeed + currSpeed) * timeElapsed;
    end
  end
  %
  methods (Access = private)
      function currSpeed = getCurrSpeed(obj, timeElapsed)
         %Calculate current speed of ball
         currSpeed = obj.initialSpeed + obj.acceleration * timeElapsed;
      end
  end
end
How should I write a test to check that the values for acceleration or that the value returned by the method getCurrSpeed is accurate? Should I just allow access to the testing functions?

Best Answer

First goggle "test private method" and read about why you should not do it (and a few ways to do it).
One way (for handle classes only) is to include the test in the class itself.
>> mc = MyClass
mc = 
  MyClass with no properties.
>> mc.test_private
Private mysort is running
>>
where
classdef    MyClass < matlab.unittest.TestCase
    %
    methods ( Test )
        function test_private( this )
            this.mysort
        end
    end
    methods ( Access = private )
        function  mysort( this )
            fprintf( 'Private mysort is running\n' )
        end
    end   
end
A better workaround based on localfunctions (added 19 hours later)
>> mc = MyClass;
>> fh = mc.get_local_function_handle;
>> fh = fh{1}
fh = 
    @the_tricky_algorithm_
>> fh( mc )
ans =
   144
>>
where in one mfile
classdef    MyClass 
    properties
        val = 12;
    end
    methods 
        function    fh = get_local_function_handle( ~ )
            fh = localfunctions();
        end
    end
    methods ( Access = private )
        function  my_private_method( this )
            fprintf( 'Private mysort is running\n' )
            the_tricky_algorithm_( this )
        end
    end   
end
function    out = the_tricky_algorithm_( this )
    out = this.val .* this.val;
end
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