Value Classes vs. Handle Classes (GNU Octave (version 8.4.0))

#### 34.6.5 Value Classes vs. Handle Classes

There are two intrinsically different types of `classdef` classes, whose major difference is the behavior regarding variable assignment. The first type are value classes:

    classdef value_class
      properties
        prop1
      endproperties

      methods
        function obj = set_prop1 (obj, val)
          obj.prop1 = val;
        endfunction
      endmethods
    endclassdef

Assigning an object of that class to another variable essentially creates a new object:

    >> a = value_class ();
    >> a.prop1 = 1;
    >> b = a;
    >> b.prop1 = 2;
    >> b.prop1
    ⇒ ans =  2
    >> a.prop1
    ⇒ ans =  1

But that also means that you might have to assign the output of a method that changes properties back to the object manually:

    >> a = value_class ();
    >> a.prop1 = 1;
    >> a.set_prop1 (3);
    ⇒ ans =

    <object value_class>

    >> ans.prop1
    ⇒ ans =  3
    >> a.prop1
    ⇒ ans =  1

The second type are handle classes. Those classes have to be derived from the abstract `handle` class:

    classdef handle_class < handle
      properties
        prop1
      endproperties

      methods
        function set_prop1 (obj, val)
          obj.prop1 = val;
        endfunction
      endmethods
    endclassdef

In the following example, the variables `a` and `b` refer to the very same object of class `handle_class`:

    >> a = handle_class ();
    >> a.prop1 = 1;
    >> b = a;
    >> b.prop1 = 2;
    >> b.prop1
    ⇒ ans =  2
    >> a.prop1
    ⇒ ans =  2

Object properties that are modified by a method of an handle class are changed persistently:

    >> a.set_prop1 (3);
    >> a.prop1
    ⇒ ans =  3