Object oriented programming: Difference between revisions

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GNU/Octave has support for object oriented programming. Below you can find some tips and tricks.
#REDIRECT [[classdef]]
 
== Creating classes ==
To create a class you need to make a folder prefixed with the symbol '''@'''. Inside the folder you need to create the ''constructor'' which is a function with the same name as the folder. For example, a class called ''dummy'' will be created in the folder ''@dummy'' and inside we should find a function (a method) ''dummy.m''. The minimal content of this method is
{{Code|A dummy class|<syntaxhighlight lang="matlab" style="font-size:13px">
function obj = dummy ()
 
  obj = struct();
 
  obj = class (obj, "dummy");
 
endfunction
</syntaxhighlight>}}
 
=== Making all fields public ===
Fields of an Octave object are by default private. This means that only methods of the object can access the field directly. For a user to access a given field directly, a method must be provided (sometimes called a ''getter'' method). If such methods is not provided, a user cannot retrieve the contents of the field. There is a work around: we can get the internal structure representing and object and from there access all the fields. The next example creates a ftp object and retrieves all the fields
 
{{Code|Retrieving underlying structure an object|<syntaxhighlight lang="matlab" style="font-size:13px">
an_object = ftp("ftp.gnu.org/gnu/");
structure = struct(an_object);
</syntaxhighlight>}}
 
From there you can get the contents of any of the fields. Although effective, this is not very elegant. The second option, and without destroying the object oriented philosophy is to provide a general ''get'' method. Find below a working example
 
{{Code|A general get method|<syntaxhighlight lang="matlab" style="font-size:13px">
function answ = get (obj,varargin)
 
  accepted = fieldnames (obj);
 
  tf = ismember (tolower (accepted), tolower (varargin));
 
  retrieve = {accepted{tf}};
  n = numel (retrieve);
  if n == 1
    answ = obj.(retrieve{1});
  else
    for i=1:n
      answ.(retrieve{i}) = obj.(retrieve{i});
    end
  end
 
  unknown = {varargin{!ismember (varargin,accepted)}};
  cellfun (@(x) warning ("Unknown field %s\n", x) , unknown);
 
endfunction
</syntaxhighlight>}}
 
Any object with this method has all the fields accessible by the user. They only need to know the name of the field and they can retrieve it. This method effectively makes all the fields of your object public, but still read-only.
 
== Packages using object oriented programming ==
* Control
* [[Geometry_package|Geometry]]
* Quaternion

Latest revision as of 08:29, 21 January 2021

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