What is the variable 'self' in MATLAB class?

Question

Gun-min Kim 2012-9-24

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回答： Neeraj Rajpurohit 2020-6-30

Hi ! While I look through the code that includes class, I found that variable 'self' behaves like 'preserved' variable.

For example...

    classdef Vocabulary < handle
       properties
            words = {};
        end
        methods
            function add_word(self, word, audio_signals)
                new_word = Word(word);
                % Concatenate and train
                new_word.train(extract_features(cell2mat(audio_signals)'));
                self.words.(char(word)) = new_word;
            end

add_word function has 3 arguments including 'self' argument. But this function is called with only 2 arguments, like 'add_word(word_labels(~~~), train_audio_signals(~~~))'.

Not only the 'add_word' fucntion, but also the other functions use similar way.

Can anyone tell me the usage of variable 'self'?

I really appreciate your help !

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Answer 1

Daniel Shub 2012-9-24

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There is nothing special about the variable "self". In MATLAB many people use "obj" instead. There is something special about the first argument to a function. The class of the first argument determines the class of the method to be used. In other words

function add_word(self, word, audio_signals)

and

function add_word(obj, word, audio_signals)

are the same, but

function add_word(word, audio_signals, self)

is very different.

I doubt you are calling

add_word(word_labels(~~~), train_audio_signals(~~~))

and bet you are calling

myVocabularyObject.add_word(word_labels(~~~), train_audio_signals(~~~))

where myVocabularyObject is an object of the Vocabulary class. This is essentially the same as calling

add_word(myVocabularyObject, word_labels(~~~), train_audio_signals(~~~))

It is not strictly identical if the subsref method has been overloaded ...

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Answer 2

Neeraj Rajpurohit 2020-6-30

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As Daniel has rightly written, obj is usually writtern instead of 'self' in definition of methods in MATLAB classes.

If the function you are calling is non-static, and it accesses the the object properties, then self is a must-have arguments.

As you can see in the below class, the function fullName will take no arguments while calling, but we are defining obj as a reference to the class object.

class Name
    properties
        Name(1, :) char
        Surname(1, :) char
    end
    methods
        function name = fullName(obj)
        name = obj.Name + obj.Surname
        end
    end
end