Object oriented programming - Constructor behavior
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I'm struggling to understand the behavior of constructor functions. Take the app designer as an example: "app" is the output variable of the constructor (i. e. the object that is created). Yet it is treated like a known (input) variable within the constructor although it is never defined. To my knowledge, this is not possible with all other functions where the return variable is unknown at the start and must be defined within the function. Hence my question: Why does that not apply to the constructor?
% App creation and deletion
methods (Access = public)
% Construct app
function app = XYZapp
% Create UIFigure and components
% Register the app with App Designer
if nargout == 0
% more code...
Steven Lord on 7 Apr 2021
Because class constructors are special. MATLAB knows exactly which method in a class is the class constructor (it has to have the same name as the class) and if the class constructor returns an output the first output must be an instance of the class. See the "Guidelines for Constructors" section on that documentation page.
Also from the "Initializing Objects in Constructor" section on that page: "The output argument is created when the constructor executes, before executing the first line of code."