Why is calling a superclass constructor after a use of the object not allowed?
I have a subclass "subclass" of two classes "classA" and "classB", which are in turn subclasses of "superclass". Here is the class tree: The superclass has two properties x and y. The classes "classA" and "classB" each do something special to initialize x and y, respectively. Then I want "subclass" to call both the "classA" and "classB" constructors to initialize x and y simultaneously. But the way the classes are built, I run into a snag. Here are the definitions of the classes: superclass.m: classdef superclass %% PROPERTIES properties x; y; end %% METHODS methods function obj = superclass(x,y) % Initialize the properties obj.x = x; obj.y = y; end end end classA.m: classdef classA < superclass %% PROPERTIES properties a; end %% METHODS methods % Constructor function obj = classA(a,y,z) x = a*z; obj@superclass(x,y); obj.a = a; obj.z = z; end end end classB.m: classdef classB < superclass %% PROPERTIES properties b; end %% METHODS methods % Constructor function obj = classB(x,b,z) y = z+b; obj@superclass(x,y); obj.b = b; obj.z = z; end end end subclass.m: classdef subclass < classA & classB %% METHODS methods % Constructor function obj = subclass(a,b,z) obj@classA(a,0,z); obj@classB(obj.x,b,z); % Why is this not allowed and how do I work around that % without redoing the calculations done in classes "classA" and "classB"? end end end This gives the following error message: "A constructor call to superclass classB appears after the object is used, or after a return." A couple of questions: 1) Why is calling a superclass constructor after a use of the object not allowed? 2) How do I work around that without redoing the calculations done in classA and classB? I feel like I might to redesign the class hierarchy. But it just bugs me that it can't be done like above. I use Matlab R2019a on Windows 10.
Prashant Kumar answered .
2025-11-20
classdef superclass
%% PROPERTIES
properties
x;
y;
end
end
classdef classA < superclass
%% PROPERTIES
properties
a
end
properties (Access=private)
z
end
%% METHODS
methods
% Constructor
function obj = classA(a,z,y)
obj.x = a*z;
obj.a = a;
obj.z = z;
if nargin>2
obj.y=y;
end
end
end
end
classdef classB < superclass
%% PROPERTIES
properties
b
end
properties (Access=private)
z
end
%% METHODS
methods
% Constructor
function obj = classB(b,z,x)
obj.y = z+b;
obj.b = b;
obj.z = z;
if nargin>2
obj.x=x;
end
end
end
end
classdef subclass < classA & classB
%% METHODS
methods
% Constructor
function obj = subclass(a,b,z)
obj@classA(a,z); %set x but not y
obj@classB(b,z); %set y but not x
end
end
end