How can i call class from a function?

Hello,

i have a class with private properties and public methods to change the properties.

I want to call the methods of the class from a function in another m.File and change the methods.

At the end a function display should return a value. Something like this:

value=dispspeed(classcar);

value=10;

How can it be done?

Thank you in advance!

[Rik]

Don't you mean something like this?

value=CarClassObject.dispspeed;
fprintf('The speed of the car is %.0f\n',value)

Or do you want to change the class itself and modify the method for all objects of that class?

Also, you should mark answers as accepted answer if they solved your problem. It is an easy way to say thank you to people investing their time in your problem.

[Michael Simonovski]

The function should call a constructor of the class. The function is not a part of this class!

The constructor has three inputs, all another values are set to zero. At the end a function of the class should be called to return values, not to print it. Something like {//... return value; } in C++. For example: value=dispspeed(classcar); a=10; value+a; ->100; How the method functionbody should look like?

Answer 1

[Rik]

You should really have a look at the documentation of how you write classes in Matlab, starting for example here.

With the code below in a file called 'car.m', you can call aCar=car(3);aCar.showSpeed and it will disp the speed.

classdef car
    properties
        speed
    end
    methods
        function obj = car(set_speed)
            if nargin>0
                obj.speed=set_speed;
            else
                obj.speed=0;
            end
        end
        function obj = setSpeed(obj,set_speed)
            obj.speed=set_speed;
        end
        function showSpeed(obj)
            disp(obj.speed)
        end
    end
end

[Michael Simonovski]

How can i call the function car(set_speed) from a function, which is in another m.file?

Would this be ok?

 function call[output]=func(input)
   set_speed=10;
   car(set_speed);
   value=showSpeed(car);
  end

[Rik]

Almost. You are calling a function, but in this case that function is just a method. And as the speed property is not hidden in any way, you can simply access it without a getter method.

function [output]=func(input)
set_speed=10;
aCar=car(set_speed);
value=aCar.speed;
end

A getter method would look something like this:

function get_speed = getSpeed(obj)
    get_speed=obj.speed;
end

And would be used like this:

value=aCar.getSpeed;

[Michael Simonovski]

The code of the car should be with private properties and public methods:

 classdef car
    properties (Access = private)
        speed
    end
    methods (Access = public)
        function obj = car(set_speed)
            if nargin>0
                obj.speed=set_speed;
            else
                obj.speed=0;
            end
        end
        function obj = setSpeed(obj,set_speed)
            obj.speed=set_speed;
        end
        function showSpeed(obj)
            disp(obj.speed)
        end
    end
end

%%Another m.File:

 function [output]=func(input)
%How to call the constructor and other methods to 
change the private properties?

 %How to setting a local variable in this function to the speed value?
For example value=showSpeed(obj);
 end

[Rik]

You don't need to explicitly call the constructor. If you want to change the private properties, just change them inside a method. The showSpeed method doesn't return a value. Just use the getter I describe in the comment above. Then you can simply use the syntax I describe.

Answer 2

[Steven Lord]

i have a class with private properties and public methods to change the properties.

I want to call the methods of the class from a function

Up to this point I follow you. That's easy.

and change the methods.

I don't understand what you're asking here. I suspect you want to change the value of the property without having to return the modified object from the method. If so, you'd want your object to be a handle class. See this documentation page for a discussion of the difference between handle and value classes.

At the end a function display should return a value. Something like this:

 value=dispspeed(classcar);
 value=10;

If what I guessed about you wanting to use a handle class is not correct, can you explain in a bit more detail exactly how you want instances of your car class to behave?

[Michael Simonovski]

1. Call a constructor

function obj = classnew(valuea, valueb)

obj.a=valuea;
obj.b=valueb;
end

%The value should not be returned at this point!

function Output(obj)

?? %At this point i do not know how the body of the function should look like!

end

The display function should output one or two of the properties. I know that in c++ it could be done with return a;

But how it could be done in Matlab?

[Steven Lord]

The constructor for a class and methods of that class are each functions, and can be called just like any other function (with a few exceptions.) The main exception is that to call a method, at least one of the inputs must be an instance of the class for which the method is defined. [I'm ignoring Static methods for purposes of this discussion.]

The easiest way to access properties of an object inside a function is to use dot notation. Basically, inside a function that is not inside the class you can only access public properties. Inside a class method (which as I said above is a function) you can access private properties as well.

function displayObject(obj)
  fprintf('Property a of the object: %d\n', obj.a);
end

If you want to return the property, just assign the property to a variable.

function y = returnPropertyA(obj)
  y = obj.a;
end

Note that you need to refer to the variable obj. This is particularly important if a method accepts two or more instances of the class, like:

function y = plus(obj1, obj2)
  y = obj1.a + obj2.a;
end