How to pass a class name as argument to a function

Hello,

I'd like to initialize an element of a class with another class instance, and I'd like to pass in the particular class as an object. For example:

classdef Aclass < handle
  properties
    thingObj
  end

    methods
       function this = Aclass(thingClassName)
         this.thingObj = thingClassName(arg1,arg2,etc);
       end
    end
  end

This code doesn't work, though ("Index exceeds matrix dimensions"). I assume the problem is you can't call the () operator on an existing object.

Instead, I do this, which does work, but isn't pretty:

 classdef SomeClass < handle
    properties
      thingObj
    end

    methods
       function this = SomeClass(thingClassName)
         this.thingObj = eval([class(thingClassName) '(arg1,arg2,etc)']);
       end
    end
  end

There's no checking in the string argument list, and errors in this line are hard to debug.

I could instantiate the object outside the constructor, but that's problematic. I'd like to instantiate the object within the "SomeClass" constructor because it knows the right way to do it.

I'm guessing this is really simple and that I haven't stumbled on the right way yet. Any ideas?

Thanks,

Charles

Answer 1

[Jeff Miller]

I think the first classdef ("Aclass") works fine, as long as you pass to its constructor the handle of the class you want instantiated. So, for example,

myobj = Aclass(@theClassToInstantiate)

[Charles]

Works great! Written a lot of Matlab since then, but never needed to do that again.

Charles

[Paul Herselman]

This works great to just create a single instance of the class passed into the constructor, but I want to create an empty array of the class objects and that does not work:

function obj = clCircularBuffer(length, classToUse)
%CLCIRCULARBUFFER Construct an instance of this class
% must pass in the handle of the class to use, eg
% clCircularBuffer(100, @classToUse)
obj.bufferLength = length;
a = classToUse();
obj.buffer = classToUse.empty(length,0);
end

The line

a = classToUse();

works fine, but on the line

obj.buffer = classToUse.empty(length,0);

I get Dot indexing is not supported for variables of this type.

[Steven Lord]

If the class in question defines one or more of the array-creation functions you could call that array-creation function using the name of the class (or an instance of the class with the 'like' option). For example, the sym class from Symbolic Math Toolbox does define the zeros method as described on that documentation page:

z = zeros(3, 'sym')

z = 

z2 = zeros(4, 'like', z)

z2 = 

whos z z2

  Name      Size            Bytes  Class    Attributes

  z         3x3                 8  sym                
  z2        4x4                 8  sym                

If it doesn't support those array-creation functions and you have to use the empty Static method, you could create a function handle to that method and call it.

name = "sym";
methodname = name + ".empty";
functionHandle = str2func(methodname);
z3 = functionHandle(3, 0)

 
z3 =
 
Empty sym: 3-by-0
 

class(z3)

ans = 'sym'