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Static class call via string variable

Asked by Jim Hokanson on 27 Apr 2013
Latest activity Edited by Cedric Wannaz
on 17 Oct 2017

Is it possible to call a static method or Constant property from a class whose name is represented by a string without using eval?

my_class_string = 'my_class'
eval([my_class_string '.MY_CONSTANT_PROPERTY'])

  1 Comment

At the point in which it is a string it might, and probably is, too late. How do you end up with a string and not an object?

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3 Answers

Answer by Matt J
on 28 Apr 2013
Edited by Matt J
on 28 Apr 2013
 Accepted Answer

Here's a way to access a constant property without creating an object of the class.

    mc=meta.class.fromName('myclass');
    mp=mc.PropertyList;
     [~,loc]=ismember('MY_CONSTANT_PROPERTY',{mp.Name});
    propertyValue = mp(loc).DefaultValue;

For a static method, I don't know why you wouldn't use EVAL, but note that you could use FEVAL as well, if it feels better,

    out = feval('myclass.MY_STATIC_METHOD');

  3 Comments

The property solution is awful, but I suspect it is probably the only way to accomplish what I was looking for without using eval. I'll probably keep using eval for now. Thanks!

The thought with avoiding eval in general is that I tend to think eval makes it more difficult to discern what is going on. I also feel like eval slows down the optimization just a bit (not tested).

The property solution is awful, but I suspect it is probably the only way to accomplish what I was looking for

Awful because of the code complexity? I wouldn't agree with that as a criticism. You can always hide the lines of code inside an mfile function of your own making.

However, I tend to agree with Daniel that your situation is already awful, needing to work with dynamic classnames and not objects... or at least "awfully peculiar" ;)

I also feel like eval slows down the optimization just a bit (not tested)

No need to test it. It's well known...

This solution works ideally for what I want, which is the same thing essentially and I have used meta class stuff in my code before so I have no aversion to the method, unlike eval!

Since the question did come up as to reasons for wanting this, I thought I would add mine:

I have a bunch of classes representing units in different domains ( e.g. Seconds, MetresPerSecond, KilogramsPerCubicMetre). Each class has a static map property listing supported units (keys being strings like 'm/s', values being function handles to the specific class constructor).

When I read data files I want to interpret the unit string of the data which may come from any one of the domains I define (e.g. TimeUnit, DepthUnit, DensityUnit), but currently only a specific domain object can take a string and convert it into the right unit class object.

So my intention was to write a function that finds all the different types of domain unit in a folder and concatenate all their supported units maps so that I can create a unit class from a string without having to know the domain of the unit a priori.

I'm not 100% sure this is the best solution, but it would allow extension to new unit domains without having to keep updating some hard coded list of these from which the full map of units is created.

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Answer by per isakson
on 27 Apr 2013
Edited by per isakson
on 27 Apr 2013

It's without eval

    foo = str2func( my_class_string );
    obj = foo();
    obj.MY_CONSTANT_PROPERTY
    ans =
        17

where

    classdef  my_class 
        properties ( Constant = true )
            MY_CONSTANT_PROPERTY = 17
        end
    end

An alternative

    obj = feval( 'my_class' )
    obj.MY_CONSTANT_PROPERTY

But, is it any better than eval? And it does neither depend on static nor constant.

  4 Comments

Show 1 older comment

Could you e.g. test if nargin==0 in the constructor and do nothing, if you meant "not ideal" because creating the object may be resource consuming?

That's a good point. Although if you require inputs for valid construction this might lead to confusion later when no inputs were passed in and the nargin check made it seem like that was ok. Like I said in the end Matt is probably right and if used somewhat frequently the ugliness should just be wrapped into a function :/

I sympathize! ;)

I am increasingly implementing more complex constructors that manage special cases though, e.g.

 methods
      function obj = Xyz( x, y, varargin )
         % x must be a num array, yet ..
         if ischar( x ) && strcmpi( x, 'test' )
             do something
             return
         end
         assert( isnumeric(x), '...' ) ;
         assert( isnumeric(y), '...' ) ;
         if ~isempty( varargin )
             parser = inputParser ;
             % .. more lengthy parsing, but no parser if not necessary
         end
         ...

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Answer by Cedric Wannaz
on 27 Apr 2013
Edited by Cedric Wannaz
on 27 Apr 2013

Yes, you can proceed the same way as with dynamic field names:

 myObject = myClass() ;
 methodName = 'foo' ;
 x = myObject.(methodName)(arg1, arg2) ;

  1 Comment

The question is when the class name is dynamic, not the method or property. Thanks though.

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