Static class call via string variable

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Jim Hokanson
Jim Hokanson 2013-4-27
编辑： Cedric 2017-10-17
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'])
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Daniel Shub
Daniel Shub 2013-4-28
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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Matt J
Matt J 2013-4-28
编辑：Matt J 2013-4-28
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');
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Matt J
Matt J 2013-4-28
编辑：Matt J 2013-4-28
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...
Adam
Adam 2015-10-15
编辑：Adam 2015-10-15
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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更多回答（2 个）

per isakson
per isakson 2013-4-27
编辑：per isakson 2013-4-27
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.
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Jim Hokanson
Jim Hokanson 2017-10-17
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 :/
Cedric
Cedric 2017-10-17
编辑：Cedric 2017-10-17
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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Cedric
Cedric 2013-4-27
编辑：Cedric 2013-4-27
Yes, you can proceed the same way as with dynamic field names:
myObject = myClass() ;
methodName = 'foo' ;
x = myObject.(methodName)(arg1, arg2) ;
  1 个评论
Jim Hokanson
Jim Hokanson 2013-4-27
The question is when the class name is dynamic, not the method or property. Thanks though.

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