How can I determine if one class is superior or inferior to another?

Matlab uses class precedence to determine which methods are called. Given two class names, is it possible to determine the class precedence relationship between the two? The new metaclass structure contains some of this information for classdef classes, but lacks information about older-style classes and fundamental types.

I'd love to find (or be able to write) a generic utility that simply asks is 'class_a' superior to 'class_b'? Is this possible?

Answer 1

[Matt J]

Instead of having 2 class names, you can do it if you have 2 objects of the class using something like the function below. Moreover, if your class constructors all support the no-argument case, you could generate an object from a class name as follows

 obj=feval(className)

but I don't know if this would always be true for you.

function tf=issuperior(A,B)
%True if class(A) superior to class(B). Otherwise false.

    try
       stupidname_neverused(A,B);
    catch ME

    end

    name=strrep(ME.message,'Undefined function ''stupidname_neverused'' for input arguments of type ','');
     name=strrep(name,'''','');
     name(end)='';

    tf=strcmp(name,class(A));

[Matt Bauman]

Very clever. What an ingenious way of getting at that information. Thanks! It's a shame Matlab doesn't expose this in a more sensible way; it'd make class precedence much more accessible and understandable.

[Matt J]

Sounds like a worthwhile enhancement request to me!

[Matt Bauman]

Already done! I also spent a bit of time and flushed out your suggestion into a complete function: https://gist.github.com/mbauman/6888607. One note: it tests the relative precedence of the arguments, not necessarily if one class is superior to another. I have found that, for example, char and double are at equal precedence and the result is dependent upon their ordering.

[Matt J]

I'm not sure of the distinction you're trying to draw, unless it's the distinction between strict superiority and "greater than or equal precendence". My function did test only the latter, so char and double which are of equal precedence should test true regardless of their order.

[Matt Bauman]

Yes, I was simply clarifying that the precedence is a function of both class and argument position. I was surprised by how many of the built-in types aren't at equal precedence... I don't think this is documented anywhere.

[Matt J]

I'm not sure what's led you to conclude that the built-ins aren't of equal precedence, but maybe the docs your looking for are

http://www.mathworks.com/help/matlab/matlab_oop/ordinary-methods.html#brd2n2o-1

and also maybe

http://www.mathworks.com/help/matlab/matlab_oop/concatenating-objects-of-different-classes.html#bsfqmmz

Answer 2

[Cedric]

You could use property SuperclassList from the output of METACLASS recursively ..

Here is an example:

 function tf = isSubclass( classObjMeta, superclassName )
    tf = true ;
    if ischar( classObjMeta ),  classObjMeta = feval( classObjMeta ) ;  end
    if ~isa( classObjMeta, 'meta.class' )
        classObjMeta = metaclass( classObjMeta ) ;
    end
    if strcmp( classObjMeta.Name, superclassName ),  return ;  end
    for scId = 1 : length( classObjMeta.SuperclassList )
        if isSubclass( classObjMeta.SuperclassList(scId), superclassName )
            return
        end
    end
    tf = false ;
 end

Note that I just took 5 minutes to write that, so you'll want to stabilize or fine tune it. In particular, I leave it to you to modify it if you don't want the output to be true when you test a class with itself, .. or to ask users to pass an object as 1st arg if class doesn't support no-argument constructor (that should be tested in try/catch statement I guess).

To test it, define e.g.

 classdef ClassA < handle
 end

 classdef ClassB < ClassA
 end

and you'll have

 >> isSubclass( 'ClassB', 'handle' )
 ans =
     1
 >> isSubclass( 'ClassA', 'handle' )
 ans =
     1
 >> isSubclass( 'ClassB', 'ClassA' )
 ans =
     1
 >> isSubclass( 'ClassA', 'ClassB' )
 ans =
     0

[Matt Bauman]

Thanks for the answer, but I'm after class precedence, not subclass hierarchy.

[Cedric]

Well, I should have taken 6 minutes instead of 5, and read completely the question!