(octave.info)Precedence of Objects

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34.4.3 Precedence of Objects
----------------------------

Many functions and operators take two or more arguments and the
situation can easily arise where these functions are called with objects
of different classes.  It is therefore necessary to determine the
precedence of which method from which class to call when there are mixed
objects given to a function or operator.  To do this the ‘superiorto’
and ‘inferiorto’ functions can be used

 -- : superiorto (CLASS_NAME, ...)
     When called from a class constructor, mark the object currently
     constructed as having a higher precedence than CLASS_NAME.

     More that one such class can be specified in a single call.  This
     function may _only_ be called from a class constructor.

     See also: Note: inferiorto.

 -- : inferiorto (CLASS_NAME, ...)
     When called from a class constructor, mark the object currently
     constructed as having a lower precedence than CLASS_NAME.

     More that one such class can be specified in a single call.  This
     function may _only_ be called from a class constructor.

     See also: Note: superiorto.

   With the polynomial class, consider the case

     2 * polynomial ([1, 0, 1]);

that mixes an object of the class "double" with an object of the class
"polynomial".  In this case the return type should be "polynomial" and
so the ‘superiorto’ function is used in the class constructor.  In
particular the polynomial class constructor would be modified to

     ## -*- texinfo -*-
     ## @deftypefn  {} {} polynomial ()
     ## @deftypefnx {} {} polynomial (@var{a})
     ## Create a polynomial object representing the polynomial
     ##
     ## @example
     ## a0 + a1 * x + a2 * x^2 + @dots{} + an * x^n
     ## @end example
     ##
     ## @noindent
     ## from a vector of coefficients [a0 a1 a2 @dots{} an].
     ## @end deftypefn
     
     function p = polynomial (a)
     
       if (nargin > 1)
         print_usage ();
       endif
     
       if (nargin == 0)
         p.poly = [0];
         p = class (p, "polynomial");
       else
         if (strcmp (class (a), "polynomial"))
           p = a;
         elseif (isreal (a) && isvector (a))
           p.poly = a(:).';  # force row vector
           p = class (p, "polynomial");
         else
           error ("@polynomial: A must be a real vector");
         endif
       endif
     
       superiorto ("double");
     
     endfunction

   Note that user classes _always_ have higher precedence than built-in
Octave types.  Thus, marking the polynomial class higher than the
"double" class is not actually necessary.

   When confronted with two objects of equal precedence, Octave will use
the method of the object that appears first in the list of arguments.