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: 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: 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 == 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.