Missing Methods and Missing Constants
is a key part of Ruby’s method lookup algorithm (see Method Lookup) and provides a powerful way to catch and
handle arbitrary invocations on an object. The
const_missing method of
Module performs a
similar function for the constant lookup algorithm and allows us to
compute or lazily initialize constants on the fly. The examples that
follow demonstrate both of these methods.
Unicode Codepoint Constants with const_missing
Example 8-3 defines a
that appears to define a constant (a UTF-8 encoded string) for every Unicode
U+10FFFF. The only practical way to support
this many constants is to use the
const_missing method. The code makes the
assumption that if a constant is referenced once, it is likely to be
used again, so the
define a real constant to refer to each value it computes.
Example 8-3. Unicode codepoint constants with const_missing
# This module provides constants that define the UTF-8 strings for # all Unicode codepoints. It uses const_missing to define them lazily. # Examples: # copyright = Unicode::U00A9 # euro = Unicode::U20AC # infinity = Unicode::U221E module Unicode # This method allows us to define Unicode codepoint constants lazily. def self.const_missing(name) # Undefined constant passed as a symbol # Check that the constant name is of the right form. # Capital U followed by a hex number between 0000 and 10FFFF. if name.to_s ...
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