Calls the given block with each successive codepoint from +self+;
each codepoint is the integer value for a character;
returns +self+:

  'hello'.each_codepoint {|codepoint| print codepoint, ' ' }
  print "\n"
  'тест'.each_codepoint {|codepoint| print codepoint, ' ' }
  print "\n"
  'こんにちは'.each_codepoint {|codepoint| print codepoint, ' ' }
  print "\n"

Output:

  104 101 108 108 111
  1090 1077 1089 1090
  12371 12435 12395 12385 12399

Returns an enumerator if no block is given.
