Default_proc_struct

The normal behaviour for a Hash is to return nil if you trying to access a key that is not set. Therefore you can check that a key is set by hsh.key?(KEY) or by hsh[KEY]. As mentioned default_proc creates a key-val (or might do any operation) just accessing the hsh[KEY]. So.. puts hsh[:ove] will trigger default_proc and populate the Hash.

In this example I will use default_proc to create a Struct.

Dat = Struct.new(:letter, :number) {
    def set(*args) = ( self.letter, self.number = args ) 
}

hsh = Hash.new {|h, k| h[k] = Dat.new }

hsh[:one].letter = 'Z'
hsh[:one].number = 123

hsh[:two].set 'H', 88

p hsh
# =>
# {
#   one: #<struct Dat letter="Z", number=123>,
#   two: #<struct Dat letter="H", number=88>
# }