ruby hash 默认值的问题

参考：http://stackoverflow.com/questions/16159370/ruby-hash-default-value-behavior

使用ruby hash 默认值为空数组，向key 对应的value 追加值然后去get一个不存在的key 时候发现value为 一个非空的arry，不是默认值[]

具体使用示例如下：

 1 One default Array with mutation
 2
 3 hsh = Hash.new([])
 4
 5 hsh[:one] << ‘one‘
 6 hsh[:two] << ‘two‘
 7
 8 hsh[:nonexistent]
 9 # => [‘one‘, ‘two‘]
10 # Because we mutated the default value, nonexistent keys return the changed value
11
12 hsh
13 # => {}
14 # But we never mutated the hash itself, therefore it is still empty!
15 One default Array without mutation
16
17 hsh = Hash.new([])
18
19 hsh[:one] += [‘one‘]
20 hsh[:two] += [‘two‘]
21 # This is syntactic sugar for hsh[:two] = hsh[:two] + [‘two‘]
22
23 hsh[:nonexistant]
24 # => []
25 # We didn‘t mutate the default value, it is still an empty array
26
27 hsh
28 # => { :one => [‘one‘], :two => [‘two‘] }
29 # This time, we *did* mutate the hash.
30 A new, different Array every time with mutation
31
32 hsh = Hash.new { [] }
33 # This time, instead of a default *value*, we use a default *block*
34
35 hsh[:one] << ‘one‘
36 hsh[:two] << ‘two‘
37
38 hsh[:nonexistent]
39 # => []
40 # We *did* mutate the default value, but it was a fresh one every time.
41
42 hsh
43 # => {}
44 # But we never mutated the hash itself, therefore it is still empty!
45
46
47 hsh = Hash.new {|hsh, key| hsh[key] = [] }
48 # This time, instead of a default *value*, we use a default *block*
49 # And the block not only *returns* the default value, it also *assigns* it
50
51 hsh[:one] << ‘one‘
52 hsh[:two] << ‘two‘
53
54 hsh[:nonexistent]
55 # => []
56 # We *did* mutate the default value, but it was a fresh one every time.
57
58 hsh
59 # => { :one => [‘one‘], :two => [‘two‘], :nonexistent => [] }