Pattern: Hash creation with mutable default value
Issue: -
Checks for Hash creation with a mutable default value. Creating a Hash in such a way will share the default value across all keys, causing unexpected behavior when modifying it.
For example, when the Hash was created with an Array as the argument, calling hash[:foo] << 'bar' will also change the value of all other keys that have not been explicitly assigned to.
# bad
Hash.new([])
Hash.new({})
Hash.new(Array.new)
Hash.new(Hash.new)
# okay -- In rare cases that intentionally have this behavior,
# without disabling the cop, you can set the default explicitly.
h = Hash.new
h.default = []
h[:a] << 1
h[:b] << 2
h # => {:a => [1, 2], :b => [1, 2]}
# okay -- beware this will discard mutations and only remember assignments
Hash.new { Array.new }
Hash.new { Hash.new }
Hash.new { {} }
Hash.new { [] }
# good - frozen solution will raise an error when mutation attempted
Hash.new([].freeze)
Hash.new({}.freeze)
# good - using a proc will create a new object for each key
h = Hash.new
h.default_proc = ->(h, k) { [] }
h.default_proc = ->(h, k) { {} }
# good - using a block will create a new object for each key
Hash.new { |h, k| h[k] = [] }
Hash.new { |h, k| h[k] = {} }