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Feature #16601

Updated by sawa (Tsuyoshi Sawada) 6 months ago

Now, `nil.to_s` returns a fixed instance: 

 ```ruby 
 nil.to_s.object_id # => 440 
 nil.to_s.object_id # => 440 
 nil.to_s.object_id # => 440 
 ... 
 ``` 

 This is useful when we have some variable `foo` which may be either `nil` or a string, and we want to check its emptiness in a condition: 

 ```ruby 
 if foo.to_s.empty?; ... end 
 ``` 

 By this feature, we do not (need to) create a new instance of an empty string each time we check `foo`, even when it happens to be `nil`. 

 There are similar situations with arrays and hashes. We may have variable `bar` which may be either `nil` or an array, or `baz` which may be either `nil` or a hash, and we want to check their emptiness in conditions as follows: 

 ```ruby 
 if bar.to_a.empty?; ... end 
 ``` 

 ```ruby 
 if baz.to_h.empty?; ... end 
 ``` 

 But unlike `nil.to_s`, the methods `nil.to_a` and `nil.to_h` create new instances of empty array or hash each time they are called: 

 ```ruby ``` 
 nil.to_a.object_id # => 540 
 nil.to_a.object_id # => 560 
 nil.to_a.object_id # => 580 
 ... 

 nil.to_h.object_id # => 460 
 nil.to_h.object_id # => 480 
 nil.to_h.object_id # => 500 
 ... 
 ``` 

 The fact that this is somewhat inefficient discourages the use of `foo.to_a` or `foo.to_h` in such use cases. 

 I request `nil.to_a` to `nil.to_h` to return a fixed empty instance.

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