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Feature #16038

Updated by sawa (Tsuyoshi Sawada) over 5 years ago

I know `ObjectSpace::WeakMap` isn't really supposed to be used, and that the blessed interface is `WeakRef`. However, `WeakRef`, however I'd like to make a case for a better public WeakMap. 

 ### Usage 

 As described in [Feature #16035], `WeakMap` is are useful for deduplicating "value objects". A typical use case is as follows: follow: 

 ```ruby 
 class Position 
   REGISTRY = {} 
   private_constant :REGISTRY 

   class << self 
     def new(*) 
       instance = super 
       REGISTRY[instance] ||= instance 
     end 
   end 

   attr_reader :x, :y, :z 

   def initialize(x, y, z) 
     @x = x 
     @y = y 
     @z = z 
     freeze 
   end 

   def hash 
     self.class.hash ^ 
       x.hash >> 1 ^ 
       y.hash >> 2 ^ 
       y.hash >> 3 
   end 

   def ==(other) 
     other.is_a?(Position) && 
       other.x == x && 
       other.y == y && 
       other.z == z 
   end 
   alias_method :eql?, :== 
 end 

 p Position.new(1, 2, 3).equal?(Position.new(1, 2, 3)) 
 ``` 

 That's pretty much the pattern [I used in Rails to deduplicate database metadata and save lots of memory](https://github.com/rails/rails/blob/f3c68c59ed57302ca54f4dfad0e91dbff426962d/activerecord/lib/active_record/connection_adapters/deduplicable.rb). 

 The big downside here is that these value objects can't be GCed anymore, so this pattern is not viable in many case. 

 ### Why not use WeakRef 

 A couple of reasons. First, reasons, first when using this pattern, pattern the goal is to reduce memory usage, so having one extra `WeakRef` for every single value object is a bit counter productive.  

 Then it's a bit annoying to work with, as you have to constantly check wether the reference is still alive, and/or rescue `WeakRef::RefError`. 

 Often, these two complications make the tradeoff not worth it. 

 ### Ruby 2.7 

 Since [Feature #13498] `WeakMap` is a bit more usable as you can now use an interned string as the unique key, e.g. 

 ```ruby 
 class Position 
   REGISTRY = ObjectSpace::WeakMap.new 
   private_constant :REGISTRY 

   class << self 
     def new(*) 
       instance = super 
       REGISTRY[instance.unique_id] ||= instance 
     end 
   end 

   attr_reader :x, :y, :z, :unique_id 

   def initialize(x, y, z) 
     @x = x 
     @y = y 
     @z = z 
     @unique_id = -"#{self.class}-#{x},#{y},#{z}" 
     freeze 
   end 

   def hash 
     self.class.hash ^ 
       x.hash >> 1 ^ 
       y.hash >> 2 ^ 
       y.hash >> 3 
   end 

   def ==(other) 
     other.is_a?(Position) && 
       other.x == x && 
       other.y == y && 
       other.z == z 
   end 
   alias_method :eql?, :== 
 end 

 p Position.new(1, 2, 3).equal?(Position.new(1, 2, 3)) 
 ``` 

 That makes the pattern much easier to work with than dealing with `WeakRef`, but there is still that an extra instance. 

 ### Proposal 

 What would be ideal would be a `WeakMap` that works by equality, so that the first snippet could simply replace `{}` by `WeakMap.new`.  

 Changing `ObjectSpace::WeakMap`'s Since changing `ObjectSpace::WeakMap` behavior would cause issues, and I can see two possibilities: possiblities: 

   - The best IMO would be to have a new top level `::WeakMap` be the equality based map, and have `ObjectSpace::WeakMap` would remain as a semi-private interface for backing up `WeakRef`. 
   - Or alternatively, alternatively `ObjectSpace::WeakMap` could have a `compare_by_equality` method (inverse of `Hash#compare_by_identity`) to change its it's behavior post instantiation. 

 I personally prefer the first one. 

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