From: matz@...
Date: 2019-08-29T06:44:04+00:00
Subject: [ruby-core:94650] [Ruby master Feature#16035] Allow non-finalizable objects such as Integer, static Symbol etc in ObjectSpace::WeakMap

Issue #16035 has been updated by matz (Yukihiro Matsumoto).


I think this is the required behavior for `WeakMap` to implement a cache for example. Accepted.

Matz.


----------------------------------------
Feature #16035: Allow non-finalizable objects such as Integer, static Symbol etc in ObjectSpace::WeakMap
https://bugs.ruby-lang.org/issues/16035#change-81249

* Author: byroot (Jean Boussier)
* Status: Assigned
* Priority: Normal
* Assignee: nobu (Nobuyoshi Nakada)
* Target version: 
----------------------------------------
This goes one step farther than what @nobu did in https://bugs.ruby-lang.org/issues/13498

With this patch, special objects such as static symbols, integers, etc can be used as either key or values inside WeakMap. They simply don't have a finalizer defined on them.

This is useful if you need to deduplicate value objects, e.g. some minimal use case:

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

  def self.new(amount)
    REGISTRY[amount] ||= super.freeze
  end

  def initialize(amount)
    @amount = amount
  end
end

if Money.new(42).eql?(Money.new(42))
  puts "Same instance"
else
  puts "Different instances"
end
```

This is a very simple example, but more complex examples can create use a dynamically created symbol as deduplication key, etc.

It also removes one weirdness introduced in the mentioned patch:

```ruby
wmap = ObjectSpace::WeakMap.new
wmap["foo".to_sym] = Object.new # works fine with dynamic symbols
wmap[:bar] = Object.new # cannot define finalizer for Symbol (ArgumentError)
```

Proposed patch: https://github.com/ruby/ruby/pull/2313



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