tycooon · nevinera · Dec 21, 2023 · Dec 21, 2023
diff --git a/README.md b/README.md
@@ -245,6 +245,10 @@ a.users {}
 
 However, this solution is kind of hacky.
 
+## Object Shape Optimization
+
+In Ruby 3.2, a new optimization was introduced that relies on tracking "object shape", and has negative interactions with dynamically added instance variables (introduced after initialization-time). See [here](https://bugs.ruby-lang.org/issues/18776) for details, or [here](https://railsatscale.com/2023-10-24-memoization-pattern-and-object-shapes/) for an explanation of the relevance, but fundamentally Memery has substantial advantages over standard memoization techniques here - it introduces only _one_ new instance variable after initialization (`@_memery_memoized_values`). For particularly performance-critical classes however, you may wish to reduce that to zero; if so, you can call `setup_memery_cache!` in the initializer of your class, and it will set the instance variable before the object shape is determined.
+
 ## Contributing
 
 Bug reports and pull requests are welcome on GitHub at https://github.com/tycooon/memery.

diff --git a/lib/memery.rb b/lib/memery.rb
@@ -115,6 +115,13 @@ def method_visibility(klass, method_name)
   end
 
   module InstanceMethods
+    # This method is not _functionally necessary_, but if called in the
+    # initializer of a class, it allows that class to avoid any issue with the
+    # "object shape optimization" introduced in Ruby 3.2.
+    def setup_memery_cache!
+      @_memery_memoized_values = {}
+    end
+
     def clear_memery_cache!
       @_memery_memoized_values = {}
     end

diff --git a/spec/memery_spec.rb b/spec/memery_spec.rb
@@ -148,6 +148,14 @@ class H
   memoize :x, :y, ttl: 3
 end
 
+class I
+  include Memery
+
+  def initialize
+    setup_memery_cache!
+  end
+end
+
 RSpec.describe Memery do
   subject(:a) { A.new }
 
@@ -475,4 +483,12 @@ class H
       include_examples "works correctly"
     end
   end
+
+  describe "#setup_memery_cache!" do
+    let(:i) { I.new }
+
+    specify do
+      expect(i).to be_instance_variable_defined("@_memery_memoized_values")
+    end
+  end
 end