I am trying to make the fastest #to_struct method in Ruby's Hash.

I am including a use case and benchmark so you can run and see if you have really improved the code.

This is my implementation and the benchmark is included. The time at the bottom is the time it takes on my machine. How can I make this faster?

require "json"
require 'benchmark'
require 'bigdecimal/math'

class Hash
  def to_struct
    k = self.keys
    klass = k.map(&:to_s).sort_by {|word| word.downcase}.join.capitalize
      Kernel.const_get("Struct::" + klass).new(*self.values_at(*k))
    rescue NameError
      Struct.new(klass, *(k)).new(*self.values_at(*k))

# You have a hash that you have built in your app
sample_hash = {
  foo_key: "foo_val",
  bar_key: "bar_val",
  baz_key: "baz_val",
  foo1_key: "foo_val",
  bar1_key: "bar_val",
  baz1_key: "baz_val",
  foo2_key: "foo_val",
  bar2_key: "bar_val",
  baz2_key: "baz_val",
  foo3_key: "foo_val",
  bar3_key: "bar_val",
  baz3_key: "baz_val",
  foo4_key: "foo_val",
  bar4_key: "bar_val",
  baz4_key: "baz_val",
  foo5_key: "foo_val",
  bar5_key: "bar_val",
  baz5_key: "baz_val",
  foo6_key: "foo_val",
  bar6_key: "bar_val",
  baz6_key: "baz_val",
  foo7_key: "foo_val",
  bar7_key: "bar_val",
  baz7_key: "baz_val",

# Then you have JSON coming from some external api
json_response = "{\"qux_key\":\"qux_val\",\"quux_key\":\"quux_val\",\"corge_key\":\"corge_val\"}"
hash_with_unknown_keys = JSON.parse(json_response)

# Merge these two together

iterations = 100_000

Benchmark.bm do |bm|
  bm.report "#to_struct" do
    iterations.times do
      # Would be super nice if I could convert this to a struct with a method
      # Somehow a bit faster than the explicit example below and much faster than open struct
      sample_struct = sample_hash.to_struct
      unless sample_struct.foo_key == "foo_val"
        raise "Wrong value"

  bm.report "Struct" do
    iterations.times do
      sample_struct = Struct.new(*sample_hash.keys)
      unless sample_struct.foo_key == "foo_val"
        raise "Wrong value"

  bm.report "OpenStruct" do
    iterations.times do
      sample_open_struct = OpenStruct.new(sample_hash)
      unless sample_open_struct.foo_key  == "foo_val"
        raise "Wrong value"
#       user     system      total        real
# #to_struct  4.030000   0.010000   4.040000 (  4.072031)
#     Struct  6.870000   0.290000   7.160000 (  7.320459)
# OpenStruct 23.550000   0.210000  23.760000 ( 23.895187)
  • \$\begingroup\$ The benchmark is testing the time it takes to run #to_struct together with the time it takes to access the structure's attributes. Is that correct? \$\endgroup\$ May 24, 2014 at 22:44
  • \$\begingroup\$ @WayneConrad The benchmark has three different ways to instantiate similar ruby objects. The first two are two different ways to create structs and the third instantiates an open struct. And yes all three benchmarks access the objects attributes. \$\endgroup\$
    – mpiccolo
    May 27, 2014 at 3:14
  • 2
    \$\begingroup\$ I think the reason that the to_struct version is faster than the plain Struct version is that it's not doing the full task on each iteration. It only constructs a new Struct subclass on the first iteration, and then re-uses it on all the subsequent iterations. It's therefore not a strictly fair comparison, as the Struct method has to create a new class for each iteration. I think you'd find that if you randomised the hash keys for each iteration, there would be little difference between the to_struct version and the Struct version. \$\endgroup\$
    – AlexT
    Jul 5, 2014 at 12:15
  • \$\begingroup\$ And why do the key joining to get a struct class name? It's optional, any reason to have it? \$\endgroup\$ Sep 4, 2016 at 16:30

1 Answer 1


Use OpenHash and Ruby >= 2.3.0

Starting with MRI 2.3.0, your benchmark using OpenHash gets fast. Very fast:

ruby-2.2.5: ruby 2.2.5p319 (2016-04-26 revision 54774) [x86_64-linux]
       user     system      total        real
#to_struct  1.780000   0.000000   1.780000 (  1.774490)
Struct  9.100000   0.000000   9.100000 (  9.099619)
OpenStruct  7.910000   0.000000   7.910000 (  7.911342)

ruby-2.3.0: ruby 2.3.0p0 (2015-12-25 revision 53290) [x86_64-linux]
       user     system      total        real
#to_struct  1.700000   0.000000   1.700000 (  1.695587)
Struct  7.660000   0.000000   7.660000 (  7.660869)
OpenStruct  0.650000   0.000000   0.650000 (  0.658817)

With the latest MRI, Your #to_struct method gets a bit of a speed boost as well.

ruby-2.4.1: ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-linux]
       user     system      total        real
#to_struct  1.460000   0.000000   1.460000 (  1.459063)
Struct  7.420000   0.000000   7.420000 (  7.416505)
OpenStruct  0.660000   0.000000   0.660000 (  0.658009)

So if you can, use Ruby >= ruby 2.3.0, and use OpenHash.

How to make #to_struct faster

I made the following changes for performance:

and these for clarity:

  • Eliminate the temporary for self.keys
  • DRY the creation of the struct instance
  • Removed self references.

With these changes, the code is:

class Hash
  def new_to_struct
    klass_name = keys.map(&:to_s).sort.join.capitalize
    klass = begin
              Kernel.const_get("Struct::" + klass_name)
            rescue NameError
              Struct.new(klass_name, *keys)

and the benchmark (run against ruby-2.4.1):

       user     system      total        real
#to_struct  1.410000   0.000000   1.410000 (  1.403908)
#new_to_struct  0.760000   0.000000   0.760000 (  0.757548)
Struct  7.060000   0.010000   7.070000 (  7.075619)
OpenStruct  0.650000   0.000000   0.650000 (  0.649057)

These changes get to_struct close to OpenStruct, but still not as fast.


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