ruby-concurrency/concurrent-ruby · error · ArgumentError

wrong number of arguments (0 for 1+)

Error message

wrong number of arguments (0 for 1+)

What it means

SettableStruct.new is the class factory that defines a new struct class, not an instance constructor: it requires at least one argument (a member list, optionally preceded by a String class name). Calling it with zero arguments raises ArgumentError('wrong number of arguments (0 for 1+)'). Instances are created afterwards by calling new (or []) on the generated class, where values are optional.

Source

Thrown at lib/concurrent-ruby/concurrent/settable_struct.rb:108

    rescue NoMethodError
      raise NameError.new("no member '#{member}' in struct")
    end

    private

    # @!visibility private
    def initialize_copy(original)
      synchronize do
        super(original)
        ns_initialize_copy
      end
    end

    # @!macro struct_new
    def self.new(*args, &block)
      clazz_name = nil
      if args.length == 0
        raise ArgumentError.new('wrong number of arguments (0 for 1+)')
      elsif args.length > 0 && args.first.is_a?(String)
        clazz_name = args.shift
      end
      FACTORY.define_struct(clazz_name, args, &block)
    end

    FACTORY = Class.new(Synchronization::LockableObject) do
      def define_struct(name, members, &block)
        synchronize do
          clazz = Synchronization::AbstractStruct.define_struct_class(SettableStruct, Synchronization::LockableObject, name, members, &block)
          members.each_with_index do |member, index|
            clazz.send :remove_method, member if clazz.instance_methods.include? member
            clazz.send(:define_method, member) do
              synchronize { @values[index] }
            end
            clazz.send(:define_method, "#{member}=") do |value|
              synchronize do
                unless @values[index].nil?

View on GitHub (pinned to 0b88d5ff75)

Solutions

1. Define the class with members: Point = Concurrent::SettableStruct.new(:x, :y)
2. Instantiate via the generated class: Point.new(1, 2) — values may be omitted there
3. If you only need a mutable record with no fixed schema, use a Hash or Struct from core Ruby instead

Example fix

# before
Struct = Concurrent::SettableStruct.new # ArgumentError

# after
Point = Concurrent::SettableStruct.new(:x, :y)
pt = Point.new   # instance constructor; values optional

raise ArgumentError, 'at least one member required' if members.empty?
Klass = Concurrent::SettableStruct.new(*members)

When it happens

Trigger: Concurrent::SettableStruct.new with no args; intending to instantiate an already-defined struct class but calling the factory by mistake (MyStruct is right, SettableStruct.new is wrong); copy-paste that drops the member symbols.

Common situations: Confusion between the definition step and the instantiation step, which differs from core Struct's ergonomics; scaffolding code generated without a member list.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• offset #{member} too large for struct(size:#{length})
• no member '#{member}' in struct
• struct size differs
• no block given
• Not all dependencies are IVars. Dependencies: #{ inputs.insp

AI-assisted analysis of ruby-concurrency/concurrent-ruby@0b88d5ff75 (2026-08-21). Data as JSON: /api/errors/3e7dba6b5ac876a6. Report an issue: GitHub.