# class Sync::Exclusive(T)

- Sync::Exclusive(T)
- Reference
- Object

##
Overview

Safely share a value `T` across fibers and execution contexts using a `Mutex`, so only one critical section can access the value at any time.

For example:

    require "sync/exclusive"

    class Queue
      @@running : Sync::Exclusive.new([] of Queue)

      def self.on_started(queue)
        @@running.lock(&.push(queue))
      end

      def self.on_stopped(queue)
        @@running.lock(&.delete(queue))
      end

      def self.each(&)
        @@running.lock do |list|
          list.each { |queue| yield queue }
        end
      end
    end

Consider an `Exclusive(T)` if your workload mostly needs to own the value, and most, if not all, critical sections need to mutate the inner state of the value for example.

##
Included Modules

- Sync::Lockable

##
Defined in:

sync/exclusive.cr

##
Constructors

- .new(value : T, type : Type = :checked)

##
Instance Method Summary

- #get : T

  Locks the mutex and returns the value.

- #lock(& : T -> U) : U forall U

  Locks the mutex and yields the value.

- #replace(& : T -> T) : Nil

  Locks the mutex, yields the value and eventually replaces the value with the one returned by the block.

- #set(value : T) : Nil

  Locks the mutex and sets the value.

- #unsafe_get : T

  Returns the value without any synchronization.

- #unsafe_set(value : T) : T

  Sets the value without any synchronization.

### Instance methods inherited from class `Reference`

==(other : self)
==(other : JSON::Any)
==(other : YAML::Any)
==(other) ==, dup dup, hash(hasher) hash, initialize initialize, inspect(io : IO) : Nil inspect, object_id : UInt64 object_id, pretty_print(pp) : Nil pretty_print, same?(other : Reference) : Bool
same?(other : Nil) same?, to_s(io : IO) : Nil to_s

### Constructor methods inherited from class `Reference`

new new, unsafe_construct(address : Pointer, *args, **opts) : self unsafe_construct

### Class methods inherited from class `Reference`

pre_initialize(address : Pointer) pre_initialize

### Instance methods inherited from class `Object`

! : Bool !, !=(other) !=, !~(other) !~, ==(other) ==, ===(other : JSON::Any)
===(other : YAML::Any)
===(other) ===, =~(other) =~, as(type : Class) as, as?(type : Class) as?, class class, dup dup, hash(hasher)
hash hash, in?(collection : Object) : Bool
in?(*values : Object) : Bool in?, inspect(io : IO) : Nil
inspect : String inspect, is_a?(type : Class) : Bool is_a?, itself itself, nil? : Bool nil?, not_nil!(message)
not_nil! not_nil!, pretty_inspect(width = 79, newline = "\n", indent = 0) : String pretty_inspect, pretty_print(pp : PrettyPrint) : Nil pretty_print, responds_to?(name : Symbol) : Bool responds_to?, tap(&) tap, to_json(io : IO) : Nil
to_json : String to_json, to_pretty_json(indent : String = " ") : String
to_pretty_json(io : IO, indent : String = " ") : Nil to_pretty_json, to_s(io : IO) : Nil
to_s : String to_s, to_yaml(io : IO) : Nil
to_yaml : String to_yaml, try(&) try, unsafe_as(type : T.class) forall T unsafe_as

### Class methods inherited from class `Object`

from_json(string_or_io : String | IO, root : String)
from_json(string_or_io : String | IO) from_json, from_yaml(string_or_io : String | IO) from_yaml

### Macros inherited from class `Object`

class_getter(*names, &block) class_getter, class_getter!(*names) class_getter!, class_getter?(*names, &block) class_getter?, class_property(*names, &block) class_property, class_property!(*names) class_property!, class_property?(*names, &block) class_property?, class_setter(*names) class_setter, def_clone def_clone, def_equals(*fields) def_equals, def_equals_and_hash(*fields) def_equals_and_hash, def_hash(*fields) def_hash, delegate(*methods, to object) delegate, forward_missing_to(delegate) forward_missing_to, getter(*names, &block) getter, getter!(*names) getter!, getter?(*names, &block) getter?, property(*names, &block) property, property!(*names) property!, property?(*names, &block) property?, setter(*names) setter

##
Constructor Detail

def self.new(value : T, type : Type = :checked) #

##
Instance Method Detail

def get : T #

Locks the mutex and returns the value. Unlocks before returning.

Always acquires the lock, so reading the value is synchronized in relation with the other methods. However, safely accessing the returned value entirely depends on the safety of `T`.

Prefer `#lock(&.dup)` or `#lock(&.clone)` to get a shallow or deep copy of the value instead.

WARNING Breaks the mutual exclusion guarantee since the returned value outlives the lock, the value can be accessed concurrently to the synchronized methods.

def lock(& : T -> U) : U forall U #

Locks the mutex and yields the value. The lock is released before returning.

The value is owned for the duration of the block, and can be safely mutated.

WARNING The value musn't be retained and accessed after the block has returned.

def replace(& : T -> T) : Nil #

Locks the mutex, yields the value and eventually replaces the value with the one returned by the block. The lock is released before returning.

The current value is now owned: it can be safely retained and mutated even after the block returned.

WARNING The new value musn't be retained and accessed after the block has returned.

def set(value : T) : Nil #

Locks the mutex and sets the value. Unlocks the mutex before returning.

Always acquires and releases the lock, so writing the value is always synchronized with the other methods.

def unsafe_get : T #

Returns the value without any synchronization.

WARNING Breaks the mutual exclusion constraint! Should only be called after acquiring the lock.

def unsafe_set(value : T) : T #

Sets the value without any synchronization.

WARNING Breaks the mutual exclusion constraint! Should only be called after acquiring the lock.