[#absl-BlockingCounter]
= xref:absl.adoc[absl]::BlockingCounter
:relfileprefix: ../
:mrdocs:

A counter that lets a thread block until a number of actions complete.

== Synopsis

Declared in `<absl/synchronization/blocking_counter.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
class BlockingCounter;
----

== Description

This class allows a thread to block for a pre‐specified number of actions. `BlockingCounter` maintains a single non‐negative abstract integer "count" with an initial value `initial_count`. A thread can then call `Wait()` on this blocking counter to block until the specified number of events occur; worker threads then call `DecrementCount()` on the counter upon completion of their work. Once the counter's internal "count" reaches zero, the blocked thread unblocks.

A `BlockingCounter` requires the following:

* its `initial_count` is non‐negative.
* the number of calls to `DecrementCount()` on it is at most `initial_count`.
* `Wait()` is called at most once on it.

Given the above requirements, a `BlockingCounter` provides the following guarantees:

* Once its internal "count" reaches zero, no legal action on the object can further change the value of "count".
* When `Wait()` returns, it is legal to destroy the `BlockingCounter`.
* When `Wait()` returns, the number of calls to `DecrementCount()` on this blocking counter exactly equals `initial_count`.

Example:

[,cpp]
----
BlockingCounter bcount(N);         // there are N items of work
... Allow worker threads to start.
... On completing each work item, workers do:
... bcount.DecrementCount();      // an item of work has been completed

bcount.Wait();                    // wait for all work to be complete
----
== Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:absl/BlockingCounter/2constructor-02.adoc[`BlockingCounter`]         [.small]#[constructor]#
| Constructors
| xref:absl/BlockingCounter/operator_assign.adoc[`operator=`]  [.small]#[deleted]#
| Deleted copy assignment; `BlockingCounter` is not copyable.
| xref:absl/BlockingCounter/DecrementCount.adoc[`DecrementCount`] 
| Decrements the counter by one.
| xref:absl/BlockingCounter/Wait.adoc[`Wait`] 
| Blocks until the counter reaches zero.
|===



[.small]#Created with https://www.mrdocs.com[MrDocs]#