[#BloombergLP-bslmt-QLock_EventFlag]
= xref:BloombergLP.adoc[BloombergLP]::xref:BloombergLP/bslmt.adoc[bslmt]::QLock_EventFlag
:relfileprefix: ../../
:mrdocs:

[*PRIVATE*]This class provides a thread‐safe mechanism for one thread to inform another thread that some event has occurred. A flag provides two primary manipulators, `set`, which indicates the event has occurred, and `waitUntilSet`, which waits until that event has occurred (or returns immediately if it has already occurred). A flag is intended to be used by only two threads: a thread setting the flag, and a thread waiting for the flag to be set, and the behavior is undefined if `set` is called while the flag is already set, or if `waitUntilSet` is called while another thread is waiting for the flag.

== Synopsis

Declared in `<bslmt_qlock.h>`

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

== Description

This class is an implementation detail of the `bslmt_qlock`, and must not be used by client code.

== Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bslmt/QLock_EventFlag/2constructor.adoc[`QLock_EventFlag`]         [.small]#[constructor]#
| Create an unset flag.
| xref:BloombergLP/bslmt/QLock_EventFlag/2destructor.adoc[`~QLock_EventFlag`] [.small]#[destructor]#
| Destroy this flag. The behavior is undefined if a thread is currently waiting for the flag to be set.
| xref:BloombergLP/bslmt/QLock_EventFlag/reset.adoc[`reset`] 
| Reset this flag to the unset state. The behavior is undefined if a thread is waiting for this flag to be set.
| xref:BloombergLP/bslmt/QLock_EventFlag/set.adoc[`set`] 
| Set this flag, and if a thread is waiting for it, signal the waiting thread. The behavior is undefined if this flag is already set.
| xref:BloombergLP/bslmt/QLock_EventFlag/waitUntilSet.adoc[`waitUntilSet`] 
| Wait until this flag has been set (returning immediately if this flag is already set), and, if this flag is not already set, spin for the specified `spinCount` iterations before waiting on a semaphore. The behavior is undefined unless there are no other threads waiting for this flag to be set.
|===



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