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

Objects of this type are used to distinguish calls to `call_once()` and ensure the provided function is only invoked once across all threads. This type is not copyable or movable. However, it has a `constexpr` constructor, and is safe to use as a namespace‐scoped global variable.

== Synopsis

Declared in `<absl/base/call_once.h>`

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

== Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:absl/once_flag/2constructor-0c.adoc[`once_flag`]         [.small]#[constructor]#
| Constructors
| xref:absl/once_flag/operator_assign.adoc[`operator=`]  [.small]#[deleted]#
| Deleted: `once_flag` is not copy‐assignable.
|===


== Non-Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:absl/call_once.adoc[`call_once`]
| For all invocations using a given `once_flag`, invokes a given `fn` exactly once across all threads. The first call to `call_once()` with a particular `once_flag` argument (that does not throw an exception) will run the specified function with the provided `args`; other calls with the same `once_flag` argument will not run the function, but will wait for the provided function to finish running (if it is still running).
|===


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