[#BloombergLP-bsls-BslOnce]
= xref:BloombergLP.adoc[BloombergLP]::xref:BloombergLP/bsls.adoc[bsls]::BslOnce
:relfileprefix: ../../
:mrdocs:

This `struct` provides a simple data type for ensuring a block of code is executed (only) once. Note that this is defined as a `struct` to allow constant initialization in a global or static context using `BSLS_BSLONCE_INITIALIZER`.

== Synopsis

Declared in `<bsls_bslonce.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
struct BslOnce;
----

== Member Functions

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bsls/BslOnce/enter.adoc[`enter`] 
| Enter the one‐time block of code. Return `true` if the one‐time block of code has been entered, and `false` if the one‐time block of code has already been executed. If this function returns `false` then the thread of execution in which `enter` returned `true` has already called `leave` ‐‐ i.e., the one‐time block of code is guaranteed to have _completed_ execution. The behavior is undefined unless this object was originally initialized to `BSLS_BSLONCE_INITIALIZER`. Note that a successful `enter` locks a spin‐lock; it is imperative that `leave` be called quickly.
| xref:BloombergLP/bsls/BslOnce/leave.adoc[`leave`] 
| Exit the one‐time block of code. The behavior is undefined unless the caller had previously called `enter`, and `enter` had returned `true`.
|===


== Data Members

[cols="1,4"]
|===
| Name| Description
| xref:BloombergLP/bsls/BslOnce/d_onceState.adoc[`d_onceState`] 
| State of the one‐time block of code managed by this object.
|===



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