[#BloombergLP-bsls-BslOnce-enter]
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:mrdocs:

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.

== Synopsis

Declared in `<bsls_bslonce.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
bool
enter();
----


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