[#BloombergLP-bslmt-Once-callOnce-03]
= xref:BloombergLP.adoc[BloombergLP]::xref:BloombergLP/bslmt.adoc[bslmt]::xref:BloombergLP/bslmt/Once.adoc[Once]::callOnce
:relfileprefix: ../../../
:mrdocs:

Call `function` at most once across all threads for this object&period;

== Synopsis

Declared in `&lt;bslmt&lowbar;once&period;h&gt;`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template&lt;class FUNC&gt;
void
callOnce(FUNC const& function);
----

== Description

If no other thread has yet called `enter` or `callOnce`, then call the specified `function` and set this object to the state where pending and future calls to `enter` or `callOnce` will return `false` or do nothing, respectively&period; Otherwise, wait for the one&hyphen;time code to complete and return without calling `function` where `function` is a function or functor that can be called with no arguments&period; Note that one&hyphen;time code is considered not to have run if `function` terminates with an exception&period;

== Parameters

[cols="1,4"]
|===
| Name| Description
| *function*
| no&hyphen;argument function or functor to invoke at most once
|===


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