[#bsl-stop_callback-06-2constructor-04]
= xref:bsl.adoc[bsl]::xref:bsl/stop_callback-06.adoc[stop_callback]::stop_callback
:relfileprefix: ../../
:mrdocs:

Create a `stop_callback` that stores a callable constructed from `arg` and registers it with the stop state of `token`, if any.

== Synopsis

Declared in `<bslstl_stoptoken.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<class t_ARG>
explicit
stop_callback(
    xref:bsl/stop_token.adoc[stop_token] const& token,
    t_ARG&& arg,
    xref:bsl/stop_callback-06/allocator_type.adoc[allocator_type] const& alloc = allocator_type());
----

== Description

If `t_CALLBACK` is allocator‐aware, the optionally specified `alloc` supplies memory; otherwise `alloc` is ignored. If `token` already refers to a stop‐requested state, invoke the callback before returning; otherwise register it with that stop state. Unlike `std::stop_callback`, these constructors are not currently `noexcept`. If `token` is an rvalue reference, it is unspecified whether this function moves from `token`.


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