# std::stop_callback<Callback>::stop_callback

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< cpp‎ | thread‎ | stop callback

template<class C> explicit stop_callback( const std::stop_token& st, C&& cb ) noexcept(/*see below*/); | (1) | (since C++20)
template<class C> explicit stop_callback( std::stop_token&& st, C&& cb ) noexcept(/*see below*/); | (2) | (since C++20)
stop_callback( const stop_callback& ) = delete; | (3) | (since C++20)
stop_callback( stop_callback&& ) = delete; | (4) | (since C++20)

Constructs a new `stop_callback` object, saving and registering the `cb` callback function into the given `std::stop_token`'s associated stop-state, for later invocation if stop is requested on the associated `std::stop_source`.

1) Constructs a `stop_callback` for the given `st` `std::stop_token` (copied), with the given invocable callback function `cb`.
2) Constructs a `stop_callback` for the given `st` `std::stop_token` (moved), with the given invocable callback function `cb`.
3-4) `stop_callback` is neither CopyConstructible nor MoveConstructible.

Both constructors participate overload resolution only if `Callback` and `C` satisfy `constructible_from` of std::constructible_from<Callback, C>. If `Callback` and `C` satisfy the concept but fail to satisfy its semantic requirement, the behavior is undefined.

### Parameters

st | - | a `std::stop_token` object to register this `stop_callback` object with
cb | - | the type to invoke if stop is requested

### Exceptions

1-2)
`noexcept` specification:
noexcept(std::is_nothrow_constructible_v<Callback, C>)
Any exception thrown by constructor-initializing the given callback into the `stop_callback` object.

### Notes

If st.stop_requested() == true for the passed-in `std::stop_token`, then the callback function is invoked in the current thread before the constructor returns.

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