# std::jthread

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< cpp‎ | thread

Defined in header `<thread>` |  |
class jthread; |  | (since C++20)

The class `jthread` represents a single thread of execution. It has the same general behavior as std::thread, except that `jthread` automatically rejoins on destruction, and can be cancelled/stopped in certain situations.

Threads begin execution immediately upon construction of the associated thread object (pending any OS scheduling delays), starting at the top-level function provided as a constructor argument. The return value of the top-level function is ignored and if it terminates by throwing an exception, std::terminate is called. The top-level function may communicate its return value or an exception to the caller via std::promise or by modifying shared variables (which may require synchronization, see std::mutex and std::atomic).

Unlike std::thread, the `jthread` logically holds an internal private member of type `std::stop_source`, which maintains a shared stop-state. The `jthread` constructor accepts a function that takes a std::stop_token as its first argument, which will be passed in by the `jthread` from its internal std::stop_source. This allows the function to check if stop has been requested during its execution, and return if it has.

`std::jthread` objects may also be in the state that does not represent any thread (after default construction, move from, detach, or join), and a thread of execution may be not associated with any `jthread` objects (after detach).

No two `std::jthread` objects may represent the same thread of execution; `std::jthread` is not CopyConstructible or CopyAssignable, although it is MoveConstructible and MoveAssignable.

### Member types

Member type | Definition
`id` | `std::thread::id`
`native_handle_type`(not always present) | `std::thread::native_handle_type`

### Member functions

(constructor) | constructs new `jthread` object (public member function)
(destructor) | if the thread is joinable, then a stop is requested and the thread joins (public member function)
operator= | moves the jthread object (public member function)
##### Observers
joinable | checks whether the thread is joinable, i.e. potentially running in parallel context (public member function)
get_id | returns the id of the thread (public member function)
native_handle | returns the underlying implementation-defined thread handle (public member function)
hardware_concurrency [static] | returns the number of concurrent threads supported by the implementation (public static member function)
##### Operations
join | waits for the thread to finish its execution (public member function)
detach | permits the thread to execute independently from the thread handle (public member function)
swap | swaps two jthread objects (public member function)
##### Stop token handling
get_stop_source | returns a stop_source object associated with the shared stop state of the thread (public member function)
get_stop_token | returns a stop_token associated with the shared stop state of the thread (public member function)
request_stop | requests execution stop via the shared stop state of the thread (public member function)

### Non-member functions

swap(std::jthread) (C++20) | specializes the std::swap algorithm (function)

### Notes

Feature-test macro | Value | Std | Comment
--- | --- | --- | ---
`__cpp_lib_jthread` | 201911L | (C++20) | Stop token and joining thread

### See also

thread (C++11) | manages a separate thread (class)

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