# Standard library header <stop_token> (C++20)

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< cpp‎ | header

This header is part of the thread support library.

### Classes
stop_token (C++20) | an interface for querying if a std::jthread cancellation request has been made (class)
stop_source (C++20) | class representing a request to stop one or more std::jthreads (class)
stop_callback (C++20) | an interface for registering callbacks on std::jthread cancellation (class template)
nostopstate_t (C++20) | placeholder type for use in `stop_source` constructor (class)
### Constants
nostopstate (C++20) | a `std::nostopstate_t` instance for use in `stop_source` constructor (constant)

### Synopsis

    namespace std {
      // class stop_token
      class stop_token;

      // class stop_source
      class stop_source;

      // no-shared-stop-state indicator
      struct nostopstate_t {
        explicit nostopstate_t() = default;
      };
      inline constexpr nostopstate_t nostopstate{};

      // class stop_callback
      template<class Callback>
      class stop_callback;
    }

#### Class std::stop_token

    namespace std {
      class stop_token {
      public:
        // constructors, copy, and assignment
        stop_token() noexcept;

        stop_token(const stop_token&) noexcept;
        stop_token(stop_token&&) noexcept;
        stop_token& operator=(const stop_token&) noexcept;
        stop_token& operator=(stop_token&&) noexcept;
        ~stop_token();
        void swap(stop_token&) noexcept;

        // stop handling
        [[nodiscard]] bool stop_requested() const noexcept;
        [[nodiscard]] bool stop_possible() const noexcept;

        [[nodiscard]]
        friend bool operator==(const stop_token& lhs, const stop_token& rhs) noexcept;
        friend void swap(stop_token& lhs, stop_token& rhs) noexcept;
      };
    }

#### Class std::stop_source

    namespace std {
      // no-shared-stop-state indicator
      struct nostopstate_t {
        explicit nostopstate_t() = default;
      };
      inline constexpr nostopstate_t nostopstate{};

      class stop_source {
      public:
        // constructors, copy, and assignment
        stop_source();
        explicit stop_source(nostopstate_t) noexcept;

        stop_source(const stop_source&) noexcept;
        stop_source(stop_source&&) noexcept;
        stop_source& operator=(const stop_source&) noexcept;
        stop_source& operator=(stop_source&&) noexcept;
        ~stop_source();
        void swap(stop_source&) noexcept;

        // stop handling
        [[nodiscard]] stop_token get_token() const noexcept;
        [[nodiscard]] bool stop_possible() const noexcept;
        [[nodiscard]] bool stop_requested() const noexcept;
        bool request_stop() noexcept;

        [[nodiscard]] friend bool
        operator==(const stop_source& lhs, const stop_source& rhs) noexcept;
        friend void swap(stop_source& lhs, stop_source& rhs) noexcept;
      };
    }

#### Class template std::stop_callback

    namespace std {
      template<class Callback>
      class stop_callback {
      public:
        using callback_type = Callback;

        // constructors and destructor
        template<class C>
        explicit stop_callback(const stop_token& st, C&& cb)
            noexcept(is_nothrow_constructible_v<Callback, C>);
        template<class C>
        explicit stop_callback(stop_token&& st, C&& cb)
            noexcept(is_nothrow_constructible_v<Callback, C>);
        ~stop_callback();

        stop_callback(const stop_callback&) = delete;
        stop_callback(stop_callback&&) = delete;
        stop_callback& operator=(const stop_callback&) = delete;
        stop_callback& operator=(stop_callback&&) = delete;

      private:
        Callback callback;      // exposition only
      };

      template<class Callback>
      stop_callback(stop_token, Callback) -> stop_callback<Callback>;
    }

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