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### Signal token requirements

A signal token is a completion token for completion signature `void(error_code, int)`.

##### Examples

A free function as a signal token:

    void signal_handler(
        const boost::system::error_code& ec,
        int signal_number)
    {
      ...
    }

A signal token function object:

    struct signal_handler
    {
      ...
      void operator()(
          const boost::system::error_code& ec,
          int signal_number)
      {
        ...
      }
      ...
    };

A lambda as a signal token:

    signal_set.async_wait(...,
        [](const boost::system::error_code& ec,
          int signal_number)
        {
          ...
        });

A non-static class member function adapted to a signal token using `std::bind()`:

    void my_class::signal_handler(
        const boost::system::error_code& ec,
        int signal_number)
    {
      ...
    }
    ...
    signal_set.async_wait(...,
        std::bind(&my_class::signal_handler,
          this, std::placeholders::_1,
          std::placeholders::_2));

A non-static class member function adapted to a signal token using `boost::bind()`:

    void my_class::signal_handler(
        const boost::system::error_code& ec,
        int signal_number)
    {
      ...
    }
    ...
    signal_set.async_wait(...,
        boost::bind(&my_class::signal_handler,
          this, boost::asio::placeholders::error,
          boost::asio::placeholders::signal_number));

Using use_future as a signal token:

    std::future<int> f = signal_set.async_wait(..., boost::asio::use_future);
    ...
    try
    {
      int signo = f.get();
      ...
    }
    catch (const system_error& e)
    {
      ...
    }

Using use_awaitable as a signal token:

    boost::asio::awaitable<void> my_coroutine()
    {
      try
      {
        ...
        int signo =
          co_await signal_set.async_wait(
              ..., boost::asio::use_awaitable);
        ...
      }
      catch (const system_error& e)
      {
        ...
      }
    }