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#### Strands: Use Threads Without Explicit Locking

A strand is defined as a strictly sequential invocation of event handlers (i.e. no concurrent invocation). Use of strands allows execution of code in a multithreaded program without the need for explicit locking (e.g. using mutexes).

Strands may be either implicit or explicit, as illustrated by the following alternative approaches:

- Calling io_context::run() from only one thread means all event handlers execute in an implicit strand, due to the io_context's guarantee that handlers are only invoked from inside run().
- Where there is a single chain of asynchronous operations associated with a connection (e.g. in a half duplex protocol implementation like HTTP) there is no possibility of concurrent execution of the handlers. This is an implicit strand.
- An explicit strand is an instance of `strand<>` or `io_context::strand`. All event handler function objects need to be bound to the strand using `boost::asio::bind_executor()` or otherwise posted/dispatched through the strand object.

In the case of composed asynchronous operations, such as `async_read()` or `async_read_until()`, if a completion handler goes through a strand, then all intermediate handlers should also go through the same strand. This is needed to ensure thread safe access for any objects that are shared between the caller and the composed operation (in the case of `async_read()` it's the socket, which the caller can `close()` to cancel the operation).

To achieve this, all asynchronous operations obtain the handler's associated executor by using the `get_associated_executor` function. For example:

    boost::asio::associated_executor_t<Handler> a = boost::asio::get_associated_executor(h);

The associated executor must satisfy the Executor requirements. It will be used by the asynchronous operation to submit both intermediate and final handlers for execution.

The executor may be customised for a particular handler type by specifying a nested type `executor_type` and member function `get_executor()`:

    class my_handler
    {
    public:
      // Custom implementation of Executor type requirements.
      typedef my_executor executor_type;

      // Return a custom executor implementation.
      executor_type get_executor() const noexcept
      {
        return my_executor();
      }

      void operator()() { ... }
    };

In more complex cases, the `associated_executor` template may be partially specialised directly:

    struct my_handler
    {
      void operator()() { ... }
    };

    namespace boost { namespace asio {

      template <class Executor>
      struct associated_executor<my_handler, Executor>
      {
        // Custom implementation of Executor type requirements.
        typedef my_executor type;

        // Return a custom executor implementation.
        static type get(const my_handler&,
            const Executor& = Executor()) noexcept
        {
          return my_executor();
        }
      };

    } } // namespace boost::asio

The `boost::asio::bind_executor()` function is a helper to bind a specific executor object, such as a strand, to a completion handler. This binding automatically associates an executor as shown above. For example, to bind a strand to a completion handler we would simply write:

    my_socket.async_read_some(my_buffer,
        boost::asio::bind_executor(my_strand,
          [](error_code ec, size_t length)
          {
            // ...
          }));

###### See Also

associated_executor, get_associated_executor, bind_executor, strand, io_context::strand, tutorial Timer.5, HTTP server 3 example.