Boost C++ Libraries of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for a snapshot of the master branch, built from commit be579b269a.


Because calls to read data may return a variable amount of bytes, the interface to calls that read data require an object that meets the requirements of DynamicBuffer. This concept is modeled on net::streambuf.

The implementation does not perform queueing or buffering of messages. If desired, these features should be provided by callers. The impact of this design is that library users are in full control of the allocation strategy used to store data and the back-pressure applied on the read and write side of the underlying TCP/IP connection.

Asynchronous Operations

Asynchronous versions are available for all functions:

flat_buffer buffer;

    [](error_code, std::size_t)
        // Do something with the buffer

Calls to asynchronous initiation functions support the extensible asynchronous model developed by the Boost.Asio author, allowing for traditional completion handlers, stackful or stackless coroutines, and even futures:

void echo(stream<tcp_stream>& ws,
    multi_buffer& buffer, net::yield_context yield)
    ws.async_read(buffer, yield);
    std::future<std::size_t> fut =
        ws.async_write(, net::use_future);

The example programs that come with the library demonstrate the usage of websocket stream operations with all asynchronous varieties.

The io_context

The creation and operation of the net::io_context associated with the underlying stream is left to the callers, permitting any implementation strategy including one that does not require threads for environments where threads are unavailable. Beast WebSocket itself does not use or require threads.

Thread Safety

Like a regular Boost.Asio socket, a stream is not thread safe. Callers are responsible for synchronizing operations on the socket using an implicit or explicit strand, as per the Asio documentation. The websocket stream asynchronous interface supports one of each of the following operations to be active at the same time:

For example, the following code is produces undefined behavior, because the program is attempting to perform two simultaneous reads:

ws.async_read(b, [](error_code, std::size_t){});
ws.async_read(b, [](error_code, std::size_t){});

However, this code is correct:

ws.async_read(b, [](error_code, std::size_t){});
ws.async_write(, [](error_code, std::size_t){});
ws.async_ping({}, [](error_code){});
ws.async_close({}, [](error_code){});

The implementation uses composed asynchronous operations; although some individual operations can perform both reads and writes, this behavior is coordinated internally to make sure the underlying stream is operated in a safe fashion. This allows an asynchronous read operation to respond to a received ping frame even while a user-initiated call to asynchronous write is active.