Boost C++ Libraries

Return the lowest layer in a stack of stream layers.

Synopsis

Defined in header <boost/beast/core/stream_traits.hpp>

template<
    class T>
lowest_layer_type< T >&
get_lowest_layer(
    T& t);

Description

If t.next_layer() is well-defined, returns get_lowest_layer(t.next_layer()). Otherwise, it returns t.

A stream layer is an object of class type which wraps another object through composition, and meets some or all of the named requirements of the wrapped type while optionally changing behavior. Examples of stream layers include net::ssl::stream or beast::websocket::stream. The owner of a stream layer can interact directly with the wrapper, by passing it to stream algorithms. Or, the owner can obtain a reference to the wrapped object by calling next_layer() and accessing its members. This is necessary when it is desired to access functionality in the next layer which is not available in the wrapper. For example, websocket::stream permits reading and writing, but in order to establish the underlying connection, members of the wrapped stream (such as connect) must be invoked directly.

Usually the last object in the chain of composition is the concrete socket object (for example, a net::basic_socket or a class derived from it). The function get_lowest_layer exists to easily obtain the concrete socket when it is desired to perform an action that is not prescribed by a named requirement, such as changing a socket option, cancelling all pending asynchronous I/O, or closing the socket (perhaps by using close_socket).

Example

// Set non-blocking mode on a stack of stream
// layers with a regular socket at the lowest layer.
template < class Stream>
void set_non_blocking (Stream& stream)
{
    error_code ec;
    // A compile error here means your lowest layer is not the right type!
    get_lowest_layer(stream).non_blocking( true , ec);
    if (ec)
        throw system_error{ec};
}

Parameters

See Also

close_socket, lowest_layer_type