us operations">asynchronous
                  operation</a> with completion signature <code class="computeroutput">void(error_code
                  ec, size_t n)</code>.<br> <br> If <code class="computeroutput">buffer_size(cb) &gt;
                  0</code>, initiates an asynchronous operation to write one or more
                  bytes of data to the stream <code class="computeroutput">a</code> from the buffer sequence
                  <code class="computeroutput">cb</code>. If successful, <code class=