where in memory, stored as exactly a void* and a size_t.">util::Blob_mutable</a>) is determined by calling the arg-supplied function at the time when at least 1 byte is available to read, instead of the buffer being given direcly as an arg. </p>
<p >By determining where to target when there are actual data available, one can avoid unnecessary copying in the meantime; among other applications.</p>
<h3>Behavior</h3>
<p >boost.asio's <code>async_read()</code> free function is generically capable of receiving i