#include <sstream>

[moxyfike]

what is #include <sstream> used for?

[tummychow]

Stringstreams. They're like streams, but they contain strings, which can be useful for employing stream-style manipulation on them. Hence, stringstream.
Read the references for what is in sstream.

[iharrold]

http://cplusplus.com/reference/iostream/stringstream/

[magnag]

it is the same as the directives iostream
which is useful for input and output
mechanisms.