Question

Or ostringstream?

istringstream a("asd");
istringstream b = a; // This does not work.

I guess memcpy won’t work either.

Answer 1

istringstream a("asd");
istringstream b(a.str());

Edit:
Based on your comment to the other reply, it sounds like you may also want to copy the entire contents of an fstream into a strinstream. You don’t want/have to do that one character at a time either (and you’re right — that usually is pretty slow).

// create fstream to read from
std::ifstream input("whatever");

// create stringstream to read the data into
std::istringstream buffer;

// read the whole fstream into the stringstream:
buffer << input.rdbuf();