What are the “string”, “stream” and “stringstream” classes in C++?

istringstream datastream("1 2 3");

int val;
datastream >> val;
cout << val << endl; // prints 1

datastream >> val;
cout << val << endl; // prints 2

datastream >> val;
cout << val << endl; // prints 3


ostringstream outstream;
outstream << 1 << "+" << 2 << "=" << 3;
cout << outstream.str() << endl; // prints "1+2=3"