std::cout << "Enter two numbers:" <<  std::endl;
std::cin >> v1 >> v2;
IN C++ every expression produces a result, which typically is the value generated by applying an operator to its operands. In the case of the output operator, the result is the value of its left-hand operans. That is, the value returned by an output operation is the output stream itself.
Well, I don't understand how does the ">>" or "<<" works in example metion aboved. Can anyone explain?