Code walkthrough

Basically, the copy algorithm accepts a range of iterators, where the first two arguments represent the source and the third argument represents the destination, which happens to be the vector:

istream_iterator<int> start_input(cin);
istream_iterator<int> end_input;

copy ( start_input, end_input, back_inserter( v ) );

The start_input iterator instance defines an istream_iterator iterator that receives input from istream and cin, and the end_input iterator instance defines an end-of-file delimiter, which is an empty string by default ("").  Hence, the input can be terminated by typing "" in the command-line input terminal.

Similarly, let's understand the following code snippet:

cout << "\nPrint the vector ..." << endl;
copy ( v.begin(), v.end(), ostream_iterator<int>(cout, "\t") );
cout << endl;

The copy algorithm is used to copy the values from a vector, one element at a time, to ostream, separating the output with a tab character (\t).