Why do I need another iterator as an argument in std::copy()?

Question

I don't understand why I need to put another iterator as the second argument in the call to std::copy() for reading through a file. How is iterator 'end' ending of a file?

    vector<Point> v;
    istream_iterator<Point> is(file), end;
    copy(is, end, back_inserter(v));

Answer 1

How is iterator 'end' ending of a file?

By convention and/or a design decision in the standard library. The iterator end is default-constructed, and on cppreference, we learn about the default std:istream_iterator constructor:

constexpr istream_iterator();

Constructs the end-of-stream iterator, value-initializes the stored value [...]

The deeper reasoning is that the standard algorithms are built around the concept of a half-open range, often denoted as [first, last). The is iterator must be distinguished from some kind of end sentinel last - otherwise, std::copy can't know when reading from the input is not meaningful anymore (i.e., when it has reached the end of the file). In your case, this is end.

Answer 2

Why?

You need to tell the algorithm somehow how many elements it should copy. Note that copy is generic and the reason to use iterators is to be agnostic of the actual container. Hence, the algorithm has no way to stop when it reaches the end of the container. All it has is the two iterators you pass it.

How?

This is just how it is defined in the language. If you look at the constructor that takes no arguments you see that it creates a special end-of-stream iterator. From cppreference:

constexpr istream_iterator();    (1) 

1) Constructs the end-of-stream iterator, value-initializes the stored value. This constructor is constexpr if the initializer in the definition auto x = T(); is a constant initializer.