Htydjtk

If I want to read all integers from standard input to a vector, I can use the handy:

vector<int> v{istream_iterator<int>(cin), istream_iterator()};

But let's assume I only want to read n integers. Is the hand-typed loop everything I got?

vector<int> v(n);

for(vector<int>::size_type i = 0; i < n; i++)

    cin >> v[i];

Or is there any more right-handed way to do this?

As given in comments, copy_n is unsafe for this job, but you can use copy_if with mutable lambda:

#include <iterator>

#include <vector>

#include <iostream>

#include <algorithm>



int main(){

    const int N = 10;

    std::vector<int> v;

    //optionally v.reserve(N);

    std::copy_if(

        std::istream_iterator<int>(std::cin),

        std::istream_iterator<int>(), 

        std::back_inserter(v), 

        [count=N] (int)  mutable {

            return count && count--;

    });



    return 0;

}

as pointed out in this answer:
std::copy n elements or to the end

You usually shouldn't do this with std::copy_n, which assumes that the provided iterator, when incremented n times, remains valid:

Copies exactly count values from the range beginning at first to the range beginning at result. Formally, for each non-negative integer i < n, performs *(result + i) = *(first + i).

^{(cppreference.com article on std::copy_n)}

If you can guarantee that, then fine, but generally with std::cin that's not possible. You can quite easily have it dereferencing an invalid iterator:

The default-constructed std::istream_iterator is known as the end-of-stream iterator. When a valid std::istream_iterator reaches the end of the underlying stream, it becomes equal to the end-of-stream iterator. Dereferencing or incrementing it further invokes undefined behavior.

^{(cppreference.com article on std::istream_iterator)}

You're pretty much there with your loop, though I'd probably use stronger termination condition to avoid excess reads from a "dead" stream:

vector<int> v(n);

for(vector<int>::size_type i = 0; i < n; i++)

    if (!cin >> v[i])

       break;

I'd be tempted actually to wrap this into something that's like std::copy_n, but accepts a full "range" whose bounds may be validated in addition to counting from 0 to N.

An implementation might look like:

template<class InputIt, class Size, class OutputIt>

OutputIt copy_atmost_n(InputIt first, InputIt last, Size count, OutputIt result)

{

   for (Size i = 0; i < count && first != last; ++i)

      *result++ = *first++;

   return result;

}

You'd use it like this:

copy_atmost_n(

   std::istream_iterator<int>(std::cin),

   std::istream_iterator<int>(),

   N,

   std::back_inserter(v)

);

Now you get M elements, where M is either the number of inputs provided or N, whichever is smaller.

(live demo)