Why is reading from an istream_iterator assigned to a variable not working?

Question

I wrote the following program that reads in 3 numbers from std::cin, and outputs them to std::cout, and does this twice:

#include <iostream>
#include <algorithm>
#include <iterator>

int main()
{
    std::copy_n(std::istream_iterator<int>(std::cin), 
                3, 
                std::ostream_iterator<int>(std::cout, " "));
    
    std::copy_n(std::istream_iterator<int>(std::cin), 
                3, 
                std::ostream_iterator<int>(std::cout, " "));              
}

For an input of 1 2 3 4 5 6, the program prints the expected 1 2 3 4 5 6.

As I found the code a bit verbose, I tried to store the iterators in variables:

#include <iostream>
#include <algorithm>
#include <iterator>

int main()
{
    auto ins = std::istream_iterator<int>(std::cin);
    auto outs = std::ostream_iterator<int>(std::cout, " ");
               
    std::copy_n(ins, 3, outs);
    std::copy_n(ins, 3, outs);
}

But now for the input 1 2 3 4 5 6, the program prints 1 2 3 1 4 5.

I don't understand the output. What's going on here, and what am I doing wrong?

Also, note that it only matters when I use ins. Whether I use outs or not doesn't affect the output.

Remy Lebeau · Accepted Answer · 2020-10-22 05:27:58Z

Per this reference:

std::istream_iterator is a single-pass input iterator that reads successive objects of type T from the std::basic_istream object for which it was constructed, by calling the appropriate operator>>. The actual read operation is performed when the iterator is incremented, not when it is dereferenced. The first object is read when the iterator is constructed. Dereferencing only returns a copy of the most recently read object.

So, when you first create the ins variable, it reads 1 from cin right away and caches it.

If you look at the declaration of copy_n(), the input iterator is passed by value, which means it is copied.

template< class InputIt, class Size, class OutputIt >
OutputIt copy_n( InputIt first, Size count, OutputIt result );

Let’s assume, for argument’s sake, that the following implementation of copy_n() is being used (check your compiler for actual implementation):

template< class InputIt, class Size, class OutputIt>
OutputIt copy_n(InputIt first, Size count, OutputIt result)
{
    if (count > 0) {
        *result++ = *first;
        for (Size i = 1; i < count; ++i) {
            *result++ = *++first;
        }
    }
    return result;
}

When you pass ins to copy_n(), that cached 1 is copied into the first parameter. When copy_n() dereferences first, it receives the cached 1 and outputs to result. Then copy_n() increments first which reads 2 from cin and caches it, then dereferences first to receive 2 and output it, then increments first which reads 3 from cin and caches it, then dereferences first to receive 3 and output it, then exits.

When you pass ins to copy_n() again, the original cached 1 is still in ins and is copied into the first parameter. When copy_n() dereferences first, it receives the cached 1 and outputs to result. Then copy_n() increments first which reads 4 from cin and caches it, then dereferences first to receive 4 and output it, then increments first which reads 5 from cin and caches it, then dereferences first to receive 5 and output it, then exits.

This is very clear, thanks. I had no idea about this caching behavior. That's what was tripping me up, i.e. once the 1 is read, how could it be read again?

Tony Delroy · Accepted Answer · 2020-10-22 05:02:05Z

2

If you look at Defect Report P0738R2 you'll see the first read for an istream_iterator should be performed by the constructor, so it's reading 1 at the line auto ins = ....

copy_n takes its arguments by value, so the first invocation doesn't move main()s ins variable past the 1 it has already read, and that's provided again to the second invocation of copy_n.

If you want concision, you could do something like:

auto mkins() = [] { return std::istream_iterator<int>(std::cin); }

std::copy_n(mkins(), 3, outs);
std::copy_n(mkins(), 3, outs);

answered Oct 22, 2020 at 5:02

Tony Delroy

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4 Comments

cigien Over a year ago

Thanks, the fix appears to work. But I'm not clear on the reason for the original code not working. Is the output unspecified?

Remy Lebeau Over a year ago

@cigien the problem is not in the output. It is because you are reusing the same input iterator object for both copy_n calls. Tony’s answer is using 2 input objects, same as your “working” code. See my answer, I explain the problem in more detail.

Tony Delroy Over a year ago

@cigien: I thought that was very clear from "the first read for an istream_iterator should be performed by the constructor"...? Your code constructs the two istream_iterators before it's finished copying from the first iterator.

cigien Over a year ago

I was confused about the 1 appearing twice in the output. The first value seems to be cached, which I didn't realize from your answer.

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