How to read arbitrary number of values using std::copy?

Question

I'm trying to code opposite action to this:

std::ostream outs; // properly initialized of course
std::set<int> my_set; // ditto

outs << my_set.size();
std::copy( my_set.begin(), my_set.end(), std::ostream_iterator<int>( outs ) );

it should be something like this:

std::istream ins;

std::set<int>::size_type size;
ins >> size;

std::copy( std::istream_iterator<int>( ins ), std::istream_iterator<int>( ins ) ???, std::inserter( my_set, my_set.end() ) );

But I'm stuck with the 'end' iterator -- input interators can't use std::advance and neither I can use two streams with the same source...

Is there any elegant way how to solve this? Of course I can use for loop, but maybe there's something nicer :)

Answer 1

You could derive from the istream_iterator<T>.
Though using Daemin generator method is another option, though I would generate directly into the set rather than use an intermediate vector.

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


template<typename T>
struct CountIter: public std::istream_iterator<T>
{
    CountIter(size_t c)
        :std::istream_iterator<T>()
        ,count(c)
    {}
    CountIter(std::istream& str)
        :std::istream_iterator<T>(str)
        ,count(0)
    {}

    bool operator!=(CountIter const& rhs) const
    {
        return (count != rhs.count) && (dynamic_cast<std::istream_iterator<T> const&>(*this) != rhs);
    }
    T operator*()
    {
        ++count;
        return std::istream_iterator<T>::operator*();
    }

    private:
        size_t  count;
};

int main()
{
    std::set<int>       x;

    //std::copy(std::istream_iterator<int>(std::cin),std::istream_iterator<int>(),std::inserter(x,x.end()));
    std::copy(
                CountIter<int>(std::cin),
                CountIter<int>(5),
                std::inserter(x,x.end())
            );
}

Answer 2

Use:

std::copy( std::istream_iterator<int>(ins),
           std::istream_iterator<int>(),
           std::inserter(my_set, my_set.end())
         );

Note the empty parameter:

std::istream_iterator<int>();