Limiting the range for std::copy with std::istream_iterator

Question

I have constructed a minimal working example to show a problem I've encountered using STL iterators. I'm using istream_iterator to read floatss (or other types) from a std::istream:

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

int main() {
   float values[4];
   std::copy(std::istream_iterator<float>(std::cin), std::istream_iterator<float>(), values);
   std::cout << "Read exactly 4 floats" << std::endl; // Not true!
}

This reads all possible floatss, until EOF into values, which is of fixed size, 4, so now clearly I want to limit the range to avoid overflows and read exactly/at most 4 values.

With more "normal" iterators (i.e. RandomAccessIterator), provided begin+4 isn't past the end you'd do:

std::copy(begin, begin+4, out);

To read exactly 4 elements.

How does one do this with std::istream_iterator? The obvious idea is to change the call to std::copy to be:

std::copy(std::istream_iterator<float>(std::cin), std::istream_iterator<float>(std::cin)+4, values);

But (fairly predictably) this doesn't compile, there are no candidates for operator+:

g++ -Wall -Wextra test.cc
test.cc: In function ‘int main()’:
test.cc:7: error: no match for ‘operator+’ in ‘std::istream_iterator<float, char, std::char_traits<char>, long int>(((std::basic_istream<char, std::char_traits<char> >&)(& std::cin))) + 4’

Any suggestions? Is there a correct, "STLified" pre-C++0x way to achieve this? Obviously I could just write it out as a for loop, but I'm looking to learn something about the STL here. I half wondered about abusing std::transform or std::merge etc. to achieve this functionality somehow, but I can't quite see how to do it.

Answer 1

As you requested a non-C++0x solution, here's an alternative that uses std::generate_n and a generator functor rather than std::copy_n and iterators:

#include <algorithm>
#include <string>
#include <istream>
#include <ostream>
#include <iostream>

template<
    typename ResultT,
    typename CharT = char,
    typename CharTraitsT = std::char_traits<CharT>
>
struct input_generator
{
    typedef ResultT result_type;

    explicit input_generator(std::basic_istream<CharT, CharTraitsT>& input)
      : input_(&input)
    { }

    ResultT operator ()() const
    {
        // value-initialize so primitives like float
        // have a defined value if extraction fails
        ResultT v((ResultT()));
        *input_ >> v;
        return v;
    }

private:
    std::basic_istream<CharT, CharTraitsT>* input_;
};

template<typename ResultT, typename CharT, typename CharTraitsT>
inline input_generator<ResultT, CharT, CharTraitsT> make_input_generator(
    std::basic_istream<CharT, CharTraitsT>& input
)
{
    return input_generator<ResultT, CharT, CharTraitsT>(input);
}

int main()
{
    float values[4];
    std::generate_n(values, 4, make_input_generator<float>(std::cin));
    std::cout << "Read exactly 4 floats" << std::endl;
}

---

If you wanted to, you could then use this generator in conjunction with boost::generator_iterator to use the generator as an input iterator.

Answer 2

Take a look at std::copy_n 

Answer 3

If you don't have std::copy_n available, it's pretty easy to write your own:

namespace std_ext { 
template<class InputIterator, class Size, class OutputIterator>
OutputIterator copy_n(InputIterator first, Size n, OutputIterator result) {
    for (int i=0; i<n; i++) {
        *result = *first;
        ++result;
        ++first;
    }
    return result;
}
}