Range-based loop over an input stream

Question

To iterate over an input stream, we would usually use a std::istream_iterator like so:

typedef std::istream_iterator input_iterato         


        

Answer 1

It doesn't matter whether they will be found by argument-dependent lookup, because you are allowed to put specializations of classes and functions in the std namespace.

Answer 2

An obvious approach is to use a simple decorator for your stream providing the type and the necessary interface. Here is how this could look like:

template <typename T>
struct irange
{
    irange(std::istream& in): d_in(in) {}
    std::istream& d_in;
};
template <typename T>
std::istream_iterator<T> begin(irange<T> r) {
    return std::istream_iterator<T>(r.d_in);
}
template <typename T>
std::istream_iterator<T> end(irange<T>) {
    return std::istream_iterator<T>();
}

for (auto const& x: irange<std::string>(std::ifstream("file") >> std::skipws)) {
    ...
}

Answer 3

I attempted to pursue the idea of specializing std::begin and std::end for classes derived from std::basic_istream (I'm not so great at this template metaprogramming business):

namespace std
{
  template <typename C>
  typename
  std::enable_if<
    std::is_base_of<std::basic_istream<typename C::char_type>, C>::value,
    std::istream_iterator<std::string>>::type
  begin(C& c)
  {
    return {c};
  }

  template <typename C>
  typename
  std::enable_if<
    std::is_base_of<std::basic_istream<typename C::char_type>, C>::value,
    std::istream_iterator<std::string>>::type
  end(C& c)
  {
    return {};
  }
}

Actually, it works pretty well. I didn't create versions that take const C& because I don't think it makes sense to extract from a const stream (and I had errors when I tried to do so). I'm also not sure if I can make this more move friendly. So now I can print out the contents of myfile like so::

std::ifstream file("myfile");
std::copy(begin(file), end(file), std::ostream_iterator<std::string>(std::cout, " "));

So these begin and end functions work as expected. However, it falls flat when used in a range-based for loop. std::basic_istream classes are derived from std::ios_base which already has a member called end (it's a flag for seeking within a stream). Once the range-based for loop finds this, it just gives up because it can't find a corresponding begin (not to mention that end is not the right kind of entity):

main.cpp:35:33: error: range-based ‘for’ expression of type ‘std::basic_ifstream’ has an ‘end’ member but not a ‘begin’

The only alternative that works in both situations, as others have mentioned, is to create a wrapper object. Unfortunately that end member in std::ios_base completely ruins any chance of implementing this in a nice way.

Answer 4

Here is one possible solution. Sadly, it does require an extra structure:

#include <iostream>
#include <fstream>
#include <iterator>
#include <algorithm>
#include <string>

struct S {
  std::istream& is;
  typedef std::istream_iterator<std::string> It;
  S(std::istream& is) : is(is) {}
  It begin() { return It(is); }
  It end() { return It(); }
};

int main () {
  std::ifstream file("myfile");
  for(auto& string : S(file)) {
    std::cout << string << "\n";
  }
}

Another solution is to derive from std::ifstream:

#include <iostream>
#include <fstream>
#include <iterator>
#include <algorithm>
#include <string>


struct ifstream : std::ifstream {
  // using std::ifstream::ifstream; I wish g++4.7 supported inheriting constructors!
  ifstream(const char* fn) : std::ifstream(fn) {}
  typedef std::istream_iterator<std::string> It;
  It begin() { return It(*this); }
  It end() { return It(); }
};

int main () {
  ifstream file("myfile");
  for(auto& string : file) {
    std::cout << string << "\n";
  }
}