Is there a range class in C++11 for use with range based for loops?

Question

I found myself writing this just a bit ago:

template 
class range_class {
 public:
   class iterator {
      friend c         


        

Answer 1

I wrote a library called range for exactly the same purpose except it is a run-time range, and the idea in my case came from Python. I considered a compile-time version, but in my humble opinion there is no real advantage to gain out the compile-time version. You can find the library on bitbucket, and it is under Boost License: Range. It is a one-header library, compatible with C++03 and works like charm with range-based for loops in C++11 :)

Features:

• A true random access container with all the bells and whistles!

• Ranges can be compared lexicographically.

• Two functions exist(returns bool), and find(returns iterator) to check the existence of a number.

• The library is unit-tested using CATCH.

• Examples of basic usage, working with standard containers, working with standard algorithms and working with range based for loops.

Here is a one-minute introduction. Finally, I welcome any suggestion about this tiny library.

Answer 2

This might be a little late but I just saw this question and I've been using this class for a while now :

#include <iostream>
#include <utility>
#include <stdexcept>

template<typename T, bool reverse = false> struct Range final {
    struct Iterator final{
        T value;
        Iterator(const T & v) : value(v) {}
        const Iterator & operator++() { reverse ? --value : ++value; return *this; }
        bool operator!=(const Iterator & o) { return o.value != value; }
        T operator*() const { return value; }
    };
    T begin_, end_;
    Range(const T & b, const T & e)  : begin_(b), end_(e) {
        if(b > e) throw std::out_of_range("begin > end");
    }

    Iterator begin() const { return reverse ? end_ -1 : begin_; }
    Iterator end() const { return reverse ? begin_ - 1: end_; }

    Range() = delete;
    Range(const Range &) = delete;
};

using UIntRange = Range<unsigned, false>;
using RUIntRange = Range<unsigned, true>;

Usage :

int main() {
    std::cout << "Reverse : ";
    for(auto i : RUIntRange(0, 10)) std::cout << i << ' ';
    std::cout << std::endl << "Normal : ";
    for(auto i : UIntRange(0u, 10u)) std::cout << i << ' ';
    std::cout << std::endl;
}

Answer 3

As far as I know, there is no such class in C++11.

Anyway, I tried to improve your implementation. I made it non-template, as I don't see any advantage in making it template. On the contrary, it has one major disadvantage : that you cannot create the range at runtime, as you need to know the template arguments at compile time itself.

//your version
auto x = range<m,n>(); //m and n must be known at compile time

//my version
auto x = range(m,n);  //m and n may be known at runtime as well!

Here is the code:

class range {
 public:
   class iterator {
      friend class range;
    public:
      long int operator *() const { return i_; }
      const iterator &operator ++() { ++i_; return *this; }
      iterator operator ++(int) { iterator copy(*this); ++i_; return copy; }

      bool operator ==(const iterator &other) const { return i_ == other.i_; }
      bool operator !=(const iterator &other) const { return i_ != other.i_; }

    protected:
      iterator(long int start) : i_ (start) { }

    private:
      unsigned long i_;
   };

   iterator begin() const { return begin_; }
   iterator end() const { return end_; }
   range(long int  begin, long int end) : begin_(begin), end_(end) {}
private:
   iterator begin_;
   iterator end_;
};

Test code:

int main() {
      int m, n;
      std::istringstream in("10 20");
      if ( in >> m >> n ) //using in, because std::cin cannot be used at coliru.
      {
        if ( m > n ) std::swap(m,n); 
        for (auto i : range(m,n)) 
        {
             std::cout << i << " ";
        }
      }
      else 
        std::cout <<"invalid input";
}

Output:

10 11 12 13 14 15 16 17 18 19

Onine demo.

Answer 4

You can easily generate an increasing sequence in C++11 using std::iota():

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

template<typename T>
std::vector<T> range(T start, T end)
{
  std::vector<T> r(end+1-start, T(0));
  std::iota(r.begin(), r.end(), T(start));//increasing sequence
  return r;
}

int main(int argc, const char * argv[])
{
  for(auto i:range<int>(-3,5))
    std::cout<<i<<std::endl;

  return 0;
}

Answer 5

have you tried using

template <class InputIterator, class Function>
   Function for_each (InputIterator first, InputIterator last, Function f);

Most of the time fits the bill.

E.g.

template<class T> void printInt(T i) {cout<<i<<endl;}
void test()
{
 int arr[] = {1,5,7};
 vector v(arr,arr+3);

 for_each(v.begin(),v.end(),printInt);

}

Note that printInt can OFC be replaced with a lambda in C++0x. Also one more small variation of this usage could be (strictly for random_iterator)

 for_each(v.begin()+5,v.begin()+10,printInt);

For Fwd only iterator

 for_each(advance(v.begin(),5),advance(v.begin(),10),printInt);

Answer 6

Here is a simpler form which is working nicely for me. Are there any risks in my approach?

r_iterator is a type which behaves, as much as possible, like a long int. Therefore many operators such as == and ++, simply pass through to the long int. I 'expose' the underlying long int via the operator long int and operator long int & conversions.

#include <iostream>
using namespace std;

struct r_iterator {
        long int value;
        r_iterator(long int _v) : value(_v) {}
        operator long int () const { return value; }
        operator long int& ()      { return value; }
        long int operator* () const { return value; }
};
template <long int _begin, long int _end>
struct range {
        static r_iterator begin() {return _begin;}
        static r_iterator end  () {return _end;}
};
int main() {
        for(auto i: range<0,10>()) { cout << i << endl; }
        return 0;
}

(Edit: - we can make the methods of range static instead of const.)