Iterating over a range of ranges [duplicate] - c++

Question

This question already has answers here:
Flattening iterator
(5 answers)
Closed 8 years ago.
If I have a range of ranges, is there a way to join them together and iterate over them as if they were a single range?
By range, I mean any kind of container or iterator pair. Basically like boost's ranges. Boost already have a function for joining two ranges together with boost::join, but that function can't join an arbitary number of ranges.
Basically, I'm looking for a way to do what the the many function does in this piece of code:
std::vector<std::vector<int>> n = {{0, 1}, {2, 3}};
for (auto i : many(n))
std::cout << i << '\n';
Which would output 0, 1, 2, 3.
In C#, I had SelectMany which could do this.

Answer 1

Here is an example of a flattening iterator used with range-based for (taken from here and added boost::make_iterator_range to show range version) (still needs work though):
#include <boost/iterator/iterator_adaptor.hpp>
#include <boost/range.hpp>
#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>
#include <string>
#include <utility>
template <typename Iter>
class flattening_iterator :
public boost::iterator_adaptor<
flattening_iterator<Iter>,
Iter,
typename Iter::value_type::iterator::value_type,
boost::forward_traversal_tag,
typename Iter::value_type::iterator::value_type
>
{
private:
using super_t = boost::iterator_adaptor<
flattening_iterator<Iter>,
Iter,
typename Iter::value_type::iterator::value_type,
boost::forward_traversal_tag,
typename Iter::value_type::iterator::value_type
>;
using inner_iterator = typename Iter::value_type::iterator;
public:
flattening_iterator(Iter it)
: super_t(it),
inner_begin(),
inner_end(),
outer_end(it)
{}
flattening_iterator(Iter begin, Iter end)
: super_t(begin),
inner_begin((*begin).begin()),
inner_end((*begin).end()),
outer_end(end)
{}
using value_type = typename Iter::value_type::iterator::value_type;
private:
friend class boost::iterator_core_access;
inner_iterator inner_begin;
inner_iterator inner_end;
Iter outer_end;
void increment()
{
if (this->base_reference() == outer_end)
return; // At the end
++inner_begin;
if (inner_begin == inner_end)
{
++this->base_reference();
inner_begin = (*this->base_reference()).begin();
inner_end = (*this->base_reference()).end();
}
}
value_type dereference() const
{
return *inner_begin;
}
};
template <typename Iter>
auto flat_iter(Iter it) -> flattening_iterator<Iter>
{
return flattening_iterator<Iter>(it);
}
template <typename Iter>
auto flat_iter(Iter begin, Iter end) -> flattening_iterator<Iter>
{
return flattening_iterator<Iter>(begin, end);
}
template <typename Cont>
auto flatten(Cont& c) -> boost::iterator_range<flattening_iterator<decltype(std::declval<Cont>().begin())>>
{
return boost::make_iterator_range(flat_iter(c.begin(), c.end()), flat_iter(c.end()));
}
int main()
{
std::vector<std::vector<int>> v1{{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
for (auto i : flatten(v1))
{
std::cout << i << ' ';
}
}
Output
1 2 3 4 5 6 7 8 9