Sorting based on frequency of elements into descending order

conseq_sort.h

#include <map>
#include <algorithm>  // fill_n, trivially replaceable

// sorts in place based on frequency
template <typename Iter>
void freq_sort(Iter begin, Iter end) {
	using T = typename Iter::value_type;

	if (begin == end) return;
	T current {*begin};
	std::map<size_t, std::vector<T>> frequency;
	// map allows in-order traversal (implemented as binary search tree)
	size_t count {1};
	auto cur = begin + 1;
	for (; cur != end; ++cur) {
		// assuming condition 1 is true (can't have multiple groups of same val)
		if (*cur == current) ++count;
		else {
			frequency[count].push_back(current);	// last value
			count = 1;
			current = *cur;
		}
	}
	// put last count back in
	frequency[count].push_back(current);
	// second pass to put values in place
	cur = begin;
	for (auto entry = frequency.rbegin(); entry != frequency.rend(); ++entry) {
		// fill frequency amount of times by the actual value starting at cur
		for (auto val : entry->second)	 // vector of values with same frequency
			cur = std::fill_n(cur, entry->first, val);
	}
}