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#include <iostream>
#include <vector>
#include <algorithm>
 
class SquarePushBackIterator
{
	std::vector<int>& dst;
public:
	SquarePushBackIterator(std::vector<int>& dst) : dst(dst) {}
	// These just return references to the iterator.
	// The only operation that matters is assignment.
	SquarePushBackIterator& operator*()     { return *this; }
	SquarePushBackIterator& operator++()    { return *this; }
	SquarePushBackIterator& operator++(int) { return *this; }
 
	// The assignment squares and inserts into the destination.
	void operator=(int val)
	{
		dst.push_back(val * val);
	}
};
 
void mergeSortedArray(std::vector<int>& data)
{
 
	// Find the iterator range for the positive values.
	using iter = std::vector<int>::const_iterator;
	iter  endPositive = std::end(data);
	iter  loopPositive = std::find_if(std::begin(data), std::end(data),
		[](int val) {return val >= 0; });
 
	// Find the iterator range for the negative values.
	using reve = std::reverse_iterator<iter>;
	reve  endNegative = reve(std::begin(data));
	reve  loopNegative = reve(loopPositive);
 
	// Create an array to put the results into.
	std::vector<int>  result;
	result.reserve(data.size());
 
	// Perform a standard merge
	std::merge(loopPositive, endPositive, loopNegative, endNegative,
		SquarePushBackIterator(result),
		[](int val1, int val2){return std::abs(val1) < std::abs(val2); });
 
	// Use move assignment to put the result in the output vector.
	// Alternatively we could return an array by value.
	data = std::move(result);
}
 
int main()
{
	std::vector<int> arr={ -3, -2, 0, 4, 5, 8 };
 
	mergeSortedArray(arr);
 
	for (const auto& i : arr)
	{
		std::cout << i << ' ';
	}
}

In file included from /usr/include/c++/4.8/bits/char_traits.h:39:0,
                 from /usr/include/c++/4.8/ios:40,
                 from /usr/include/c++/4.8/ostream:38,
                 from /usr/include/c++/4.8/iostream:39,
                 from prog.cpp:1:
/usr/include/c++/4.8/bits/stl_algobase.h: In instantiation of ‘_OI std::__copy_move_a(_II, _II, _OI) [with bool _IsMove = false; _II = const int*; _OI = SquarePushBackIterator]’:
/usr/include/c++/4.8/bits/stl_algobase.h:428:38:   required from ‘_OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove = false; _II = __gnu_cxx::__normal_iterator<const int*, std::vector<int> >; _OI = SquarePushBackIterator]’
/usr/include/c++/4.8/bits/stl_algobase.h:460:17:   required from ‘_OI std::copy(_II, _II, _OI) [with _II = __gnu_cxx::__normal_iterator<const int*, std::vector<int> >; _OI = SquarePushBackIterator]’
/usr/include/c++/4.8/bits/stl_algo.h:5640:19:   required from ‘_OIter std::merge(_IIter1, _IIter1, _IIter2, _IIter2, _OIter, _Compare) [with _IIter1 = __gnu_cxx::__normal_iterator<const int*, std::vector<int> >; _IIter2 = std::reverse_iterator<__gnu_cxx::__normal_iterator<const int*, std::vector<int> > >; _OIter = SquarePushBackIterator; _Compare = mergeSortedArray(std::vector<int>&)::__lambda1]’
prog.cpp:44:67:   required from here
/usr/include/c++/4.8/bits/stl_algobase.h:382:57: error: no type named ‘value_type’ in ‘struct std::iterator_traits<SquarePushBackIterator>’
       typedef typename iterator_traits<_OI>::value_type _ValueTypeO;
                                                         ^
/usr/include/c++/4.8/bits/stl_algobase.h:387:9: error: no type named ‘value_type’ in ‘struct std::iterator_traits<SquarePushBackIterator>’
         && __are_same<_ValueTypeI, _ValueTypeO>::__value);
         ^
/usr/include/c++/4.8/bits/stl_algobase.h: In instantiation of ‘_OI std::__copy_move_a(_II, _II, _OI) [with bool _IsMove = false; _II = std::reverse_iterator<__gnu_cxx::__normal_iterator<const int*, std::vector<int> > >; _OI = SquarePushBackIterator]’:
/usr/include/c++/4.8/bits/stl_algobase.h:428:38:   required from ‘_OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove = false; _II = std::reverse_iterator<__gnu_cxx::__normal_iterator<const int*, std::vector<int> > >; _OI = SquarePushBackIterator]’
/usr/include/c++/4.8/bits/stl_algobase.h:460:17:   required from ‘_OI std::copy(_II, _II, _OI) [with _II = std::reverse_iterator<__gnu_cxx::__normal_iterator<const int*, std::vector<int> > >; _OI = SquarePushBackIterator]’
/usr/include/c++/4.8/bits/stl_algo.h:5640:20:   required from ‘_OIter std::merge(_IIter1, _IIter1, _IIter2, _IIter2, _OIter, _Compare) [with _IIter1 = __gnu_cxx::__normal_iterator<const int*, std::vector<int> >; _IIter2 = std::reverse_iterator<__gnu_cxx::__normal_iterator<const int*, std::vector<int> > >; _OIter = SquarePushBackIterator; _Compare = mergeSortedArray(std::vector<int>&)::__lambda1]’
prog.cpp:44:67:   required from here
/usr/include/c++/4.8/bits/stl_algobase.h:382:57: error: no type named ‘value_type’ in ‘struct std::iterator_traits<SquarePushBackIterator>’
       typedef typename iterator_traits<_OI>::value_type _ValueTypeO;
                                                         ^
/usr/include/c++/4.8/bits/stl_algobase.h:387:9: error: no type named ‘value_type’ in ‘struct std::iterator_traits<SquarePushBackIterator>’
         && __are_same<_ValueTypeI, _ValueTypeO>::__value);
         ^

Standard output is empty