There is a nice article by John Torjo which deals with this very question in a systematic way. The result he comes up with seems more generic and more efficient than any of the solutions suggested here so far:
http://www.builderau.com.au/program/java/soa/C-Removing-duplicates-from-a-range/0,339024620,320271583,00.htm
https://web.archive.org/web/1/http://articles.techrepublic%2ecom%2ecom/5100-10878_11-1052159.html
Unfortunately, the complete code of John's solution seems to be no longer available, and John did not respond to may email. Therefore, I wrote my own code which is based on similar grounds like his, but intentionally differs in some details. Feel free to contact me (vschoech think-cell com) and discuss the details if you wish.
To make the code compile for you, I added some of my own library stuff which I use regularly. Also, instead of going with plain stl, I use boost a lot to create more generic, more efficient, and more readable code.
Have fun!
#include <vector>
#include <functional>
#include <boost/bind.hpp>
#include <boost/range.hpp>
#include <boost/iterator/counting_iterator.hpp>
/////////////////////////////////////////////////////////////////////////////////////////////
// library stuff
template< class Rng, class Func >
Func for_each( Rng& rng, Func f ) {
return std::for_each( boost::begin(rng), boost::end(rng), f );
};
template< class Rng, class Pred >
Rng& sort( Rng& rng, Pred pred ) {
std::sort( boost::begin( rng ), boost::end( rng ), pred );
return rng; // to allow function chaining, similar to operator+= et al.
}
template< class T >
boost::iterator_range< boost::counting_iterator<T> > make_counting_range( T const& tBegin, T const& tEnd ) {
return boost::iterator_range< boost::counting_iterator<T> >( tBegin, tEnd );
}
template< class Func >
class compare_less_impl {
private:
Func m_func;
public:
typedef bool result_type;
compare_less_impl( Func func )
: m_func( func )
{}
template< class T1, class T2 > bool operator()( T1 const& tLeft, T2 const& tRight ) const {
return m_func( tLeft ) < m_func( tRight );
}
};
template< class Func >
compare_less_impl<Func> compare_less( Func func ) {
return compare_less_impl<Func>( func );
}
/////////////////////////////////////////////////////////////////////////////////////////////
// stable_unique
template<class forward_iterator, class predicate_type>
forward_iterator stable_unique(forward_iterator itBegin, forward_iterator itEnd, predicate_type predLess) {
typedef std::iterator_traits<forward_iterator>::difference_type index_type;
struct SIteratorIndex {
SIteratorIndex(forward_iterator itValue, index_type idx) : m_itValue(itValue), m_idx(idx) {}
std::iterator_traits<forward_iterator>::reference Value() const {return *m_itValue;}
index_type m_idx;
private:
forward_iterator m_itValue;
};
// {1} create array of values (represented by iterators) and indices
std::vector<SIteratorIndex> vecitidx;
vecitidx.reserve( std::distance(itBegin, itEnd) );
struct FPushBackIteratorIndex {
FPushBackIteratorIndex(std::vector<SIteratorIndex>& vecitidx) : m_vecitidx(vecitidx) {}
void operator()(forward_iterator itValue) const {
m_vecitidx.push_back( SIteratorIndex(itValue, m_vecitidx.size()) );
}
private:
std::vector<SIteratorIndex>& m_vecitidx;
};
for_each( make_counting_range(itBegin, itEnd), FPushBackIteratorIndex(vecitidx) );
// {2} sort by underlying value
struct FStableCompareByValue {
FStableCompareByValue(predicate_type predLess) : m_predLess(predLess) {}
bool operator()(SIteratorIndex const& itidxA, SIteratorIndex const& itidxB) {
return m_predLess(itidxA.Value(), itidxB.Value())
// stable sort order, index is secondary criterion
|| !m_predLess(itidxB.Value(), itidxA.Value()) && itidxA.m_idx < itidxB.m_idx;
}
private:
predicate_type m_predLess;
};
sort( vecitidx, FStableCompareByValue(predLess) );
// {3} apply std::unique to the sorted vector, removing duplicate values
vecitidx.erase(
std::unique( vecitidx.begin(), vecitidx.end(),
!boost::bind( predLess,
// redundand boost::mem_fn required to compile
boost::bind(boost::mem_fn(&SIteratorIndex::Value), _1),
boost::bind(boost::mem_fn(&SIteratorIndex::Value), _2)
)
),
vecitidx.end()
);
// {4} re-sort by index to match original order
sort( vecitidx, compare_less(boost::mem_fn(&SIteratorIndex::m_idx)) );
// {5} keep only those values in the original range that were not removed by std::unique
std::vector<SIteratorIndex>::iterator ititidx = vecitidx.begin();
forward_iterator itSrc = itBegin;
index_type idx = 0;
for(;;) {
if( ititidx==vecitidx.end() ) {
// {6} return end of unique range
return itSrc;
}
if( idx!=ititidx->m_idx ) {
// original range must be modified
break;
}
++ititidx;
++idx;
++itSrc;
}
forward_iterator itDst = itSrc;
do {
++idx;
++itSrc;
// while there are still items in vecitidx, there must also be corresponding items in the original range
if( idx==ititidx->m_idx ) {
std::swap( *itDst, *itSrc ); // C++0x move
++ititidx;
++itDst;
}
} while( ititidx!=vecitidx.end() );
// {6} return end of unique range
return itDst;
}
template<class forward_iterator>
forward_iterator stable_unique(forward_iterator itBegin, forward_iterator itEnd) {
return stable_unique( itBegin, itEnd, std::less< std::iterator_traits<forward_iterator>::value_type >() );
}
void stable_unique_test() {
std::vector<int> vecn;
vecn.push_back(1);
vecn.push_back(17);
vecn.push_back(-100);
vecn.push_back(17);
vecn.push_back(1);
vecn.push_back(17);
vecn.push_back(53);
vecn.erase( stable_unique(vecn.begin(), vecn.end()), vecn.end() );
// result: 1, 17, -100, 53
}
