Question

For some reason the following code fails. You can’t simply erase a reverse_iterator by using its base() method.

#include <set> #include <iostream>  int main() {     std::set<int> setOfInts;     setOfInts.insert(1);     setOfInts.insert(2);     setOfInts.insert(3);      std::set<int>::reverse_iterator rev_iter = setOfInts.rbegin();     std::set<int>::reverse_iterator nextRevIter = setOfInts.rbegin();     ++nextIter;      while ( rev_iter != setOfInts.rend())     {         // Find 3 and try to erase         if (*rev_iter == 3)         {             // SEGFAULT HERE             setOfInts.erase( rev_iter.base());         }         rev_iter = nextRevIter;         ++nextRevIter;     }  } 

How does one go about correctly doing the above? Given a reverse_iterator that corresponds to something you want to erase, how do you erase it?

Note, erase won’t take reverse_iterators unfortunately. It wants the real thing.

Answer 1

Apparently the solution is what base() returns is 1 off. The following identity holds for a reverse_iterator:

&*(reverse_iterator(i)) == &*(i - 1)  

Or in other words, the reverse_iterator is always one pass the regular iterator it is the base of. Not sure why.

In GCC

Simply change

        // SEGFAULT HERE         setOfInts.erase( rev_iter.base()); 

to

        // WORKS!         setOfInts.erase( --rev_iter.base()); 

I’m definitely curious though as to why the identity above makes sense.

In Visual Studio

Coming back into work and trying this in visual studio, I see the above solution doesn’t quite work. The ‘nextIter’ becomes invalid on the erase. Instead, you need to save away the temporary from the erase to get the next iterator instead of keeping around a nextIter like above.

  set<int>::iterator tempIter = setOfInts.erase(--rev_iter.base());   rev_iter = setOfInts.erase(tempIter); 

So the final solution is

int main() {     using namespace std;      set<int> setOfInts;     setOfInts.insert(1);     setOfInts.insert(2);     setOfInts.insert(3);      set<int>::reverse_iterator rev_iter = setOfInts.rbegin();      while ( rev_iter != setOfInts.rend())     {         // Find 3 and try to erase         if (*rev_iter == 3)         {             cout << 'Erasing : ' << *rev_iter;             set<int>::iterator tempIter = setOfInts.erase( --rev_iter.base());             rev_iter = set<int>::reverse_iterator(tempIter);                     }         else         {             ++rev_iter;         }     }     } 

Note, associative containers do not return an iterator from erase. So this solution wouldn’t work for map, multimap, etc.