Why is it not possible to remove elements from a std::map using reverse iterators?

Question

I was looking for the most efficient and expressive way to remove the last element from a std::map. I tried:

#include <map>

int main()
{
    std::map<int, int> m;
    m.insert(std::make_pair(1,1));
    m.erase(m.crbegin());
    return 0;
}

The code does not compile, since std::map::erase can take only std::map::const_iterator.

Moreover, prior to C++11 it could take std::map::iterators as well, but for some reason, this functionality was removed too.

What is the motivation behind these restrictions?

Answer 1

erase() now take const_iterators to make const_iterator actually useful. iterator is convertible to const_iterator, so the original functionality is not affected.

reverse_iterator is an iterator adapter; it exposes a .base() member function to get the underlying iterator, which you can pass to the container member functions. That said, crbegin().base() is end(), and passing end() to erase() is UB.