C++ std::set::erase with std::remove_if

You cannot use std::remove_if() with sequences which have const parts. The sequence of std::set<T> elements are made up of T const objects. We actually discussed this question just yesterday at the standard C++ committee and there is some support to create algorithms dealing specifically with the erase()ing objects from containers. It would look something like this (see also N4009):

template <class T, class Comp, class Alloc, class Predicate>
void discard_if(std::set<T, Comp, Alloc>& c, Predicate pred) {
    for (auto it{c.begin()}, end{c.end()}; it != end; ) {
        if (pred(*it)) {
            it = c.erase(it);
        }
        else {
            ++it;
        }
    }
}

(it would probably actually delegate to an algorithm dispatching to the logic above as the same logic is the same for other node-based container).

For you specific use, you can use

a.erase(10);

but this only works if you want to remove a key while the algorithm above works with arbitrary predicates. On the other hand, a.erase(10) can take advantage of std::set<int>'s structure and will be O(log N) while the algorithm is O(N) (with N == s.size()).

std::remove_if re-orders elements, so it cannot be used with std::set. But you can use std::set::erase:

std::set<int> a;
a.erase(10);