void put(const K& k, U v)
{
if (!v)
impl.erase(k);
else
impl.insert(k, std::move(v));
}
void put(const K& k, V v)
{
if (v == V())
impl.erase(k);
else
impl.insert(k, std::move(v));
}
void insert(const K& k, V v)
{
// As far as I can tell, this is the simplest way to
// implement move-only insert-with-replacement.
iterator it = impl.lower_bound(k);
// invariant: if it is valid, it->first >= k
if (it != impl.end() && it->first == k)
it->second = std::move(v);
else
it = impl.insert(std::pair<K, V>(std::move(k), std::move(v))).first;
return (void)&it->second;
}
