How do I remove from a map while iterating it? like:
std::map<K, V> map;
for(auto i : map)
if(needs_removing(i))
// remove it from the map
If I use map.erase
it will invalidate the iterators
The standard associative-container erase idiom:
for (auto it = m.cbegin(); it != m.cend() /* not hoisted */; /* no increment */)
{
if (must_delete)
{
m.erase(it++); // or "it = m.erase(it)" since C++11
}
else
{
++it;
}
}
Note that we really want an ordinary for
loop here, since we are modifying the container itself. The range-based loop should be strictly reserved for situations where we only care about the elements. The syntax for the RBFL makes this clear by not even exposing the container inside the loop body.
Edit. Pre-C++11, you could not erase const-iterators. There you would have to say:
for (std::map<K,V>::iterator it = m.begin(); it != m.end(); ) { /* ... */ }
Erasing an element from a container is not at odds with constness of the element. By analogy, it has always been perfectly legitimate to delete p
where p
is a pointer-to-constant. Constness does not constrain lifetime; const values in C++ can still stop existing.