return iterator from std::set::insert() is const? [duplicate]

Question

According to the C++ reference, set::insert is supposed to return pair where the iterator points to either the newly inserted element, or the existing element if such existed.

But I'm having a problem assigning to the iterator, as this simple examples shows:

int main() {
  set<int> set;
  *set.insert(5).first = 5;
  return 0;
} 

I've tried G++ and Clang and neither works.

set.cc:7:24: error: read-only variable is not assignable
  *set.insert(5).first = 5;
  ~~~~~~~~~~~~~~~~~~~~ ^

I can't find anything in the documentation that indicates that the iterator should deference to a const object, and nothing in the type signature would indicate this either. Can someone help me understand why this doesn't work?

Answer 1

For std::set, both its associated iterator and const_iterator types are constant bidirectional iterators. The reason for this is because std::set is ordered. If you were to modify an element of the set though an iterator, you would be breaking that ordering.

Consider a std::set with ordered elements {1, 4, 8}. If you then did something like *set.insert(5).first = 10; (if it were allowed), first 5 would be inserted to get {1, 4, 5, 8} and then the inserted element would be set to 10 to get {1, 4, 10, 8}. The ordering invariant has now been broken.

Since you're inserting 5 with insert(5), there's no reason to dereference the iterator and assign 5 to it.

Answer 2

You can't modify a set's members in place. It's an ordered container. Its iterators are not assignable.

Answer 3

In C++11, set iterators refer to const types (see set reference). If you think about it it makes sense, as a set stores its element ordered, and simply changing a certain element would most likely violate the ordering constraints.