Why is this iterator const?

Question

Possible Duplicate:
C++ STL set update is tedious: I can't change an element in place

Why does this code complain that my argument is a const_iterator when it's not declared that way?

void foo(std::set<int>::iterator it)
{
  *it=2;
}

I get error: assignment of read-only location ‘it.std::_Rb_tree_const_iterator<_Tp>::operator* with _Tp = int’

After reading the link, I understand. I don't like it, but it makes sense. — John Gordon, Mar 14 '12 at 20:28
They did this because otherwise they cannot maintain "the invaraints" (they cannot guarantee it's still sorted), which would make `set` fail to work. — Mooing Duck, Mar 14 '12 at 20:41

score 3 · Answer 1 · answered Mar 14 '12 at 20:23

3

Because it's a set. Sets use const iterators because you can't change the values without messing up the order of the set.

answered Mar 14 '12 at 20:23

Fred Larson

60,987
18
112
174

score 3 · Answer 2 · answered Mar 14 '12 at 20:24

3

All set iterators are const. You can't update the value of a set member by assigning through the iterator. You must erase the old value and insert a new one.

answered Mar 14 '12 at 20:24

Robᵩ

163,533
20
239
308

score 1 · Answer 3 · answered Mar 14 '12 at 20:24

1

Because in C++11 set<>::iterator is the same as set<>::const_iterator. I.e. you cannot modify elements inside a set.

answered Mar 14 '12 at 20:24

Eugene

6,194
1
20
31

Why is this iterator const?

3 Answers3