std::list Iterator doesn't get assigned

Sorry about the vague question title, but I have these typedefs here:

typedef std::list<AnimationKeyframe> Timeline;
typedef Timeline::iterator Keyframe;

and let's say a class like this:

class foo
{
Timeline timeline;
Keyframe left,right;
};

I also have a copy/assignment constructor because I'm storing iterators (a bad idea it seems), and the trouble lies just there.

Keyframe myTemp, hisTemp;
this->left = this->timeline.begin(); // assigns
myTemp = this->timeline.begin(); // assigns
hisTemp = that.timeline.begin() // does not assign

as soon as I am trying to assign a Keyframe with one of 'that' (the other foo) I get errors that look like an ambiguity problem.

binary '=' : no operator found which takes a right-hand operand of type 'std::_List_const_iterator<_Mylist>' (or there is no acceptable conversion)

Additional info:

  with
 [
     _Mylist=std::_List_val<AnimationKeyframe,std::allocator<AnimationKeyframe>>
 ]
  could be 'std::_List_iterator<_Mylist> &std::_List_iterator<_Mylist>::operator =(const std::_List_iterator<_Mylist> &)'
 with
 [
     _Mylist=std::_List_val<AnimationKeyframe,std::allocator<AnimationKeyframe>>
 ]
 while trying to match the argument list '(Keyfr开发者_如何学Pythoname, std::_List_const_iterator<_Mylist>)'
 with
 [
     _Mylist=std::_List_val<AnimationKeyframe,std::allocator<AnimationKeyframe>>
 ]

What causes this odd behavior? I'm suspecting my bad style for using iterators as states...but I don't know how else I could do, aside from keeping pointers.

It looks as though that is const (perhaps a const reference). begin() const returns a const_iterator, not an iterator.

The reason there is no conversion from const_iterator to iterator is as follows:

void foo(const std::vector<int> &vec) {
    std::vector<int>::iterator it = vec.begin(); // if this compiled...
    *it = 5;  // then this would attempt to modify a vector taken by const ref
}