Getting index of a std::vector::iterator

Question

So that's what I have tried so far:

class menu_item
{
  private:

    // ....

    std::vector<std::string> options_;
    std::vector<std::string>::iterator current_;

  public:
    menu_item(std::string name, std::vector<std::string> options)
        : name_(name), options_(options)
    {
        current_ = begin(options_);
    }

    // ....

    const int curr_opt_id()
    {
        return current_ - begin(options_);
    }
};

But curr_opt_id() returns -24. Does anybody know what I am doing wrong here?

How are you using your `menu_item` class? Are you adding elements to `options_` after initialization? — Andy Prowl, May 03 '13 at 15:38
`menu_item`s are added to a `std::vector>` in my `list_menu` class. And no, I'm not adding any elements to options_ after initialization. — , May 03 '13 at 15:52
Then you should prepare an [`SSCCE`](http://sscce.org). I cannot reproduce the behavior based on the code you provided — Andy Prowl, May 03 '13 at 15:53
My `list_menu` class invalidated the iterators. Thanks for all answers, problem solved! — , May 03 '13 at 16:06

score 3 · Accepted Answer · edited May 23 '17 at 12:28

3

When you add to a vector, there's a chance that the internal storage will be reallocated which will invalidate all existing iterators. Doing arithmetic on an invalid iterator isn't going to end well.

See Iterator invalidation rules

edited May 23 '17 at 12:28

Community

1
1

answered May 03 '13 at 15:41

Mark Ransom

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score 2 · Answer 2 · answered May 03 '13 at 15:41

Iterators of a vector get invalidated upon reallocation, which happens when the current capacity is not sufficient to hold the actual content plus a newly added element.

What is most likely happening here is that the current_ iterator, which is initialized at construction time, gets invalidated by subsequent insertions into options_, which gives you undefined behavior when evaluating the expression:

current_ - begin(options_)

Getting index of a std::vector::iterator

2 Answers2