C++ why this code doesn't work?

Question

vector<int> v = { 1,2,3,4,5 };
for (auto beg = v.begin(); beg != v.end();++beg)
{
    if (beg == v.begin())
        v.push_back(50);
}

During run time, it says : "vector iterator not incrementable".

Possible duplicate of [Why is this vector iterator not incrementable?](http://stackoverflow.com/questions/3779227/why-is-this-vector-iterator-not-incrementable) — Jongware, Jan 09 '17 at 20:22
Please provide [SSCCE](http://sscce.org). Also, you probably want `auto&` and a range based loop. — Jesper Juhl, Jan 09 '17 at 20:22
Possible duplicate of [How to use vector iterators when using vector<>::push\_back()](http://stackoverflow.com/questions/24879979/how-to-use-vector-iterators-when-using-vectorpush-back) — Superlokkus, Jan 09 '17 at 20:23
@Mohsen_Fatemi no, that's *exactly* the time when `auto` really shines for me, when I just want an iterator but really don't care much what type it needs to be. — Mark Ransom, Jan 09 '17 at 20:23
@Slava why do you think this *isn't* a minimal, complete, and verifiable example? — Mark Ransom, Jan 09 '17 at 20:25
@Slava obviously the question was answerable without it, so your complaint was pointless. — Mark Ransom, Jan 09 '17 at 20:31
@MarkRansom the fact that we assumed what type used there does not make this question good and does not make example [mcve] either. — Slava, Jan 09 '17 at 20:33

score 9 · Answer 1 · answered Jan 09 '17 at 20:22

See std::vector::push_back.

If the new size() is greater than capacity() then all iterators and references (including the past-the-end iterator) are invalidated. Otherwise only the past-the-end iterator is invalidated.

In your example, beg is an iterator. It is being invalidated by the push_back, you cannot use it anymore.

score 1 · Accepted Answer · answered Jan 09 '17 at 20:26

1

As mentioned std::vector::push_back() may invalidate your iterator. Possible, but pretty ugly solution could be:

for (auto beg = v.begin(); beg != v.end();++beg)
{
    if (beg == v.begin()) {
        v.push_back(50);
        beg = v.begin();
    }
}

but your logic seems convoluted, why not push back just before the loop?

answered Jan 09 '17 at 20:26

Slava

43,454
1
47
90

I didn't thought about the code, I just wanted to know why this is an error. – gigi Jan 09 '17 at 20:33
Answers on SO are not only for you, and that is the reason I asked you to fix your code. – Slava Jan 09 '17 at 20:35
I think that the reason why push_back() invalidates all iterators is because push_back() change the memory address occupied by the vector, am I right? – gigi Jan 09 '17 at 20:39
@gigi `push_back` **may** invalidate iterators. And actually in your updated example most probably it would not to. Now you should see why [mcve] is asked. But yes, vector may have to reallocate memory for storage if `capacity()` will be not big enough to hold new data size. – Slava Jan 09 '17 at 20:42

C++ why this code doesn't work?

2 Answers2