Custom iterator range function invalid output

Question

I want to implement range function to traverse container

template<class IterT> struct iteratorRange { IterT begin_; IterT end_; IterT begin() { return begin_; } IterT end() { return end_; } }; template<class T> iteratorRange<typename vector<T>::iterator>Head(vector<T> v, size_t step) { return {begin(v), next(begin(v), min(step, v.size()))}; } int main() { vector<int> v {1, 2, 3, 4, 5, 6, 7}; for (int& i : Head(v, 4)) { cout << i << ' '; } }

I expected

1 2 3 4

But received

1103219 1109239 3 4

What is the error?

I think the problem is that vector<T> v is a copy and your iterators refer to that copy's elements. But the copy doesn't exist anymore when you cout. — François Andrieux
– François Andrieux, Commented Jan 31, 2020 at 20:53

TKDmn · Accepted Answer · 2020-01-31 21:26:28Z

template<class T> iteratorRange<typename vector<T>::iterator>Head(vector<T>& v, size_t step) { return {begin(v), next(begin(v), min(step, v.size()))}; }

Instead of Head taking vector v by value, it should take a reference. This fixes your problem.

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