CODEWITHSUNDEEP
c++
q0987
q0987

Reputation: 36012

usage of the returned iterator from std::unique

template <typename T>
int eliminate_duplicate(vector<T> &A) {
  sort(A.begin(), A.end());  // makes identical elements become neighbors
  auto it = unique(A.begin(), A.end());  // removes neighboring duplicates
  A.resize(it - A.cbegin());  // truncates the unnecessary trailing part
  return it - A.cbegin(); // Question> Is this line valid?
}

Is the last line valid?

Here is my concern: after the calling of resize, the iterator it will point to a invalidate location, so can we still use it as it - A.cbegin() in the return line?

Upvotes: 0

Views: 162

Answers (1)

hmjd
hmjd

Reputation: 122011

From section 23.3.6.3 vector capacity of the C++11 standard (draft n3337), clause 9 (the bolded text is my emphasis as it is the case in the posted code that sz <= size() is true):

void resize(size_type sz);

Effects: If sz <= size(), equivalent to erase(begin() + sz, end());. If size() < sz, appends sz - size() value-initialized elements to the sequence.

and from section 23.3.6.5 vector modifiers, clause 3:

iterator erase(const_iterator position);
iterator erase(const_iterator first, const_iterator last);

Effects: Invalidates iterators and references at or after the point of the erase.

begin() + sz is equal to it, therefore it is invalidated.

To correct, just return A.size() (if the caller really requires it as that information is available from A anyway). Suggest using A.erase(it, A.end()); (as commented by juanchopanza) as the intent of the code is clearer.

Upvotes: 3

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