Are STL predicates allowed to use their argument's address?

Question

When writing a custom predicate function/functor to pass to an STL algorithm, is the predicate allowed to use the address of its argument?

Here's the problem that inspired the question. I have a vector vec, and a vector inds, which contains some indices into vec. I'd like to remove those elements in vec whose indices are listed in inds.

One approach is to use remove_if with a predicate functor InInds that determines its argument's index in vec by taking its address:

class InInds {
  private:

  const vector<Element>& vec_;
  const vector<int>& inds_;

  public:

  InInds(const vector<Element>& vec, const vector<int>& inds) 
    : vec_(vec), inds_(inds) {}

  bool operator()(const Element& element) {
    // WARNING: uses the ADDRESS of element, not its value. May not be kosher?
    int index = &element - &vec[0];
    return std::find(inds_.begin(), inds_.end(), index) != inds_.end();
  }

}

InInds works if called directly on an element in vec. It will break if it's called on a copy of an element, since the copy's address will not be useful to determine element's index.

My question is: will this predicate work in remove_if for any standards-compliant compiler? Or are predicates strictly meant to operate only on values, not addresses?

Just use a loop. Trying to make algorithms fit where they don't fit isn't going to help anything. — Nicol Bolas, Apr 06 '12 at 21:14
@NicolBolas Should I infer from your answer that using addresses in predicates is definitely not standards-compliant? — SuperElectric, Apr 06 '12 at 21:29
Whether it is or not is irrelevant; it's easier and much clearer to all involved to just write a loop. — Nicol Bolas, Apr 06 '12 at 22:57
I would say that whether it's clearer is irrelevant to the question as asked ;) It was, as you'll note, a question about standards, not style. — SuperElectric, Apr 26 '12 at 00:47

score 1 · Answer 1 · answered Apr 06 '12 at 21:37

1

Use of remove_if with your predicate is wrong since remove_if usually moves undeleted elements toward the beginning of the range. You need to copy undeleted elements to another container instead of removing them in place.

answered Apr 06 '12 at 21:37

Eugene

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Are STL predicates allowed to use their argument's address?

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