How do I loop over consecutive pairs in an STL con

问题:

How do I create a custom class to loop over consecutive pairs of items in a STL container using a range-based loop?

This is the syntax and output I want:

std::list<int> number_list;
number_list.push_back(1);
number_list.push_back(2);
number_list.push_back(3);

auto paired_list = Paired(number_list);
for (const auto & pair : paired_list) {
  std::printf("The pair is (%d, %d)\n", *(pair[0]), *(pair[1]));
  // or
  //std::printf("The pair is (%d, %d)\n", *(pair.first), *(pair.second));
}
// output:
// The pair is (1, 2)
// The pair is (2, 3)

I know these (and more) are needed, but I can't figure it out:

template <class T>
class Paired {
  ???
  class iterator {
    ???
  }
  iterator begin() {
    ...
  }
  iterator end() {
    ...
  }
}

Don't worry about const modifiers.

No boost.

Do not modify or copy objects in the container.

回答1:

Here's what I would do.

#include <iterator>
#include <utility>

template <typename FwdIt> class adjacent_iterator {
public:
    adjacent_iterator(FwdIt first, FwdIt last)
        : m_first(first), m_next(first == last ? first : std::next(first)) { }

    bool operator!=(const adjacent_iterator& other) const {
        return m_next != other.m_next; // NOT m_first!
    }

    adjacent_iterator& operator++() {
        ++m_first;
        ++m_next;
        return *this;
    }

    typedef typename std::iterator_traits<FwdIt>::reference Ref;
    typedef std::pair<Ref, Ref> Pair;

    Pair operator*() const {
        return Pair(*m_first, *m_next); // NOT std::make_pair()!
    }

private:
    FwdIt m_first;
    FwdIt m_next;
};

template <typename FwdIt> class adjacent_range {
public:
    adjacent_range(FwdIt first, FwdIt last)
        : m_first(first), m_last(last) { }

    adjacent_iterator<FwdIt> begin() const {
        return adjacent_iterator<FwdIt>(m_first, m_last);
    }

    adjacent_iterator<FwdIt> end() const {
        return adjacent_iterator<FwdIt>(m_last, m_last);
    }

private:
    FwdIt m_first;
    FwdIt m_last;
};

template <typename C> auto make_adjacent_range(C& c) -> adjacent_range<decltype(c.begin())> {
    return adjacent_range<decltype(c.begin())>(c.begin(), c.end());
}

#include <iostream>
#include <vector>
using namespace std;

void test(const vector<int>& v) {
    cout << "[ ";

    for (const auto& p : make_adjacent_range(v)) {
        cout << p.first << "/" << p.second << " ";
    }

    cout << "]" << endl;
}

int main() {
    test({});
    test({11});
    test({22, 33});
    test({44, 55, 66});
    test({10, 20, 30, 40});
}

This prints:

[ ]
[ ]
[ 22/33 ]
[ 44/55 55/66 ]
[ 10/20 20/30 30/40 ]

Notes:

• I haven't exhaustively tested this, but it respects forward iterators (because it doesn't try to use operations beyond ++, !=, and *).

• range-for has extremely weak requirements; it doesn't require all of the things that forward iterators are supposed to provide. Therefore I have achieved range-for's requirements but no more.

• The "NOT m_first" comment is related to how the end of the range is approached. An adjacent_iterator constructed from an empty range has m_first == m_next which is also == last. An adjacent_iterator constructed from a 1-element range has m_first pointing to the element and m_next == last. An adjacent_iterator constructed from a multi-element range has m_first and m_next pointing to consecutive valid elements. As it is incremented, eventually m_first will point to the final element and m_next will be last. What adjacent_range's end() returns is constructed from (m_last, m_last). For a totally empty range, this is physically identical to begin(). For 1+ element ranges, this is not physically identical to a begin() that has been incremented until we don't have a complete pair - such iterators have m_first pointing to the final element. But if we compare iterators based on their m_next, then we get correct semantics.

• The "NOT std::make_pair()" comment is because make_pair() decays, while we actually want a pair of references. (I could have used decltype, but iterator_traits will tell us the answer too.)

• The major remaining subtleties would revolve around banning rvalues as inputs to make_adjacent_range (such temporaries would not have their lives prolonged; the Committee is studying this issue), and playing an ADL dance to respect non-member begin/end, as well as built-in arrays. These exercises are left to the reader.

回答2:

edit I was using transform.

Use adjacent_difference.

The second version takes a binary function which transforms the two values into a new (different) value:

string make_message(int first, int second) {
    ostringstream oss;
    oss << "The pair is (" << first << ", " << second << ")";
    return oss.str();
}

We can now transform the adjacent pairs into a third range. We'll use the ostream_iterator to use cout like a range:

list<int> numbers;
//...
adjacent_difference(numbers.begin(), numbers.end(),
                    ostream_iterator<string>(cout, "\n"),
                    make_message);

2nd edit

I found a question on comp.lang.c++.moderated asking why there aren't more 'adjacent' functions in the standard library such as for_each_adjacent. The reply said they were trivial to implement using std::mismatch.

I think this would be a better direction to go than implementing a special adjacent iterator.

回答3:

Try this.

#include <list>
#include <iostream>

template<class T, class TIter = typename T::iterator, class TVal = typename T::value_type>
class PairedImpl {
    T& m_t;
public:
    class Iter {
        TIter m_it;
    public:
        Iter(const TIter & it) : m_it(it)  {}

        bool  operator!=(const Iter& it)   {         return m_it != it.m_it; }
        Iter& operator++()                 { ++m_it; return *this; }
        const Iter & operator *() const    {         return *this; }
        const TVal & first()      const    {         return *m_it; }
        const TVal & second()     const    {         return *std::next(m_it); }
    };

    PairedImpl(T& t) : m_t(t) {}

    Iter begin() { return Iter(m_t.begin()); }

    Iter end() {
        TIter end = m_t.end();
        return Iter(m_t.empty() ? end : --end);
    }
};

template<class T>
PairedImpl<T> Paired(T& t) {
    return PairedImpl<T>(t);
}

Usage

int main()
{
    std::list<int> lst;
    lst.push_back(1);
    lst.push_back(2);
    lst.push_back(3);
    lst.push_back(4);
    lst.push_back(5);

    for (const auto & pair : Paired(lst)) {
        std::cout << "(" << pair.first() << ", " << pair.second() << ")" << std::endl;
    }
    return 0;
}

回答4:

Okay, an hour with no answers, I've come up with a solution which works. Note that this uses my own FixedLengthVector which is exactly what it sounds like.

template <class T>
class Grouped {
private:
  // length of grouped objects
  static const unsigned length_ = 2;
  // hold pointer to base container to avoid comparing incompatible iterators
  T * base_container_;
public:
  // constructor
  Grouped(T & base_container) :
      base_container_(&base_container) {
  }
  // iterator
  class iterator {
  private:
    // hold pointer to base container to avoid comparing incompatible iterators
    T * base_container_;
    // hold pointers to objects in base container
    FixedLengthVector<length_, typename T::value_type *> ptr_;
    // hold iterator to last object
    typename T::iterator last_iterator_;
  public:
    // constructor
    iterator(T & base_container, typename T::iterator & it)
        : base_container_(&base_container),
          last_iterator_(it) {
      // set up pointers if possible
      unsigned i = 0;
      // check for end iterator
      if (last_iterator_ == base_container_->end()) {
        ptr_.fill(NULL);
        return;
      }
      // set up first object
      ptr_[0] = &*last_iterator_;
      // set up next objects
      for (unsigned i = 1; i < length_; ++i) {
        ++last_iterator_;
        if (last_iterator_ == base_container_->end()) {
          ptr_.fill(NULL);
          return;
        }
        ptr_[i] = &*last_iterator_;
      }
    }
    // dereference operator
    FixedLengthVector<length_, typename T::value_type *> & operator * (void) {
      assert(ptr_[0] != NULL);
      return ptr_;
    }
    // pre-increment
    iterator & operator ++ (void) {
      // can't increase past end
      assert(last_iterator_ != base_container_->end());
      // find next iterator
      ++last_iterator_;
      if (last_iterator_ == base_container_->end()) {
        ptr_.fill(NULL);
        return * this;
      }
      // cycle pointers left
      for (unsigned i = 1; i < length_; ++i) {
        ptr_[i - 1] = ptr_[i];
      }
      ptr_[length_ - 1] = &*last_iterator_;
      return * this;
    }
    // equality comparison
    bool operator == (const iterator & that) const {
      return base_container_ == that.base_container_ &&
             last_iterator_ == that.last_iterator_;
    }
    // inequality comparison
    bool operator != (const iterator & that) const {
      return !(*this == that);
    }
  };
  // end iterator
  iterator end() {
    return iterator(*base_container_, base_container_->end());
  }
  // begin iterator
  iterator begin() {
    return iterator(*base_container_, base_container_->begin());
  }
};