Document number:	J16/01-0011 = WG21 N1297
Date:	March 21, 2001
Author:	Jeremy Siek, University of Notre Dame
	jsiek@lsc.nd.edu

Improved Iterator Categories and Requirements

Introduction

Examples of useful iterators that do not ``fit''

Because of the mixing of iterator traversal and dereference return type, many useful iterators can not be appropriately categorized. For example, vector<bool>::iterator is almost a random access iterator, but the return type is not bool& (see issue 96 and Herb Sutter's paper J16/99-0008 = WG21 N1185). Therefore, the iterators only meet the requirements of input iterator and output iterator. This is so nonintuitive that at least one implementation erroneously assigns random_access_iterator_tag as its iterator_category. Also, vector<bool> is not the only example of useful iterators that do not return true references: there is the often cited example of disk-based collections.

Another example is a counting iterator, an iterator the returns a sequence of integers when incremented and dereferenced (see boost::counting_iterator). There are two ways to implement this iterator, 1) make the reference type be a true reference (a reference to an integer data member of the counting iterator) or 2) make the reference type be the same as the value_type. Option 1) runs into the problems discussed in Issue 198, the reference will not be valid after the iterator is destroyed. Option 2) is therefore a better choice, but then we have a counting iterator that cannot be a random access iterator.

Yet another example is a transform iterator, an iterator adaptor that applies a unary function object to the dereference value of the wrapped iterator (see boost::transform_iterator). For unary functions such as std::times the return type of operator* clearly needs to be the result_type of the function object, which is typically not a reference. However, with the current iterator requirements, if you wrap int* with a transform iterator, you do not get a random access iterator as expected, but an input iterator.

A fourth example is found in the vertex and edge iterators of the Boost Graph Library. These iterators return vertex and edge descriptors, which are lightweight handles created on-the-fly. They must be returned by-value. As a result, their current standard iterator category is std::input_iterator_tag, which means that, strictly speaking, you could not use these iterators with algorithms like std::min_element(). As a temporary solution, we introduced the concept Multi-Pass Input Iterator to describe the vertex and edge descriptors, but as the design notes for concept suggest, a better solution is needed.

In short, there are many useful iterators that do not fit into the current standard iterator categories. As a result, the following bad things happen:

• Iterators are often miss-categorized.
• Algorithm requirements are more strict than necessary, because they can not separate out the need for random-access from the need for a true reference return type.

Proposal for new iterator categories and requirements

• Readable Iterator
• Writable Iterator
• Swappable Iterator
• Constant Lvalue Iterator
• Mutable Lvalue Iterator

• Forward Traversal Iterator
• Bidirectional Traversal Iterator
• Random Access Traversal Iterator

Note: we considered defining a Single-Pass Iterator, which could be combined with Readable or Writable Iterator to replace the Input and Output Iterator requirements. We rejected this idea because there are several differences between Input and Output Iterators that make it hard to merge them: Input Iterator requires Equality Comparable while Output Iterator does not and Input Iterator requires Assignable while Output Iterator does not.

New categories and traits classes

namespace std {

  // Return Type Categories
  struct readable_iterator_tag { };
  struct writable_iterator_tag { };
  struct swappable_iterator_tag { };
  struct mutable_lvalue_iterator_tag : virtual public writable_iterator_tag,
    virtual public readable_iterator_tag { };
  struct constant_lvalue_iterator_tag : public readable_iterator_tag { };

  // Traversal Categories
  struct forward_traversal_tag { };
  struct bidirectional_traversal_tag : public forward_traversal_tag { };
  struct random_access_traversal_tag : public bidirectional_traversal_tag { };

}

namespace std {

  struct new_iterator_base { };

  template <typename Iterator>
  struct return_category
  {
    // Pseudo-code
    if (Iterator inherits from new_iterator_base) {
      typedef typename Iterator::return_category type;
    } else {
      typedef std::iterator_traits<Iterator> OldTraits;
      typedef typename OldTraits::iterator_category Cat;
      if (Cat inherits from std::forward_iterator_tag)
	if (is-const(T))
	  typedef boost::constant_lvalue_iterator_tag type;
	else
	  typedef boost::mutable_lvalue_iterator_tag type;
      else if (Cat inherits from std::input_iterator_tag)
	typedef boost::readable_iterator_tag type;
      else if (Cat inherits from std::output_iterator_tag)
	typedef boost::writable_iterator_tag type;
    }
  };

  template <typename T>
  struct return_category<T*>
  {
    // Pseudo-code
    if (is-const(T))
      typedef boost::constant_lvalue_iterator_tag type;
    else
      typedef boost::mutable_lvalue_iterator_tag type;
  };

  template <typename Iterator>
  struct traversal_category
  {
    // Pseudo-code
    if (Iterator inherits from new_iterator_base) {
      typedef typename Iterator::traversal_category type;
    } else {
      typedef std::iterator_traits<Iterator> OldTraits;
      typedef typename OldTraits::iterator_category Cat;

      if (Cat inherits from std::random_access_iterator_tag)
	typedef boost::random_access_traversal_tag type;
      else if (Cat inherits from std::bidirectional_iterator_tag)
	typedef boost::bidirectional_traversal_tag type;
      else if (Cat inherits from std::forward_iterator_tag)
	typedef boost::forward_traversal_tag type;
    }
  };

  template <typename T>
  struct traversal_category<T*>
  {
    typedef boost::random_access_traversal_tag type;
  };

}

Impact on the Standard Algorithms

The New Iterator Requirements

Notation

Readable Iterator

Associated Types

Refinement of

Valid expressions

Writable Iterator

Definitions

Associated Types

Refinement of

Valid expressions

Swappable Iterator

Note: the requirements for Swappable Iterator are dependent on the issues surrounding std::swap() being resolved. Here we assume that the issue will be resolved by allowing the overload of std::swap() for user-defined types.

Note: Readable Iterator and Writable Iterator combined implies Swappable Iterator because of the fully templated std::swap(). However, Swappable Iterator does not imply Readable Iterator nor Writable Iterator.

Associated Types

Valid expressions

Name	Expression	Return type
Iterator Swap	std::iter_swap(x, y)	void
Dereference and Swap	std::swap(*x, *y)	void

Constant Lvalue Iterator

Refinement of

Associated Types

Mutable Lvalue Iterator

Refinement of

Associated Types

Forward Traversal Iterator

Refinement of

Associated types

Valid expressions

Bidirectional Traversal Iterator

Refinement of

Associated types

Valid expressions

Random Access Traversal Iterator

Refinement of

Associated types

Valid expressions