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The standard iterator concepts (the iterator requirements defined -in the C++ Standard) have a flaw. They glom together two separate -issues into a single set of concepts. The two issues are iterator -traversal and dereference return type semantics. These two issues are -inherently orthogonal and therefore ought to be represented by two -separate sets of concepts. The concepts described here do just -that.
+The standard iterator categories and requirements are flawed because +they use a single hierarchy of requirements to address two orthogonal +issues: iterator traversal and dereference return +type. The current iterator requirement hierarchy is mainly +geared towards iterator traversal (hence the category names), while +requirements that address dereference return type sneak in at various +places. +
+The iterator requirements should be separated into two hierarchies. One set of concepts handles the return type semantics:
+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 +some differences between Input and Output Iterators that make it hard +to merge them: for example Input Iterator requires Equality Comparable +while Output Iterator does not. + +
+-std::Input Iterator refines boost::Single-Pass Iterator and -boost::ReadableIterator. +std::Input Iterator implies boost::ReadableIterator.
-std::Output Iterator refines boost::Single-Pass Iterator and -boost::Writable Iterator. +std::Output Iterator implies boost::Writable Iterator.
std::Forward Iterator refines boost::Forward Iterator and @@ -106,106 +121,99 @@ boost::Constant Lvalue Iterator or boost::Mutable Lvalue Iterator.
| X | -The iterator type. | -
| T | -The value type of X. | -
| x, y | -An object of type X. | -
| t | -An object of type T. | +
| X | +The iterator type. | +
| T | +The value type of X, i.e., std::iterator_traits<X>::value_type. | +
| x, y | +An object of type X. | +
| t | +An object of type T. |
- template <class Readable Iterator>
- void foo(Readable Iterator x)
- {
- typedef std::iterator_traits<Readable Iterator>::value_type T;
- T t = *x; // Read a value. This is OK.
- T& s = *x; // Bind to a reference. This is a bad idea.
- *x = t; // Try to assign. This is a really bad idea.
- }
-
-
+iterator.
| Value type | -boost::iterator_traits<X>::value_type | -- The type of the value obtained by dereferencing a LvalueIterator - | +
| Value type | +std::iterator_traits<X>::value_type | +The type of the objects pointed to by the iterator. |
| Return Category | -boost::iterator_traits<X>::return_category | -- A type convertible to boost::readable_iterator_tag - | +
| Reference type | +std::iterator_traits<X>::reference | ++ The return type of dereferencing the iterator. This + type must be convertible to T. + | +
| Return Category | +std::return_category<X>::type | ++ A type convertible to std::readable_iterator_tag + |
| Name | Expression | Type requirements | Return type |
|---|---|---|---|
| Dereference | -*x | -- | Convertible to T. | -
| Member access | -x->m | -T is a type with a member named m. | -- - | +
| Name | Expression | Type requirements | Return type |
| Dereference | +*x | ++ | std::iterator_traits<X>::reference | +
| Member access | +x->m | +T is a type with a member named m. | ++If m is a data member, the type of m. +If m is a member function, the return type of m. + |
| Return Category | -boost::iterator_traits<X>::return_category | -- A type convertible to boost::writable_iterator_tag - | +
| Return Category | +std::return_category<X>::type | ++ A type convertible to std::writable_iterator_tag + |
| Name | Expression | Return type | -
|---|---|---|
| Dereference assignment | -*x = a | -unspecified | -
| Dereference assignment | +*x = a | +unspecified | +
-
+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. + +
| Return Category | +std::return_category<X>::type | ++ A type convertible to std::swappable_iterator_tag + | +
+
| Name | Expression | Return type | +
|---|---|---|
| Iterator Swap | +std::iter_swap(x, y) | +void | +
| Dereference and Swap | +std::swap(*x, *y) | +void | +
+ + +
| Value type | -boost::iterator_traits<X>::value_type | -- The type of the value obtained by dereferencing a Constant Lvalue Iterator. - | -
| Reference type | -boost::iterator_traits<X>::reference | -- The return type of operator*(), which must be + |
| Reference type | +std::iterator_traits<X>::reference | ++ The return type of dereferencing the iterator, which must be const T&. - | +
| POinter type | -boost::iterator_traits<X>::pointer | -+ - |
| Return Category | -boost::iterator_traits<X>::return_category | -- A type convertible to boost::constant_lvalue_iterator_tag - | +
| Return Category | +std::return_category<X>::type | ++ A type convertible to std::constant_lvalue_iterator_tag + |
+ +
| Value type | -boost::iterator_traits<X>::value_type | -- The type of the value obtained by dereferencing a Mutable Lvalue Iterator. - | +
| Reference type | +std::iterator_traits<X>::reference | +The return type of dereferencing the iterator, which must be + T&. |
| Reference type | -boost::iterator_traits<X>::reference | -- The return type of operator*(), which is - T&. - | -
| Pointer type | -boost::iterator_traits<X>::pointer | -+ - |
| Return Category | -boost::iterator_traits<X>::return_category | -- A type convertible to boost::mutable_lvalue_iterator_tag - | +
| Return Category | +std::return_category<X>::type | ++ A type convertible to std::mutable_lvalue_iterator_tag + |
| Difference type | -boost::iterator_traits<X>::difference_type | -- A signed integral type used to represent the distance from one - iterator to another, or the number of elements in a range. - | -
| Traversal Category | -boost::iterator_traits<X>::traversal_category | -- A type convertible to boost::single_pass_iterator_tag - | +
| Difference Type | +std::iterator_traits<X>::difference_type | ++ A signed integral type used for representing distances + between iterators that point into the same range. + |
| Name | Expression | Type requirements | Return type | -
|---|---|---|---|
| Preincrement | -++i | X& | -|
| Postincrement | -(void)i++ | - |
-
| Traversal Category | -boost::iterator_traits<X>::traversal_category | -- A type convertible to boost::forward_iterator_tag - | +|
| Preincrement | +++i | X& | +|
| Postincrement | +i++ | convertible to const X& |
| Traversal Category | -boost::iterator_traits<X>::traversal_category | -- A type convertible to boost::bidirectional_iterator_tag - | +
| Traversal Category | +std::traversal_category<X>::type | ++ A type convertible to std::bidirectional_traversal_tag + |
| Name | Expression | Type requirements | Return type | -
|---|---|---|---|
| Predecrement | ---i | X& | -|
| Postdecrement | -i-- | X | +|
| Predecrement | +--i | X& | +|
| Postdecrement | +i-- | convertible to const X& |
| Traversal Category | -boost::iterator_traits<X>::traversal_category | -- A type convertible to boost::random_access_iterator_tag - | +
| Traversal Category | +std::traversal_category<X>::type | ++ A type convertible to std::random_access_traversal_tag + |
| Name | Expression | Type requirements | +|
|---|---|---|---|
| Name | Expression | Type requirements | Return type | -
| Iterator addition | -i += n | X& | -|
| Iterator addition | -i + n or n + i | X | -|
| Iterator subtraction | -i -= n | X& | -|
| Iterator subtraction | -i - n | X | -|
| Difference | -i - j | boost::iterator_traits<X>::difference_type | -|
| Element operator | -i[n] | -X must be a model of - Readable Iterator. | -The same return type as *i. | -
| Element assignment | -i[n] = t | -X must be a model of - Writable Iterator. | -unspecified | +
| Iterator addition | +i += n | X& | +|
| Iterator addition | +i + n or n + i | X | +|
| Iterator subtraction | +i -= n | X& | +|
| Iterator subtraction | +i - n | X | +|
| Difference | +i - j | std::iterator_traits<X>::difference_type | +|
| Element operator | +i[n] | +X must also be a model of + Readable Iterator. | +std::iterator_traits<X>::reference | +
| Element assignment | +i[n] = t | +X must also be a model of + Writable Iterator. | +unspecified |