boost/iterator/iterator_facade.hpp

   1 // (C) Copyright David Abrahams 2002.
   2 // (C) Copyright Jeremy Siek    2002.
   3 // (C) Copyright Thomas Witt    2002.
   4 // Distributed under the Boost Software License, Version 1.0. (See
   5 // accompanying file LICENSE_1_0.txt or copy at
   6 // http://www.boost.org/LICENSE_1_0.txt)
   7 #ifndef BOOST_ITERATOR_FACADE_23022003THW_HPP
   8 #define BOOST_ITERATOR_FACADE_23022003THW_HPP
   9
  10 #include <boost/iterator.hpp>
  11 #include <boost/iterator/interoperable.hpp>
  12 #include <boost/iterator/iterator_traits.hpp>
  13
  14 #include <boost/iterator/detail/facade_iterator_category.hpp>
  15 #include <boost/iterator/detail/enable_if.hpp>
  16
  17 #include <boost/implicit_cast.hpp>
  18 #include <boost/static_assert.hpp>
  19
  20 #include <boost/type_traits/is_same.hpp>
  21 #include <boost/type_traits/add_const.hpp>
  22 #include <boost/type_traits/add_pointer.hpp>
  23 #include <boost/type_traits/remove_const.hpp>
  24 #include <boost/type_traits/remove_reference.hpp>
  25 #include <boost/type_traits/is_convertible.hpp>
  26 #include <boost/type_traits/is_pod.hpp>
  27
  28 #include <boost/mpl/eval_if.hpp>
  29 #include <boost/mpl/if.hpp>
  30 #include <boost/mpl/or.hpp>
  31 #include <boost/mpl/and.hpp>
  32 #include <boost/mpl/not.hpp>
  33 #include <boost/mpl/always.hpp>
  34 #include <boost/mpl/apply.hpp>
  35 #include <boost/mpl/identity.hpp>
  36
  37 #include <boost/iterator/detail/config_def.hpp> // this goes last
  38
  39 namespace boost
  40 {
  41   // This forward declaration is required for the friend declaration
  42   // in iterator_core_access
  43   template <class I, class V, class TC, class R, class D> class iterator_facade;
  44
  45   namespace detail
  46   {
  47     // A binary metafunction class that always returns bool.  VC6
  48     // ICEs on mpl::always<bool>, probably because of the default
  49     // parameters.
  50     struct always_bool2
  51     {
  52         template <class T, class U>
  53         struct apply
  54         {
  55             typedef bool type;
  56         };
  57     };
  58
  59     //
  60     // enable if for use in operator implementation.
  61     //
  62     template <
  63         class Facade1
  64       , class Facade2
  65       , class Return
  66     >
  67     struct enable_if_interoperable
  68 #if BOOST_WORKAROUND(BOOST_MSVC, <= 1300)
  69     {
  70         typedef typename mpl::if_<
  71             mpl::or_<
  72                 is_convertible<Facade1, Facade2>
  73               , is_convertible<Facade2, Facade1>
  74             >
  75           , Return
  76           , int[3]
  77         >::type type;
  78     };
  79 #else
  80       : ::boost::iterators::enable_if<
  81            mpl::or_<
  82                is_convertible<Facade1, Facade2>
  83              , is_convertible<Facade2, Facade1>
  84            >
  85          , Return
  86         >
  87     {};
  88 #endif
  89
  90     //
  91     // Generates associated types for an iterator_facade with the
  92     // given parameters.
  93     //
  94     template <
  95         class ValueParam
  96       , class CategoryOrTraversal
  97       , class Reference
  98       , class Difference
  99     >
 100     struct iterator_facade_types
 101     {
 102         typedef typename facade_iterator_category<
 103             CategoryOrTraversal, ValueParam, Reference
 104         >::type iterator_category;
 105
 106         typedef typename remove_const<ValueParam>::type value_type;
 107
 108         typedef typename mpl::eval_if<
 109             boost::detail::iterator_writability_disabled<ValueParam,Reference>
 110           , add_pointer<const value_type>
 111           , add_pointer<value_type>
 112         >::type pointer;
 113
 114 # if defined(BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION)                          \
 115     && (BOOST_WORKAROUND(_STLPORT_VERSION, BOOST_TESTED_AT(0x452))              \
 116         || BOOST_WORKAROUND(BOOST_DINKUMWARE_STDLIB, BOOST_TESTED_AT(310)))     \
 117     || BOOST_WORKAROUND(BOOST_RWSTD_VER, BOOST_TESTED_AT(0x20101))              \
 118     || BOOST_WORKAROUND(BOOST_DINKUMWARE_STDLIB, <= 310)
 119
 120         // To interoperate with some broken library/compiler
 121         // combinations, user-defined iterators must be derived from
 122         // std::iterator.  It is possible to implement a standard
 123         // library for broken compilers without this limitation.
 124 #  define BOOST_ITERATOR_FACADE_NEEDS_ITERATOR_BASE 1
 125
 126         typedef
 127            iterator<iterator_category, value_type, Difference, pointer, Reference>
 128         base;
 129 # endif
 130     };
 131
 132     // iterators whose dereference operators reference the same value
 133     // for all iterators into the same sequence (like many input
 134     // iterators) need help with their postfix ++: the referenced
 135     // value must be read and stored away before the increment occurs
 136     // so that *a++ yields the originally referenced element and not
 137     // the next one.
 138     template <class Iterator>
 139     class postfix_increment_proxy
 140     {
 141         typedef typename iterator_value<Iterator>::type value_type;
 142      public:
 143         explicit postfix_increment_proxy(Iterator const& x)
 144           : stored_value(*x)
 145         {}
 146
 147         // Returning a mutable reference allows nonsense like
 148         // (*r++).mutate(), but it imposes fewer assumptions about the
 149         // behavior of the value_type.  In particular, recall taht
 150         // (*r).mutate() is legal if operator* returns by value.
 151         value_type&
 152         operator*() const
 153         {
 154             return this->stored_value;
 155         }
 156      private:
 157         mutable value_type stored_value;
 158     };
 159
 160     //
 161     // In general, we can't determine that such an iterator isn't
 162     // writable -- we also need to store a copy of the old iterator so
 163     // that it can be written into.
 164     template <class Iterator>
 165     class writable_postfix_increment_proxy
 166     {
 167         typedef typename iterator_value<Iterator>::type value_type;
 168      public:
 169         explicit writable_postfix_increment_proxy(Iterator const& x)
 170           : stored_value(*x)
 171           , stored_iterator(x)
 172         {}
 173
 174         // Dereferencing must return a proxy so that both *r++ = o and
 175         // value_type(*r++) can work.  In this case, *r is the same as
 176         // *r++, and the conversion operator below is used to ensure
 177         // readability.
 178         writable_postfix_increment_proxy const&
 179         operator*() const
 180         {
 181             return *this;
 182         }
 183
 184         // Provides readability of *r++
 185         operator value_type&() const
 186         {
 187             return stored_value;
 188         }
 189
 190         // Provides writability of *r++
 191         template <class T>
 192         T const& operator=(T const& x) const
 193         {
 194             *this->stored_iterator = x;
 195             return x;
 196         }
 197
 198         // This overload just in case only non-const objects are writable
 199         template <class T>
 200         T& operator=(T& x) const
 201         {
 202             *this->stored_iterator = x;
 203             return x;
 204         }
 205
 206         // Provides X(r++)
 207         operator Iterator const&() const
 208         {
 209             return stored_iterator;
 210         }
 211
 212      private:
 213         mutable value_type stored_value;
 214         Iterator stored_iterator;
 215     };
 216
 217 # ifdef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION
 218
 219     template <class Reference, class Value>
 220     struct is_non_proxy_reference_impl
 221     {
 222         static Reference r;
 223
 224         template <class R>
 225         static typename mpl::if_<
 226             is_convertible<
 227                 R const volatile*
 228               , Value const volatile*
 229             >
 230           , char[1]
 231           , char[2]
 232         >::type& helper(R const&);
 233
 234         BOOST_STATIC_CONSTANT(bool, value = sizeof(helper(r)) == 1);
 235     };
 236
 237     template <class Reference, class Value>
 238     struct is_non_proxy_reference
 239       : mpl::bool_<
 240             is_non_proxy_reference_impl<Reference, Value>::value
 241         >
 242     {};
 243 # else
 244     template <class Reference, class Value>
 245     struct is_non_proxy_reference
 246       : is_convertible<
 247             typename remove_reference<Reference>::type
 248             const volatile*
 249           , Value const volatile*
 250         >
 251     {};
 252 # endif
 253
 254     // A metafunction to choose the result type of postfix ++
 255     //
 256     // Because the C++98 input iterator requirements say that *r++ has
 257     // type T (value_type), implementations of some standard
 258     // algorithms like lexicographical_compare may use constructions
 259     // like:
 260     //
 261     //          *r++ < *s++
 262     //
 263     // If *r++ returns a proxy (as required if r is writable but not
 264     // multipass), this sort of expression will fail unless the proxy
 265     // supports the operator<.  Since there are any number of such
 266     // operations, we're not going to try to support them.  Therefore,
 267     // even if r++ returns a proxy, *r++ will only return a proxy if
 268     // *r also returns a proxy.
 269     template <class Iterator, class Value, class Reference, class CategoryOrTraversal>
 270     struct postfix_increment_result
 271       : mpl::eval_if<
 272             mpl::and_<
 273                 // A proxy is only needed for readable iterators
 274                 is_convertible<Reference,Value const&>
 275
 276                 // No multipass iterator can have values that disappear
 277                 // before positions can be re-visited
 278               , mpl::not_<
 279                     is_convertible<
 280                         typename iterator_category_to_traversal<CategoryOrTraversal>::type
 281                       , forward_traversal_tag
 282                     >
 283                 >
 284             >
 285           , mpl::if_<
 286                 is_non_proxy_reference<Reference,Value>
 287               , postfix_increment_proxy<Iterator>
 288               , writable_postfix_increment_proxy<Iterator>
 289             >
 290           , mpl::identity<Iterator>
 291         >
 292     {};
 293
 294     // operator->() needs special support for input iterators to strictly meet the
 295     // standard's requirements. If *i is not a reference type, we must still
 296     // produce a lvalue to which a pointer can be formed. We do that by
 297     // returning an instantiation of this special proxy class template.
 298     template <class T>
 299     struct operator_arrow_proxy
 300     {
 301         operator_arrow_proxy(T const* px) : m_value(*px) {}
 302         T* operator->() const { return &m_value; }
 303         // This function is needed for MWCW and BCC, which won't call operator->
 304         // again automatically per 13.3.1.2 para 8
 305         operator T*() const { return &m_value; }
 306         mutable T m_value;
 307     };
 308
 309     // A metafunction that gets the result type for operator->.  Also
 310     // has a static function make() which builds the result from a
 311     // Reference
 312     template <class ValueType, class Reference, class Pointer>
 313     struct operator_arrow_result
 314     {
 315         // CWPro8.3 won't accept "operator_arrow_result::type", and we
 316         // need that type below, so metafunction forwarding would be a
 317         // losing proposition here.
 318         typedef typename mpl::if_<
 319             is_reference<Reference>
 320           , Pointer
 321           , operator_arrow_proxy<ValueType>
 322         >::type type;
 323
 324         static type make(Reference x)
 325         {
 326             return implicit_cast<type>(&x);
 327         }
 328     };
 329
 330 # if BOOST_WORKAROUND(BOOST_MSVC, < 1300)
 331     // Deal with ETI
 332     template<>
 333     struct operator_arrow_result<int, int, int>
 334     {
 335         typedef int type;
 336     };
 337 # endif
 338
 339     // A proxy return type for operator[], needed to deal with
 340     // iterators that may invalidate referents upon destruction.
 341     // Consider the temporary iterator in *(a + n)
 342     template <class Iterator>
 343     class operator_brackets_proxy
 344     {
 345         // Iterator is actually an iterator_facade, so we do not have to
 346         // go through iterator_traits to access the traits.
 347         typedef typename Iterator::reference  reference;
 348         typedef typename Iterator::value_type value_type;
 349
 350      public:
 351         operator_brackets_proxy(Iterator const& iter)
 352           : m_iter(iter)
 353         {}
 354
 355         operator reference() const
 356         {
 357             return *m_iter;
 358         }
 359
 360         operator_brackets_proxy& operator=(value_type const& val)
 361         {
 362             *m_iter = val;
 363             return *this;
 364         }
 365
 366      private:
 367         Iterator m_iter;
 368     };
 369
 370     // A metafunction that determines whether operator[] must return a
 371     // proxy, or whether it can simply return a copy of the value_type.
 372     template <class ValueType, class Reference>
 373     struct use_operator_brackets_proxy
 374       : mpl::not_<
 375             mpl::and_<
 376                 // Really we want an is_copy_constructible trait here,
 377                 // but is_POD will have to suffice in the meantime.
 378                 boost::is_POD<ValueType>
 379               , iterator_writability_disabled<ValueType,Reference>
 380             >
 381         >
 382     {};
 383
 384     template <class Iterator, class Value, class Reference>
 385     struct operator_brackets_result
 386     {
 387         typedef typename mpl::if_<
 388             use_operator_brackets_proxy<Value,Reference>
 389           , operator_brackets_proxy<Iterator>
 390           , Value
 391         >::type type;
 392     };
 393
 394     template <class Iterator>
 395     operator_brackets_proxy<Iterator> make_operator_brackets_result(Iterator const& iter, mpl::true_)
 396     {
 397         return operator_brackets_proxy<Iterator>(iter);
 398     }
 399
 400     template <class Iterator>
 401     typename Iterator::value_type make_operator_brackets_result(Iterator const& iter, mpl::false_)
 402     {
 403       return *iter;
 404     }
 405
 406     struct choose_difference_type
 407     {
 408         template <class I1, class I2>
 409         struct apply
 410           :
 411 # ifdef BOOST_NO_ONE_WAY_ITERATOR_INTEROP
 412           iterator_difference<I1>
 413 # elif BOOST_WORKAROUND(BOOST_MSVC, < 1300)
 414           mpl::if_<
 415               is_convertible<I2,I1>
 416             , typename I1::difference_type
 417             , typename I2::difference_type
 418           >
 419 # else
 420           mpl::eval_if<
 421               is_convertible<I2,I1>
 422             , iterator_difference<I1>
 423             , iterator_difference<I2>
 424           >
 425 # endif
 426         {};
 427
 428     };
 429   } // namespace detail
 430
 431
 432   // Macros which describe the declarations of binary operators
 433 # ifdef BOOST_NO_STRICT_ITERATOR_INTEROPERABILITY
 434 #  define BOOST_ITERATOR_FACADE_INTEROP_HEAD(prefix, op, result_type)       \
 435     template <                                                              \
 436         class Derived1, class V1, class TC1, class Reference1, class Difference1 \
 437       , class Derived2, class V2, class TC2, class Reference2, class Difference2 \
 438     >                                                                       \
 439     prefix typename mpl::apply2<result_type,Derived1,Derived2>::type \
 440     operator op(                                                            \
 441         iterator_facade<Derived1, V1, TC1, Reference1, Difference1> const& lhs   \
 442       , iterator_facade<Derived2, V2, TC2, Reference2, Difference2> const& rhs)
 443 # else
 444 #  define BOOST_ITERATOR_FACADE_INTEROP_HEAD(prefix, op, result_type)   \
 445     template <                                                          \
 446         class Derived1, class V1, class TC1, class Reference1, class Difference1 \
 447       , class Derived2, class V2, class TC2, class Reference2, class Difference2 \
 448     >                                                                   \
 449     prefix typename boost::detail::enable_if_interoperable<             \
 450         Derived1, Derived2                                              \
 451       , typename mpl::apply2<result_type,Derived1,Derived2>::type       \
 452     >::type                                                             \
 453     operator op(                                                        \
 454         iterator_facade<Derived1, V1, TC1, Reference1, Difference1> const& lhs   \
 455       , iterator_facade<Derived2, V2, TC2, Reference2, Difference2> const& rhs)
 456 # endif
 457
 458 #  define BOOST_ITERATOR_FACADE_PLUS_HEAD(prefix,args)              \
 459     template <class Derived, class V, class TC, class R, class D>   \
 460     prefix Derived operator+ args
 461
 462   //
 463   // Helper class for granting access to the iterator core interface.
 464   //
 465   // The simple core interface is used by iterator_facade. The core
 466   // interface of a user/library defined iterator type should not be made public
 467   // so that it does not clutter the public interface. Instead iterator_core_access
 468   // should be made friend so that iterator_facade can access the core
 469   // interface through iterator_core_access.
 470   //
 471   class iterator_core_access
 472   {
 473 # if defined(BOOST_NO_MEMBER_TEMPLATE_FRIENDS)
 474       // Tasteless as this may seem, making all members public allows member templates
 475       // to work in the absence of member template friends.
 476    public:
 477 # else
 478
 479       template <class I, class V, class TC, class R, class D> friend class iterator_facade;
 480
 481 #  define BOOST_ITERATOR_FACADE_RELATION(op)                                \
 482       BOOST_ITERATOR_FACADE_INTEROP_HEAD(friend,op, boost::detail::always_bool2);
 483
 484       BOOST_ITERATOR_FACADE_RELATION(==)
 485       BOOST_ITERATOR_FACADE_RELATION(!=)
 486
 487       BOOST_ITERATOR_FACADE_RELATION(<)
 488       BOOST_ITERATOR_FACADE_RELATION(>)
 489       BOOST_ITERATOR_FACADE_RELATION(<=)
 490       BOOST_ITERATOR_FACADE_RELATION(>=)
 491 #  undef BOOST_ITERATOR_FACADE_RELATION
 492
 493       BOOST_ITERATOR_FACADE_INTEROP_HEAD(
 494           friend, -, boost::detail::choose_difference_type)
 495       ;
 496
 497       BOOST_ITERATOR_FACADE_PLUS_HEAD(
 498           friend inline
 499           , (iterator_facade<Derived, V, TC, R, D> const&
 500            , typename Derived::difference_type)
 501       )
 502       ;
 503
 504       BOOST_ITERATOR_FACADE_PLUS_HEAD(
 505           friend inline
 506         , (typename Derived::difference_type
 507            , iterator_facade<Derived, V, TC, R, D> const&)
 508       )
 509       ;
 510
 511 # endif
 512
 513       template <class Facade>
 514       static typename Facade::reference dereference(Facade const& f)
 515       {
 516           return f.dereference();
 517       }
 518
 519       template <class Facade>
 520       static void increment(Facade& f)
 521       {
 522           f.increment();
 523       }
 524
 525       template <class Facade>
 526       static void decrement(Facade& f)
 527       {
 528           f.decrement();
 529       }
 530
 531       template <class Facade1, class Facade2>
 532       static bool equal(Facade1 const& f1, Facade2 const& f2, mpl::true_)
 533       {
 534           return f1.equal(f2);
 535       }
 536
 537       template <class Facade1, class Facade2>
 538       static bool equal(Facade1 const& f1, Facade2 const& f2, mpl::false_)
 539       {
 540           return f2.equal(f1);
 541       }
 542
 543       template <class Facade>
 544       static void advance(Facade& f, typename Facade::difference_type n)
 545       {
 546           f.advance(n);
 547       }
 548
 549       template <class Facade1, class Facade2>
 550       static typename Facade1::difference_type distance_from(
 551           Facade1 const& f1, Facade2 const& f2, mpl::true_)
 552       {
 553           return -f1.distance_to(f2);
 554       }
 555
 556       template <class Facade1, class Facade2>
 557       static typename Facade2::difference_type distance_from(
 558           Facade1 const& f1, Facade2 const& f2, mpl::false_)
 559       {
 560           return f2.distance_to(f1);
 561       }
 562
 563       //
 564       // Curiously Recurring Template interface.
 565       //
 566       template <class I, class V, class TC, class R, class D>
 567       static I& derived(iterator_facade<I,V,TC,R,D>& facade)
 568       {
 569           return *static_cast<I*>(&facade);
 570       }
 571
 572       template <class I, class V, class TC, class R, class D>
 573       static I const& derived(iterator_facade<I,V,TC,R,D> const& facade)
 574       {
 575           return *static_cast<I const*>(&facade);
 576       }
 577
 578    private:
 579       // objects of this class are useless
 580       iterator_core_access(); //undefined
 581   };
 582
 583   //
 584   // iterator_facade - use as a public base class for defining new
 585   // standard-conforming iterators.
 586   //
 587   template <
 588       class Derived             // The derived iterator type being constructed
 589     , class Value
 590     , class CategoryOrTraversal
 591     , class Reference   = Value&
 592     , class Difference  = std::ptrdiff_t
 593   >
 594   class iterator_facade
 595 # ifdef BOOST_ITERATOR_FACADE_NEEDS_ITERATOR_BASE
 596     : public boost::detail::iterator_facade_types<
 597          Value, CategoryOrTraversal, Reference, Difference
 598       >::base
 599 #  undef BOOST_ITERATOR_FACADE_NEEDS_ITERATOR_BASE
 600 # endif
 601   {
 602    private:
 603       //
 604       // Curiously Recurring Template interface.
 605       //
 606       Derived& derived()
 607       {
 608           return *static_cast<Derived*>(this);
 609       }
 610
 611       Derived const& derived() const
 612       {
 613           return *static_cast<Derived const*>(this);
 614       }
 615
 616       typedef boost::detail::iterator_facade_types<
 617          Value, CategoryOrTraversal, Reference, Difference
 618       > associated_types;
 619
 620    protected:
 621       // For use by derived classes
 622       typedef iterator_facade<Derived,Value,CategoryOrTraversal,Reference,Difference> iterator_facade_;
 623
 624    public:
 625
 626       typedef typename associated_types::value_type value_type;
 627       typedef Reference reference;
 628       typedef Difference difference_type;
 629       typedef typename associated_types::pointer pointer;
 630       typedef typename associated_types::iterator_category iterator_category;
 631
 632       reference operator*() const
 633       {
 634           return iterator_core_access::dereference(this->derived());
 635       }
 636
 637       typename boost::detail::operator_arrow_result<
 638           value_type
 639         , reference
 640         , pointer
 641       >::type
 642       operator->() const
 643       {
 644           return boost::detail::operator_arrow_result<
 645               value_type
 646             , reference
 647             , pointer
 648           >::make(*this->derived());
 649       }
 650
 651       typename boost::detail::operator_brackets_result<Derived,Value,reference>::type
 652       operator[](difference_type n) const
 653       {
 654           typedef boost::detail::use_operator_brackets_proxy<Value,Reference> use_proxy;
 655
 656           return boost::detail::make_operator_brackets_result<Derived>(
 657               this->derived() + n
 658             , use_proxy()
 659           );
 660       }
 661
 662       Derived& operator++()
 663       {
 664           iterator_core_access::increment(this->derived());
 665           return this->derived();
 666       }
 667
 668 # if BOOST_WORKAROUND(BOOST_MSVC, < 1300)
 669       typename boost::detail::postfix_increment_result<Derived,Value,Reference,CategoryOrTraversal>::type
 670       operator++(int)
 671       {
 672           typename boost::detail::postfix_increment_result<Derived,Value,Reference,CategoryOrTraversal>::type
 673           tmp(this->derived());
 674           ++*this;
 675           return tmp;
 676       }
 677 # endif
 678
 679       Derived& operator--()
 680       {
 681           iterator_core_access::decrement(this->derived());
 682           return this->derived();
 683       }
 684
 685       Derived operator--(int)
 686       {
 687           Derived tmp(this->derived());
 688           --*this;
 689           return tmp;
 690       }
 691
 692       Derived& operator+=(difference_type n)
 693       {
 694           iterator_core_access::advance(this->derived(), n);
 695           return this->derived();
 696       }
 697
 698       Derived& operator-=(difference_type n)
 699       {
 700           iterator_core_access::advance(this->derived(), -n);
 701           return this->derived();
 702       }
 703
 704       Derived operator-(difference_type x) const
 705       {
 706           Derived result(this->derived());
 707           return result -= x;
 708       }
 709
 710 # if BOOST_WORKAROUND(BOOST_MSVC, < 1300)
 711       // There appears to be a bug which trashes the data of classes
 712       // derived from iterator_facade when they are assigned unless we
 713       // define this assignment operator.  This bug is only revealed
 714       // (so far) in STLPort debug mode, but it's clearly a codegen
 715       // problem so we apply the workaround for all MSVC6.
 716       iterator_facade& operator=(iterator_facade const&)
 717       {
 718           return *this;
 719       }
 720 # endif
 721   };
 722
 723 # if !BOOST_WORKAROUND(BOOST_MSVC, < 1300)
 724   template <class I, class V, class TC, class R, class D>
 725   inline typename boost::detail::postfix_increment_result<I,V,R,TC>::type
 726   operator++(
 727       iterator_facade<I,V,TC,R,D>& i
 728     , int
 729   )
 730   {
 731       typename boost::detail::postfix_increment_result<I,V,R,TC>::type
 732           tmp(*static_cast<I*>(&i));
 733
 734       ++i;
 735
 736       return tmp;
 737   }
 738 # endif
 739
 740
 741   //
 742   // Comparison operator implementation. The library supplied operators
 743   // enables the user to provide fully interoperable constant/mutable
 744   // iterator types. I.e. the library provides all operators
 745   // for all mutable/constant iterator combinations.
 746   //
 747   // Note though that this kind of interoperability for constant/mutable
 748   // iterators is not required by the standard for container iterators.
 749   // All the standard asks for is a conversion mutable -> constant.
 750   // Most standard library implementations nowadays provide fully interoperable
 751   // iterator implementations, but there are still heavily used implementations
 752   // that do not provide them. (Actually it's even worse, they do not provide
 753   // them for only a few iterators.)
 754   //
 755   // ?? Maybe a BOOST_ITERATOR_NO_FULL_INTEROPERABILITY macro should
 756   //    enable the user to turn off mixed type operators
 757   //
 758   // The library takes care to provide only the right operator overloads.
 759   // I.e.
 760   //
 761   // bool operator==(Iterator,      Iterator);
 762   // bool operator==(ConstIterator, Iterator);
 763   // bool operator==(Iterator,      ConstIterator);
 764   // bool operator==(ConstIterator, ConstIterator);
 765   //
 766   //   ...
 767   //
 768   // In order to do so it uses c++ idioms that are not yet widely supported
 769   // by current compiler releases. The library is designed to degrade gracefully
 770   // in the face of compiler deficiencies. In general compiler
 771   // deficiencies result in less strict error checking and more obscure
 772   // error messages, functionality is not affected.
 773   //
 774   // For full operation compiler support for "Substitution Failure Is Not An Error"
 775   // (aka. enable_if) and boost::is_convertible is required.
 776   //
 777   // The following problems occur if support is lacking.
 778   //
 779   // Pseudo code
 780   //
 781   // ---------------
 782   // AdaptorA<Iterator1> a1;
 783   // AdaptorA<Iterator2> a2;
 784   //
 785   // // This will result in a no such overload error in full operation
 786   // // If enable_if or is_convertible is not supported
 787   // // The instantiation will fail with an error hopefully indicating that
 788   // // there is no operator== for Iterator1, Iterator2
 789   // // The same will happen if no enable_if is used to remove
 790   // // false overloads from the templated conversion constructor
 791   // // of AdaptorA.
 792   //
 793   // a1 == a2;
 794   // ----------------
 795   //
 796   // AdaptorA<Iterator> a;
 797   // AdaptorB<Iterator> b;
 798   //
 799   // // This will result in a no such overload error in full operation
 800   // // If enable_if is not supported the static assert used
 801   // // in the operator implementation will fail.
 802   // // This will accidently work if is_convertible is not supported.
 803   //
 804   // a == b;
 805   // ----------------
 806   //
 807
 808 # ifdef BOOST_NO_ONE_WAY_ITERATOR_INTEROP
 809 #  define BOOST_ITERATOR_CONVERTIBLE(a,b) mpl::true_()
 810 # else
 811 #  define BOOST_ITERATOR_CONVERTIBLE(a,b) is_convertible<a,b>()
 812 # endif
 813
 814 # define BOOST_ITERATOR_FACADE_INTEROP(op, result_type, return_prefix, base_op) \
 815   BOOST_ITERATOR_FACADE_INTEROP_HEAD(inline, op, result_type)                   \
 816   {                                                                             \
 817       /* For those compilers that do not support enable_if */                   \
 818       BOOST_STATIC_ASSERT((                                                     \
 819           is_interoperable< Derived1, Derived2 >::value                         \
 820       ));                                                                       \
 821       return_prefix iterator_core_access::base_op(                              \
 822           *static_cast<Derived1 const*>(&lhs)                                   \
 823         , *static_cast<Derived2 const*>(&rhs)                                   \
 824         , BOOST_ITERATOR_CONVERTIBLE(Derived2,Derived1)                         \
 825       );                                                                        \
 826   }
 827
 828 # define BOOST_ITERATOR_FACADE_RELATION(op, return_prefix, base_op) \
 829   BOOST_ITERATOR_FACADE_INTEROP(                                    \
 830       op                                                            \
 831     , boost::detail::always_bool2                                   \
 832     , return_prefix                                                 \
 833     , base_op                                                       \
 834   )
 835
 836   BOOST_ITERATOR_FACADE_RELATION(==, return, equal)
 837   BOOST_ITERATOR_FACADE_RELATION(!=, return !, equal)
 838
 839   BOOST_ITERATOR_FACADE_RELATION(<, return 0 >, distance_from)
 840   BOOST_ITERATOR_FACADE_RELATION(>, return 0 <, distance_from)
 841   BOOST_ITERATOR_FACADE_RELATION(<=, return 0 >=, distance_from)
 842   BOOST_ITERATOR_FACADE_RELATION(>=, return 0 <=, distance_from)
 843 # undef BOOST_ITERATOR_FACADE_RELATION
 844
 845   // operator- requires an additional part in the static assertion
 846   BOOST_ITERATOR_FACADE_INTEROP(
 847       -
 848     , boost::detail::choose_difference_type
 849     , return
 850     , distance_from
 851   )
 852 # undef BOOST_ITERATOR_FACADE_INTEROP
 853 # undef BOOST_ITERATOR_FACADE_INTEROP_HEAD
 854
 855 # define BOOST_ITERATOR_FACADE_PLUS(args)           \
 856   BOOST_ITERATOR_FACADE_PLUS_HEAD(inline, args)     \
 857   {                                                 \
 858       Derived tmp(static_cast<Derived const&>(i));  \
 859       return tmp += n;                              \
 860   }
 861
 862 BOOST_ITERATOR_FACADE_PLUS((
 863   iterator_facade<Derived, V, TC, R, D> const& i
 864   , typename Derived::difference_type n
 865 ))
 866
 867 BOOST_ITERATOR_FACADE_PLUS((
 868     typename Derived::difference_type n
 869     , iterator_facade<Derived, V, TC, R, D> const& i
 870 ))
 871 # undef BOOST_ITERATOR_FACADE_PLUS
 872 # undef BOOST_ITERATOR_FACADE_PLUS_HEAD
 873
 874 } // namespace boost
 875
 876 #include <boost/iterator/detail/config_undef.hpp>
 877
 878 #endif // BOOST_ITERATOR_FACADE_23022003THW_HPP