______________________________________________________________________
24 Iterators library [lib.iterators]
______________________________________________________________________
1 This clause describes components that C++ programs may use to perform
iterations over containers (clause _lib.containers_), streams
(_lib.iostream.format_), and stream buffers (_lib.stream.buffers_).
2 The following subclauses describe iterator requirements, and compo-
nents for iterator primitives, predefined iterators, and stream itera-
tors, as summarized in Table 1:
Table 1--Iterators library summary
+-----------------------------------------------------------+
| Subclause Header(s) |
+-----------------------------------------------------------+
|_lib.iterator.requirements_ Requirements |
+-----------------------------------------------------------+
|_lib.iterator.primitives_ Iterator primitives |
|_lib.predef.iterators_ Predefined iterators <iterator> |
|_lib.stream.iterators_ Stream iterators |
+-----------------------------------------------------------+
24.1 Iterator requirements [lib.iterator.requirements]
1 Iterators are a generalization of pointers that allow a C++ program to
work with different data structures (containers) in a uniform manner.
To be able to construct template algorithms that work correctly and
efficiently on different types of data structures, the library formal-
izes not just the interfaces but also the semantics and complexity
assumptions of iterators. All iterators i support the expression *i,
resulting in a value of some class, enumeration, or built-in type T,
called the value type of the iterator. All iterators i for which the
expression (*i).m is well-defined, support the expression i->m with
the same semantics as (*i).m. For every iterator type X for which
equality is defined, there is a corresponding signed integral type
called the difference type of the iterator.
2 Since iterators are an abstraction of pointers, their semantics is a
generalization of most of the semantics of pointers in C++. This
ensures that every template function that takes iterators works as
well with regular pointers. This International Standard defines five
categories of iterators, according to the operations defined on them:
input iterators, output iterators, forward iterators, bidirectional
iterators and random access iterators, as shown in Table 2.
Table 2--Relations among iterator categories
+----------------------------------------------------------+
|Random access -> Bidirectional -> Forward -> Input |
| -> Output |
+----------------------------------------------------------+
3 Forward iterators satisfy all the requirements of the input and output
iterators and can be used whenever either kind is specified; Bidirec-
tional iterators also satisfy all the requirements of the forward
iterators and can be used whenever a forward iterator is specified;
Random access iterators also satisfy all the requirements of bidirec-
tional iterators and can be used whenever a bidirectional iterator is
specified.
4 Besides its category, a forward, bidirectional, or random access iter-
ator can also be mutable or constant depending on whether the result
of the expression *i behaves as a reference or as a reference to a
constant. Constant iterators do not satisfy the requirements for out-
put iterators, and the result of the expression *i (for constant iter-
ator i) cannot be used in an expression where an lvalue is required.
5 Just as a regular pointer to an array guarantees that there is a
pointer value pointing past the last element of the array, so for any
iterator type there is an iterator value that points past the last
element of a corresponding container. These values are called past-
the-end values. Values of an iterator i for which the expression *i
is defined are called dereferenceable. The library never assumes that
past-the-end values are dereferenceable. Iterators can also have sin-
gular values that are not associated with any container. [Example:
After the declaration of an uninitialized pointer x (as with int* x;),
x must always be assumed to have a singular value of a pointer. ]
Results of most expressions are undefined for singular values; the
only exception is an assignment of a non-singular value to an iterator
that holds a singular value. In this case the singular value is over-
written the same way as any other value. Dereferenceable and past-
the-end values are always non-singular.
6 An iterator j is called reachable from an iterator i if and only if
there is a finite sequence of applications of the expression ++i that
makes i == j. If j is reachable from i, they refer to the same con-
tainer.
7 Most of the library's algorithmic templates that operate on data
structures have interfaces that use ranges. A range is a pair of
iterators that designate the beginning and end of the computation. A
range [i, i) is an empty range; in general, a range [i, j) refers to
the elements in the data structure starting with the one pointed to by
i and up to but not including the one pointed to by j. Range [i, j)
is valid if and only if j is reachable from i. The result of the
application of functions in the library to invalid ranges is unde-
fined.
8 All the categories of iterators require only those functions that are
realizable for a given category in constant time (amortized). There-
fore, requirement tables for the iterators do not have a complexity
column.
9 In the following sections, a and b denote values of X, n denotes a
value of the difference type Distance, u, tmp, and m denote identi-
fiers, r denotes a value of X&, t denotes a value of value type T.
24.1.1 Input iterators [lib.input.iterators]
1 A class or a built-in type X satisfies the requirements of an input
iterator for the value type T if the following expressions are valid,
where U is the type of any specified member of type T, as shown in
Table 3.
2 In Table 3, the term the domain of == is used in the ordinary mathe-
matical sense to denote the set of values over which == is (required
to be) defined. This set can change over time. Each algorithm places
additional requirements on the domain of == for the iterator values it
uses. These requirements can be inferred from the uses that algorithm
makes of == and !=. [Example: the call find(a,b,x) is defined only if
the value of a has the property p defined as follows: b has property p
and a value i has property p if (*i==x) or if (*i!=x and ++i has prop-
erty p). ]
Table 3--Input iterator requirements
+----------------------------------------------------------------------------------------+
|operation type semantics, pre/post-conditions |
+----------------------------------------------------------------------------------------+
|X u(a); X post: u is a copy of a |
| A destructor is assumed to be present and accessible. |
+----------------------------------------------------------------------------------------+
|u = a; X& result: u |
| post: u is a copy of a |
+----------------------------------------------------------------------------------------+
|a == b convertible to bool == is an equivalence relation over its domain. |
+----------------------------------------------------------------------------------------+
|a != b convertible to bool bool(a==b) != bool(a!=b) over the domain of == |
+----------------------------------------------------------------------------------------+
|*a convertible to T pre: a is dereferenceable. |
| If a==b and (a,b) is in the domain of == |
| then *a is equivalent to *b. |
+----------------------------------------------------------------------------------------+
|a->m pre: (*a).m is well-defined |
| Equivalent to (*a).m |
+----------------------------------------------------------------------------------------+
|++r X& pre: r is dereferenceable. |
| post: r is dereferenceable or r is past-the-end. |
| post: any copies of the previous value of r are no |
| longer required either to be dereferenceable or to be |
| in the domain of ==. |
+----------------------------------------------------------------------------------------+
|(void)r++ equivalent to (void)++r |
+----------------------------------------------------------------------------------------+
|*r++ T { T tmp = *r; ++r; return tmp; } |
+----------------------------------------------------------------------------------------+
3 [Note: For input iterators, a == b does not imply ++a == ++b. (Equal-
ity does not guarantee the substitution property or referential trans-
parency.) Algorithms on input iterators should never attempt to pass
through the same iterator twice. They should be single pass algo-
rithms. Value type T is not required to be an Assignable type
(_lib.container.requirements_). These algorithms can be used with
istreams as the source of the input data through the istream_iterator
class. ]
24.1.2 Output iterators [lib.output.iterators]
1 A class or a built-in type X satisfies the requirements of an output
iterator if X is an Assignable type (_lib.container.requirements_) and
also the following expressions are valid, as shown in Table 4:
Table 4--Output iterator requirements
+---------------------------------------------------------------------------------+
|expression return type operational assertion/note |
| semantics pre/post-condition |
+---------------------------------------------------------------------------------+
|X(a) a = t is equivalent to X(a) = |
| t. |
| note: a destructor is assumed. |
+---------------------------------------------------------------------------------+
|X u(a); |
|X u = a; |
+---------------------------------------------------------------------------------+
|*a = t result is not used |
+---------------------------------------------------------------------------------+
|++r X& &r == &++r. |
+---------------------------------------------------------------------------------+
|r++ convertible to { X tmp = r; |
| const X& ++r; |
| return tmp; |
| } |
+---------------------------------------------------------------------------------+
|*r++ = t result is not used |
+---------------------------------------------------------------------------------+
2 [Note: The only valid use of an operator* is on the left side of the
assignment statement. Assignment through the same value of the itera-
tor happens only once. Algorithms on output iterators should never
attempt to pass through the same iterator twice. They should be sin-
gle pass algorithms. Equality and inequality might not be defined.
Algorithms that take output iterators can be used with ostreams as the
destination for placing data through the ostream_iterator class as
well as with insert iterators and insert pointers. --end note]
24.1.3 Forward iterators [lib.forward.iterators]
1 A class or a built-in type X satisfies the requirements of a forward
iterator if the following expressions are valid, as shown in Table 5:
Table 5--Forward iterator requirements
+-------------------------------------------------------------------------------------+
|expression return type operational assertion/note |
| semantics pre/post-condition |
+-------------------------------------------------------------------------------------+
|X u; note: u might have a singular |
| value. |
| note: a destructor is assumed. |
+-------------------------------------------------------------------------------------+
|X() note: X() might be singular. |
+-------------------------------------------------------------------------------------+
|X(a) a == X(a). |
+-------------------------------------------------------------------------------------+
|X u(a); X u; u = a; post: u == a. |
|X u = a; |
+-------------------------------------------------------------------------------------+
|a == b convertible to bool == is an equivalence relation. |
+-------------------------------------------------------------------------------------+
|a != b convertible to bool !(a == b) |
+-------------------------------------------------------------------------------------+
|r = a X& post: r == a. |
+-------------------------------------------------------------------------------------+
|*a T& pre: a is dereferenceable. |
| a == b implies *a == *b. |
| If X is mutable, *a = t is valid. |
+-------------------------------------------------------------------------------------+
|a->m U& (*a).m pre: (*a).m is well-defined. |
+-------------------------------------------------------------------------------------+
|++r X& pre: r is dereferenceable. |
| post: r is dereferenceable or r |
| is past-the-end. |
| r == s and r is dereferenceable |
| implies ++r == ++s. |
| &r == &++r. |
+-------------------------------------------------------------------------------------+
|r++ convertible to con- { X tmp = r; |
| st X& ++r; |
| return tmp; |
| } |
+-------------------------------------------------------------------------------------+
|*r++ T& |
+-------------------------------------------------------------------------------------+
--If a and b are equal, then either a and b are both dereferenceable
or else neither is dereferenceable.
--If a and b are both dereferenceable, then a == b if and only if *a
and *b are the same object.
2 [Note: The condition that a == b implies ++a == ++b (which is not true
for input and output iterators) and the removal of the restrictions on
the number of the assignments through the iterator (which applies to
output iterators) allows the use of multi-pass one-directional algo-
rithms with forward iterators. --end note]
24.1.4 Bidirectional iterators [lib.bidirectional.iterators]
1 A class or a built-in type X satisfies the requirements of a bidirec-
tional iterator if, in addition to satisfying the requirements for
forward iterators, the following expressions are valid as shown in
Table 6:
Table 6--Bidirectional iterator requirements (in addition to forward iterator)
+----------------------------------------------------------------------------+
|expression return type operational assertion/note |
| semantics pre/post-condition |
+----------------------------------------------------------------------------+
|--r X& pre: there exists s such |
| that r == ++s. |
| post: s is dereferenceable. |
| --(++r) == r. |
| --r == --s implies r == s. |
| &r == &--r. |
+----------------------------------------------------------------------------+
|r-- convertible to { X tmp = r; |
| const X& --r; |
| return tmp; |
| } |
+----------------------------------------------------------------------------+
|*r-- convertible to T |
+----------------------------------------------------------------------------+
2 [Note: Bidirectional iterators allow algorithms to move iterators
backward as well as forward. --end note]
24.1.5 Random access iterators [lib.random.access.iterators]
1 A class or a built-in type X satisfies the requirements of a random
access iterator if, in addition to satisfying the requirements for
bidirectional iterators, the following expressions are valid as shown
in Table 7:
Table 7--Random access iterator requirements (in addition to bidirectional iterator)
+------------------------------------------------------------------------------------+
|expression return type operational assertion/note |
| semantics pre/post-condition |
+------------------------------------------------------------------------------------+
|r += n X& { Distance m = |
| n; |
| if (m >= 0) |
| while (m--) |
| ++r; |
| else |
| while (m++) |
| --r; |
| return r; } |
+------------------------------------------------------------------------------------+
|a + n { X tmp = a; |
| X return tmp += a + n == n + a. |
| n; } |
|n + a |
+------------------------------------------------------------------------------------+
|r -= n X& return r += -n; |
+------------------------------------------------------------------------------------+
|a - n X { X tmp = a; |
| return tmp -= |
| n; } |
+------------------------------------------------------------------------------------+
|b - a Distance (a<b)? dis- pre: there exists a value n of |
| tance(a,b): Distance such that a + n == b. |
| -distance(b,a) b == a + (b - a). |
+------------------------------------------------------------------------------------+
|a[n] convertible to T *(a + n) |
+------------------------------------------------------------------------------------+
|a < b convertible to bool b - a > 0 < is a total ordering relation |
+------------------------------------------------------------------------------------+
|a > b convertible to bool b < a > is a total ordering relation |
| opposite to <. |
+------------------------------------------------------------------------------------+
|a >= b convertible to bool !(a < b) |
+------------------------------------------------------------------------------------+
|a <= b convertible to bool !(a > b) |
+------------------------------------------------------------------------------------+
24.2 Header <iterator> synopsis [lib.iterator.synopsis]
namespace std {
// _lib.iterator.primitives_, primitives:
template<class Iterator> struct iterator_traits;
template<class T> struct iterator_traits<T*>;
template<class Category, class T, class Distance = ptrdiff_t,
class Pointer = T*, class Reference = T&> struct iterator;
struct input_iterator_tag {};
struct output_iterator_tag {};
struct forward_iterator_tag: public input_iterator_tag {};
struct bidirectional_iterator_tag: public forward_iterator_tag {};
struct random_access_iterator_tag: public bidirectional_iterator_tag {};
// _lib.iterator.operations_, iterator operations:
template <class InputIterator, class Distance>
void advance(InputIterator& i, Distance n);
template <class InputIterator>
typename iterator_traits<InputIterator>::difference_type
distance(InputIterator first, InputIterator last);
// _lib.predef.iterators_, predefined iterators:
template <class Iterator> class reverse_iterator;
template <class Iterator>
bool operator==(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
bool operator<(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
bool operator!=(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
bool operator>(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
bool operator>=(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
bool operator<=(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
typename reverse_iterator<Iterator>::difference_type operator-(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
reverse_iterator<Iterator>
operator+(
typename reverse_iterator<Iterator>::difference_type n,
const reverse_iterator<Iterator>& x);
template <class Container> class back_insert_iterator;
template <class Container>
back_insert_iterator<Container> back_inserter(Container& x);
template <class Container> class front_insert_iterator;
template <class Container>
front_insert_iterator<Container> front_inserter(Container& x);
template <class Container> class insert_iterator;
template <class Container, class Iterator>
insert_iterator<Container> inserter(Container& x, Iterator i);
// _lib.stream.iterators_, stream iterators:
template <class T, class charT = char, class traits = char_traits<charT>,
class Distance = ptrdiff_t>
class istream_iterator;
template <class T, class charT, class traits, class Distance>
bool operator==(const istream_iterator<T,charT,traits,Distance>& x,
const istream_iterator<T,charT,traits,Distance>& y);
template <class T, class charT, class traits, class Distance>
bool operator!=(const istream_iterator<T,charT,traits,Distance>& x,
const istream_iterator<T,charT,traits,Distance>& y);
template <class T, class charT = char, class traits = char_traits<charT> >
class ostream_iterator;
template<class charT, class traits = char_traits<charT> >
class istreambuf_iterator;
template <class charT, class traits>
bool operator==(const istreambuf_iterator<charT,traits>& a,
const istreambuf_iterator<charT,traits>& b);
template <class charT, class traits>
bool operator!=(const istreambuf_iterator<charT,traits>& a,
const istreambuf_iterator<charT,traits>& b);
template <class charT, class traits = char_traits<charT> >
class ostreambuf_iterator;
}
24.3 Iterator primitives [lib.iterator.primitives]
1 To simplify the task of defining iterators, the library provides sev-
eral classes and functions:
24.3.1 Iterator traits [lib.iterator.traits]
1 To implement algorithms only in terms of iterators, it is often neces-
sary to determine the value and difference types that correspond to a
particular iterator type. Accordingly, it is required that if Itera-
tor is the type of an iterator, the types
iterator_traits<Iterator>::difference_type
iterator_traits<Iterator>::value_type
iterator_traits<Iterator>::iterator_category
be defined as the iterator's difference type, value type and iterator
category, respectively. In the case of an output iterator, the types
iterator_traits<Iterator>::difference_type
iterator_traits<Iterator>::value_type
are both defined as void.
2 The template iterator_traits<Iterator> is defined as
template<class Iterator> struct iterator_traits {
typedef typename Iterator::difference_type difference_type;
typedef typename Iterator::value_type value_type;
typedef typename Iterator::pointer pointer;
typedef typename Iterator::reference reference;
typedef typename Iterator::iterator_category iterator_category;
};
It is specialized for pointers as
template<class T> struct iterator_traits<T*> {
typedef ptrdiff_t difference_type;
typedef T value_type;
typedef T* pointer;
typedef T& reference;
typedef random_access_iterator_tag iterator_category;
};
and for pointers to const as
template<class T> struct iterator_traits<const T*> {
typedef ptrdiff_t difference_type;
typedef T value_type;
typedef const T* pointer;
typedef const T& reference;
typedef random_access_iterator_tag iterator_category;
};
[Note: If there is an additional pointer type __far such that the dif-
ference of two __far is of type long, an implementation may define
template<class T> struct iterator_traits<T __far*> {
typedef long difference_type;
typedef T value_type;
typedef T __far* pointer;
typedef T __far& reference;
typedef random_access_iterator_tag iterator_category;
};
--end note]
3 [Example: To implement a generic reverse function, a C++ program can
do the following:
template <class BidirectionalIterator>
void reverse(BidirectionalIterator first, BidirectionalIterator last) {
typename iterator_traits<BidirectionalIterator>::difference_type n =
distance(first, last);
--n;
while(n > 0) {
typename iterator_traits<BidirectionalIterator>::value_type
tmp = *first;
*first++ = *--last;
*last = tmp;
n -= 2;
}
}
--end example]
24.3.2 Basic iterator [lib.iterator.basic]
1 The iterator template may be used as a base class to ease the defini-
tion of required types for new iterators.
namespace std {
template<class Category, class T, class Distance = ptrdiff_t,
class Pointer = T*, class Reference = T&>
struct iterator {
typedef T value_type;
typedef Distance difference_type;
typedef Pointer pointer;
typedef Reference reference;
typedef Category iterator_category;
};
}
24.3.3 Standard iterator tags [lib.std.iterator.tags]
1 It is often desirable for a template function to find out what is the
most specific category of its iterator argument, so that the function
can select the most efficient algorithm at compile time. To facili-
tate this, the library introduces category tag classes which are used
as compile time tags for algorithm selection. They are: input_itera-
tor_tag, output_iterator_tag, forward_iterator_tag, bidirec-
tional_iterator_tag and random_access_iterator_tag. For every itera-
tor of type Iterator, iterator_traits<Iterator>::iterator_category
must be defined to be the most specific category tag that describes
the iterator's behavior.
namespace std {
struct input_iterator_tag {};
struct output_iterator_tag {};
struct forward_iterator_tag: public input_iterator_tag {};
struct bidirectional_iterator_tag: public forward_iterator_tag {};
struct random_access_iterator_tag: public bidirectional_iterator_tag {};
}
2 [Example: For a program-defined iterator BinaryTreeIterator, it could
be included into the bidirectional iterator category by specializing
the iterator_traits template:
template<class T> struct iterator_traits<BinaryTreeIterator<T> > {
typedef ptrdiff_t difference_type;
typedef T value_type;
typedef T* pointer;
typedef T& reference;
typedef bidirectional_iterator_tag iterator_category;
};
Typically, however, it would be easier to derive BinaryTreeIterator<T>
from iterator<bidirectional_iterator_tag,T,ptrdiff_t,T*,T&>. --end
example]
3 [Example: If evolve() is well defined for bidirectional iterators, but
can be implemented more efficiently for random access iterators, then
the implementation is as follows:
template <class BidirectionalIterator>
inline void
evolve(BidirectionalIterator first, BidirectionalIterator last) {
evolve(first, last,
typename iterator_traits<BidirectionalIterator>::iterator_category());
}
template <class BidirectionalIterator>
void evolve(BidirectionalIterator first, BidirectionalIterator last,
bidirectional_iterator_tag) {
// ... more generic, but less efficient algorithm
}
template <class RandomAccessIterator>
void evolve(RandomAccessIterator first, RandomAccessIterator last,
random_access_iterator_tag) {
// ... more efficient, but less generic algorithm
}
--end example]
4 [Example: If a C++ program wants to define a bidirectional iterator
for some data structure containing double and such that it works on a
large memory model of the implementation, it can do so with:
class MyIterator :
public iterator<bidirectional_iterator_tag, double, long, T*, T&> {
// code implementing ++, etc.
};
5 Then there is no need to specialize the iterator_traits template.
--end example]
24.3.4 Iterator operations [lib.iterator.operations]
1 Since only random access iterators provide + and - operators, the
library provides two template functions advance and distance. These
functions use + and - for random access iterators (and are, therefore,
constant time for them); for input, forward and bidirectional itera-
tors they use ++ to provide linear time implementations.
template <class InputIterator, class Distance>
void advance(InputIterator& i, Distance n);
Requires:
n may be negative only for random access and bidirectional itera-
tors.
Effects:
Increments (or decrements for negative n) iterator reference i by n.
template<class InputIterator>
typename iterator_traits<InputIterator>::difference_type
distance(InputIterator first, InputIterator last);
Effects:
Returns the number of increments or decrements needed to get from
first to last.
Requires:
last must be reachable from first.
24.4 Predefined iterators [lib.predef.iterators]
24.4.1 Reverse iterators [lib.reverse.iterators]
1 Bidirectional and random access iterators have corresponding reverse
iterator adaptors that iterate through the data structure in the oppo-
site direction. They have the same signatures as the corresponding
iterators. The fundamental relation between a reverse iterator and
its corresponding iterator i is established by the identity:
&*(reverse_iterator(i)) == &*(i - 1).
2 This mapping is dictated by the fact that while there is always a
pointer past the end of an array, there might not be a valid pointer
before the beginning of an array.
24.4.1.1 Template class reverse_iterator [lib.reverse.iterator]
namespace std {
template <class Iterator>
class reverse_iterator : public
iterator<typename iterator_traits<Iterator>::iterator_category,
typename iterator_traits<Iterator>::value_type,
typename iterator_traits<Iterator>::difference_type,
typename iterator_traits<Iterator>::pointer,
typename iterator_traits<Iterator>::reference> {
protected:
Iterator current;
public:
typedef Iterator
iterator_type;
typedef typename iterator_traits<Iterator>::difference_type
difference_type;
typedef typename iterator_traits<Iterator>::reference
reference;
typedef typename iterator_traits<Iterator>::pointer
pointer;
reverse_iterator();
explicit reverse_iterator(Iterator x);
template <class U> reverse_iterator(const reverse_iterator<U>& u);
Iterator base() const; // explicit
reference operator*() const;
pointer operator->() const;
reverse_iterator& operator++();
reverse_iterator operator++(int);
reverse_iterator& operator--();
reverse_iterator operator--(int);
reverse_iterator operator+ (difference_type n) const;
reverse_iterator& operator+=(difference_type n);
reverse_iterator operator- (difference_type n) const;
reverse_iterator& operator-=(difference_type n);
reference operator[](difference_type n) const;
};
template <class Iterator>
bool operator==(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
bool operator<(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
bool operator!=(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
bool operator>(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
bool operator>=(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
bool operator<=(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
typename reverse_iterator<Iterator>::difference_type operator-(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
template <class Iterator>
reverse_iterator<Iterator> operator+(
typename reverse_iterator<Iterator>::difference_type n,
const reverse_iterator<Iterator>& x);
}
24.4.1.2 reverse_iterator [lib.reverse.iter.requirements]
requirements
1 The template parameter Iterator shall meet all the requirements of a
Bidirectional Iterator (_lib.bidirectional.iterators_).
2 Additionally, Iterator shall meet the requirements of a Random Access
Iterator (_lib.random.access.iterators_) if any of the members opera-
tor+ (_lib.reverse.iter.op+_), operator- (_lib.reverse.iter.op-_),
operator+= (_lib.reverse.iter.op+=_), operator-=
(_lib.reverse.iter.op-=_), operator[] (_lib.reverse.iter.opindex_), or
the global operators operator< (_lib.reverse.iter.op<_), operator>
(_lib.reverse.iter.op>_), operator<= (_lib.reverse.iter.op<=_), opera-
tor>= (_lib.reverse.iter.op>=_), operator- (_lib.reverse.iter.opdiff_)
or operator+ (_lib.reverse.iter.opsum_). is referenced in a way that
requires instantiation (_temp.inst_).
24.4.1.3 reverse_iterator operations [lib.reverse.iter.ops]
24.4.1.3.1 reverse_iterator constructor [lib.reverse.iter.cons]
explicit reverse_iterator(Iterator x);
Effects:
Initializes current with x.
template <class U> reverse_iterator(const reverse_iterator<U> &u);
Effects:
Initializes current with u.current.
24.4.1.3.2 Conversion [lib.reverse.iter.conv]
Iterator base() const; // explicit
Returns:
current
24.4.1.3.3 operator* [lib.reverse.iter.op.star]
reference operator*() const;
Effects:
Iterator tmp = current;
return *--tmp;
24.4.1.3.4 operator-> [lib.reverse.iter.opref]
pointer operator->() const;
Effects:
return &(operator*());
24.4.1.3.5 operator++ [lib.reverse.iter.op++]
reverse_iterator& operator++();
Effects:
--current;
Returns:
*this
reverse_iterator operator++(int);
Effects:
reverse_iterator tmp = *this;
--current;
return tmp;
24.4.1.3.6 operator-- [lib.reverse.iter.op--]
reverse_iterator& operator--();
Effects:
++current
Returns:
*this
reverse_iterator operator--(int);
Effects:
reverse_iterator tmp = *this;
++current;
return tmp;
24.4.1.3.7 operator+ [lib.reverse.iter.op+]
reverse_iterator
operator+(typename reverse_iterator<Iterator>::difference_type n) const;
Returns:
reverse_iterator(current-n)
24.4.1.3.8 operator+= [lib.reverse.iter.op+=]
reverse_iterator&
operator+=(typename reverse_iterator<Iterator>::difference_type n);
Effects:
current -= n;
Returns:
*this
24.4.1.3.9 operator- [lib.reverse.iter.op-]
reverse_iterator
operator-(typename reverse_iterator<Iterator>::difference_type n) const;
Returns:
reverse_iterator(current+n)
24.4.1.3.10 operator-= [lib.reverse.iter.op-=]
reverse_iterator&
operator-=(typename reverse_iterator<Iterator>::difference_type n);
Effects:
current += n;
Returns:
*this
24.4.1.3.11 operator[] [lib.reverse.iter.opindex]
reference
operator[](typename reverse_iterator<Iterator>::difference_type n) const;
Returns:
current[-n-1]
24.4.1.3.12 operator== [lib.reverse.iter.op==]
template <class Iterator>
bool operator==(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
Returns:
x.current == y.current
24.4.1.3.13 operator< [lib.reverse.iter.op<]
template <class Iterator>
bool operator<(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
Returns:
x.current > y.current
24.4.1.3.14 operator!= [lib.reverse.iter.op!=]
template <class Iterator>
bool operator!=(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
Returns:
x.current != y.current
24.4.1.3.15 operator> [lib.reverse.iter.op>]
template <class Iterator>
bool operator>(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
Returns:
x.current < y.current
24.4.1.3.16 operator>= [lib.reverse.iter.op>=]
template <class Iterator>
bool operator>=(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
Returns:
x.current <= y.current
24.4.1.3.17 operator<= [lib.reverse.iter.op<=]
template <class Iterator>
bool operator<=(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
Returns:
x.current >= y.current
24.4.1.3.18 operator- [lib.reverse.iter.opdiff]
template <class Iterator>
typename reverse_iterator<Iterator>::difference_type operator-(
const reverse_iterator<Iterator>& x,
const reverse_iterator<Iterator>& y);
Returns:
y.current - x.current
24.4.1.3.19 operator+ [lib.reverse.iter.opsum]
template <class Iterator>
reverse_iterator<Iterator> operator+(
typename reverse_iterator<Iterator>::difference_type n,
const reverse_iterator<Iterator>& x);
Returns:
reverse_iterator<Iterator> (x.current - n)
24.4.2 Insert iterators [lib.insert.iterators]
1 To make it possible to deal with insertion in the same way as writing
into an array, a special kind of iterator adaptors, called insert
iterators, are provided in the library. With regular iterator
classes,
while (first != last) *result++ = *first++;
2 causes a range [first, last) to be copied into a range starting with
result. The same code with result being an insert iterator will
insert corresponding elements into the container. This device allows
all of the copying algorithms in the library to work in the insert
mode instead of the regular overwrite mode.
3 An insert iterator is constructed from a container and possibly one of
its iterators pointing to where insertion takes place if it is neither
at the beginning nor at the end of the container. Insert iterators
satisfy the requirements of output iterators. operator* returns the
insert iterator itself. The assignment operator=(const T& x) is
defined on insert iterators to allow writing into them, it inserts x
right before where the insert iterator is pointing. In other words,
an insert iterator is like a cursor pointing into the container where
the insertion takes place. back_insert_iterator inserts elements at
the end of a container, front_insert_iterator inserts elements at the
beginning of a container, and insert_iterator inserts elements where
the iterator points to in a container. back_inserter, front_inserter,
and inserter are three functions making the insert iterators out of a
container.
24.4.2.1 Template class [lib.back.insert.iterator]
back_insert_iterator
namespace std {
template <class Container>
class back_insert_iterator :
public iterator<output_iterator_tag,void,void,void,void> {
protected:
Container* container;
public:
typedef Container container_type;
explicit back_insert_iterator(Container& x);
back_insert_iterator<Container>&
operator=(typename Container::const_reference value);
back_insert_iterator<Container>& operator*();
back_insert_iterator<Container>& operator++();
back_insert_iterator<Container> operator++(int);
};
template <class Container>
back_insert_iterator<Container> back_inserter(Container& x);
}
24.4.2.2 back_insert_iterator [lib.back.insert.iter.ops]
operations
24.4.2.2.1 back_insert_iterator [lib.back.insert.iter.cons]
constructor
explicit back_insert_iterator(Container& x);
Effects:
Initializes container with &x.
24.4.2.2.2 [lib.back.insert.iter.op=]
back_insert_iterator::operator=
back_insert_iterator<Container>&
operator=(typename Container::const_reference value);
Effects:
container->push_back(value);
Returns:
*this.
24.4.2.2.3 [lib.back.insert.iter.op*]
back_insert_iterator::operator*
back_insert_iterator<Container>& operator*();
Returns:
*this.
24.4.2.2.4 [lib.back.insert.iter.op++]
back_insert_iterator::operator++
back_insert_iterator<Container>& operator++();
back_insert_iterator<Container> operator++(int);
Returns:
*this.
24.4.2.2.5 back_inserter [lib.back.inserter]
template <class Container>
back_insert_iterator<Container> back_inserter(Container& x);
Returns:
back_insert_iterator<Container>(x).
24.4.2.3 Template class [lib.front.insert.iterator]
front_insert_iterator
namespace std {
template <class Container>
class front_insert_iterator :
public iterator<output_iterator_tag,void,void,void,void> {
protected:
Container* container;
public:
typedef Container container_type;
explicit front_insert_iterator(Container& x);
front_insert_iterator<Container>&
operator=(typename Container::const_reference value);
front_insert_iterator<Container>& operator*();
front_insert_iterator<Container>& operator++();
front_insert_iterator<Container> operator++(int);
};
template <class Container>
front_insert_iterator<Container> front_inserter(Container& x);
}
24.4.2.4 front_insert_iterator [lib.front.insert.iter.ops]
operations
24.4.2.4.1 front_insert_iterator [lib.front.insert.iter.cons]
constructor
explicit front_insert_iterator(Container& x);
Effects:
Initializes container with &x.
24.4.2.4.2 [lib.front.insert.iter.op=]
front_insert_iterator::operator=
front_insert_iterator<Container>&
operator=(typename Container::const_reference value);
Effects:
container->push_front(value);
Returns:
*this.
24.4.2.4.3 [lib.front.insert.iter.op*]
front_insert_iterator::operator*
front_insert_iterator<Container>& operator*();
Returns:
*this.
24.4.2.4.4 [lib.front.insert.iter.op++]
front_insert_iterator::operator++
front_insert_iterator<Container>& operator++();
front_insert_iterator<Container> operator++(int);
Returns:
*this.
24.4.2.4.5 front_inserter [lib.front.inserter]
template <class Container>
front_insert_iterator<Container> front_inserter(Container& x);
Returns:
front_insert_iterator<Container>(x).
24.4.2.5 Template class insert_iterator [lib.insert.iterator]
namespace std {
template <class Container>
class insert_iterator :
public iterator<output_iterator_tag,void,void,void,void> {
protected:
Container* container;
typename Container::iterator iter;
public:
typedef Container container_type;
insert_iterator(Container& x, typename Container::iterator i);
insert_iterator<Container>&
operator=(typename Container::const_reference value);
insert_iterator<Container>& operator*();
insert_iterator<Container>& operator++();
insert_iterator<Container>& operator++(int);
};
template <class Container, class Iterator>
insert_iterator<Container> inserter(Container& x, Iterator i);
}
24.4.2.6 insert_iterator operations [lib.insert.iter.ops]
24.4.2.6.1 insert_iterator constructor [lib.insert.iter.cons]
insert_iterator(Container& x, typename Container::iterator i);
Effects:
Initializes container with &x and iter with i.
24.4.2.6.2 insert_iterator::operator= [lib.insert.iter.op=]
insert_iterator<Container>&
operator=(typename Container::const_reference value);
Effects:
iter = container->insert(iter, value);
++iter;
Returns:
*this.
24.4.2.6.3 insert_iterator::operator* [lib.insert.iter.op*]
insert_iterator<Container>& operator*();
Returns:
*this.
24.4.2.6.4 insert_iterator::operator++ [lib.insert.iter.op++]
insert_iterator<Container>& operator++();
insert_iterator<Container>& operator++(int);
Returns:
*this.
24.4.2.6.5 inserter [lib.inserter]
template <class Container, class Inserter>
insert_iterator<Container> inserter(Container& x, Inserter i);
Returns:
insert_iterator<Container>(x,typename Container::iterator(i)).
24.5 Stream iterators [lib.stream.iterators]
1 To make it possible for algorithmic templates to work directly with
input/output streams, appropriate iterator-like template classes are
provided.
2 [Example:
partial_sum_copy(istream_iterator<double, char>(cin),
istream_iterator<double, char>(),
ostream_iterator<double, char>(cout, "\n"));
reads a file containing floating point numbers from cin, and prints
the partial sums onto cout. --end example]
24.5.1 Template class istream_iterator [lib.istream.iterator]
1 istream_iterator reads (using operator>>) successive elements from the
input stream for which it was constructed. After it is constructed,
and every time ++ is used, the iterator reads and stores a value of T.
If the end of stream is reached ( operator void*() on the stream
returns false), the iterator becomes equal to the end-of-stream itera-
tor value. The constructor with no arguments istream_iterator()
always constructs an end of stream input iterator object, which is the
only legitimate iterator to be used for the end condition. The result
of operator* on an end of stream is not defined. For any other itera-
tor value a const T& is returned. The result of operator-> on an end
of stream is not defined. For any other iterator value a const T* is
returned. It is impossible to store things into istream iterators.
The main peculiarity of the istream iterators is the fact that ++
operators are not equality preserving, that is, i == j does not guar-
antee at all that ++i == ++j. Every time ++ is used a new value is
read.
2 The practical consequence of this fact is that istream iterators can
be used only for one-pass algorithms, which actually makes perfect
sense, since for multi-pass algorithms it is always more appropriate
to use in-memory data structures.
3 Two end-of-stream iterators are always equal. An end-of-stream itera-
tor is not equal to a non-end-of-stream iterator. Two non-end-of-
stream iterators are equal when they are constructed from the same
stream.
namespace std {
template <class T, class charT = char, class traits = char_traits<charT>,
class Distance = ptrdiff_t>
class istream_iterator:
public iterator<input_iterator_tag, T, Distance, const T*, const T&> {
public:
typedef charT char_type
typedef traits traits_type;
typedef basic_istream<charT,traits> istream_type;
istream_iterator();
istream_iterator(istream_type& s);
istream_iterator(const istream_iterator<T,charT,traits,Distance>& x);
~istream_iterator();
const T& operator*() const;
const T* operator->() const;
istream_iterator<T,charT,traits,Distance>& operator++();
istream_iterator<T,charT,traits,Distance> operator++(int);
private:
//basic_istream<charT,traits>* in_stream; exposition only
//T value; exposition only
};
template <class T, class charT, class traits, class Distance>
bool operator==(const istream_iterator<T,charT,traits,Distance>& x,
const istream_iterator<T,charT,traits,Distance>& y);
template <class T, class charT, class traits, class Distance>
bool operator!=(const istream_iterator<T,charT,traits,Distance>& x,
const istream_iterator<T,charT,traits,Distance>& y);
}
24.5.1.1 istream_iterator [lib.istream.iterator.cons]
constructors and destructor
istream_iterator();
Effects:
Constructs the end-of-stream iterator.
istream_iterator(istream_type& s);
Effects:
Initializes in_stream with s. value may be initialized during con-
struction or the first time it is referenced.
istream_iterator(const istream_iterator<T,charT,traits,Distance>& x);
Effects:
Constructs a copy of x.
~istream_iterator();
Effects:
The iterator is destroyed.
24.5.1.2 istream_iterator operations [lib.istream.iterator.ops]
const T& operator*() const;
Returns:
value
const T* operator->() const;
Returns:
&(operator*())
istream_iterator<T,charT,traits,Distance>& operator++();
Effects:
*in_stream >> value
Returns:
*this
istream_iterator<T,charT,traits,Distance>& operator++(int);
Effects:
istream_iterator<T,charT,traits,Distance> tmp = *this;
*in_stream >> value;
return (tmp);
template <class T, class charT, class traits, class Distance>
bool operator==(const istream_iterator<T,charT,traits,Distance> &x,
const istream_iterator<T,charT,traits,Distance> &y);
Returns:
(x.in_stream == y.in_stream)
24.5.2 Template class ostream_iterator [lib.ostream.iterator]
1 ostream_iterator writes (using operator<<) successive elements onto
the output stream from which it was constructed. If it was con-
structed with char* as a constructor argument, this string, called a
delimiter string, is written to the stream after every T is written.
It is not possible to get a value out of the output iterator. Its
only use is as an output iterator in situations like
while (first != last) *result++ = *first++;
2 ostream_iterator is defined as:
namespace std {
template <class T, class charT = char, class traits = char_traits<charT> >
class ostream_iterator:
public iterator<output_iterator_tag, void, void, void, void> {
public:
typedef charT char_type;
typedef traits traits_type;
typedef basic_ostream<charT,traits> ostream_type;
ostream_iterator(ostream_type& s);
ostream_iterator(ostream_type& s, const charT* delimiter);
ostream_iterator(const ostream_iterator<T,charT,traits>& x);
~ostream_iterator();
ostream_iterator<T,charT,traits>& operator=(const T& value);
ostream_iterator<T,charT,traits>& operator*();
ostream_iterator<T,charT,traits>& operator++();
ostream_iterator<T,charT,traits>& operator++(int);
private:
// basic_ostream<charT,traits>* out_stream; exposition only
// const char* delim; exposition only
};
}
24.5.2.1 ostream_iterator [lib.ostream.iterator.cons.des]
constructors and destructor
ostream_iterator(ostream_type& s);
Effects:
Initializes out_stream with s and delim with null.
ostream_iterator(ostream_type& s, const charT* delimiter);
Effects:
Initializes out_stream with s and delim with delimiter.
ostream_iterator(const ostream_iterator& x);
Effects:
Constructs a copy of x.
~ostream_iterator();
Effects:
The iterator is destroyed.
24.5.2.2 ostream_iterator operations [lib.ostream.iterator.ops]
ostream_iterator& operator=(const T& value);
Effects:
*out_stream << value;
if(delim != 0) *out_stream << delim;
return (*this);
ostream_iterator& operator*();
Returns:
*this
ostream_iterator& operator++();
ostream_iterator& operatot++(int);
Returns:
*this
24.5.3 Template class [lib.istreambuf.iterator]
istreambuf_iterator
namespace std {
template<class charT, class traits = char_traits<charT> >
class istreambuf_iterator
: public iterator<input_iterator_tag, charT,
typename traits::off_type, charT*, charT&> {
public:
typedef charT char_type;
typedef traits traits_type;
typedef typename traits::int_type int_type;
typedef basic_streambuf<charT,traits> streambuf_type;
typedef basic_istream<charT,traits> istream_type;
class proxy; // exposition only
public:
istreambuf_iterator() throw();
istreambuf_iterator(istream_type& s) throw();
istreambuf_iterator(streambuf_type* s) throw();
istreambuf_iterator(const proxy& p) throw();
charT operator*() const;
istreambuf_iterator<charT,traits>& operator++();
proxy operator++(int);
bool equal(istreambuf_iterator& b);
private:
streambuf_type* sbuf_; exposition only
};
template <class charT, class traits>
bool operator==(const istreambuf_iterator<charT,traits>& a,
const istreambuf_iterator<charT,traits>& b);
template <class charT, class traits>
bool operator!=(const istreambuf_iterator<charT,traits>& a,
const istreambuf_iterator<charT,traits>& b);
}
1 The template class istreambuf_iterator reads successive characters
from the streambuf for which it was constructed. operator* provides
access to the current input character, if any. Each time operator++
is evaluated, the iterator advances to the next input character. If
the end of stream is reached (streambuf_type::sgetc() returns
traits::eof()), the iterator becomes equal to the end of stream itera-
tor value. The default constructor istreambuf_iterator() and the con-
structor istreambuf_iterator(0) both construct an end of stream itera-
tor object suitable for use as an end-of-range.
2 The result of operator*() on an end of stream is undefined. For any
other iterator value a char_type value is returned. It is impossible
to assign a character via an input iterator.
3 Note that in the input iterators, ++ operators are not equality pre-
serving, that is, i == j does not guarantee at all that ++i == ++j.
Every time ++ is evaluated a new value is used.
4 The practical consequence of this fact is that an istreambuf_iterator
object can be used only for one-pass algorithms. Two end of stream
iterators are always equal. An end of stream iterator is not equal to
a non-end of stream iterator.
24.5.3.1 Template class [lib.istreambuf.iterator::proxy]
istreambuf_iterator::proxy
namespace std {
template <class charT, class traits = char_traits<charT> >
class istreambuf_iterator<charT, traits>::proxy {
charT keep_;
basic_streambuf<charT,traits>* sbuf_;
proxy(charT c,
basic_streambuf<charT,traits>* sbuf);
: keep_(c), sbuf_(sbuf) {}
public:
charT operator*() { return keep_; }
};
}
1 Class istreambuf_iterator<charT,traits>::proxy is for exposition only.
An implementation is permitted to provide equivalent functionality
without providing a class with this name. Class istreambuf_itera-
tor<charT,traits>::proxy provides a temporary placeholder as the
return value of the post-increment operator operator++). It keeps the
character pointed to by the previous value of the iterator for some
possible future access to get the character.
24.5.3.2 istreambuf_iterator [lib.istreambuf.iterator.cons]
constructors
istreambuf_iterator() throw();
Effects:
Constructs the end-of-stream iterator.
istreambuf_iterator(basic_istream<charT,traits>& s) throw();
istreambuf_iterator(basic_streambuf<charT,traits>* s) throw();
Effects:
Constructs an istreambuf_iterator<> that uses the basic_streambuf<>
object *(s.rdbuf()), or *s, respectively. Constructs an end-of-
stream iterator if s.rdbuf() is null.
istreambuf_iterator(const proxy& p) throw();
Effects:
Constructs a istreambuf_iterator<> that uses the basic_streambuf<>
object pointed to by the proxy object's constructor argument p.
24.5.3.3 [lib.istreambuf.iterator::op*]
istreambuf_iterator::operator*
charT operator*() const
Returns:
The character obtained via the streambuf member sbuf_->sgetc().
24.5.3.4 [lib.istreambuf.iterator::op++]
istreambuf_iterator::operator++
istreambuf_iterator<charT,traits>&
istreambuf_iterator<charT,traits>::operator++();
Effects:
sbuf_->sbumpc().
Returns:
*this.
proxy istreambuf_iterator<charT,traits>::operator++(int);
Returns:
proxy(sbuf_->sbumpc(), sbuf_).
istreambuf_iterator<charT,traits> tmp = *this;
sbuf_->sbumpc();
return(tmp);
24.5.3.5 [lib.istreambuf.iterator::equal]
istreambuf_iterator::equal
bool equal(istreambuf_iterator<charT,traits>& b);
Returns:
true if and only if both iterators are at end-of-stream, or neither
is at end-of-stream, regardless of what streambuf object they use.
24.5.3.6 operator== [lib.istreambuf.iterator::op==]
template <class charT, class traits>
bool operator==(const istreambuf_iterator<charT,traits>& a,
const istreambuf_iterator<charT,traits>& b);
Returns:
a.equal(b).
24.5.3.7 operator!= [lib.istreambuf.iterator::op!=]
template <class charT, class traits>
bool operator!=(const istreambuf_iterator<charT,traits>& a,
const istreambuf_iterator<charT,traits>& b);
Returns:
!a.equal(b).
24.5.4 Template class [lib.ostreambuf.iterator]
ostreambuf_iterator
namespace std {
template <class charT, class traits = char_traits<charT> >
class ostreambuf_iterator:
public iterator<output_iterator_tag, void, void, void, void> {
public:
typedef charT char_type;
typedef traits traits_type;
typedef basic_streambuf<charT,traits> streambuf_type;
typedef basic_ostream<charT,traits> ostream_type;
public:
ostreambuf_iterator(ostream_type& s) throw();
ostreambuf_iterator(streambuf_type* s) throw();
ostreambuf_iterator& operator=(charT c);
ostreambuf_iterator& operator*();
ostreambuf_iterator& operator++();
ostreambuf_iterator& operator++(int);
bool failed() const throw();
private:
streambuf_type* sbuf_; exposition only
};
}
1 The template class ostreambuf_iterator writes successive characters
onto the output stream from which it was constructed. It is not pos-
sible to get a character value out of the output iterator.
24.5.4.1 ostreambuf_iterator [lib.ostreambuf.iter.cons]
constructors
ostreambuf_iterator(ostream_type& s) throw();
Requires:
s is not null.
Effects:
: sbuf_(s.rdbuf()) {}
ostreambuf_iterator(streambuf_type* s) throw();
Effects:
: sbuf_(s) {}
24.5.4.2 ostreambuf_iterator [lib.ostreambuf.iter.ops]
operations
ostreambuf_iterator<charT,traits>&
operator=(charT c);
Effects:
If failed() yields false, calls sbuf_->sputc(c); otherwise has no
effect.
Returns:
*this.
ostreambuf_iterator<charT,traits>& operator*();
Returns:
*this.
ostreambuf_iterator<charT,traits>& operator++();
ostreambuf_iterator<charT,traits>& operator++(int);
Returns:
*this.
bool failed() const throw();
Returns:
true if in any prior use of member operator=, the call to
sbuf_->sputc() returned traits::eof(); or false otherwise.