Standard Template Library Programmer's Guide

[3] After executing ++i , it is not required that copies of the old value of i be dereferenceable or that they be in the domain of operator== .

[4] It is not guaranteed that it is possible to pass through the same input iterator twice.

Output Iterator, Iterator overview

Category: iterators

Component type: concept

An Output Iterator is a type that provides a mechanism for storing (but not necessarily accessing) a sequence of values. Output Iterators are in some sense the converse of Input Iterators, but they have a far more restrictive interface: they do not necessarily support member access or equality, and they do not necessarily have either an associated distance type or even a value type [1]. Intuitively, one picture of an Output Iterator is a tape: you can write a value to the current location and you can advance to the next location, but you cannot read values and you cannot back up or rewind.

Assignable, DefaultConstructible

None. [1]

XA type that is a model of Output Iterator

x, yObject of type X

If x is an Output Iterator of type X , then the expression *x = t; stores the value t into x . Note that operator= , like other C++ functions, may be overloaded; it may, in fact, even be a template function. In general, then, t may be any of several different types. A type T belongs to the set of value types of X if, for an object t of type T , *x = t; is well-defined and does not require performing any non-trivial conversions on t . [1]

An Output Iterator may be singular , meaning that the results of most operations, including copying and dereference assignment, are undefined. The only operation that is guaranteed to be supported is assigning a nonsingular iterator to a singular iterator.

An Output Iterator may be dereferenceable , meaning that assignment through it is defined. Dereferenceable iterators are always nonsingular, but nonsingular iterators are not necessarily dereferenceable.

The complexity of operations on output iterators is guaranteed to be amortized constant time.

• ostream_iterator

• insert_iterator

• front_insert_iterator

• back_insert_iterator

Notes

[1] Other iterator types, including Trivial Iterator and Input Iterator, define the notion of a value type , the type returned when an iterator is dereferenced. This notion does not apply to Output Iterators, however, since the dereference operator (unary operator* ) does not return a usable value for Output Iterators. The only context in which the dereference operator may be used is assignment through an output iterator: *x = t . Although Input Iterators and output iterators are roughly symmetrical concepts, there is an important sense in which accessing and storing values are not symmetrical: for an Input Iterator ы operator* must return a unique type, but, for an Output Iterator, in the expression *x = t , there is no reason why operator= must take a unique type. [5] Consequently, there need not be any unique "value type" for Output Iterators.

[2] There should be only one active copy of a single Output Iterator at any one time. That is: after creating and using a copy x of an Output Iterator y , the original output iterator y should no longer be used.

[3] Assignment through an Output Iterator x is expected to alternate with incrementing x , and there must be an assignment through x before x is ever incremented. Any other order of operations results in undefined behavior. That is: {*x = t ; ++x; *x = t2; ++x} is acceptable, but {*x = t; ++x; ++x; *x = t2;} is not.

[4] Note that an Output Iterator need not define comparison for equality. Even if an operator== is defined, x == y need not imply ++x == ++y .

[5] If you are implementing an Output Iterator class X , one sensible way to define *x = t is to define X::operator*() to return an object of some private class X_proxy , and then to define X_proxy::operator= . Note that you may overload X_proxy::operator= , or even define it as a member template; this allows assignment of more than one type through Output Iterators of class X .

Trivial Iterator, Input Iterator, Iterator overview

Category: iterators

Component type: concept

A Forward Iterator is an iterator that corresponds to the usual intuitive notion of a linear sequence of values. It is possible to use Forward Iterators (unlike Input Iterators and Output Iterators) in multipass algorithms. Forward Iterators do not, however, allow stepping backwards through a sequence, but only, as the name suggests, forward.