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Output Iterator

Category: iterators Component type: concept

Description

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.

Refinement of

Assignable, DefaultConstructible

Associated types

None. [1]

Notation

X A type that is a model of Output Iterator
x, y Object of type X

Definitions

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.

Valid expressions

Name Expression Type requirements Return type
Default constructor
X x;
X()
   
Copy constructor X(x)   X
Copy constructor X y(x); or X y = x;    
Dereference assignment *x = t t is convertible to a type in the set of value types of X. [1] Result is not used
Preincrement ++x   X&
Postincrement (void) x++   void
Postincrement and assign *x++ = t;   Result is not used

Expression semantics

Name Expression Precondition Semantics Postcondition
Default constructor
X x;
X()
    x may be singular
Copy constructor X(x) x is nonsingular   *X(x) = t is equivalent to *x = t [2]
Copy constructor X x(y); or X x = y; y is nonsingular   *y = t is equivalent to *x = t [2]
Dereference assignment *x = t x is dereferenceable. If there has been a previous assignment through x, then there has been an intervening increment. [3]    
Preincrement ++x x is dereferenceable. x has previously been assigned through. If x has previously been incremented, then there has been an intervening assignment through x [3] [4]   x points to the next location into which a value may be stored
Postincrement (void) x++ x is dereferenceable. x has previously been assigned through. Equivalent to (void) ++x x points to the next location into which a value may be stored
Postincrement and assign *x++ = t; x is dereferenceable. If there has been a previous assignment through x, then there has been an intervening increment. [3] [4] Equivalent to {*x = t; ++x; } x points to the next location into which a value may be stored

Complexity guarantees

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

Invariants

Models

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 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.

See also

Trivial Iterator, Input Iterator, Iterator overview


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