Silicon Graphics, Inc.

Trivial Iterator

Category: iterators Component type: concept


A Trivial Iterator is an object that may be dereferenced to refer to some other object. Arithmetic operations (such as increment and comparison) are not guaranteed to be supported.

Refinement of

Assignable, Equality Comparable, Default Constructible

Associated types

Value type The type of the value obtained by dereferencing a Trivial Iterator


X A type that is a model of Trivial Iterator
T The value type of X
x, y Object of type X
t Object of type T


A type that is a model of Trivial Iterator may be mutable, meaning that the values referred to by objects of that type may be modified, or constant, meaning that they may not. For example, int* is a mutable iterator type and const int* is a constant iterator type. If an iterator type is mutable, this implies that its value type is a model of Assignable; the converse, though, is not necessarily true.

A Trivial Iterator may have a singular value, meaning that the results of most operations, including comparison for equality, are undefined. The only operation that a is guaranteed to be supported is assigning a nonsingular iterator to a singular iterator.

A Trivial Iterator may have a dereferenceable value, meaning that dereferencing it yields a well-defined value. Dereferenceable iterators are always nonsingular, but the converse is not true. For example, a null pointer is nonsingular (there are well defined operations involving null pointers) even thought it is not dereferenceable.

Invalidating a dereferenceable iterator means performing an operation after which the iterator might be nondereferenceable or singular. For example, if p is a pointer, then delete p invalidates p.

Valid expressions

In addition to the expressions defined in Assignable, Equality Comparable, and Default Constructible, the following expressions must be valid.
Name Expression Type requirements Return type
Default constructor X x    
Dereference *x   Convertible to T [1]
Dereference assignment *x = t X is mutable  
Member access x->m [2] T is a type for which x.m is defined  

Expression semantics

Name Expression Precondition Semantics Postcondition
Default constructor X x     x is singular
Dereference *x x is dereferenceable    
Dereference assignment *x = t x is dereferenceable   *x is a copy of t
Member access x->m x is dereferenceable Equivalent to (*x).m  

Complexity guarantees

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


Identity x == y if and only if &*x == &*y



[1] The requirement for the return type of *x is specified as "convertible to T", rather than simply T, because it sometimes makes sense for an iterator to return some sort of proxy object instead of the object that the iterator conceptually points to. Proxy objects are implementation details rather than part of an interface (one use of them, for example, is to allow an iterator to behave differently depending on whether its value is being read or written), so the value type of an iterator that returns a proxy is still T.

[2] Defining operator-> for iterators depends on a feature that is part of the C++ language but that is not yet widely implemented by C++ compilers. It is therefore not implemented in this release; once compilers that support it become more common, future releases of the STL will provide operator-> for iterators. In the mean time, the workaround is to use (*it).m instead of it->m.

See also

Input Iterator, Output Iterator, Forward Iterator, Bidirectional Iterator, Random Access Iterator, Iterator Overview
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