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Range Concepts

Boost.Range

Range concepts

Overview

A Range is a concept similar to the STL Container concept. A Range provides iterators for accessing a half-open range [first,one_past_last) of elements and provides information about the number of elements in the Range. However, a Range has fewer requirements than a Container.

The motivation for the Range concept is that there are many useful Container-like types that do not meet the full requirements of Container, and many algorithms that can be written with this reduced set of requirements. In particular, a Range does not necessarily

Because of the second requirement, a Range object must be passed by (const or non-const) reference in generic code.

The operations that can be performed on a Range is dependent on the traversal category of the underlying iterator type. Therefore the range concepts are named to reflect which traversal category its iterators support. See also terminology and style guidelines. for more information about naming of ranges.

The concepts described below specifies associated types as metafunctions and all functions as free-standing functions to allow for a layer of indirection.

Single Pass Range

Notation

X A type that is a model of Single Pass Range.
a Object of type X.

Description

A range X where boost::range_iterator<X>::type is a model of Single Pass Iterator

Associated types

Value type boost::range_value<X>::type The type of the object stored in a Range.
Iterator type boost::range_iterator<X>::type The type of iterator used to iterate through a Range's elements. The iterator's value type is expected to be the Range's value type. A conversion from the iterator type to the const iterator type must exist.
Const iterator type boost::range_const_iterator<X>::type A type of iterator that may be used to examine, but not to modify, a Range's elements.

Valid expressions

The following expressions must be valid.

Name Expression Return type
Beginning of range boost::begin(a) boost::range_iterator<X>::type if a is mutable, boost::range_const_iterator<X>::type otherwise
End of range boost::end(a) boost::range_iterator<X>::type if a is mutable, boost::range_const_iterator<X>::type otherwise
Is range empty? boost::empty(a) Convertible to bool

Expression semantics

Expression Semantics Postcondition
boost::begin(a) Returns an iterator pointing to the first element in the Range. boost::begin(a) is either dereferenceable or past-the-end. It is past-the-end if and only if boost::size(a) == 0.
boost::end(a) Returns an iterator pointing one past the last element in the Range. boost::end(a) is past-the-end.
boost::empty(a) Equivalent to boost::begin(a) == boost::end(a). (But possibly faster.)  - 

Complexity guarantees

All three functions are at most amortized linear time. For most practical purposes, one can expect boost::begin(a), boost::end(a) and boost::empty(a) to be amortized constant time.

Invariants

Valid range For any Range a, [boost::begin(a),boost::end(a)) is a valid range, that is, boost::end(a) is reachable from boost::begin(a) in a finite number of increments.
Completeness An algorithm that iterates through the range [boost::begin(a),boost::end(a)) will pass through every element of a.

See also

Container

implementation of metafunctions

implementation of functions

Forward Range

Notation

X A type that is a model of Forward Range.
a Object of type X.

Description

A range X where boost::range_iterator<X>::type is a model of Forward Traversal Iterator

Refinement of

Single Pass Range

Associated types

Distance type boost::range_difference<X>::type A signed integral type used to represent the distance between two of the Range's iterators. This type must be the same as the iterator's distance type.
Size type boost::range_size<X>::type An unsigned integral type that can represent any nonnegative value of the Range's distance type.

Valid expressions

Name Expression Return type
Size of range boost::size(a) boost::range_size<X>::type

Expression semantics

Expression Semantics Postcondition
boost::size(a) Returns the size of the Range, that is, its number of elements. Note boost::size(a) == 0u is equivalent to boost::empty(a). boost::size(a) >= 0

Complexity guarantees

boost::size(a) is at most amortized linear time.

Invariants

Range size boost::size(a) is equal to the distance from boost::begin(a) to boost::end(a).

See also

implementation of metafunctions

implementation of functions

Bidirectional Range

Notation

X A type that is a model of Bidirectional Range.
a Object of type X.

Description

This concept provides access to iterators that traverse in both directions (forward and reverse). The boost::range_iterator<X>::type iterator must meet all of the requirements of Bidirectional Traversal Iterator.

Refinement of

Forward Range

Associated types

Reverse Iterator type boost::range_reverse_iterator<X>::type The type of iterator used to iterate through a Range's elements in reverse order. The iterator's value type is expected to be the Range's value type. A conversion from the reverse iterator type to the const reverse iterator type must exist.
Const reverse iterator type boost::range_const_reverse_iterator&ltX>::type A type of reverse iterator that may be used to examine, but not to modify, a Range's elements.

Valid expressions

Name Expression Return type Semantics
Beginning of range boost::rbegin(a) boost::range_reverse_iterator<X>::type if a is mutable, boost::range_const_reverse_iterator<X>::type otherwise. Equivalent to boost::range_reverse_iterator<X>::type(boost::end(a)).
End of range boost::rend(a) boost::range_reverse_iterator<X>::type if a is mutable, boost::range_const_reverse_iterator<X>::type otherwise. Equivalent to boost::range_reverse_iterator<X>::type(boost::begin(a)).

Complexity guarantees

boost::rbegin(a) has the same complexity as boost::end(a) and boost::rend(a) has the same complexity as boost::begin(a) from Forward Range.

Invariants

Valid reverse range For any Bidirectional Range a, [boost::rbegin(a),boost::rend(a)) is a valid range, that is, boost::rend(a) is reachable from boost::rbegin(a) in a finite number of increments.
Completeness An algorithm that iterates through the range [boost::rbegin(a),boost::rend(a)) will pass through every element of a.

See also

implementation of metafunctions

implementation of functions

Random Access Range

Description

A range X where boost::range_iterator<X>::type is a model of Random Access Traversal Iterator

Refinement of

Bidirectional Range

Concept Checking

Each of the range concepts has a corresponding concept checking class in the file boost/range/concepts.hpp. These classes may be used in conjunction with the Boost Concept Check library to insure that the type of a template parameter is compatible with a range concept. If not, a meaningful compile time error is generated. Checks are provided for the range concepts related to iterator traversal categories. For example, the following line checks that the type T models the ForwardRange concept. 
    function_requires<ForwardRangeConcept<T> >();
An additional concept check is required for the value access property of the range based on the range's iterator type. For example to check for a ForwardReadableRange, the following code is required. 
    function_requires<ForwardRangeConcept<T> >();
    function_requires<
        ReadableIteratorConcept<
            typename range_iterator<T>::type
        >
    >();
The following range concept checking classes are provided.

See also

Range Terminology and style guidelines

Iterator Concepts

Boost Concept Check library

Copyright &copy 2000 Jeremy Siek
Copyright &copy 2004 Thorsten Ottosen. Use, modification and distribution is subject to the Boost Software License, Version 1.0.