Boost.Range

Range concepts

Overview
Single Pass Range
Forward Range
Bidirectional Range
Random Access Range
Concept Checking

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 much 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

own the elements that can be accessed through it,
have copy semantics,

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 their 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

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_iterator<const 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_iterator<const X>::type otherwise

End of range boost::end(a) boost::range_iterator<X>::type if a is mutable, boost::range_iterator<const X>::type otherwise

Name	Expression	Return type
Beginning of range	`boost::begin(a)`	`boost::range_iterator<X>::type` if `a` is mutable, `boost::range_iterator<const X>::type` otherwise
End of range	`boost::end(a)`	`boost::range_iterator<X>::type` if `a` is mutable, `boost::range_iterator<const X>::type` otherwise

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::distance(a) == 0`.
`boost::end(a)`	Returns an iterator pointing one past the last element in the Range.	`boost::end(a)` is past-the-end.

Complexity guarantees

boost::end(a) is at most amortized linear time, boost::begin(a) is amortized constant time. For most practical purposes, one can expect both 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`.

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

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

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.

Class SinglePassRangeConcept checks for Single Pass Range
Class ForwardRangeConcept checks for Forward Range
Class BidirectionalRangeConcept checks for Bidirectional Range
Class RandomAccessRangeConcept checks for Random Access Range