| # Boost.Range

## Range concepts

- Overview
- Single Pass Range
- Forward Range
- Bidirectional Range
- Random Access Range

### Overview

A Range is a concept similar to the STL Container concept. A Range provides iterators for accessing a closed-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

- 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 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 `range_iterator<X>::type` is a model of Single Pass Iterator

### Associated types

Value type | `range_value<X>::type` | The type of the object stored in a Range.
Iterator type | `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 | `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 | `begin(a)` | `range_iterator<X>::type` if `a` is mutable, `range_const_iterator<X>::type` otherwise
End of range | `end(a)` | `range_iterator<X>::type` if `a` is mutable, `range_const_iterator<X>::type` otherwise
Is range empty? | `empty(a)` | Convertible to `bool`

### Expression semantics

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

### Complexity guarantees

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

### Invariants

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

### See also

Container

## Forward Range

### Notation

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

### Description

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

### Refinement of

Single Pass Range

### Associated types

Distance type | `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 | `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 | `size(a)` | `range_size<X>::type`

### Expression semantics

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

### Complexity guarantees

`size(a)` is at most amortized linear time.

### Invariants

Range size | `size(a)` is equal to the distance from `begin(a)` to `end(a)`.

## 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 `range_iterator<X>::type` iterator must meet all of the requirements of Bidirectional Traversal Iterator.

### Refinement of

Forward Range

### Associated types

Reverse Iterator type | `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 | `range_const_reverse_iterator<X>::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 | `rbegin(a)` | `range_reverse_iterator<X>::type` if `a` is mutable, `range_const_reverse_iterator<X>::type` otherwise. | Equivalent to `range_reverse_iterator<X>::type(end(a))`.
End of range | `rend(a)` | `range_reverse_iterator<X>::type` if `a` is mutable, `range_const_reverse_iterator<X>::type` otherwise. | Equivalent to `range_reverse_iterator<X>::type(begin(a))`.

### Complexity guarantees

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

### Invariants

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

## Random Access Range

### Description

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

### Refinement of

Bidirectional Range

Copyright © 2000 | Jeremy Siek
Copyright © 2004 | Thorsten Ottosen.