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A array is wrapper class that provides ]-like interface to standard fixed-size C-arrays. It also overcomes several limitations of standard array.


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== Creation History ==
{{R with history}}

In his book, ''Generic Programming and the STL'', Matthew H. Austern introduces a wrapper class for ordinary arrays with static size, called <tt>block</tt>. It is safer and has no worse performance than ordinary arrays. In ''The C++ Programming Language, 3rd edition'', ] introduces a similar class, called <tt>c_array</tt>, which Nicolai Josuttis present slightly modified in his book ''The C++ Standard Library - A Tutorial and Reference'', called <tt>carray</tt>.

Under the name <tt>array</tt> this class is introduced in ] libraries by Nicolai Josuttis. Later this class was introduced in C++ standard library in ].

== Motivation ==

Standard C arrays has several principal limitation:
* They aren't ]s. They can not be copied like any other object.
* They do not obey standard <tt>operator &</tt> semantics.
* They do not provide STL-like interface.

The second item means that in the following code
<source lang="cpp">
int a;
int * b1 = a;
int * b2 = &a;
</source>
<tt>b1</tt> and <tt>b2</tt> receive the same value. This behavior differs from one of any other standard type.

== Design ==

Array template class is defined in header <tt>&lt;array&gt;</tt> in C++ standard library and in header <tt>&lt;boost/array.hpp&gt;</tt> in boost. It can resides in namespaces <tt>std::</tt> (in C++0x), <tt>std::tr1::</tt> (in C++03 with TR1) or <tt>boost::</tt>.

The <tt>array</tt> class template is parametrized with a type of element and a number of elements. It can be instantiated with any type that fulfills the <tt>CopyConstructible</tt> and <tt>Assignable</tt> requirements. It also itself fulfills <tt>CopyConstructible</tt> and <tt>Assignable</tt> requirements.

If <tt>array</tt> class template is instantiated with a type that fulfills <tt>EqualityComparable</tt> or <tt>LessThanComparable</tt> requirements, it fulfills <tt>EqualityComparable</tt> or <tt>LessThanComparable</tt> correspondingly.

Class also provides standard iterators and element access functions.

=== Implementation as aggregate ===

<tt>array</tt> class is implemented as aggregate class. This allow array to be initialized with a brace-enclosing, comma-separated list of initializers for the elements of the container, written in increasing subscript order:

<source lang="cpp">
array<int,4> a = { { 1, 2, 3 } };
</source>

Note that if there are fewer elements in the initializer list, then each remaining element gets default-initialized (thus, it has a defined value).

However, this approach has its drawbacks: passing no initializer list means that the elements have an indetermined initial value, because the rule says that aggregates may have:

* No user-declared constructors.
* No private or protected non-static data members.
* No base classes.
* No virtual functions.

Note that for standard conforming compilers it is possible to use fewer braces (according to 8.5.1 (11) of the Standard). That is, array can be initialized as follows:

<source lang="cpp">
array<int,4> a = { 1, 2, 3 };
</source>

== Differences from standard array ==

* <tt>array</tt> class is value type. It satisfy <tt>CopyConstructable</tt> and <tt>Assignable</tt> requirements.
* <tt>array</tt> class can not be implicitly casted to <tt>T *</tt> or <tt>T const *</tt>. However there is member function <tt>data()</tt> that returns pointer to first element.
* <tt>array</tt> implementation is not required to do bound check. However implementation in boost do that for <tt>operator</tt>, but not for iterators.

=== Zero-sized arrays ===

Unlike standard arrays <tt>array</tt> class can have zero size. The effect of calling <tt>front()</tt> or <tt>back()</tt> for a zero-sized array is implementation defined. And <tt>begin() == end()</tt> shall be unique value. The return value of <tt>data()</tt> is unspecified.

== Differences from standard containers ==

* <tt>array</tt> class do not provides constant-time swap. Instead it provides linear-time swap.
* Because <tt>array</tt> class is aggregate it do not provides fill and range constructors. Its default constructor also does not initialize elements with zeros.
* size() is always constant, based on the second template argument of the type.
* The container provides no allocator support.

== Overview of functions ==

Object of array class can be created using default constructor, copy constructor or initializer list syntax.

{| class="wikitable" style="text-align: center"|
|-
! style="text-align: left" | expression
! description
! computational complexity
|-
| <tt>array a</tt> || create a array object, elements of that have undetermined values || ''O''(1)
|-
| <tt>array a1(a2)</tt> || create a copy of other array object || ''O''(N)
|-
| <tt>array a = {/*...*/}</tt> || create a array object initialized with specified values || ''O''(N)
|}

Array class provides swap function and assignment operator. The only difference from other containers is that <tt>swap</tt> takes linear time.

{| class="wikitable" style="text-align: center"|
|-
! style="text-align: left" | expression
! return type
! description
! computational complexity
|-
| <tt>swap(a1, a2)</tt> || <tt>void</tt> || swap content of arrays || ''O''(N)
|-
| <tt>a1 = a2</tt> || <tt>array &</tt> || copy content of a2 to a1 || ''O''(N)
|}

Although TR1 define function <tt>assign</tt> to fill array with specified value, C++0x has function <tt>fill</tt> intended for the same purpose. Boost implementation of array supports both functions.

{| class="wikitable" style="text-align: center"|
|-
! style="text-align: left" | expression
! return type
! description
! computational complexity
|-
| <tt>a.assign(u)</tt> || <tt>void</tt> || fill a with u (TR1 only) || ''O''(N)
|-
| <tt>a.fill(u)</tt> || <tt>void</tt> || fill a with u (C++0x only) || ''O''(N)
|}

Array class provides standard iterator interface.

{| class="wikitable" style="text-align: center"|
|-
! style="text-align: left" | expression
! return type
! description
! computational complexity
|-
| <tt>a.begin()</tt> || <tt>iterator</tt> || returns iterator to first element of array || ''O''(1)
|-
| <tt>a.end()</tt> || <tt>iterator</tt> || returns iterator to the one after the last element of array || ''O''(1)
|-
| <tt>a.rbegin()</tt> || <tt>reverse_iterator</tt> || returns reverse iterator to first element of array || ''O''(1)
|-
| <tt>a.rend()</tt> || <tt>reverse_iterator</tt> || returns reverse iterator to the one after the last element of array || ''O''(1)
|}

Array class provides standard query-capacity functions.

{| class="wikitable" style="text-align: center"|
|-
! style="text-align: left" | expression
! return type
! description
! computational complexity
|-
| <tt>a.size()</tt> || <tt>size_t</tt> || returns size of array || ''O''(1)
|-
| <tt>a.max_size()</tt> || <tt>size_t</tt> || returns size of array || ''O''(1)
|-
| <tt>a.empty()</tt> || <tt>bool</tt> || <tt>a.size() == 0</tt> || ''O''(1)
|}

Array class provides standard set of element access functions.

{| class="wikitable" style="text-align: center"|
|-
! style="text-align: left" | expression
! return type
! description
! computational complexity
|-
| <tt>a</tt> || <tt>T &</tt> || returns reference to i-th element || ''O''(1)
|-
| <tt>a.at(i)</tt> || <tt>T &</tt> || returns reference to i-th element or throws out_of_range exception || ''O''(1)
|-
| <tt>a.front()</tt> || <tt>T &</tt> || returns reference to first element of array || ''O''(1)
|-
| <tt>a.back()</tt> || <tt>T &</tt> || returns reference to last element of array || ''O''(1)
|}

Array class has six comparison operators.

{| class="wikitable" style="text-align: center"|
|-
! style="text-align: left" | expression
! return type
! description
! computational complexity
|-
| <tt>a1 < a2</tt> || <tt>bool</tt> || compare arrays lexicographically || ''O''(N)
|-
| <tt>a1 == a2</tt> || <tt>bool</tt> || compare arrays lexicographically || ''O''(N)
|-
| <tt>a1 > a2</tt> || <tt>bool</tt> || compare arrays lexicographically || ''O''(N)
|-
| <tt>a1 <= a2</tt> || <tt>bool</tt> || compare arrays lexicographically || ''O''(N)
|-
| <tt>a1 != a2</tt> || <tt>bool</tt> || compare arrays lexicographically || ''O''(N)
|-
| <tt>a1 >= a2</tt> || <tt>bool</tt> || compare arrays lexicographically || ''O''(N)
|}

Raw data access functions.

{| class="wikitable" style="text-align: center"|
|-
! style="text-align: left" | expression
! return type
! description
! computational complexity
|-
| <tt>a.data()</tt> || <tt>T *</tt> || returns pointer to first element of array || ''O''(1)
|}

== Links ==

*
*

]
]

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