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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. | |||
| {{R to section}} | |||
| == 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><array></tt> in C++ standard library and in header <tt><boost/array.hpp></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. | |||
| * Unlike standard arrays <tt>array</tt> class can have zero size. | |||
| == 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) | |||
| |} | |||
| {| 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) | |||
| |- | |||
| | <tt>a.assign(u)</tt> || <tt>void</tt> || fill a with u || ''O''(N) | |||
| |} | |||
| {| class="wikitable" style="text-align: center"| | |||
| |- | |||
| ! style="text-align: left" | expression | |||
| ! description | |||
| ! computational complexity | |||
| |- | |||
| | <tt>a.begin()</tt> || returns iterator to first element of array || ''O''(1) | |||
| |- | |||
| | <tt>a.end()</tt> || returns iterator to the one after the last element of array || ''O''(1) | |||
| |- | |||
| | <tt>a.rbegin()</tt> || returns reverse iterator to first element of array || ''O''(1) | |||
| |- | |||
| | <tt>a.rend()</tt> || returns reverse iterator to the one after the last element of array || ''O''(1) | |||
| |} | |||
| {| 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 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) | |||
| |} | |||
| == Links == | |||
| * | |||
| * | |||
| ] | |||
| ] | |||
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