Array (C++) - Misplaced Pages

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

Creation History

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

Under the name array this class is introduced in boost libraries by Nicolai Josuttis. Later this class was introduced in C++ standard library in TR1.

Motivation

Standard C arrays has several principal limitation:

They aren't value types. They can not be copied like any other object.
They do not obey standard operator & semantics.
They do not provide STL-like interface.

The second item means that in the following code

int a;
int * b1 = a;
int * b2 = &a;

b1 and b2 receive the same value. This behavior differs from one of any other standard type.

Design

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

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

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

Class also provides standard iterators and element access functions.

Implementation as aggregate

array 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:

array<int,4> a = { { 1, 2, 3 } };

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:

array<int,4> a = { 1, 2, 3 };

Differences from standard array

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

Differences from standard containers

array class do not provides constant-time swap. Instead it provides linear-time swap.
Because array 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.

expression	description	computational complexity
`array a`	create a array object, elements of that have undetermined values	O(1)
`array a1(a2)`	create a copy of other array object	O(N)
`array a = {/.../}`	create a array object initialized with specified values	O(N)

expression	return type	description	computational complexity
`swap(a1, a2)`	`void`	swap content of arrays	O(N)
`a1 = a2`	`array &`	copy content of a2 to a1	O(N)
`a.assign(u)`	`void`	fill a with u	O(N)

expression	description	computational complexity
`a.begin()`	returns iterator to first element of array	O(1)
`a.end()`	returns iterator to the one after the last element of array	O(1)
`a.rbegin()`	returns reverse iterator to first element of array	O(1)
`a.rend()`	returns reverse iterator to the one after the last element of array	O(1)

expression	return type	description	computational complexity
`a.size()`	`size_t`	returns size of array	O(1)
`a.max_size()`	`size_t`	returns size of array	O(1)
`a.empty()`	`bool`	`a.size() == 0`	O(1)

Array class has six comparison operators.

expression	return type	description	computational complexity
`a1 < a2`	`bool`	compare arrays lexicographically	O(N)
`a1 == a2`	`bool`	compare arrays lexicographically	O(N)
`a1 > a2`	`bool`	compare arrays lexicographically	O(N)
`a1 <= a2`	`bool`	compare arrays lexicographically	O(N)
`a1 != a2`	`bool`	compare arrays lexicographically	O(N)
`a1 >= a2`	`bool`	compare arrays lexicographically	O(N)