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A array is wrapper class that provide STL-like interface to standard fixed-size C-arrays. It also overcome 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.

Nicolai Josuttis introduced it under the name array in boost libraries. 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.

And several minor drawbacks:

• They do not provide STL-like interface.
• They do not made bound-checks even in debug builds.
• They have weird syntax.

The second item means that in the following code

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

b1 and b2 receives the same value. That is impossible for object 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, you can initialize an array 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. It default construct also do not initialize elements to zero.
• size() is always constant, based on the second template argument of the type.
• The container provides no allocator support.

Links

• A Proposal to Add a Fixed Size Array Wrapper to the Standard Library Technical Report
• boost::array documentation