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## Chapter 5. Boost.Array

### Nicolai Josuttis

Copyright © 2001-2004 Nicolai M. Josuttis

Copyright © 2012 Marshall Clow

Distributed under the Boost Software License, Version 1.0. (See accompanying file `LICENSE_1_0.txt` or copy at http://www.boost.org/LICENSE_1_0.txt)

## Introduction

The C++ Standard Template Library STL as part of the C++ Standard Library provides a framework for processing algorithms on different kind of containers. However, ordinary arrays don't provide the interface of STL containers (although, they provide the iterator interface of STL containers).

As replacement for ordinary arrays, the STL provides class `std::vector`. However, `std::vector<>` provides the semantics of dynamic arrays. Thus, it manages data to be able to change the number of elements. This results in some overhead in case only arrays with static size are needed.

In his book, Generic Programming and the STL, Matthew H. Austern introduces a useful 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 I (Nicolai Josuttis) present slightly modified in my book The C++ Standard Library - A Tutorial and Reference, called `carray`. This is the essence of these approaches spiced with many feedback from boost.

After considering different names, we decided to name this class simply `array`.

Note that this class is suggested to be part of the next Technical Report, which will extend the C++ Standard (see http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1548.htm).

Update: `std::array` is (as of C++11) part of the C++ standard. The differences between `boost::array` and `std::array` are minimal. If you are using C++11, you should consider using `std::array` instead of `boost::array`.

Class `array` fulfills most but not all of the requirements of "reversible containers" (see Section 23.1, [lib.container.requirements] of the C++ Standard). The reasons array is not an reversible STL container is because:

- No constructors are provided.
- Elements may have an undetermined initial value (see the section called “Design Rationale”).
- `swap`() has no constant complexity.
- `size`() is always constant, based on the second template argument of the type.
- The container provides no allocator support.

It doesn't fulfill the requirements of a "sequence" (see Section 23.1.1, [lib.sequence.reqmts] of the C++ Standard), except that:

- `front`() and `back`() are provided.
- `operator[]` and `at`() are provided.