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## Array Example Boostbook XML Documentation

### 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://std.dkuug.dk/jtc1/sc22/wg21/docs/papers/2003/n1548.htm).

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.

### Reference

#### Header <boost/array.hpp>

    namespace boost {
      template<typename T, std::size_t N> class array;
      template<typename T, std::size_t N> void swap(array<T, N>&, array<T, N>&);
      template<typename T, std::size_t N>
        bool operator==(const array<T, N>&, const array<T, N>&);
      template<typename T, std::size_t N>
        bool operator!=(const array<T, N>&, const array<T, N>&);
      template<typename T, std::size_t N>
        bool operator<(const array<T, N>&, const array<T, N>&);
      template<typename T, std::size_t N>
        bool operator>(const array<T, N>&, const array<T, N>&);
      template<typename T, std::size_t N>
        bool operator<=(const array<T, N>&, const array<T, N>&);
      template<typename T, std::size_t N>
        bool operator>=(const array<T, N>&, const array<T, N>&);
    }

### Design Rationale

There was an important design tradeoff regarding the constructors: We could implement array as an "aggregate" (see Section 8.5.1, [dcl.init.aggr], of the C++ Standard). This would mean:

- An array can be initialized with a brace-enclosing, comma-separated list of initializers for the elements of the container, written in increasing subscript order:

                     boost::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.

Nevertheless, The current implementation uses this approach.

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:

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

I'd appreciate any constructive feedback. Please note: I don't have time to read all boost mails. Thus, to make sure that feedback arrives to me, please send me a copy of each mail regarding this class.

The code is provided "as is" without expressed or implied warranty.

### For more information...

To find more details about using ordinary arrays in C++ and the framework of the STL, see e.g.

The C++ Standard Library - A Tutorial and Reference
by Nicolai M. Josuttis
Addison Wesley Longman, 1999
ISBN 0-201-37926-0

Home Page of Nicolai Josuttis

### Acknowledgements

Doug Gregor ported the documentation to the BoostBook format.