Class array, an STL Container (as Wrapper) for Arrays of Constant Size

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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 vector<>. However, 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.

The class provides the following interface:

Types: |
value_type | type of the elements
iterator | type of the iterator (random-access iterator)
const_iterator | type of iterator that considers elements as being constant
reference | type of element reference
const_reference | type of element reference that considers elements as being constant
size_type | type for signed size values
difference_type | type for unsigned difference values
Operations: |
array<type,num> | default constructor, creates array of num element of type, see comment below
array<type,num>(a) | copy constructor, copies all elements of a (a must have same type and num)
operator= | assignment, assigns all elements
assign(val) | assigns val to all elements
begin() | returns iterator for the first element
end() | returns iterator for position after the last element
rbegin() | returns reverse iterator for position of first element of reverse iteration
rend() | returns reverse iterator for position behind last element of reverse iteration
operator[i] | returns element with index i (no range checking)
at(i) | returns element with index i (throw std::range_error if i is not valid)
front() | returns first element (caller has to ensure that it exists)
back() | returns last element (caller has to ensure that it exists)
data() | returns raw element array for read-only element access
size() | returns number of elements
empty() | returns whether array is empty
max_size() | returns maximum possible number of elements (same as size())
swap(a) | swap elements with array a
==, != | checks for equality
<, <=, >, >= | compares array
Values: |
static_size | yields size at compile time

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 below)
- 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

Regarding the constructors there was an important design tradeoff: 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, passing no initializer list means that the elements have an indetermined initial value.
- It has no user-declared constructors.
- It has no private or protected non-static data members.
- It has no base classes.
- It has no virtual functions.

The current implementation uses this approach. However, being able to have indeterminate initial values is a big drawback. So, please give me some feedback, how useful you consider this feature to be. This leads to the list of Open issues:

- Do we want initializer list support or would the following be OK?:

  > int data[] = { 1, 2, 3, 4 }

  > array<int,5> x(data); or array<int,data> x;

- Could "{ ... }" be used portably instead of "{ { ... } }" to initialize values?

> 8.5.1 (11) of the Standard seems to allow it; however, gcc 2.95.2 prints a warning message.

- Any way to have determinate initial values and initializer list support?
- Static_casts for reverse iterator stuff?

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 me, please send me a copy of each mail regarding this class.

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

array.hpp, the implementation of array<>:

- as HTML file
- as plain file

array1.cpp, a simple example for using array<>:

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- as plain file

array2.cpp, another example for using array<>:

- as HTML file
- as plain file

array3.cpp, a third example for using array<>:

- as HTML file
- as plain file

array4.cpp, an example for using arrays of arrays:

- as HTML file
- as plain file

array5.cpp, an example for testing other operations of array<>:

- as HTML file
- as plain file
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

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