An array is a collection of members that are all created as members of the collection at the same time. Existing member values may be replaced with new values, but the members themselves are fixed at constant offsets within the collection. The fixed structure of an array permits very fast access to members by integer offset positions, since the location of each member may be directly calculated.
The Array type is one of the simplest collection types in the collections library, and the closest to a data structure directly supported in C. Unlike C arrays, the group of members belonging to the array is not necessarily fixed for the lifetime of the array, but may be dynamically resized to contain a different number of members. When an array is dynamically resized, existing member values are preserved as much as possible.
The Array type adds few messages to the generic messages inherited from Collection. This type is provided partly so that a fixed-structure array can be accessed with the same uniform set of basic messages as any other kind of object collection. It also handles all required memory allocation within the collection. As an option, however, the Array type can be used to wrap an existing C array for external access as an object collection. It can also provide access to an internal C array for direct manipulation using C expressions. These forms of low-level access support hybrid modes of use in which advantages of both low-level manipulation and external object access can be combined.
The Array type is directly creatable, and supports all standard messages of Collection except removal of individual members. The messages based on an integer offset, either on the collection (atOffset:, atOffset:put:), or an index (setOffset:) all execute in fast constant time. Members of an array are fully ordered according to these integer offsets. Sequential access to members through its members is also fully supported. The Array type disables the remove message inherited from Collection; the message is defined, but any attempt to remove a member will raise an error that the operation is not supported.
The default value of the ReplaceOnly option is true, and cannot be overridden.
The type of index returned by begin: on an array is simply Index. There is no special index type for Array because there are no additional messages beyond those already defined by Index.
All the Array create-time options can also be set after the array is already created, subject to restrictions noted below.