Data Abstraction Definition/Meaning:
The principle of defining a data type in terms of the
operations that apply to objects of the type, with the constraint that the
values of such objects can be modified and observed only by use of these
operations. This application of the general principle of abstraction leads to
the concept of an abstract data type. Data abstraction is of very considerable
importance in modern programming, especially for the coarse structuring of
programs. Such use yields several benefits. The abstract data type provides a
natural unit for specification and verification purposes (see module
specification). It provides some basis for high-level design, and is consistent
with the principles of information hiding. The specification of the data type
in terms of available operations provides all the information needed to make use
of the data type while leaving maximum freedom of implementation, which indeed
can be changed if required without affecting the users. There is also the
possibility of developing a "library" of useful data abstractions - stacks,
queues, etc.
The typical implementation of an abstract data type within a program is by means
of a multiprocedure module. This module has local data that can be used to
represent a value of the type, and each procedure implements one of the
operations associated with the type. The local data of the module can only be
accessed by these procedures, so that the user of the data type can only access
the operations and has no direct access to the representation. The implementer
is therefore free to choose the representation, which remains "invisible" to the
users and can be changed if required. Each instance of the abstract data type
employs one instance of the local data of the module to represent its value.
Proper support for such multiprocedure modules demands that the concept be
recognized by the programming language, which must, for example, allow for the
clustering of modules and data and have scope rules reflecting the desired
restrictions on access. The first language to provide such support was SIMULA
with its CLASS. Many modern languages now offer a similar facility, e.g. the
MODULE of Modula and the PACKAGE of Ada.
|
