Software Engineering for Students A Programming Approach

190 Chapter 14 ■ The basics
Ada does not specify whether they are to be implemented using sharing or copying.
Though beneficial to the language implementer, since the space requirements of the
parameter can be used to determine whether sharing or copying should be used, this decision
can be troublesome to the programmer. In the presence of aliases, call by valueresult
and call by reference may return different results.
14.10 ● Primitive data types
Programmers are accustomed to being provided with a rudimentary set of primitive
data types. These are provided built in and ready made by the programming language.
They usually include:
■ Boolean
■ char
■ integer
■ real or floating point.
These data types are accompanied by a supporting cast of operations (relational,
arithmetic, etc.). For each type, it should be possible to clearly define the form of the
literals or constants which make up the type. For example, the constants true and
false make up the set of constants for the type Boolean. Similarly, we should be able
to define the operations for each type. For the type Boolean, these might include the
operations =, , not, and, and or.
In most languages the primitive data types are not true objects (in the sense of
objects created from classes). But in Eiffel and Smalltalk, every data type is a proper
object and can be treated just like any other object.
For certain application domains, advanced computation facilities, such as extended
precision real numbers or long integers, are essential. The ability to specify the range of
integers and reals and the precision to which reals are represented reduces the dependence
on the physical characteristics, such as the word size, of a particular machine. This
increases the portability of programs. However, some languages (for example C and
C++) leave the issue of the precision and range of numbers to the compiler writer for
the particular target machine. Java gets around this sloppiness by precisely defining the
representation of all its built-in data types. Whatever machine a program is executed on,
the expectation is that data is represented in exactly the same manner. Thus the program
will produce exactly the same behavior, whatever the machine.
14.11 ● Data typing
A data type is a set of data objects and a set of operations applicable to all objects of
that type. Almost all languages can be thought of as supporting this concept to some
extent. Many languages require the programmer to define explicitly the type (e.g. integer
or character) of all objects to be used in a program, and, to some extent or another,
depending on the individual language, this information prescribes the operations that