Encapsulation
When the size of a program reaches beyond a few thousand lines, two practical
problems appear. From the programmer’s point of view, having such a program appear as
a single collection of subprograms does not impose an adequate level of organization on
the program to keep it intellectually manageable. On solution is to organize the program
into syntactic containers that include groups of logically related subprograms and data.
These syntactic containers are often called modules, and the process of designing them is
called modularization. The second practical problem for larger programs is
recompilation. In case of a small program, recompiling the whole program after each
modification is not costly. But when programs grow beyond a few thousand lines, the
cost of recompilation ceases to be insignificant. So there is an obvious need to find ways
to avoid recompilation of the parts of a program that are not affected by a change. This
can be provided by organizing programs into collection of subprograms and data, each of
which can be compiled without recompilation of the rest of the program. Such a
collection is called a compilation unit.
An encapsulation is a grouping of subprograms and the data they manipulate. An
encapsulation, which is either separately or independently compilable, provides an
abstracted system and a logical organization for a collection of related computations.
Therefore, an encapsulation solves both of the practical problems described above.
Encapsulation are often placed in libraries and made available for reusing programs other
than those for which they were written.
Abstract Data Types in C++
C++ provides the class, which more directly supports abstract data types. C++
classes are types. A C++ program, variables are declared to be of class types. A C++
program unit that declares an instance of a class can also access any of the public entities
in that class, but only through an instance of the class.
Encapsulation
The classes of C++ are based on the structure in C. The data defined in a class are
called data members; the functions defined in a class are called member functions. All of
the instances of a class share a single set of member functions, but each instance gets its
own set of the class’s data members. Although class instances can also be static and heap-
dynamic, we consider only stack-dynamic classes here. The instances of such classes are
always created by elaboration of an object declaration. Classes can have heap-dynamic
data members, so that even though a class instance is stack-dynamic, it can include data
members that are heap-dynamic and allocated from the heap. C++ provides the new and
delete operators to manage the heap.
A member function of a class can be defined in two distinct ways: The complete
definition can appear in the class, or only in its header. When both the header and the
body of a member function appear in the class definition, the member function is
implicitly inlined.
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