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1. Abstract Data Types
• Data abstraction, or abstract data types, is a programming
methodology where one defines not only the data
structure to be used, but the processes to manipulate the
structure
– like process abstraction, ADTs can be supported directly by
programming languages
• To support it, there needs to be mechanisms for
– defining data structures
– encapsulation of data structures and their routines to manipulate
the structures into one unit
• by placing all definitions in one unit, it can be compiled at one time
– information hiding to protect the data structure from outside
interference or manipulation
• the data structure should only be accessible from code encapsulated with
it so that the structure is hidden and protected from the outside
• objects are one way to implement ADTs, but because objects have
additional properties, we defer discussion of them until the next chapter

2. ADT Design Issues
• Encapsulation: it must be possible to define a unit that
contains a data structure and the subprograms that
access (manipulate) it
– design issues:
• will ADT access be restricted through pointers?
• can ADTs be parameterized (size and/or type)?
• Information hiding: controlling access to the data
structure through some form of interface so that it
cannot be directly manipulated by external code
– this is often done by using two sections of an ADT definition
• public part (interface) constitutes those elements that can be accessed
externally (often the interface permits only access to subprograms and
constants)
• the private part, which remains secure because it is only accessible by
subprograms of the ADT itself

3. Modula-2 ADTs
• Unit for encapsulation called a module
– modules can be combined to form libraries of ADTs
• To define a module:
– definition module: the interface containing a partial or
complete type definition (data structure) and the subprogram
headers and parameters
– implementation module: the portion of the data structure that
is to be hidden, along with all operation subprograms
• If the complete type declaration is given in the definition
module, the type is “transparent” otherwise it is
“opaque”
– opaque types represent true ADTs and must be accessed
through pointers
• this restriction allows the ADT to be entirely hidden from user
programs since the user program need only define a pointer

4. ADTs in Ada
• The encapsulation construct is the package
• Packages consist of two parts:
– specification package (the public or interface part)
– body package (the hidden or private part)
• The two packages can be compiled separately
– but only if specification package is compiled first
• The specification package must include details of the data
structure itself
– to preserve information hiding, the data structure’s definition can follow
the word private denoting that the following is hidden
• Ada offers three forms of ADTs
– those without information hiding, and thus are not true ADTs
– those that preserve information hiding by specifying that the data structure
is private
– those that specify that the data structure is limited private
• all ADTs have built-in operations for assignment and equality except for
limited private ADTs which have no built-in operations at all

5. Example Part I
package Stack_Pack is
type Stack_Type is limited private;
Max_Size : constant := 100;
function Empty(Stk : in Stack_Type) return Boolean;
procedure Push(Stk : in out Stack_Type; Element : in Integer);
procedure Pop(Stk : in out Stack_Type);
function Top(Stk : in Stack_Type) return Integer;
private
type List_Type is array (1..Max_Size) of Integer;
type Stack_Type is
record
List : List_Type;
Topsub : Integer range 0..Max_Size := 0;
end record;
end Stack_Pack;
The specification package
for a stack ADT – see the next
slide for the body package
The actual ADT
definition must either
appear in the open
section (e.g., the public
part) or in the private
section
An alternative implementation to this approach is to
define a pointer in the private section of this package and
define the actual Stack_Type ADT in the body package.
This is discussed in more detail in the notes section of this
slide.

6. Example Part II
with Ada.Text_IO; use Ada.Text_IO;
package body Stack_Pack is
function Empty(Stk : in Stack_Type) return Boolean is
begin
return Stk.Topsub = 0;
end Empty;
procedure Push(Stk : in out Stack_Type; Element : in Integer) is
begin
if Stk.Topsub >= Max_Size then
Put_Line(“ERROR – Stack overflow”);
else
Stk.Topsub := Stk.Topsub +1; Stk.List(Topsub):=Element;
end if;
end Push;
procedure Pop(Stk : in out Stack_Type) is
begin … end Pop;
function Top(Stk : in Stack_Type) return Integer is
begin … end Top;
end Stack_Pack;
The rest of the implementation
can be found on page 481

7. C++ ADTs
• C++ offers two mechanisms for building data structures:
the struct and the class
– because the struct does not have a mechanism for information
hiding, it can only offer encapsulation, so for a true ADT, we
must use C++s object
– C++ classes contain both visible (public) and hidden (private)
components (as well as protected)
– C++ instances can be static, heap-dynamic and stack-dynamic
• the lifetime of an instance ends when it reaches the end of the scope of
where it was declared
• a stack-dynamic object may have heap-dynamic data so that parts of the
object may continue even though the instant is deallocated
– we defer most of our discussion of objects in C++ to the next
chapter, but we will see an example next

8. C++ Example
#include <iostream.h>
class stack {
private:
int *stackPtr;
int max;
int topPtr;
public:
stack( ) { // constructor
stackPtr = new int [100];
max = 99;
topPtr = -1;
}
~stack( ) {delete [ ] stackPtr;} // destructor
void push(int number) {…} // details omitted
void pop( ) {…}
int top( ) {…}
int empty( ) {…}
Unlike the Ada example, in C++, the
entire definition is encapsulated in one
location
Information hiding is preserved through
the use of a private part with the interface
being defined in the public part
Any methods that are to be defined in this
class but not accessible outside of the
class would also be defined in the private
section

9. Java, C# and Ruby ADTs
• All three languages support ADTs through classes
– Java permits no stand-alone functions, only methods defined
in class definitions and unlike C++, referenced through
reference variables (pointers), therefore, in Java, every data
structure is an ADT
• it is up to the programmer as to whether information hiding is enforced
or not
– C# borrows from both C++ and Java but primarily from Java,
where all objects are heap dynamic, modifiers are private,
public, protected, but C# also offers
• internal and protected internal modifiers which are used for assemblies
(cross-platform objects), and methods that can serve as both accessors
and mutators (see the example on page 500-501)
– Ruby requires that all class variables be private, and all
methods default to being public (but the programmer can
change this)
• class variables do not have to be explicitly declared in Ruby, see the
example on page 502-04
• we look at Ruby in more detail in chapter 12

10. Parameterized ADTs
• The ability to define an ADT
where the type and/or size is
specified generically so that a
specific version can be
generated later
– a stack defined without
specifying the element type
(integer vs. string vs. float, etc)
– a stack defined without a
restriction on the size of the stack
– Ada, C++, Java and C# all have
this capability
• The approach is to replace the
type definition with a place
holder that is filled in later
In ADA:
generic
Max_Size : positive;
type Element_Type is private;
… rest of ADT as before except that
Element_Type replaces Integer
and Max_Size as a constant is
removed
now we instantiate our ADT:
package Integer_Stack is new
Generic_Stack(100, Integer);

11. Parameterized ADTs Continued
• In C++, parameterized ADTs are implemented as
templated classes
– to change the stack class’ size, only change the constructor to
receive the size as a parameter, which is used to establish the
size of the array
– to change the stack’s type, the class is now defined as a
template using template <class Type> where Type is the
place-holder to be filled in by a specific type at run-time
• In both Ada and C++, the parameterized ADT
definition is generated at compile-time
– the new statement signals that a new package should be
generated by the compiler
• in C++, if two definitions ask for the same type of ADT, only 1 set of
source code is generated, in Ada, the same source code is generated
twice!
• In Java and C#, parameterized ADTs are implemented
as generic classes (you should have covered this in 360
for Java, so we skip it here)

12. Encapsulation Constructs
• For large programs, to avoid having to recompile all code when
one section changes
– code can be grouped into logically related chunks called encapsulations
• one approach is the nested subprogram, place logically related subprograms
inside of the subprograms that commonly call them, although this approach is
not available in C-languages since nesting of subprograms is not possible
• use a header file (C, C++) and place logically related functions in the same
file, distributing the program across multiple files
– C++ goes beyond simple header files and includes the notation of a friend which
has access to private definitions
• Ada packages (which can be compiled separately) can include any number of
data and subprogram declarations so that they can contain interfaces for
multiple ADTs
• C# assemblies that can share code with other software written in the .NET
environment
• Each language has some technique for then using the named
encapsulation, sometimes called a namespace
– see the notes section of this slide for details in various languages