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

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

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

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

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 packagefor a stack ADT – see the nextslide for the body package

The actual ADTdefinition must eitherappear in the opensection (e.g., the publicpart) or in the privatesection

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.

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 implementationcan be found on page 481

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

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

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

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);

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)

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