Abstract Data Types

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. 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 of data being stored)? Information hiding: controlling access to the data structure through some form of interface so that it cannot be directly manipulated by external code often implemented via two sections of code public part (interface) constitutes those elements that can be accessed externally (often limited to subprograms and constants) private part, which remains secure because it is only accessible by subprograms of the ADT itself ADT Design Issues

3. 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 Modula-2 ADTs

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 is omitted

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 (and encapsulation is not enforced when using structs, merely available), 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 for inheritance) • 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 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 #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( ) {…}

9. 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 in section 11.4) 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 in section 11.4 we look at Ruby in more detail in chapter 12 Java, C# and Ruby ADTs

10. 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 Parameterized ADTs 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 • using nested subprograms we can place logically related subprograms inside of the subprograms that commonly call them • approach not available in C-languages • use header files to place logically related functions’ prototypes in the same header, C++ also adds the “friend” (we look at this in chapter 12) • 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