Classes: Part II

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Chapter 7

• Classes Part II

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1. Introduction

• In this chapter we continue our study of classes
  and data abstraction
• We discuss many more advanced topics and lay the
  groundwork for the discussion of classes and
  operator overloading in Chapter 8.
• We us the C-style strings to help the reader
  master the complex topic of C pointers.

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2. const (Constant) Objects and const Member
Functions

• const Objects
• Some objects need to be modifiable and some do
  not. The programmer may use the keyword const to
  specify that an object is not modifiable, and
  that any attempt to modify the object is a syntax
  error. const Time noon(12, 0, 0)

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2. const (Constant) Objects and const Member
Functions (cont.)

• C compilers disallow any member functions calls
  for const objects unless the member functions
  themselves are also declared const.
• This is true even for get member functions that
  do not modify the object.
• Member functions declared const cannot modify the
  object -- the compiler disallows this.

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2. const (Constant) Objects and const Member
Functions (cont.)

• const Member Functions
• A function is specified as const both in its
  prototype and in its definition.
• Example
• prototype int getValue() const
• definition int AgetValue() const
  return privateDataMember

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2. const (Constant) Objects and const Member
Functions (cont.)

• An interesting problem arises here for
  constructors and destructors, each of which often
  need to modify objects.
• The const declaration is not required for
  constructors and destructors of const objects.
• A constructor must be allowed to modify an object
  so that the object can be initialized properly.
• A destructor must be able to perform its
  termination housekeeping chores before an object
  is destroyed.

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2. const (Constant) Objects and const Member
Functions (cont.)

• Member Initializer Lists
• This example classclass (int c, int i )
  privData(i) count c is
  equivalent to classclass (int c,
  int i ) privData i count c

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2. const (Constant) Objects and const Member
Functions (cont.)

• Member Initializer Lists (cont.)
• All data members can be initialized using member
  initializer lists.
• but consts and references must be initialized in
  this manner.
• not providing a member initializer list for these
  is usually a syntax error.

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3. Composition Objects as Members of Classes

• An AlarmClock class object needs to know when it
  is supposed to sound its alarm, so why not
  include a Time object as a member of the
  AlarmClock object?
• Such a capability is called composition. A class
  can have objects of other classes as members.

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3. Composition Objects as Members of Classes
(cont.)

• Member objects are constructed in the order in
  which they are declared (not the order in the
  initializer list) and before their host object is
  constructed.
• A member object does not need to be initialized
  explicitly through a member initializer. If it is
  not provided, the member objects default
  constructor will be called implicitly.

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4. friend Functions and friend Classes

• A friend function of a class is defined outside
  that classs scope, yet has the right to access
  the private members of the class.
• A function or an entire class may be declared to
  be a friend of another class.

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4. friend Functions and friend Classes (cont.)

• To declare a function as a friend of a class,
  precede the function prototype in the class
  definition with the keyword friend.
• To declare class ClassTwo as a friend of class
  ClassOne place a declaration of the form
  friend class ClassTwo in the definition of
  class ClassOne.

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4. friend Functions and friend Classes (cont.)

• Friendship is granted, not taken.
• for class B to be a friend of class A class A
  must explicitly declare class B is it s friend.
• Also, friendship is neither symmetric or
  transitive
• if A is a friend of B and B is a friend of C
• That does not imply that B is a friend of A
• That also does not imply that A is a friend of C.

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5. Using the this Pointer

• this is a pointer to the object you are in.
• this is the object itself.
• One use of the this pointer is in enabling
  cascading member function calls, and in cascading
  overloaded input and output. a b c
  cout ltlt a ltlt b ltlt c

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6. Dynamic Memory Allocation with Operators new
and delete

• The new and delete operators provide a means for
  performing dynamic memory allocation.
• new
• The new operator automatically creates an object
  of the proper size, calls the constructor for the
  object and returns a pointer of the correct type.

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6. Dynamic Memory Allocation with Operators new
and delete (cont.)

• new (cont.)
• Example TypeName typeNamePtr
  typeNamePtr new TypeName
• If new is unable to find space, it returns a NULL
  pointer.
• In the new ANSI/ISO draft new throws an
  exception -- Chapter 13 discusses this, along
  with how to catch the exception.

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6. Dynamic Memory Allocation with Operators new
and delete (cont.)

• delete
• To free space for this object you must use the
  delete operator as follows delete
  typeNamePtr
• C allows you to provide an initializer for a
  newly created object as in float thingPtr
  new float (3.14159)

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6. Dynamic Memory Allocation with Operators new
and delete (cont.)

• A 10 element integer array can be created as
  follows int arrayPtr new int 10
• This array can be deleted with the statement
  delete arrayPtr
• forgetting the in the previous statement can
  lead to runtime logic errors on some compilers.

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7. static Class Members

• A static class variable represents class-wide
  information.
• Every class instance can see the static member as
  if it were its data member, but there is only one
  copy shared among them.
• Although static data members may seem like global
  variables, static data members have class scope
  ad can be public or private (or protected).

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7. static Class Members (cont.)

• A classs static members exist even when no
  objects of that class exist.
• You typically initialize static data members in
  the definition file of the class int
  Employeecount 0
• If there is a static member function (getCount)
  you can call it with the class name and scope
  resolution operator the same way
  EmployeegetCount( )

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7. static Class Members (cont.)

• Referring to the this pointer within a static
  member function is a syntax error.
• Declaring a static member function const is a
  syntax error.
• A classs static data members and static member
  functions exist and can be used even if no
  objects of that class are instantiated.
• static member functions can only access static
  data members of the class.

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7. static Class Members (cont.)

• assert
• The assert macro -- defined in assert.h --
  tests the value of a condition
• if the value is false, then assert issues an
  error message and calls the function abort (part
  of stdlib.h) and an error message containing the
  following is printed
• the line number
• the file name
• the condition tested

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7. static Class Members (cont.)

• assert (cont.)
• Assertions do not have to be removed from the
  program when debugging is completed. When
  assertions are no longer needed the line
  define NDEBUG is inserted at the
  beginning of the program file. This causes the
  preprocessor to ignore all assertions instead of
  the programmer having to delete each assertion
  manually.

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8. Data Abstraction and Information Hiding

• Classes normally hide their implementation
  details from the clients of the classes. This is
  called information hiding.
• Describing the functionality of a class
  independent of its implementation is called data
  abstraction.

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8. Data Abstraction and Information Hiding (cont.)

• C classes allow us to define Abstract Data
  Types (ADTs). An ADT has a few properties
• data (and its representation)
• operations on that data
• encapsulation

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9. Container Classes and Iterators

• Container classes
• Among the most popular types of classes are
  container classes (classes designed to hold
  collections of objects)
• Container classes typically provide services such
  as insertion, deletion, searching, sorting and
  testing an item for membership.
• Arrays, stacks, queues, trees, and linked lists
  are examples of container classes.

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9. Container Classes and Iterators (cont.)

• Iterators
• It is common to associate iterator objects -- or
  more simply iterators -- with container classes.
• An iterator is an object that returns the next
  item of a collection (or performs some operation
  on the next item)
• Iterators are normally written as friends so they
  can have direct access to the private data.

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10. Proxy Classes

• It is desirable to hide the implementation
  details of a class to prevent access to
  proprietary information (private data) and
  proprietary program logic.
• Providing clients of your class with a proxy
  class that know only the public interface to your
  class enables the clients to use your classs
  services without giving the client access to your
  classs implementation details.

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10. Proxy Classes (cont.)

• Implementing a proxy class requires several
  steps.
• First we have our Implementation class we want to
  hide.
• Second we create an Interface class we want to
  allow or customers to use.
• You provide the client with the Interface.h file.
  and compiled versions of everything else.

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• Interface.h class Implementation
  class Interface public
  Interface( int) void setValue(
  int) int getValue() const private
  Implementation ptr

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• Interface.cpp include interface.h
  include implementation.h Interface
  Interface (int v) ptr(new
  Implementation(v)) void
  InterfacesetValue(int v)
  ptr-gtsetValue(v) int
  InterfacegetValue() const return
  ptr-gtgetValue()

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• Client.cpp include ltiostream.hgt include
  interface.h int main()
  Interface i(5) cout ltlt
  Interface contains ltlt i.getValue()
  ltltendl i.setValue(10) cout ltlt
  Interface contains ltlt i.getValue()
  ltltendl return 0