C Overview, Part II

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C Overview, Part II
Reference Chapter 2, Sections 2.1.1-2.1.5
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Terminology

• Abstraction
• The specification of a data types
  logical properties (data values
• and operations) versus the details of
  its implementation
• Abstract Data Type (ADT)
• The specification of a data types data
  values, operations, and
• the rules governing these operations
• Encapsulation
• Hiding a data types implementation in a
  separate block with a
• formally specified interface
• What tool do we have in C to implement an ADT
  and
• promote encapsulation? Answer Classes

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C Classes

• A construct that supports data abstraction and
  encapsulation
• A class defines a new data type
• It encapsulates the data types data and
  operations
• General format
• class Name
• class body
• Dont forget the closing semicolon!
• Notice that a class looks very much like a
  struct -- but there
• is a difference (next slide)

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Class Example (Account.h)

• class Account
• public
• Account(unsigned n, double b) //
  constructor
• void deposit(double amt) //
  deposit amt into this account
• int withdraw(double amt) //
  withdraw amt from this account
• double balance()
  // balance inquiry
• private
• unsigned acct_no
  // account number
• double acct_bal
  // current balance
• Account is now a data type
• Operations and data (methods and attributes)
  are
• encapsulated
• Public and private members
• All class members are private by default
• With a struct, all members are public by
  default

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Objects

• Making a new object is essentially the
  declaration of
• a new variable
• Account sally(5551234, 600.00) //
  object sally
• Account bob(5556789, 540.00) //
  object bob
• Declaring an object is called instantiation
• Therefore, an object is an instance of a class
• Space is allocated for an objects attributes
• sally

acct_no acct_bal
5551234
600.00
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Accessing an Objects Members

• General syntax
• object.member
• Can access public members only
• sally.acct_bal 150.00 // illegal
  (private attribute)
• sally.deposit(150.00) //
  deposit via member function
• bob.acct_bal - 25.00 // illegal
  (private attribute)
• bob.withdraw(25.00) //
  withdrawal via member function
• balance sally.acct_bal // illegal
  (private attribute)
• balance sally.balance() // using
  member function

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Pointers to Objects

• Pointers to objects are useful
• Account bob(5556789, 540.00) //
  object bob
• Account acct_ptr bob //
  pointer to object bob
• Accessing members
• (pointer).member OR
  pointer-gtmember
• (acct_ptr).deposit(150.00)
• OR
• acct_ptr-gtdeposit(150.))

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Class Member Functions

• Code for member functions is not included in
  the class
• definition (except for inline functions)
• Place the class definition (specification) in
  a file called
• class.h (example Account.h)
• The the code for the member functions in a
  file called
• class.C (example Account.C)

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Class Member Functions (continued)

• Syntax for a member function
• type classfunction-name(parameters)
• function body
• is the scope resolution operator

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Class Member Functions (continued)

• Example
• // File Account.C
• include Account.h
• void Accountdeposit(double amt)
• if (amt gt 0)
• acct_bal amt
• int Accountwithdraw(double amt)
• if (amt gt acct_bal)
• acct_bal - amt
• return(acct_bal)

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Constructors

• Are used to initialize the attributes of an
  object upon
• instantiation
• Example
• AccountAccount(unsigned id, double amt)
• acct_no id
• if (amt gt 0) acct_bal amt
• Has same name as its class
• Has no return type

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Student Exercise

• Copy the code for Account.h and Account.C from
  the
• diskette in your text.
• Write a main function that
• - Instantiates an Account object with an
  id number and
• a beginning balance
• - Checks the balance
• - Makes a deposit of 50.75
• - Checks the balance again
• - Makes a withdrawal of 25.19
• - Checks the balance again
• - And anything else that you would like to try!

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