Structures/Classes

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Structures/Classes

• CS 308 Data Structures

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What is a structure?

• It is an aggregate data type built using
  elements of other types.
• Declaring a structure requires declaring its
  members and their data types.
• struct rectangle
• float height
• float width
• int xpos
• int ypos

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Structure variables

• They are declared like variables of any other
  type.
•  
• rectangle rc
• rectangle rcRef rc
• rectangle rcPtr rc

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Accessing members of structures

• dot operator (.)
• rc.height 15.89
• rcRef.height 15.89
• arrow operator (-gt)
• rcPtr -gt height 15.89 or
• (rcPtr).height 15.89
• Important the parentheses around rcPtr are
  necessary since the member operator . takes
  precedence over the dereference operator.

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Member functions (methods)

• Functions which operate on the data of the
  structure.
• The prototype of a member function appears within
  the structure definition.
•  
• struct rectangle
• float height
• float width
• int xpos
• int ypos
• void draw() // draw member function
• void posn(int, int) // position member
  function
• void move(int, int) // move member function

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Member function declaration

• Usually, they are declared outside the structure.
•  
• data_type structure_namefunction_name(arguments)
  
•   ( is the "scope resolution operator")
•  
• void rectangledraw()
• cout ltlt "position is " ltlt xpos ltlt ypos ltlt endl
•  
• void rectangleposn(int x, int y)
• xpos x
• ypos y
•  
• void rectanglemove(int dx, int dy)
• xpos dx

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Referring to a member function

• We refer to a member function just as any other
  variable of the structure.
•  
• rc.draw()
• rc.posn(100, 100)
• rc.move(50, 50)

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Information Hiding
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Philosophy behind information hiding

• The actual data representation used within a
  structure is of no concern to the structure's
  clients.
• Protects data members from receiving invalid
  values.
• It promotes program modifiability (if the
  representation of data changes, only the member
  functions need to change).

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Controlling access to members

• Most common member access specifiers are public
  and private
•  
• struct rectangle
• private
• float height
• float width
• int xpos
• int ypos
•  
• The private keyword specifies that the structure
  members following it are private to the structure
  and can only be accessed by member functions (and
  by friend functions)

public void draw() // draw member
function void posn(int, int) // position
member function void move(int, int) // move
member function
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Controlling access to members
(cont.)

• The public keyword specifies that the structure
  members following it are public to the structure
  and may be accessed from outside the structure.
•  
• void main()
• rectangle rc
•  
• rc.height 20 // Error not accessible
•  
• Another way to access private data members is by
  using get-set member functions which are public
  to the structure.

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Classes
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What is a class?

• Practically, there are no differences between
  structures and classes
• (i) A class is a structure which has all of its
  members private by default.
• (ii) Structures have all of their members
  public by default.
•  
• class rectangle
• private // not needed but
  included for clarity
• float height
• float width
• int xpos
• int ypos
• public
• void draw() // draw member
  function
• void posn(int, int) // position member
  function
• void move(int, int) // move member function

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What is an object?

• An instance or an object is a variable of type
  class.

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Class-based programming

• Data and functions co-exist inside a class.
• Member functions are called without passing the
  data members of the class to them.
• There is far less chance of misusing functions by
  passing them the wrong data.