CS-2303 System Programming Concepts

1
More about C Classes

• CS-2303System Programming Concepts
• (Slides include materials from The C
  Programming Language, 2nd edition, by Kernighan
  and Ritchie and from C How to Program, 5th and
  6th editions, by Deitel and Deitel)

2
Reading Assignment

• Deitel Deitel, 5th edition, Chapter 20

3
Preprocessor Wrappers

• Prevent code from being included more than once.
• ifndef if not defined
• Skip this code if it has been included already
• define
• Define a name so this code will not be included
  again
• endif
• If the header has been included previously
• Name is defined already and the header file is
  not included again.
• Prevents multiple-definition errors
• Example
• ifndef TIME_Hdefine TIME_H // codeendif

Exactly as in C
Figure 20.1 in DD
4
Example Class

• class TreeNode
• public
• ... / other methods /
• TreeNode(const string newWord) //constructor
• TreeNode() //destructor
• private
• const string word
• int count
• TreeNode left, right

In this design, once a word is assigned to a
TreeNode, that word never changes.
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Example Class

• class TreeNode
• public
• ... / other methods /
• TreeNode(const string newWord) //constructor
• TreeNode() //destructor
• private
• const string word
• int count
• TreeNode left, right

What does the constructor methodof TreeNode have
to do?
6
What Must Constructor Do?

• (Allocate memory for object)
• Done before constructor method is called
• Initialize left, right
• To NULL
• Initialize count
• To 1
• Create a string object word and initialize
• How?

7
Creating Member Objects
A string object is a member ofTreeNode Therefore,
string constructoris automatically called to
initialize object word before TreeNodeconstructo
r method is called

• class TreeNode
• ...
• string word
• ...
• TreeNodeTreeNode (string newWord)
• ...
• word newWord
• ...

TreeNode constructor can thenbe programmed to
set theinitial value of word to benewWord (the
parameter)
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What About const Members?

• class TreeNode
• public
• ... / other methods /
• TreeNode(const string newWord) //constructor
• TreeNode() //destructor
• private
• const string word
• int count
• TreeNode left, right

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Constructor with const Member

• TreeNodeTreeNode(const string newWord)left
  right NULLcount 1
• word newWord
• class TreeNode
• ...
• private
• const string word
• int count
• TreeNode left, right

10
Solution Initializer List

• A list of member-value pairs
• Or member-constructor pairs
• Between the () of the constructor header and the
  of the constructor body
• Preceded by '' and separated by ','

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Example Initializer List

• TreeNodeTreeNode(const string newWord)
• word(newWord), //initialize word
• count(1), //initialize count
• left(NULL),
• right(NULL)
• / rest of constructor body /
• // TreeNode constructor

Widely used in the design of C classes
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Common Programming Error 21.5

• Not providing a member initializer for a const
  data member is a compilation error.

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Common Programming Error 21.6

• A compilation error occurs if
• a member object is not initialized with a member
  initializer
• and
• the member objects class does not provide a
  default constructor
• Even if member objects class defines one or more
  constructors, but none is a default constructor.

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Questions?
15
Destructors

• The opposite of constructors
• Called to clean up objects before deleting them
• Very important if your class contains objects of
  other classes inside of it.
• E.g., string
• Dynamically allocates array of characters to hold
  the string itself
• Must be freed before string object can be deleted

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Example

• class TreeNode
• public
• ... / other methods /
• TreeNode(const string newWord) //constructor
• TreeNode() //destructor
• private
• const string word
• int count
• TreeNode left, right

• TreeNodeTreeNode() if (left) delete
  left left NULLif (right) delete
  right right NULL

Note destructor for string word andfor count
are called automatically(because these are in
class scope). Destructors for left and right have
to be called forcibly, because thoseobject were
allocated dynamically
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When are Constructors Called?
Deitel Deitel, 20.8

• Global Scope I.e., objects declared outside of
  any function
• Before main() is called!
• Function or Block Scope I.e., automatic
  variables and constants
• When execution reaches point where object is
  declared
• For static objects, the first time execution
  reaches point where object is declared
• Class Scope I.e., data members of a class
• When class constructor executes initialization
  list (or enters block scope of constructor
  function)
• Dynamic objects I.e., objects created by new
• Constructor is invoked by new operator

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When are Destructors Called?

• In opposite order of constructors (mostly)
• Dynamic objects
• Invoked by delete operator
• Class scope I.e., data members of a class
• Invoked by destructor of class object
• Function or Block Scope
• When just before leaving the scope
• Global Scope
• After main() has returned

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When are Destructors Called Exceptions

• static objects in function or block scope
• After main() has returned but before calling
  destructors of objects in Global Scope
• exit() function
• Destructors of automatic objects are not called
• Usually means abnormal termination of program,
  file cannot be opened, etc.
• abort() function
• No destructors are called
• Usually means serious error, etc.

Deitel Deitel, 20.8
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Dynamically Allocated Objects

• Always use new operator (as in Java)
• Returns pointer to object (as with malloc() in C)
• Never use malloc()
• There is a lot more to creating an object in C
  than simply allocating memory
• new calls malloc() to allocate memory, calls
  constructor, sets up inheritance, finds the right
  polymorphic functions, etc.
• Always use delete operator
• Invokes destructor, cleans up, calls free(), etc.
• Takes pointer to object
• Never call free() directly
• For same reasons not to call malloc() memory
  leaks, etc.

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Questions?
22
Dynamically Allocated Arrays
Deitel Deitel, 21.6

• new can be used to dynamically allocate arrays
• Examples
• int myArray new int10double dArray new
  double2n1TreeNode silly new TreeNodek
• Calls default constructor for each element
• No arguments for initializing individual elements!

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Using Dynamically Allocated Arrays

• Examples
• int myArray new int10double dArray new
  double2n1TreeNode silly new TreeNodek
• Example usage
• myArray0, ... myArray9 dArrayi,
  dArrayj, ...int c sillyk-1.getCount()

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Deleting Dynamically Allocated Arrays

• delete myArraydelete dArraydelete
  silly
• Calls the destructor for each element before
  deallocating memory!
• Note
• delete myArray only calls destructor for first
  element!

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Common Programming Error 21.9

• Using delete instead of delete can lead to
  runtime logic errors.
• To ensure that every element receives a
  destructor call, be sure to use delete
• To call the destructor for an individual element,
  use delete for that element only
• Why would anyone want to do this?

26
Questions?
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Stream Manipulators

• setfill(), setw()
• Sets fill character and field width of stream
• Examples
• cout ltlt setfill('0') ltlt setw(2) ltlt hour ltlt "" ltlt
  setw(2) ltlt minute ltlt "" ltlt setw(2) ltlt second ltlt
  endl
• cout.setw(2) // sets all field widths

Deitel Deitel, Fig 20.2
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More Stream Manipulators

• setprecision()
• Sets precisions (in decimal places) of floating
  point numbers
• eof()
• Returns a bool indicating whether the end of the
  stream has been reached
• True only after program attempts to read past
  last character of the stream
• Usage cin.eof()

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Other Useful Stream (Member) Functions

• getline()
• Reads stream to next newline, returns string
• get()
• Reads one character from input stream, returns
  int
• put()
• Writes one character to output stream
• A long list of manipulators at
• http//www.cplusplus.com/reference/iostream/
• Deitel Deitel, Chapter 26

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Questions?
31
Review Constructors

• Initialize an object of a class
• I.e., set all data members to their initial
  values
• Especially members of other classes that need
  initialization
• Initializer List
• List of member-value pairs for setting values
• Follows header of constructor method, precedes
  body of constructor method
• Strongly recommended for good C programming
  style