Constructors and Destructors

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Constructors and Destructors
ECE230 Lectures Series

• Ying Wu
• Electrical Engineering Computer Science
• Northwestern University
• yingwu_at_ece.northwestern.edu

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A Class is a blueprint

• Class is the blueprint of a class of packages

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Class vs. Object

• A blueprint can be instantiated to different
  packages or objects
• Example
• Class COffer
• Title
• Base_salary
• Bonus rate
• Benefit
• Allowance
• Working_load
• Different people may get different offers, so we
  have
• COffer John_offer, Mike_offer, Joe_offer
• Then, you may need to specify the values in the
  offer for each people

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Question?

• Can we have a way to specify a default one
  instead of setting them one by one?
• Is there is way to initialize a package in our
  need when I create it?
• E.g.,
• COffer John_Offer(manager, 80K)?
• Instead of
• Coffer John_Offer
• John_Offer.SetTitle(Manager)
• John_Offer.SetBaseSalary(80K)

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What to learn today?

• Constructor
• Default constructor
• Destructor
• When constructor and destructor are called
• Copy an object
• How to pass an object to a function

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Initializing Class Objects Constructors

• Constructors
• Initialize class members
• Same name as the class
• No return type
• Member variables can be initialized by the
  constructor or set afterwards
• Passing arguments to a constructor
• When an object of a class is declared,
  initializers can be provided
• Format of declaration with initializers
• Class-type ObjectName( value1,value2,)
• Default arguments may also be specified in the
  constructor prototype

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Example

• CVariableCVariable(const char name, const
  double v)
• m_dValue v
• m_sName new charstrlen(name)1
• strcpy(m_sName, name)

1.0
Void main() CVariable a(var_a,
1.0) CVariable b(v, 3.3)
3.3
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Default Constructor

• Pay more attention!
• Wed better define a default constructor for a
  class
• i.e., a constructor CVariable() exists
• So that we can use something like
• CVariable var
• On the other hand, if we only have a constructor
  CVariable(const charname, double v)
• We can only instantiate an object by using
• CVariable var(a,2.3)
• If you do not use a default constructor, the
  compiler will create one for you, but it will not
  guarantee it is what you want!

!! Can you guess what the compiler will do for
you?
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A safe way!

• What the compiler will do for you

• It is safer to do it yourself by have a default
  constructor

• CVariableCVariable()
• m_dValue 0.0
• m_sName NULL

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Another Way member initializer

• CVariableCVariable()
• m_dValue(0.0),
• m_sName(NULL)
• // empty

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You might wonder

• In our previous lectures, we learnt DMA
• If you need memory, you new some
• If you dont want them anymore, you should delete
  them, and the O/S will recycle them
• We know a rule
• Always pair new and delete
• Otherwise, those allocated memory will never be
  able to be used by other programs until your
  program ends
• Then your program might have eaten all the
  memories!
• Then, lets see

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Memory Leak
I have no idea
I have no idea

• void myFunc()
• CVariable tmp(I have no idea!, 1.0)
• void main()
• for(int k0 klt1000k)
• for(int j0jlt1000j)
• myFunc()

What will happen?
At each myFunc() call, you will waste 16Bytes.
Then before your program ends, you will eat 16M
in total!!!
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How can we solve it?

• Can we have an automatic mechanism
• When an object is no longer needed, if it has
  some memory allocated by me, I should recycle
  them.
• How to do it automatically?

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Destructors

• Destructors
• Are member function of class
• Perform termination housekeeping before the
  system reclaims the objects memory
• Complement of the constructor
• Name is tilde () followed by the class name
• Recall that the constructors name is the class
  name
• Receives no parameters, returns no value
• One destructor per class
• No overloading allowed

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Housekeeping!

• CVariableCVariable()
• if(m_sName!NULL)
• delete m_sName

Note (1) Each class has only ONE destructor
(2) Generally, we dont call the destructor
explicitly. (3) It will be called
ATUOMATICALLY.
Question when constructor and destructors are
called?
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When Constructors and Destructors Are Called

• Constructors and destructors called automatically
• Order depends on scope of objects
• Global scope objects
• Constructors called before any other function
  (including main)
• Destructors called when main terminates (or exit
  function called)
• Destructors not called if program terminates with
  abort
• Automatic local objects
• Constructors called when objects are defined
• Destructors called when objects leave scope
• i.e., when the block in which they are defined is
  exited
• Destructors not called if the program ends with
  exit or abort

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• Class CTest
• CTest()
• CTest()
• CTestCTest() cout ltlt Constructor
  called!\n)
• CTestCTest() cout ltlt Destructor called!\n)
  
• CTest tg
• void myFunc()
• CTest tf
• void main()
• CTest tm

constructor called! (tg) Constructor
called! (tm) Constructor called! (tf) Destructor
called! (tf) Constructor called! (tm_2) Destructo
r called! (tm_2) Destructor called! (tm) Destructo
r called! (tg)
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Can I have sth like this?

• void main()
• CVariable a(var_a, 1.5)
• CVariable b
• b a

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Memberwise Copy

• Assigning objects
• An object can be assigned to another object of
  the same type using the assignment operator ()
• Member by member copy
• Objects may be
• Passed as function arguments
• Returned from functions (call-by-value default)

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Lets go deep!
CVariable a(var_a, 1.5)
CVariable b
b a
1.5
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A problem!

• void main()
• CVariable a(var_a, 1.5)
• CVariable b
• b a
• a.SetName(change)
• cout ltlt b.Name() ltlt endl

?
How to solve this problem? Keep this question
and well see it next week!
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How to pass an object to functions

• Call-by-value
• involves copying objects (memberwise)
• is good for security (safe), but bad for
  performance
• Call-by-reference
• passes the reference or the pointer
• does not copy anything
• is good for performance, but bad for security,
  because the function can change the object
• Call-by-const-reference
• passes a const reference or a const pointer
• is good for both!
• WHY? The function will not be able to change the
  object, because it is a const object.

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A BIG Problem!

• void myFunc(CVariable t)
• cout ltlt something happen?\n)
• void main()
• CVariable a(var_a, 1.5)
• myFunc(a)
• cout ltlt a.Name() ltlt endl

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Another BIG Problem!

• CVariable Create()
• CVariable t(var_a, 0.0)
• return t
• void main()
• CVariable a
• a Create()
• cout ltlt a.Name() ltlt endl

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Save them for next week!

• Yes, we have big problems!
• Lets solve them next week.

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Questions for today

• Finding errors
• void Time( int )
• Definition of class Time
• class Time
• public
• // function prototypes
• private
• int hour 0
• int mintue 0
• int second 0