CMPUT 201: Lab 6

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CMPUT 201 Lab 6
Constructors Destructors
October 24th, 2002
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Constructors

• When using C-style structs, you must remember
  to initialize every field in a dedicated function
  after declaration
• This is error-prone you might forget to do
  this
• When using C classes, you can write a special
  function called a Constructor that gets called
  every time an object of that class is created
• In the Constructor, you can initialize the
  objects variables in the appropriate manner so
  declaration and initialization are done in one
  line

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Declaration of Constructors

• Constructors always have the same name as the
  class and must not return anything
• You can have multiple constructors for a class,
  each of which initializes a new objects
  variables in a different way
• Constructors that take arguments are called
  Value Constructors
• A constructor that takes no arguments is called
  the Default Constructor (only one per class)

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Declaration of Constructors

• Consider the following class in hobbit.h
• class Hobbit public // Default
  constructor Hobbit() // Value
  constructors Hobbit(char hname)
  Hobbit(int hp, int st) Hobbit(char hname,
  int hp, int st)private char name16
  int health_points int stamina

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Implementation of Constructors

• Constructors are implemented in hobbit.cpp
• // Creates a new hobbit and initializes its
  nameHobbitHobbit(char hname) name new
  char strlen(hname) 1 strcpy(name,
  hname)// Creates a new hobbit and
  initializes its health and stamina// Uses an
  alternative mechanism for doing
  soHobbitHobbit(int hp, int st)
  health_points(hp), stamina(st)

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Constructor Invocation

• Constructors are invoked in three cases
• 1. When you declare an object
• // calls first value constructorHobbit
  hob1(frodo) // calls default
  constructorHobbit hobPtr new Hobbit
• When you create a new object and assign it to an
  existing variable
• Creature c1...// calls third value
  constructorc1 Hobbit(samwise,40,50)

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Constructor Invocation

• When you pass an object to a function by value
• promoteLevel(hob1)
• Passing an object by value to a function invokes
  a special constructor called the Copy Constructor
  (only one per class)
• The Copy Constructor takes one reference argument
  that is the same type as the class
• HobbitHobbit( const Hobbit source )
• name source.name
• health_points source.health_points
• stamina source.stamina

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Passing User-Defined Objects by Value

• When an argument is passed by value, a local
  variable of the given type is created in the
  invoked function
• The variable is initialized with the value of the
  corresponding argument in the function call
• // in main function
• Hobbit hob1(frodo)
• ...
• // frodo is passed by value
• promoteLevel( hob1 )

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Passing User-Defined Objects by Value

• // in another part of the code
• promoteLevel( Hobbit hobToPromote ) ...
• A new Hobbit object hobToPromote is created
• and initialized with the object hob1
• The argument binding in the above function is
  equivalent to
• Hobbit hobToPromote hob1
• The particular constructor called depends on what
  the local variable is initialized with

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The Assignment Operator

• Useful to overload the assignment operator
  when you wish to assign one instance of a class
  to another
• The assignment statement has the form hob3
  hob1
• Where hob3 and hob1 are existing objects of
  type Hobbit
• Initialization is not the same as assignment
  since a new object is being created in the first
  case and an existing redefined in the second case

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Destructors

• In the example class, there was a private
  instance variable of type char, which means the
  programmer cannot access it and do a delete
• The Destructor function handles this
• A Destructor is a special function that gets
  invokes when an object leaves scope
• Destructors have the same name as the class
  preceded by a tilda and must not return
  anything

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Declaration Implementation of Destructor

• In hobbit.h
• Class Hobbit public // Default
  constructor Hobbit() ...
• // Destructor Hobbit() ...

• In hobbit.cpp
• HobbitHobbit() // return dynamic memory
  associated // with the name field
  delete(name) health_points -1 stamina
  -1

The Destructor is called at the end of a
functionwith a local variable of type Hobbit
or when the programmer issues a delete hob1
command