159'234 Lecture 10

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159.234 LECTURE 11
Constructors and destructors Copy
constructor Textbook p. 156-166, 183
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Constructor

• PointPoint() x(0), y(0)
• PointPoint(int i,int j)x(i), y(j)
• A default constructor may be created as a
  function having the same Class name that
• does not have any argument, or
• comes with arguments with default values set
• Const and Reference data members can only be
  initialized using an initializer list
• Constructor initializers are in fact the
  preferred way of setting values.

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Constructors with Default Arguments
It is possible to construct constructors with
default arguments. Example Default
Constructor PointPoint() x(0),
y(0) Default Argument Constructor PointPoin
t(int i0,int j0)x(i), y(j) However, the
default argument constructor becomes a default
constructor when called without any arguments.
When both forms appear in a class, ambiguity
arises when the following statement is
issued Point p
Initializer list is not always declared at the
class interface (See ch_stack.cpp for an example)
No arguments
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Calling the Constructor
Explicit Call
Point p Point(5, 10)
Creates a Point object p and passes the values 5
and 10 to it
Implicit Call
Point p(5, 10)
Short-hand method looks better and easier to
implement
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What is wrong in the following code? include
ltiostreamgt using namespace std int main()
int p, r p new int p10 r
new int r25 r p cout
ltlt(p)ltlt" "ltlt(r) delete p cout
ltlt (r)ltltendl delete r // will cause
double delete error return 0
Memory Leak.cpp
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Destructor
Each class has exactly one destructor When an
object is destroyed - the objects destructor is
called. Destructors are used when the object
contains pointers and new has been used to
allocate memory to them. If we dont free that
memory before the object disappears, then the
memory will never be freed - a memory leak.
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Destructor
class str public str(void) str()
char s strstr(void) s new
char128 strstr() delete s
The destructor function has the same name as the
class with a at the front.
Destructors do not have return values, and cannot
have arguments not even void.
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Destructor
Destructors are used to release any resources
allocated by the object. The most common
example is when the constructor uses new, and the
destructor uses delete. Destructors are a
"prepare to die" member function. They are
often abbreviated "dtor". Constructors are
ctor.
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Destructor

• Destructor
• called by the system for you when an object is
  destroyable (e.g. about to go out of scope)
• has the same name as the class
• with a at the front
• does not have return values
• cannot have arguments.

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When are constructors/destructors called?
Constructors and destructors are called
automatically. The order in which they are
called depends on the order in which execution
enters and leaves the scope in which objects are
instantiated and the type of storage for
objects. General rule destructor calls are
made in the reverse order of the constructor
calls.
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class C public C(int) //constructor
C() //destructor private int data
CC(int value) data value
coutltlt\nCtor called "ltlt data CC()
coutltlt\nDtor called "ltlt data
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void createFc() C one(1) //global
object int main() cout ltlt"Main starts
here."ltltendl static C two(2) //local
static object coutltlt"After two(local static)in
main."ltltendl createFc() //function
call void createFc() cout ltltendlltlt" FC
STARTS HERE. "ltltendl C ten(77) //local
object coutltlt "LAST IN FC. "ltltendlltltendl
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Output Ctor called 1 Main starts here. Ctor
called 2 After two (local static) in main. FC
STARTS HERE. Ctor called 77 LAST IN FC. Dtor
called 77 Dtor called 2 Dtor called 1
global
Local static
Local
Constructor and Destructor Call.cpp
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When are constructors/destructors called?
Destructor called
For stack objects defined
Constructors called
Before any other function (including main)
When main terminates, or exit is called
In global scope
When the object enters scope.
When the object leaves scope
Local objects
Static local objects
Once, when the object enters scope the first time.
When main terminates, or exit is called
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Problems when passing objects
Objects can be passed as function arguments.
class Str public Str() s new
char128 assert(s!0) // Str()
delete s void print() //... private
char s void display( Str s)
Passing Object Using Call by Value.cpp
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Problems when passing objects
int main() Str a a.print() //... if
(value gt 100) display(a) a.print() void
display( Str s)s.print()
Assume that the value stored in a was
aaaaaaaaaa Output Data is aaaaaaaaaa Data
is aaaaaaaaaa Data is ?A ?A aa
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• Problem
• The destructor is called when display(a)finishes
  
• It damages the original object a.
• How to solve it?
• Use call by reference display(a) so no
  copying is done
• OR
• 2. Write a copy constructor for the class which
  does a proper (deep) copy

Passing object using Call by Reference.cpp
Copy Constructor.cpp
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Copy Constructor
If we pass an object as an argument to a
function Str p display(p) The object
is copied to the called routine. The copy
constructor of the class is called to perform the
copy. When the class does not provide its copy
constructor the copy constructor provided by the
system is used, but this is not appropriate for
classes with pointer data members! System copy
constructor only does a shallow job!
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Copy Constructor
The copy constructor has the form StrStr(const
Str x) The original object is referred to by
x. Str has to copy it into the new
object. StrStr(const Str x) s new
char128 strcpy(s,x.s) This performs a
deep copy. A new string is created in the
object, and the original string copied into it.
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Copy Constructor
If we omit the copy constructor, or write our own
one like this strstr(const str x) s
x.s Then we get a shallow copy. s is set to
be the same pointer as x.s - there is only one
string, both objects point to it. When using a
shallow copy, it is important to know how many
objects refer to the same memory location. Only
when the last object is destroyed can that memory
be freed safely.
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Copy Constructor
The copy constructor is also called when a
function returns a class, and when an object is
initialised with another object. str s1
str s2(s1) str sp new str(s2)
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When is a copy constructor called?
SUMMARY
1. When a copy of an object is required, such as
in call-by-value. Str p display(p) 2.
When returning an object by value from a
function. Point findMiddlePt ( Point p1,Point
p2) Point temp //---do something
return temp 3. When initializing an
object to be a copy of another object of the same
class. Str x //...other statements
here Str y(x) //y declared and initialized
with x or Str y x //y declared and
initialized with x
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Constructor for Conversion
There is one final use for constructors to
convert between types. class point public
point(int i0, int j0) x(i),y(j)
... This point constructor allows us to
create objects like point a //x,y defaults so
a (0,0) point b(3) //y defaults so b
(3,0) point c(2,3) //no defaults so c (2,3)
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Constructor for Conversion
If in our code we write a 4 at first
sight, this appears to be wrong, a is a point and
4 is an integer. However the compiler allows it,
as the constructor can be used to do the
conversion. The constructor tells the compiler
how to create a point out of an integer.
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Constructor for Conversion
If we do not want the constructor to be used in
this fashion, then we use the keyword
explicit class point public explicit
point(int i0, int j0) x(i),y(j)
...
See pr_char.cpp
See ch_stack.cpp
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Sample Program ch_stack
See ch_stack.cpp
Str
h
T
s
i
Initially,
99
99
..
..
..
..
2
2
1
1
0
T
0
Top
s
Push

• Increment Top
• Put character into element
• of stack pointed to by Top

Top-1 Max_len100
Data members
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• include ltiostreamgt
• include ltcstringgt
• include ltassert.hgt
• using namespace std
• void display( Str s) // prototype
• class Str
• public
• Str(char c ' ')
• p new charMAXLEN
• assert(p!0)
• p0 c
• p1 '\0'
• Str() delete p
• Str( const Str src ) // copy
  constructor
• p new charMAXLEN // makes p point to
  another memory address
• assert( p ! 0 )

• int main()
• Str a('A') // a now in scope
• a.print()
• int value 101
• cerr ltlt "Debug line 1" ltlt endl
• if (value gt 100)
• display(a) // passes a copy of a
• cerr ltlt "Debug line 2" ltlt endl
• a.print()
• cerr ltlt "Debug line 3" ltlt endl
• return 0
• Example Output
• ContentsA
• Debug line 1
• ContentsA
• Debug line 2

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Summary

• A constructor constructs objects of its class
  type. This process may involve data members and
  allocating free store, using operator new.
• A default constructor is a constructor requiring
  no arguments (initialising arrays).
• A copy constructor is used to copy one value into
  another when
• a type variable is initialized by a type value
• a type value is passed as an argument to a
  function
• a type value is returned from a function.

A destructor releases any resources allocated
by the object, typically by using delete.
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Next Friend functions/classes Textbook
p.200-203