Classes and Objects:

1
Classes and Objects
Object and its encapsulation
Chapter 8, Slide 1
2
Inheritance is facilitated by subclasses (derived
classes) Polymorphism is characterized by
overloading of names of methods and
operators.In C data containers can be either
in static memory, on the heap, or on the stack.
C objects can span all three
Consider chapter8_1 program
Chapter 8, Slide 2
3
Create some global objects Sample sample
("hey")Sample sample1("good bye")
Chapter 8, Slide 3
4
Analyze program chapter8_2 program
Destructor is missing, memory is leaking!
Chapter 8, Slide 4
5
Constructors and destructors are not inherited
from the base class. The base constructor may be
invoked implicitly or explicitly. The base
destructor will be invoked implicitly.
Analyze chapter8_3 program
The program will display hey,Joe and then (the
main purpose of this illustration) CC() invoked
followed by C() invoked. Notice the reverse
order of destructors called. All C
allocators/deallocator can be used in C. On top
of it, there are two new operators -- allocator
new (and new) and deallocator delete (and
delete).
Chapter 8, Slide 5
6
When new fails, it either uses new_handler or
throws an exception of type bad_alloc. The
operator new is involved in object
creation class X public X(..) .... //
constructor .. //end class X .. X
ptr .. ptr new X(..)
Chapter 8, Slide 6
7
Placement syntax of the operator newinclude
ltnewgt .. class X public void operator
new(size_t s,int a1,int a2) ... .. //end
class X int main () X ptr new(1,2) X
.. return 0
Chapter 8, Slide 7
8
Even though destructors cannot be called
explicitly, a placement new should have a
corresponding placement delete (used only when
construction fails midway)include
ltnewgt .. class X public void operator
new(size_t s,int a1,int a2) ... void
operator delete(void p,int a1,int a2) ...
.. //end class X
Chapter 8, Slide 8
9
Arrays of objects must be created by new using
default constructors only!
Analyze chapter 8_4 program
They have to be deallocated by delete
Analyze chapter8_5 program

• A few guidelines to make sure that the process
  memory manager does not get corrupted
• Never pass a pointer to free() that has not
  been returned previously by malloc(), calloc(),
  or realloc().
• Deallocate segments allocated by malloc(),
  calloc(), and realloc() using exclusively
  free().

Chapter 8, Slide 9
10

• Never pass a pointer to delete that has not
  been returned previously by new.
• Deallocate segments allocated by new using
  exclusively delete.
• Never pass a pointer to delete that has not
  been returned previously by new.
• Deallocate segments allocated by new using
  exclusively delete.
• If your program uses placement-new, it should
  have a corresponding placement-delete, even
  though it will be only called implicitly by the
  compiler when an exception is thrown during the
  construction of an object.

Chapter 8, Slide 10
11
Copy constructor
Analyze chapter8_6 program
The will work fine displaying hey Joe on the
screen. The object c created in main() using
constructor C(char) will be copied to the
activation frame of doit() using the copy
constructor C(const C). When doit() terminates,
the object d --- as built in the activation frame
of doit() --- is destructed using C(). Without
the copy constructor (the memberwise copy
constructor would have been used), the program
would have crashed, as no deep copy of the
salutation would have been provided, yet C()
would still try to deallocate it.
Chapter 8, Slide 11
12
Assignment operator --- operator
This method specifies how to perform assignments
of type o1o2 between two objects (which is in a
sense again a form of copying, from o1 to o2). In
the absence of an explicit assignment, the
memberwise assignment is performed. The same
problems may ensue as discussed for the missing
copy constructor. However, a missing assignment
is even more dangerous as it can lead to memory
leaks class C ... //end class C int
doit() C c1("hey Joe"), c2 c1c2 return
0 //end doit
Chapter 8, Slide 12
13
c1 is created and c1.salutation points to a
dynamically created string "hey Joe". c2 is
created using the default constructor and hence
the value of c2.salutation is set to NULL. When
c1c2 assignment is performed, due to the absence
of an explicit assignment method in the
definition of the class C, the memberwise copy is
performed and thus c1.salutation is set to NULL.
When c1 and c2 are destructed, none of them is
linked to the string "hey Joe" and so the
string is never deallocated and leaks. The
difference between copy and assignment results
from the fact that the copy is concerned with
forming the raw memory into an object while the
assignment must deal with a well-constructed
object, thus in essence the assignment must
de-construct the object before it can do the
copying
Analyze chapter8_7 program
Chapter 8, Slide 13
14
End of slides for chapter 8
Chapter 8, Slide 14