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Classes and Objects: Chapter 8, Slide 1 Object and its encapsulation.

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Presentation on theme: "Classes and Objects: Chapter 8, Slide 1 Object and its encapsulation."— Presentation transcript:

1 Classes and Objects: Chapter 8, Slide 1 Object and its encapsulation

2 Chapter 8, Slide 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:chapter8_1

3 Chapter 8, Slide 3 Create some global objects: Sample sample ("hey"); Sample sample1("good bye");

4 Chapter 8, Slide 4 Analyze program chapter8_2 program: chapter8_2 Destructor is missing, memory is leaking!

5 Chapter 8, Slide 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:chapter8_3 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[]).

6 Chapter 8, Slide 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(..);

7 Chapter 8, Slide 7 Placement syntax of the operator new : #include.. 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; }

8 Chapter 8, Slide 8 Even though destructors cannot be called explicitly, a placement new should have a corresponding “placement” delete (used only when construction fails midway): #include.. 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

9 Chapter 8, Slide 9 Arrays of objects must be created by new[] using default constructors only! Analyze chapter8_5 program: They have to be deallocated by delete[] Analyze chapter 8_4 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().

10 Chapter 8, Slide 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.

11 Chapter 8, Slide 11 Copy constructor: Analyze chapter8_6 program:chapter8_6 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.

12 Chapter 8, Slide 12 Assignment operator --- operator= This method specifies how to perform assignments of type o1=o2 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; c1=c2; return 0; } //end doit

13 Chapter 8, Slide 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 c1=c2 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:chapter8_7

14 Chapter 8, Slide 14 End of slides for chapter 8

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