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Chapter 15 Pointers, Dynamic Data, and Reference Types

Chapter 15 Pointers, Dynamic Data, and Reference Types. Dale/Weems/Headington. Chapter 15 Topics. Using the Address-Of Operator & Declaring and Using Pointer Variables Using the Indirection (Dereference) Operator * The NULL Pointer Using C++ Operators new and delete

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Chapter 15 Pointers, Dynamic Data, and Reference Types

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  1. Chapter 15Pointers, Dynamic Data, and Reference Types Dale/Weems/Headington

  2. Chapter 15 Topics • Using the Address-Of Operator & • Declaring and Using Pointer Variables • Using the Indirection (Dereference) Operator * • The NULL Pointer • Using C++ Operators new and delete • Meaning of an Inaccessible Object • Meaning of a Dangling Pointer • Use of a Class Destructor • Shallow Copy vs. Deep Copy of Class Objects • Use of a Copy Constructor

  3. 6000 ‘H’ ‘e’ ‘l’ ‘l’ ‘o’ ‘\0’ str [0] [1] [2] [3] [4] [5] [6] [7] Recall that . . . char str [ 8 ]; stris the base address of the array. We say str is a pointer because its value is an address. It is a pointer constant because the value of str itself cannot be changed by assignment. It “points” to the memory location of a char.

  4. Addresses in Memory • when a variable is declared, enough memory to hold a value of that type is allocated for it at an unused memory location. This is the address of the variable int x; float number; char ch; 2000 2002 2006 x number ch

  5. Obtaining Memory Addresses • the address of a non-array variable can be obtained by using the address-of operator & int x; float number; char ch; cout << “Address of x is “ << &x << endl; cout << “Address of number is “ << &number << endl; cout << “Address of ch is “ << &ch << endl;

  6. What is a pointer variable? • A pointer variable is a variable whose value is the address of a location in memory. • to declare a pointer variable, you must specify the type of value that the pointer will point to,for example, int* ptr; // ptr will hold the address of an int char* q;// q will hold the address of a char

  7. Using a Pointer Variable 2000 12 x 3000 2000 ptr int x; x = 12; int* ptr; ptr = &x; NOTE: Because ptr holds the address of x, we say that ptr “points to” x

  8. Unary operator * is the indirection (deference) operator 2000 12 x 3000 2000 ptr int x; x = 12; int* ptr; ptr = &x; cout << *ptr; NOTE: The value pointed to by ptr is denoted by *ptr

  9. Using the Dereference Operator 2000 12 5 x 3000 2000 ptr int x; x = 12; int* ptr; ptr = &x; *ptr = 5; // changes the value // at address ptr to 5

  10. Another Example 4000 A Z ch 5000 6000 4000 4000 q p char ch; ch = ‘A’; char* q; q = &ch; *q = ‘Z’; char* p; p = q; // the rhs has value 4000 // now p and q both point to ch

  11. Using a Pointer to Access the Elements of a String 3000 char msg[ ] = “Hello”; ‘M’ ‘a’ 3001 char* ptr; 3000 ptr = msg; // recall that msg == &msg[ 0 ] *ptr = ‘M’ ; ptr++; // increments the address in ptr *ptr = ‘a’; ‘H’ ‘e’ ‘l’ ‘l’ ‘o’ ‘\0’ ptr

  12. int StringLength ( /* in */ const char str[ ] ) // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - // Precondition: str is a null-terminated string // Postcondition: FCTVAL == length of str (not counting ‘\0’) // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - { char* p ; int count = 0; p = str; while ( *p != ‘\0’ ) { count++ ; p++ ; // increments the address p by sizeof char } return count; } 12

  13. floating address float double long double pointer reference C++ Data Types simple structured integral enum array struct union class char short int long bool

  14. Some C++ Pointer Operations Precedence Higher->Select member of class pointed to Unary: ++ -- ! * new delete Increment, Decrement, NOT, Dereference, Allocate, Deallocate + - Add Subtract < <= > >= Relational operators == != Tests for equality, inequality Lower = Assignment 14

  15. Operator new Syntax new DataType new DataType [IntExpression] If memory is available, in an area called the heap (or free store) new allocates the requested object or array, and returns a pointerto (address of ) the memory allocated. Otherwise, program terminates with error message. The dynamically allocated object exists until the delete operator destroys it. 15

  16. The NULL Pointer There is a pointer constant 0 called the “null pointer” denoted by NULL in header file cstddef. But NULL is not memory address 0. NOTE: It is an error to dereference a pointer whose value is NULL. Such an error may cause your program to crash, or behave erratically. It is the programmer’s job to check for this. while (ptr != NULL) { . . .// ok to use *ptr here }

  17. 3 Kinds of Program Data • STATIC DATA: memory allocation exists throughout execution of program static long currentSeed; • AUTOMATIC DATA: automatically created at function entry, resides in activation frame of the function, and is destroyed when returning from function • DYNAMIC DATA: explicitly allocated and deallocated during program execution by C++ instructions written by programmer using operators new and delete

  18. Allocation of Memory STATIC ALLOCATION Static allocation is the allocation of memory space at compile time. DYNAMIC ALLOCATION Dynamic allocation is the allocation of memory space at run time by using operator new.

  19. Dynamically Allocated Data 2000 ptr char* ptr; ptr = new char; *ptr = ‘B’; cout << *ptr;

  20. Dynamically Allocated Data 2000 ptr char* ptr; ptr = new char; *ptr = ‘B’; cout << *ptr; NOTE: Dynamic data has no variable name

  21. Dynamically Allocated Data 2000 ptr ‘B’ char* ptr; ptr = new char; *ptr = ‘B’; cout << *ptr; NOTE: Dynamic data has no variable name

  22. Dynamically Allocated Data 2000 ptr NOTE: delete deallocates the memory pointed to by ptr char* ptr; ptr = new char; *ptr = ‘B’; cout << *ptr; delete ptr; ?

  23. Referencing Arrays with pointers • An array name is a constant pointer int b[5]; int *bPtr; bPtr = b; or bPtr = &b[0] b += 2; this is an invalid statement (constant ptr) bPtr += 2; valid statement, bPtr now points to b[2] *(bPtr +2) references the value at b[2] *(b + 2) also references the value at b[2]

  24. // Fig. 5.26: fig05_26.cpp // Multipurpose sorting program using function pointers void bubble( int [], const int, bool (*)( int, int ) ); bool ascending( int, int ); bool descending( int, int ); . . . if ( order == 1 ) { bubble( a, arraySize, ascending ); cout << "\nData items in ascending order\n"; } else { bubble( a, arraySize, descending ); cout << "\nData items in descending order\n"; } . . . void bubble( int work[], const int size, bool (*compare)( int, int ) ) { void swap( int * const, int * const ); // prototype for ( int pass = 1; pass < size; pass++ ) for ( int count = 0; count < size - 1; count++ ) if ( (*compare)( work[ count ], work[ count + 1 ] ) ) swap( &work[ count ], &work[ count + 1 ] ); } Fig 5.26 Function prototype - parameter list: => array of int, const int, pointer to a function which accepts two integers and returns a bool (true or false) Function call for bubble using function => ascending Function call for bubble using function => descending bool ascending( int a, int b ) { return b < a; // swap if b is less than a } bool descending( int a, int b ) { return b > a; // swap if b is greater than a } Function call for ascending => Function call for descending =>

  25. Using Operator delete Operator delete returns to the free store memory which was previously allocated at run-time by operator new. The object or array currently pointed to by the pointer is deallocated, and the pointer is considered unassigned. 25

  26. Dynamic Array Allocation char *ptr;// ptr is a pointer variable that // can hold the address of a char ptr = new char[ 5 ]; // dynamically, during run time, allocates // memory for a 5 character array // and stores the base address into ptr 6000 6000 ptr

  27. Dynamic Array Allocation char *ptr ; ptr = new char[ 5 ]; strcpy( ptr, “Bye” ); ptr[ 1 ] = ‘u’; // a pointer can be subscripted cout << ptr[ 2] ; 6000 ‘u’ 6000 ‘B’ ‘y’ ‘e’ ‘\0’ ptr

  28. Operator delete Syntax delete Pointer delete [ ] Pointer If the value of the pointer is 0 there is no effect. Otherwise, the object or array currently pointed to by Pointer is deallocated, and the value of Pointer is undefined. The memory is returned to the free store. Square brackets are used with delete to deallocate a dynamically allocated array. 28

  29. Dynamic Array Deallocation char *ptr ; ptr = new char[ 5 ]; strcpy( ptr, “Bye” ); ptr[ 1 ] = ‘u’; delete ptr;// deallocates array pointed to by ptr // ptr itself is not deallocated // the value of ptr is undefined. ? ptr

  30. What happens here? 3 ptr int* ptr = new int; *ptr = 3; ptr = new int; // changes value of ptr *ptr = 4; 3 ptr 4

  31. Inaccessible Object An inaccessible object is an unnamed object that was created by operator new and which a programmer has left without a pointer to it. int* ptr = new int; *ptr = 8; int* ptr2 = new int; *ptr2 = -5; How else can an object become inaccessible? 8 ptr -5 ptr2

  32. Making an Object Inaccessible 8 ptr -5 ptr2 int* ptr = new int; *ptr = 8; int* ptr2 = new int; *ptr2 = -5; ptr = ptr2;// here the 8 becomes inaccessible 8 ptr -5 ptr2

  33. Memory Leak A memory leak is the loss of available memory space that occurs when dynamic data is allocated but never deallocated.

  34. A Dangling Pointer • is a pointer that points to dynamic memory that has been deallocated int* ptr = new int; *ptr = 8; int* ptr2 = new int; *ptr2 = -5; ptr = ptr2; 8 ptr -5 ptr2 FOR EXAMPLE,

  35. Leaving a Dangling Pointer 8 ptr -5 ptr2 int* ptr = new int; *ptr = 8; int* ptr2 = new int; *ptr2 = -5; ptr = ptr2; delete ptr2; // ptr is left dangling ptr2 = NULL; 8 ptr NULL ptr2

  36. // SPECIFICATION FILE (dynarray.h) // Safe integer array class allows run-time specification // of size, prevents indexes from going out of bounds, // allows aggregate array copying and initialization. class DynArray { public: DynArray( /* in */ int arrSize ); // Constructor. // PRE: arrSize is assigned // POST: IF arrSize >= 1 && enough memory THEN // Array of size arrSize is created with // all elements == 0 ELSE error message. DynArray( const DynArray& otherArr ); // Copy constructor. // POST: this DynArray is a deep copy of otherArr // Is implicitly called for initialization.

  37. // SPECIFICATION FILE continued (dynarray.h) ~DynArray( ); // Destructor. // POST: Memory for dynamic array deallocated. int ValueAt ( /* in */ int i ) const; // PRE: i is assigned. // POST: IF 0 <= i < size of this array THEN // FCTVAL == value of array element at index i // ELSE error message. void Store ( /* in */ int val, /* in */ int i ) // PRE: val and i are assigned // POST: IF 0 <= i < size of this array THEN // val is stored in array element i // ELSE error message. 37

  38. // SPECIFICATION FILE continued (dynarray.h) void CopyFrom ( /* in */ DynArray otherArr); // POST: IF enough memory THEN // new array created (as deep copy) // with size and contents // same as otherArr // ELSE error message. private: int* arr ; int size ; }; 38

  39. class DynArray Free store 6000 DynArray 80 40 90 ? ? Private data: size5 arr6000 ~DynArray DynArray ValueAt Store CopyFrom

  40. DynArray beta(5); //constructor beta Free store 2000 DynArray ? ? ? ? ? Private data: size5 arr 2000 ~DynArray DynArray ValueAt Store CopyFrom

  41. DynArray::DynArray( /* in */ int arrSize ) // Constructor. // PRE: arrSize is assigned // POST: IF arrSize >= 1 && enough memory THEN // Array of size arrSize is created with // all elements == 0 ELSE error message. { int i; if ( arrSize < 1 ) { cerr << “DynArray constructor - invalid size: “ << arrSize << endl; exit(1); } arr = new int[arrSize] ; // allocate memory size = arrSize; for (i = 0; i < size; i++) arr[i] = 0; } 41

  42. beta.Store(75, 2); beta Free store 2000 DynArray ? ? 75 ? ? Private data: size 5 arr 2000 ~DynArray DynArray ValueAt Store CopyFrom

  43. void DynArray::Store ( /* in */ int val, /* in */ int i ) // PRE: val and i are assigned // POST: IF 0 <= i < size of this array THEN // arr[i] == val // ELSE error message. { if ( i < 0 || i >= size ) { cerr << “Store - invalid index : “ << i << endl; exit(1) ; } arr[i] = val ; } 43

  44. DynArray gamma(4);//constructor gamma beta 3000 2000 DynArray DynArray ? ? ? ? ? ? 75 ? ? Private: size 4 arr 3000 Private: size 5 arr 2000 ~DynArray ~DynArray DynArray DynArray ValueAt ValueAt Store Store CopyFrom CopyFrom

  45. gamma.Store(-8,2); gamma beta 3000 2000 DynArray DynArray ? ? -8 ? ? ? 75 ? ? Private: size 4 arr 3000 Private: size 5 arr 2000 ~DynArray ~DynArray DynArray DynArray ValueAt ValueAt Store Store CopyFrom CopyFrom

  46. int DynArray::ValueAt ( /* in */ int i ) const // PRE: i is assigned. // POST: IF 0 <= i < size THEN // FCTVAL == arr[i] // ELSE halt with error message. { if ( i < 0 || i >= size ) { cerr << “ValueAt - invalid index : “ << i << endl; exit(1) ; } return arr[i]; } 46

  47. Why is a destructor needed? When a DynArray class variable goes out of scope, the memory space for data members size and pointer arr is deallocated. But the dynamic array that arr points to is not automatically deallocated. A class destructor is used to deallocate the dynamic memory pointed to by the data member.

  48. class DynArray Destructor DynArray::~DynArray( ); // Destructor. // POST: Memory for dynamic array deallocated. { delete [ ] arr ; } 48

  49. What happens . . . • When a function is called that uses pass by value for a class object of DynArray type? 2000 DynArray ? ? 75 ? ? Private: size 5 arr 2000 ~DynArray DynArray ValueAt Store CopyFrom

  50. Passing a Class Object by Value // FUNCTION CODE void SomeFunc( DynArray someArr ) // Uses pass by value { . . . . } 50

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