Pointers, Dynamic Data, and Reference Types in C++

1. Pointers, Dynamic Data, and Reference Types • Review on Pointers • Reference Variables • Dynamic Memory Allocation • The new operator • The delete operator • Dynamic Memory Allocation for Arrays

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

3. Addresses in Memory • In general, memory is byte addressable • At declaration, memory is allocated for a variable • How much? Depends on the data type • Address of first byte is the address of the variable int i; float f; char i; 200 201 202 203 … 500 … 700 701 702 703 • For an array, address of the first element is its address • Address-of-operator: & (for non-array variable) • &i gives address of i (= 200)

4. c: p: &c ‘a’ char c = ‘a’; char *p = &c; Pointers • All addresses are integer values (but not of type int) • Pointer is a datatype whose value refers directly to (“points to”) another value stored at some memory location. • Declaration of pointer variable needs a type (why?) • For a type T, T* is the type “pointer to T” int *pi; // pointer to integer char *pc; // pointer to character float *pf; // pointer to floating point void *pv; // no type

5. 5000 1000 i pi 20 1000 30 Operators & - Address of operator * - Dereferencing operator • & can be applied onto any variable • & on a pointer variable? (ex: &p where p is int *p;) • * can only be applied on pointer variable int i = 20; int *pi = &i; // pi points to i *pi = 30; // changes the value of i to 30 int i = 20, j = 10; int *pi = &i; // pi points to i int **ppi = π // ppi points to pi **ppi = 30; // changes the value of i to 30

6. Arrays • For a type T, T[size] is the type “array of size elements of type T” • Elements are indexed from 0 … size-1 • [] operator references each element • Array name points to first element (i.e., 0th) (base address) • and it is a constant pointer int arr[4] = {10, 20, 30, 40}, i = 50; int *pi = &i; // pi points to i arr[0] = 11; // changes the first element // arr and &arr[0] gives the same value *arr = 12; // arr[0] changes to 12 arr = pi; // error: arr is a constant pointer pi = arr; // valid 100 104 108 112 arr : arr[2] arr[0] arr[1] arr[3] pi: 100

7. Operations on pointers • Pointer assignment • Pointer arithmetic • Result of ++, --, +, - depends on pointer type • value of P+1 = value of P + sizeof (T) • Result of == is implementation-defined int v[5] = {1, 2, 3, 4, 0}; for (int i=0; v[i] != 0; i++) cout << v[i] << endl; int v[5] = {1, 2, 3, 4, 0}; int *p = v; for (; *p != 0; p++) cout << *p << endl; v[i] == *(v+i) int v[4], u[4]; int i1 = &v[2] - &v[0]; // i1 = 2 int i2 = &v[2] - &u[0]; // undefined

8. Pointers and constants • const int *p;(same as int const *p;) • p is a pointer to integer which is a constant • int *const p; • p is a constant pointer to integer • Example: arr in int arr[10]; • Right-to-left reading of declarations int i = 10, j =20; const int *pi = &i; *pi = 30; // error: piis pointer to const pi = &j; // valid: pointer is not const int i = 10, j =20; int *const pi = &i; *pi = 30; // valid: piis pointer to variable pi = &j; // error: constant pointer

9. 1000 pi: 1000 10 i: 11 r: References • Alternate names to variables • For type T, T&defines “reference to T” int i = 10; int &r = i; // r is reference to i r++; // same as i++ int *pi = &r; // same as &i • We can not have pointer to a reference • Hence, can not have array of references

10. Usage of references • Can be specified as function arguments • Call-by-reference void increment ( int& rr ) { rr++; } int x = 1; increment (x); // x = 2 int &rr = x; rr++; • Can be used to define functions that can be used on both LHS and RHS of an assignment int arr[5] = {1, 2, 3, 4, 5}; // say, global array int & foo( int x ) { return arr[x]; } // return xth element foo(2)++; // increments arr[2]

11. More power over the memory

12. Dynamic Memory Allocation In C and C++, three types of memory are used by programs: • Static memory - where global and static variables live • Heap memory -dynamically allocated at execution time -"small" amount of “self-managed" memory accessed using pointers • Stack memory - used by automatic variables • Call stack or Function stack

13. 3 Kinds of Program Data • STATIC DATA: Allocated at compiler time • DYNAMIC DATA: explicitly allocated and deallocated during program execution by C++ instructions written by programmer using operators new and delete • AUTOMATIC DATA: automatically created at function entry, resides in activation frame of the function, and is destroyed when returning from function

14. Dynamic Memory Allocation Diagram

15. Dynamic Memory Allocation • In C, functions such as malloc() are used to dynamically allocate memory from the Heap. • void * malloc (size_t size); (explicit type cast required) • size bytes are allocated • In C++, this is accomplished using the new and delete operators • new is used to allocate memory during execution time • returns a pointer to the address where the object is to be stored • always returns a pointer to the type that follows the new

16. Operator new Syntax new DataType new DataType [IntExpression] • If memory is available, new allocates the memory for requested object (or array) in heap, and returns a pointerto the memory allocated i.e., address of object (or first element) • Otherwise, program terminates with error message • The dynamically allocated object exists until the delete operator destroys it 16

17. 1000 ptr: 1000 ‘B’ Operator new char *ptr; ptr = new char; // allocate memory for one character *ptr = ‘B’; cout << *ptr; pi: int *ptr; ptr = new int [4]; // memory for fourintegers ptr[0] = 10; ptr[1] = 20; … 100 100 104 108 112 • Dynamic memory has no variable names

18. The NULLPointer • 0 can be used as constant of integral, floating-point, or pointer type (determined by context) • No object is allocated at address 0 • Hence, 0 is pointer literal • Can not dereference a NULL pointer GOOD BAD int *ptr = NULL; … if ( ptr != NULL ) { use ptr } int *ptr; … use ptr

19. Operator delete Syntax int *ptr = new int; int *arr = new int [4]; … delete ptr; delete [] arr; arr = NULL; delete Pointer delete [ ] Pointer • 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 (heap). • Good idea to set the pointer to the released memory to NULL • Square brackets are used with delete to deallocate a dynamically allocated array. 19

20. Take Home Message • Be aware where a pointer points to, and what is the size of that space. • Have the same perspective in mind when you use reference variables. • Always check if a pointer points to NULL before accessing it (unless you are sure of it)