Copyright 2005, The Ohio State University 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

Copyright 2005, The Ohio State University 2 C++ Data Types structured array struct union class address pointer reference simple integral enum char short int long bool floating float double long double

Copyright 2005, The Ohio State University 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; char i; float f; … 500 … For an array, address of the first element is its address Address-of-operator: & (for non-array variable) –&i gives address of i (= 200)

Copyright 2005, The Ohio State University 4 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” &c p: ‘a’ c:c: char c = ‘a’; char *p = & c; int *pi; // pointer to integer char *pc; // pointer to character float *pf; // pointer to floating point void *pv; // no type

Copyright 2005, The Ohio State University 5 Operators & can be applied onto any variable –& on a pointer variable? (ex: &p where p is int *p;) * can only be applied on pointer variable & - Address of operator * - Dereferencing operator int i = 20; int *pi = &i; // pi points to i *pi = 30; // changes the value of i to i pi 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

Copyright 2005, The Ohio State University 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., 0 th ) (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 arr : arr[0] arr[1] arr[2] arr[3] pi: 100

Copyright 2005, The Ohio State University 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

Copyright 2005, The Ohio State University 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: pi is pointer to const pi = &j; // valid: pointer is not const int i = 10, j =20; int *const pi = &i; *pi = 30; // valid: pi is pointer to variable pi = &j; // error: constant pointer

Copyright 2005, The Ohio State University 9 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 i: pi: r: 11

Copyright 2005, The Ohio State University 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]

Copyright 2005, The Ohio State University 11 More power over the memory

Copyright 2005, The Ohio State University 12 Dynamic Memory Allocation 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 In C and C++, three types of memory are used by programs:

Copyright 2005, The Ohio State University 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

Copyright 2005, The Ohio State University 14 Dynamic Memory Allocation Diagram

Copyright 2005, The Ohio State University 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

Copyright 2005, The Ohio State University 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 pointer to 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

Copyright 2005, The Ohio State University 17 Operator new char *ptr; ptr = new char; // allocate memory for one character *ptr = ‘B’; cout << *ptr; 1000 ptr: ‘B’  Dynamic memory has no variable names pi: 100 int *ptr; ptr = new int [4]; // memory for four integers ptr[0] = 10; ptr[1] = 20; …

Copyright 2005, The Ohio State University 18 The NULL Pointer 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 int *ptr; … use ptr int *ptr = NULL; … if ( ptr != NULL ) { use ptr } BAD GOOD

Copyright 2005, The Ohio State University Operator delete Syntax 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 int *ptr = new int; int *arr = new int [4]; … delete ptr; delete [] arr; arr = NULL;

Copyright 2005, The Ohio State University 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)