Storage Allocation CSE 143. Pointers, Arrays, and Dynamic Storage Allocation. Pointer Variables. Pointers: Review. Pointers and Types

Size: px

Start display at page:

Download "Storage Allocation CSE 143. Pointers, Arrays, and Dynamic Storage Allocation. Pointer Variables. Pointers: Review. Pointers and Types"

Garey Lloyd
5 years ago
Views:

1 CSE 143 Pointers, Arrays, and Dynamic Storage Allocation [Chater 4, , ] Storage Allocation Storage (memory) is a linear array of cells (bytes) Objects of different tyes often reuire differing amounts of storage Built-in tyes: imlementation deendent PC (tyical): char: 1 byte (8 bits) int: 4 bytes (2 bytes on older systems) double: 8 bytes Programmer defined tyes: deends on size of data members could be few bytes or thousands of bytes Pointers: Review Every memory cell in a comuter s memory has two characteristics: A value (contents) A uniue address Every object in the rogram occuies a range of memory cells Suose we have: int x; Value (contents) of x: x Address (location) of x: &x 159 Pointer Variables By "address of an object" we mean the address of the first memory cell occuied by the object A ointer variable is one that contains the address of another data object as its value. Pointer variable declaration: Tye* name; Examles: int* intptr; char* charptr; BigNat* bignatptr; 160 Pointers and Tyes Pointers to different tyes themselves have different tyes double *dt; Cell * c; In C/C++, dt and c have different tyes even though under the hood they are both just memory addresses Tyes have to match in many contexts e.g. arameter and argument tyes in a function call ointers are no excetions Pointer Oerations Oerations valid on ointers of all tyes. We ll cover only a subset: = (ointer assignment) int* = &someint; = (ointer assignment) int* = ; * (dereference) * = * + 1; cout << *; CSE 143 1

2 More Pointer Oerations == (euality test) if (tr1 == tr2) {...!= (test for ineuality) if (tr1!= tr2) {... -> (select a member of a ointed-to struct or object) void foo (Comlex* b) { b->re = -1; Pointers and Arrays In C, there is a strong relationshi between ointers and arrays, strong enough that ointers and arrays should be discussed simultaneously. Any oeration that can be achieved by array subscriting can also be done with ointers. Brian Kernighan and Dennis Ritchie The C Programming Language In C++, ointers and arrays are very closely related. Bjarne Stroustru The C++ Programming Language Arrays vs. Pointers An array name refers to the address of the first element of the array Array notation and ointer notation can be mixed int a[10]; // int array int *a, *b; // int ointers a = &a[0]; *a = 5; b = a; j = *b; Arrays Pointers! int * i; int iarr[10]; // what haens now? iarr[0] = 100; i[0] = 200; i = iarr; iarr = i; i = new int[20]; iarr = new int[20]; Arrays Parameters Array and ointer arameters are interchangeable These four functions are exactly the same void seta0(char a[ ]) { a[0] = 17; void seta0(char a[ ]) { *a = 17; void seta0(char * a) { a[0] = 17; void seta0(char * a) { *a = 17; Use whichever makes your code easier to read and understand Variable-Size Data Problem: All of our data structures so far have a maximum size. This size is fixed at comile time. Sometimes this is ok, but most real alications need to grow and shrink the amount of memory consumed by a data structure during execution. Most languages rovide some form of dynamic memory. C++ rovides an interface to dynamic memory via two oerators: new and delete CSE 143 2

3 Storage Classes Static - allocated at rogram startu time, exists throughout the execution of the entire rogram Automatic - imlicitly allocated on function entry, deallocated on exit void foo (char x) { int tem;... // x and tem are deallocated here Dynamic - exlicitly allocated and deallocated by the rogrammer Allocation & Deallocation Allocate dynamic memory with oerator new: The exression new Tye returns a ointer to a newly created object of tye Tye: int* ; char* str; // allocate a single int str = new char[10]; // allocate an array of chars Deallocate memory with oerator delete: delete Pointer deallocates the object ointed to by Pointer delete ; delete [] str; // deallocate a simle item // deallocate an array The Hea Objects created by new come from a region of memory set aside for dynamic objects (called the, or free store). The new oerator obtains a chunk of memory from the ; delete returns that memory to the. In C++ the rogrammer must manage the. Dynamic data is unnamed (anonymous) and can only be accessed through ointers. Hea Memory int *v, *w; v = new int; w = new int[5]; IntSet * s; s = new IntSet; delete v; delete [] w; delete s; Constructors, arrays, & new Whenever a class instance is created, a constructor is called IntSet s; // initialized by IntSet::IntSet( ) This is true for all elements in an object array IntSet a[3]; // IntSet::IntSet( ) executed 3x to // initialize each of s[0],s[1],s[2] The same rules aly when objects are allocated dynamically. IntSet *s1 = new IntSet; // constructor called once IntSet *s2 = new IntSet[3]; // constructor called for // each array element The NULL ointer During rogram execution, a ointer variable can be in one of the following states: Pointing to a data object Unassigned (never initialized or ointing to deleted storage) Contain the secial value NULL The constant NULL is usually defined as 0 in <stddef.h>, and is used to mean a ointer that does not oint to any object. NULL is comatible with all ointer tyes CSE 143 3

4 Dynamic Memory Dangers Uninitialized Pointer Failure to return objects to Run out of resources IntSet *s; for (int i = 0; i < ; i++) s = new IntSet; Pointers to non-allocated objects Accessing garbage values Overwriting valid data Causes: Non initialized or null ointers Keeing references to deleted objects Pointer arithmetic Misusing delete Deleting deleted data Confusing delete and delete [ ] 175 Examle int* ; * = 45; 176 Garbage (memory leak) Garbage (memory leak) Examle 1 int* ; * = 45; * = 55; Examle 2 int *, *; = new int; * = 45; * = 55; = ; Dangling Pointers Dangling Pointers Examle 1 int * int *; * = 45; = ; delete ; * = 55; Examle 2 char* broken( ) { char buffer[80]; cin >> buffer; return buffer; charptr = broken( ); // charptr is dangling Destroyed when function exits CSE 143 4

5 Abusing delete Abusing delete Examle 1 int * * = 45; delete ; delete ; Examle 2 int *, *; * = 45; = ; delete ; delete ; Abusing delete Dynamic Data Guidelines Examle 3 int *, *; = new int[3]; delete [ ] ; delete ; Avoid creating garbage when invoking new or moving ointers. Oerator new returns a ointer to an object. If the object s tye is a class, then the constructor has been called. If not, the object is still uninitialized. Don t dereference an unassigned ointer. Don t dereference a NULL ointer. After delete, the ointer value is unassigned (don't assume NULL) CSE 143 5

Pointers and Memory Allocation p. 1. Brooklyn College. Michael Lampis. CISC 3130 Notes. Pointers and Memory Allocation

Pointers and Memory Allocation CISC 3130 Notes Michael Lamis mlamis@cs.ntua.gr Brooklyn College Pointers and Memory Allocation. 1 int x; Pointers x Pointers and Memory Allocation. 2 Pointers int x; int