Copyright © 2006 Pearson Addison-Wesley. All rights reserved This Weeks Topics: Pointers (continued)  Modify C-String through a function call  Dynamic variable  new keyword and delete keyword  Arrays and pointers  Pointer arithmetic  Dynamic array  Size not specified at programming time  Determined while program running

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Modify C-String in a function call or exchange the value of two C- Strings  Pass by value of the c-string (char *)?  Won’t’ work  Pass by the reference of the c-string (char * &)?  Yes, it works  Pass by pointer to the c-string (char **)?  Yes, it works  How to implement?

Copyright © 2006 Pearson Addison-Wesley. All rights reserved The new Operator  Since pointers can refer to variables…  No "real" need to have a standard variable  Can dynamically allocate variables  Operator new creates variables int * p1; p1 = new int;  Creates new "nameless" variable, and assigns p1 to "point to" it  Can access with *p1  Use just like ordinary variable

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Basic Pointer Manipulations Example: Display 10.2 Basic Pointer Manipulations (1 of 2)

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Basic Pointer Manipulations Example: Display 10.2 Basic Pointer Manipulations (2 of 2)

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Memory Management  Heap  Reserved for dynamically-allocated variables  All new dynamic variables consume heap memory  If too many  could use all heap memory  Problem: future "new" operations will fail if heap is "full“  Resolution: use delete operator to de-allocate spaces

Copyright © 2006 Pearson Addison-Wesley. All rights reserved delete Operator  De-allocate dynamic memory  When no longer needed  Returns memory to heap  Example: int *p; p = new int; … //Some processing… delete p;  De-allocates dynamic memory "pointed to by pointer p"  Literally "destroys" memory

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Dynamic and Regular Variables  Dynamic variables  Created with new operator; destroyed with delete operator  Created and destroyed while program runs  Local variables  Declared within function definition  Not dynamic  Created when function is called  Destroyed when function call completes

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Recall: Array Variables  Arrays stored in memory addresses sequentially  Array variable "refers to" first indexed variable  So array variable is a kind of pointer variable!

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Array Variables  Pointers  Example: int a[10]; int * p;  and p are pointer variables  Can perform assignments: p = a;// Legal.  p now points where a points  To first indexed variable of array a  a = p;// ILLEGAL!  Array pointer is CONSTANT pointer!

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Pointer Arithmetic  Can perform arithmetic on pointers  "Address" arithmetic  Example: double f[4] = {1.1, 2.2, 3.3, 4.4}; double * d = f;  d contains address of d[0]  d + 1 evaluates to address of d[1]  d + 2 evaluates to address of d[2]  Equates to "address" at these locations

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Alternative Array Manipulation  Use pointer arithmetic!  “Step through” array without indexing: for (int i = 0; i < arraySize; i++) cout << *(d + i) << " " ;  Equivalent to: for (int i = 0; i < arraySize; i++) cout << d[i] << " " ;

Array and Pointers Copyright © 2006 Pearson Addison-Wesley. All rights reserved  Array can be accessed using pointers; passing array as a parameter in a function is passing the address of the first element int main() { int n[4]={0,0,0,0}; int *p; p=&n[0]; // same as p =n; *p = 1; // same as p[0] = 1; *(p+1) = 1 // same as p[1] = 1; for (int i=0; i<4; i++) cout << n[i] << " "; return 0; }

Array and Pointers (continued) Copyright © 2006 Pearson Addison-Wesley. All rights reserved C-String is a special character array int main() { char s[] = "Hi There!"; char *p; p=&s[0]; // same as p = s; *p = 'B'; // what will happen here? *(p+1) = ‘C’; // what will happen here? p = "Bye Bye!"; cout << s << endl; return 0; }

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Dynamic Arrays  Array limitations  Must specify size first  May not know until program runs!  Must "estimate" maximum size needed  Sometimes OK, sometimes not  "Wastes" memory  Dynamic arrays  Can determine size at runtime

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Creating Dynamic Arrays  Use new operator  Dynamically allocate with pointer variable  Treat like standard arrays  Example: double *d; d = new double[10]; //Size in brackets, can be variable  Creates dynamically allocated array variable d,with ten elements, base type double

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Deleting Dynamic Arrays  Allocated dynamically at run-time  So should be destroyed at run-time  Simple again. Recall Example: double * d = new double[10]; … //Processing delete [] d;  De-allocates all memory for dynamic array  Brackets indicate "array" is there

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Multidimensional Dynamic Arrays  Recall: "arrays of arrays" int **m = new int *[3];  Creates array of three pointers  Make each allocate array of 4 ints for (int i = 0; i < 3; i++) m[i] = new int[4];  Results in three-by-four dynamic array!

Copyright © 2006 Pearson Addison-Wesley. All rights reserved Multidimensional Dynamic Arrays: De-allocate memories  In the reverse order allocating spaces for (int i = 0; i < 3; i++) delete [] m[i]; delete[] m;