CS 210 - Fundamentals of Programming I

Spring 2006 - In-class exercise for 4/27/06

20 points

This exercise should be completed in pairs. The purpose of this exercise is to work with pointers and dynamic memory allocation. Recall that there are three distinct entities to be aware of when using pointers: the pointer variable (that stores a pointer), the pointer value (an address), and the location being pointed to (the ``pointee'', which may either be a dynamically allocated anonymous variable or a statically allocated named variable).

For each of the following programs, draw the memory picture created by each program at the points indicated in the comments (under the comment) and predict the output. Then create a project and type in the source code, and build and run the program. (Take turns typing in the code.) Observe the actual output and explain any discrepancies between your prediction and the actual output. When you have completed this exercise, hand in a hardcopy of your programs and these answer sheets

Program pointer1.cpp: Memory pictures:

#include <iostream>
using namespace std;

int main ()
{
   int *ptr1, *ptr2;                     // (a.) what is the picture here




   ptr1 = new int;
   *ptr1 = 20;                           // (b.) what is the picture here




   cout << (*ptr1) << endl;
   ptr2 = ptr1;
   *ptr2 = *ptr2 + 5;                    // (c.) what is the picture here



   cout << (*ptr1) << "  " 
        << (*ptr2) << endl;
   ptr1 = new int;
   *ptr1 = *ptr2;                        // (d.) what is the picture here





   cout << (*ptr1) << "  " 
        << (*ptr2) << endl;
   *ptr2 = 20;                           // (e.) what is the picture here




   cout << (*ptr1) << "  " 
        << (*ptr2) << endl;
   delete ptr1;
   delete ptr2;                          // (f.) what is the picture here
   return 0;
}  // end main

Predicted output: Actual output:

Explain any discrepancies:

Pointer variables and values may be used as formal parameters and actual arguments of functions and returned objects, just like any other data object. They may be value parameters or reference parameters. The rules for correspondence are exactly the same.

Program pointer2.cpp: Memory pictures:

#include<iostream>
using namespace std;

void AddOne (int *ptrParameter);

int main ()
{
   int *ptr1;

   ptr1 = new int; 
   *ptr1 = 10;                    // (a.) what is the picture here




   cout << (*ptr1) << endl;
   AddOne (ptr1);                 // (b.) what is the picture here 
                                  //      (after the call)

   cout << (*ptr1) << endl;
   delete ptr1;
   return 0;
} // end main

void AddOne (int *ptrParameter)   // (c.) show where ptrParameter points
                                  //      to when it is called in the
                                  //      program above
{
   *ptrParameter = *ptrParameter + 1;
} // end AddOne

Predicted output: Actual output:

Explain any discrepancies:

Program pointer3.cpp: Memory pictures:

#include <iostream>
using namespace std;

const int MAX = 10;

int main ()
{
   int *ptr1,  // for arrays.
       *ptr2;                      // (a.) what it the picture here






   ptr1 = new int [MAX];           // (b.) what is the picture here






   for (int i = 0; i < 6; i++)
   {
      ptr1[i] = i * 10;
   } // end for
   ptr2 = ptr1;                    // (c.) what is the picture here
   for (int j = 0; j < 6; j++)
   {
      cout << ptr2[j] << endl;  
   } // end for

   delete [] ptr1;
   return 0;
} // end main

Predicted output: Actual output:

Explain any discrepancies: