This set of notes is adapted from that provided by “Computer Science – A Structured Programming Approach Using C++”, B.A. Forouzan & R.F. Gilberg, Thomson Learning, 2000.

Pointers

Dynamic Memory

MEMORY MANAGEMENT

STATIC MEMORY

•Until now, we have only learned about static memory•Memory is allocated at compilation time•Declaration of large memory variables (arrays) must be determined before the program is compiled.

• int students[10]; • what if we don’t know the exact number of students in advance?

DYANMIC MEMORYWe can “ask” for memory while the program is running.

We can allocate and release memory ourselves!More powerful programming ability (also more potential for errors!)

Memory Allocation

{ int a[200]; int *ptr = a; …}

int *ptr = new int[200];…delete [] ptr;

Conceptual View of Memory

(DYNAMIC MEMORY)

Static vs. Dynamic Objects Static object

Memory is acquired automatically

Memory is returned automatically when object goes out of scope

Dynamic object Memory is acquired

by program with an allocation request

new operation Dynamic objects can

exist beyond the function in which they were allocated

Object memory is returned by a deallocation request

delete operation

Dynamic Memory

C++ “new” keywordnew <type>; // allocate size of the type new <type>[size]; // allocate an array

Asks the Operating System to return you a pointer to a “chunk” of data.

Allocating MemoryOperating System

Manages dynamicmemory.

You have:

int *p;

p = new int[1000];

Ask OS to find a segmentOf memory of 1000*4 btyes

OS returnsthe address.

So, you need a pointer to the type you requested.If you request <int>, you need an <int> pointer.

If you request <float>, you need a <float> pointer.

Allocating memory using “new”

8002

8006

8010

8014

8018

8022

8026

new returns the address to the“start” of the memory it allocated

int *p;p = new int[7];//p is assigned 8002

p[0] = *(p) = *(p+0) = ?p[1] = *(p+1) = ?p[2] = *(p+2) = ?

p[0]

p[1]

p[2]

Dynamic Memory Allocation

Request for “unused” memory from the Operating System

int *p, n=10;

p = new int;

p = new int[100];

p = new int[n];

pnew

pnew

pnew

Dynamic Memory Allocation Example

Need an array of unknown sizeint main(){ cout << “How many students? “; cin >> n;

int *grades = new int[n];

for( int i = 0; i < n; i++) { cout << “Input Grade for Student” << (i+1) << “ ? :”; cin >> grades[i] ;

} . . . // call a function with dynamic array printMean( grades, n ); . . . }

Dynamic Memory

C++ “delete” keyword

delete address; // delete one element at address

delete [] address; // delete array at address

How can we specify what the address is?We have pointer. Pointer variables storeaddresses.

Freeing (or deleting) Memory

The memory you request by “new” can not be used again until you explicitly release it via “delete”. We call this “freeing” or “releasing” or “deleting” memory.

int *p;p = new int [1000];delete [] p; // delete (or free) the array at p

NOTE, the value of p will not change! However, it is now no longer points to a valid memory address.

Freeing (or deleting) Memory

Freeing (or Deleting) memory “ delete” is a keyword

delete <address, i.e. pointer variable>; // a variable

example: int *p= new int; delete p;

delete [] <address, i.e. pointer variable>; // an array

example: int *p = new int[1000]; delete [] p;

Dynamic Memory Dynamic Memory is very powerful

C++ trusts you as the programmer to manage you own memory

While this gives you lots of control, it is easy to make mistakes.

Dynamic memory bugs is common in large software projects written in C++.

The Dangling Pointer problem

Be careful when you delete memory pointed to by p, you are not erasing a location that some other pointer q is pointing to.int *p, *q;P = new int;q = p;delete p;// q is a dangling pointer

p

qint

p

qint delete p

Memory Leak Problem

Make sure to delete memory when finished

int *p;

p = new int[100];

int a;

NO ONE HAS ACCESS TO THIS MEMORYNO ONE HAS ACCESS TO THIS MEMORY 100 integersp = &a;

aa

100 integers

aa

This memory is un-addressableAnd it is reserved until yourprogram terminates. It is garbage(it has leaked from the system)

Exercise 1

What is wrong with the following program?.

int main( ){ int x =20; int* p; p = new int; *p = 30; cout << *p << " " << x << endl; delete p; p = &x; delete p; cout << *p << " " << x << endl;

return 0; }

Exercise 2 What is wrong in the following code? int main( ){

const int SIZE = 10000000;int i;int A[SIZE];

for(i=0; i<SIZE; i++)A[i] = i;

int *B;B = new int[SIZE];

for(i=0; i<SIZE; i++)B[i] = A[i];

for(i=0; i<SIZE; i++)cout << A[i] << " " << B[i] << endl;

return 0; }

Exercise 3What is the output of the following program? int main( ){

int i, *p, *r, t[4]={0,1,2,3};

p = new int[4]; for(i=0; i<4; i++) p[i] = 6*(i+1); r = t; for(i=3; i>=0; i--) r[i] -= 1; r[2] = 8; for(i=0; i<4; i++) t[i] = p[i] + r[i]; for(i=0; i<4; i++) cout << t[i] << " "; delete [] p; return 0;

}

Exercise 4

What is the output of the following program? void F1(int* temp){ *temp = 99; } void main( ){

int *p1, *p2;p1 = new int;*p1 = 50;p2 = p1;F1(p2);cout << *p1 << " " << *p2 << endl;p1 = new int;*p1 = 88;cout << *p1 << " " << *p2 << endl;delete p1;delete p2;

}

Exercise 5

What is wrong with the following program?

int main( ){int *p, *r;p = new int;r = new int;p = 1;r = p;

return 0;}