12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 1 Engineering Problem Solving with C++, Etter/Ingber Chapter 9 An Introduction to Pointers

4 Addresses and Pointers 4 Pointers to Array Elements 4 Dynamic Memory Allocation 4 Data Structures and the STL. 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 2

ADDRESSES AND POINTERS address operator pointer assignment pointer arithmetic 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 3

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 4 Addresses and Pointers 4 A pointer is an object that holds the memory address of another object.  If a variable p contains the address of another variable q, then p is said to point to q.  If q is a variable at location 100 in memory, then p would have the value 100 ( q ’s address). –Memory snapshot: qp

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 5 Address Operator 4 The operator & is called the address operator. 4 When the & operator is applied to an object, the result is the address of the object. 4 Example: int x=75; cout << "x is " << x; cout << "\nthe addres of x is " << &x; 75 x[0x7fff8164] OUTPUT: x is 75 the address of x is 0x7fff8164

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 6 Pointer Assignment 4 Pointer types are declared using the pointer operator *, also called the dereferencing operator. 4 Syntax - –type *variable_name, *variable_name; –type* variable_name; 4 When declaring more than one pointer variable, the * operator must precede each identifier.

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 7 Example int *iPtr; double* dPtr;  The variable iPtr is declared to be of type pointer to int.  The variable dPtr is declared to be of type pointer to double. 4 Neither variable in this example has been initialized. iPtrdPtr ??

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 8 Example int *iPtr, i=6; char* s, str[] = "example"; double *dPtr, d=1.25; iPtr s dPtr 6 "example" 1.25 i str d

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 9 Initialization and Assignment 4 Pointer types may be initialized at the time they are declared. 4 Pointer types may be assigned new values using the assignment operator.

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 10 Example – Initial Pointers to NULL int *iPtr=0; char *s=NULL; //predefined constant in iostream double *dPtr=NULL; iPtr s dPtr

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 11 Assignment 4 The assignment operator (=) is defined for pointers of the same base type. 4 The right operand of the assignment operator can be any expression that evaluates to the same type as the left operand. Example: int x, *xp, *ip; xp = &x; ip = xp; x xp ip

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 12 The Base Type 4 The base type of a pointer refers to the type of object the pointer is referencing. 4 The base type of a pointer defines the size of the object the pointer is referencing. 4 The size of a pointer is independent of its base type. –p and q are the same ( 4 bytes**), but p points to 4 bytes** and q points to 8 bytes** ** compiler dependent

Example: 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 13 int p(5), *iPtr=&p; double q, *dPtr=&q; dPtr p q 5 ? ? iPtr  sizeof p is 4  sizeof q is 8  sizeof iPtr is 4  sizeof dPtr is 4

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 14 Base Type 4 A pointers base type determines how the object referenced by the pointer will be interpreted. 4 The declaration: int *p; declares p to be a pointer to int. What ever p points to will be interpreted as an int, ie 4 bytes. 4 Base type also defines pointer arithmetic.

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 15 Pointer Arithmetic 4 Four arithmetic operations are supported: +, -, ++, -- 4 Arithmetic is performed relative to the base type of the pointer. 4 When applied to pointers, ++ means increment pointer to point to next object. Example: p++; –if p is defined as int *p, p will be incremented by 4 (bytes). –if p is defined as double *p, p will be incremented by 8(bytes).

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 16 Example int *p; cout << "size of char is " << sizeof(char) << endl; cout << "size of int is " << sizeof(int) << endl; cout << "size of double is " << sizeof(double) << endl; cout << "size of float is " << sizeof(float) << endl; cout << "the size of p is " << sizeof(p) << endl; Output: size of char is 1 size of int is 4 size of double is 8 size of float is 4 the size of p is 4

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 17 Practice! int q=6; int *iPtr = &q; cout << "iPtr is " << iPtr << endl; cout << "*iPtr is " << *iPtr << endl; cout << "++*iPtr, is " << ++*iPtr << endl; cout << "q is " << q << endl; cout << "iPtr is " << iPtr << endl; cout << "*iPtr++ is " << *iPtr++ << endl; cout << "iPtr is " << iPtr << endl; cout << "q is " << q << endl; Complete the output: iPtr is 0x7fff2f14

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 18 Result of Practice iPtr is 0x7fff2f14 *iPtr is 6 ++*iPtr is 7 q is 7 iPtr is 0x7fff2f14 *iPtr++ is 7 iPtr is 0x7fff2f18 q is 7

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 19 Comparing Pointers 4 You may compare pointers using relational operators 4 Common comparisons are: –check for null pointer (p == NULL) –Note: since NULL evaluates as false, and any other pointer evaluates as true, checking for a null pointer can be done as (!p) –check if two pointers are pointing to the same object (p == q) –Note: (*p == *q) means they are pointing to equivalent, but not necessarily the same data.

POINTERS TO ARRAY ELEMENTS one-dimensional arrays pointers as arguments to functions 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 20

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 21 One Dimensional Arrays 4 The name of an array is the address of the first element (i.e. a pointer to the first element). 4 Arrays and pointers may often be used interchangeably. Example int num[4] = {1,2,3,4}, *p; p = num;//same as p = &num[0]; cout << *p <<endl; ++p; cout << *p; output: 1 2

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 22 Arrays and Pointers 4 You can index a pointer using [] operator. Example: char myString[] = "This is a string"; char *str; str = myString; for(int i =0; str[i]; i++)//look for null cout << str[i]; What does this segment print? This is a string

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 23 Arrays and Pointers 4 When an array is defined, memory is allocated according to the specified size of the array. 4 The name of an array is a pointer to the first element. However, the value of the pointer can not be changed. It will always point to the same memory location. 4 When a pointer is defined, 4 bytes* are allocated to store a memory address. 4 The value assigned to a pointer can be modified (reassigned to point to a different object).

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 24 Practice! What does this print? char myString[ ] = "This is a string"; char *strPtr; strPtr = myString; cout << *myString << endl; cout<<myString << endl; cout << *(myString + 1) << endl; strPtr++; cout << *++strPtr << endl; myString++;//not legal 0xfff4c252 strPtr T This is a string h i 0xfff4c252 myString Thisisastring\0

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 25 Arrays of Pointers 4 You may define arrays of pointers like any other data type in C++ int num=8; //declare an array of 10 pointers to int int *iPtrs[10]; //first element is assigned a value iPtrs[0] = &num; //output the value of num cout << *iPtrs[0];

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 26 Pointers As Arguments to Functions 4 Pointers may be passed either "by value" or "by reference". 4 In either case, the pointer can be used to modify the object to which it is pointing. 4 Only when passed by reference can the pointer argument itself be modified.

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 27 Common Pointer Problems 4 Using uninitialized pointers int *iPtr; *iPtr = 100; iPtr has not been initialized. The value 100 will be assigned to some memory location. Which one determines the error. 4 Failing to reset a pointer after altering it’s value. 4 Incorrect/unintended syntax.

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 28 #include int main() { char *aString = "What happens here?"; int len=0; while(*aString++ != '\0') len++; std::cout << len << ": " << aString; return 0; } Does this compile? If not, why? If it does, what is the output? Explain? Practice! Yes 18:

DYNAMIC MEMORY ALLOCATION new delete dynamically allocated arrays 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 29

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 30 Dynamic Allocation Using new and delete 4 Storage for data is allocated as needed from the free memory area known as the heap. 4 Run-time stack –Local variables –Formal parameters –Managed by the compiler 4 Heap –Dynamic storage –Managed by storage allocator Stack Heap Global data Program code

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 31 Dynamic Memory Allocation 4 Dynamically allocated memory is defined at runtime. 4 Dynamic allocation of memory allows for more efficient use of a finite resource. 4 Dynamic allocation is often used to support dynamic data structures such as stacks, queues, linked lists and binary trees. 4 Dynamically allocated memory should be freed during execution when it is no longer needed.

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 32 Operator new 4 Replaces malloc() used in C 4 Allocates a block of memory from the heap. 4 Returns the address of the first byte.  If allocation fails, new throws and exception that terminates the program (or returns NULL on older compilers).

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 33 Example int *iPtr; iPtr = new int; // 4 bytes are allocated // iPtr points to 1 st byte double *dPtr; dPtr = new double[20]; // 160 bytes allocated // dPtr points to 1 st byte ? ? ? ? ? … ? ? heap iPtr dPtr

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 34 Initializing Dynamically Allocated Memory 4 To initialize a dynamically allocated object, the initial value is provided inside parentheses following the type. 4 Example: int *ptr; ptr = new int(100); //4 bytes allocated //initial value: 100

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 35 int main() { int *iPtr, *jPtr, i; iPtr = new int; jPtr = new int(3); double *dPtr; dPtr = new double[6]; *iPtr = 7; cout << *iPtr << ',' << *jPtr << endl; for(i=0; i<6; i++) dPtr[i] = 5; for(i=0; i<6; i++) cout << (*dPtr)++ << ' '; cout << endl; for(i=0; i<6; i++) cout << dPtr[i] << ' '; return 0; } OUTPUT 7, EXPLAIN Practice:

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 36 Operator delete 4 Replaces free() used in C.  The delete operator frees memory allocated by new.  Using delete to attempt to free any other type of address will result in errors.

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 37 delete Example int *ptr; ptr = new int (100); cout << *ptr;//see if it worked value of ptr is now undefined 100 ptr delete ptr;//free the memory ? ptr

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 38 Example: Dynamically Allocated Arrays double *dptr; const int SIZE = 10; dptr = new double[SIZE];//80 bytes for(int i=0; i<SIZE; ++i) cin >> dptr[i]; fun1(dptr, SIZE); // pass array to fun1 delete [] dptr; //free all 10 elements

12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 39 Example using delete int main() { int *iPtr, *jPtr; iPtr = new int(2); jPtr = new int(3); cout << *iPtr << ',' << *jPtr << endl; delete iPtr; delete jPtr; cout << *iPtr << ',' << *jPtr << endl; int *dPtr; dPtr = new int(5); cout << iPtr << ',' << jPtr << endl; cout << *iPtr << ',' << *jPtr << endl; cout << dPtr << endl; return; } What output would you expect from this code?

DATA STRUCTURES AND THE STL list stack queue 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 40

List 4 A list is a data structure organized as a collection of elements, or nodes, that are linked by pointers. 4 Elements can be added at any position in a list in constant time by reassigning pointers. 4 List have many useful applications in the organization large amounts of data. 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 41

The list class 4 list is a class template defined in the header file list. #include list word; //list of strings 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 42

list Methods o Elements can be added to and removed from any position in a list using the appropriate method and an iterator. o An iterator is similar to a pointer, but is usually implemented as a class object. o Common Methods: bool empty() iterator insert() iterator remove() iterator begin() iterator end() 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 43

Example list wordList; list ::iterator iter = wordList.begin(); iter = wordList.insert(iter,"hello"); wordList.insert(iter,"world"); for(iter=wordList.begin(); iter!= wordList.end(); iter++) cout << *iter << " "; 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 44 output: hello world

Queue 4 A queue is known as a first-in-first-out (FIFO) data structure. 4 Items are added to the end of a queue and removed from the front of the queue. 4 Queues have many useful applications, including processing requests. 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 45

The queue class 4 queue is a class template defined in the header file queue. #include queue printIDs; //queue of ints 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 46

queue Methods bool empty() void pop()//remove from front void push()//add to end data-type front()//return front data-type back()//return end 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 47

Example #include … queue theQueue; theQueue.push(10); theQueue.push(20); cout << theQueue.front(); //output 10 cout << theQueue.back(); //output 20 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 48

Stack 4 A stack is known as a last-in-first-out (LIFO) data structure. 4 Items are added to and removed from the top of the stack. 4 Stacks are essential in the design of compilers and for implementing recursion. 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 49

The stack class 4 stack is a class template defined in the header file stack. #include stack theStack; //stack of ints 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 50

stack Methods bool empty() void pop() //remove from top void push() //add to top data-type top()//return top 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 51

Example #include … stack theStack; theStack.push(10); theStack.push(20); cout << theStack.top(); //output 20 theStack.pop(); cout << theStack.top(); //output 10 12/23/2015Engineering Problem Solving with C++, second edition, J. Ingber 52