CS352-Week 2

Topics

• Heap allocation• References • Pointers

Previous knowledge

• vi/vim information http://troll.cs.ua.edu/cs150/

• Concept of an array allocating the data in a sequence that is all the same type

• Functions - http://troll.cs.ua.edu/cs150/book/index_9.html

– Understanding of passing read only parameters to a function

– Understanding of returning a value from a function

– Denote type of variable in formal parameter list

Objectives

• Dynamically allocate an array of any native type• Understand how/where local variables, global variables

and dynamically allocated variables are stored• Multiple ways to refer to an array member and iterate

over an array• Understanding of pass by reference vs pass by value• Motivation for using references instead of pointers• Motivation for using references to reduce processing

Function to add values//add.cc#include <iostream>using namespace std;int add(int i, int j){

return i+j;}

int main() {int m = 5, n =10;

cout<<"inputs: “<< m <<“ “<<n<<“endl;cout<<"inputs: “<< add(m,n) <<“endl;

}

What is the output generated by this code?

static_cast is used toconvert x and y to integers via truncation

inputs 5 10outputs 15

Program to swap values//swap1.cc#define <iostream>using namespace std;int main() {

int m = 5, n =10;

cout<<"inputs: “<< m <<“ “<<n<<“endl;swap(m, n);cout<<"inputs: “<< m <<“ “<<n<<“endl;

}void swap(int i, int j){

int temp = i;i = j;j = temp;

}We wish to change the parameters that are passed (m and n)This is called mutation or mutating the parameters

inputs 5 10outputs 5 10

No compile errors.Why doesn’t this work?

Call by valueNative types are passed to a function using call by value

int m=5, n=10;swap(m,n)

swap(int i, int j)

0101 1010

m n

0101

1010

Values are copied to stack (special memory section to store function parms)i

j

special code to save place in calling function and get ready to execute function

The i = j statement only affects the memory that holds the copy, not the original

If we send the address of the variables in memory, we can change the original values

20 21

Call by valueNative types are passed to a function using call by value

int m=5, n=10;swap(&m,&n)

swap(int * i, int * j)

0101 1010

m n

21

20

Addresses are copied to stack

i j

special code to save place in calling function and get ready to execute function

20 21

Must let function know that we passed addresses not values

C style program to swap values//swap2.cc#define <iostream>using namespace std;int main() {

int m = 5, n =10;

cout<<"inputs: “<< m <<“ “<<n<<“endl;swap(&m, &n);cout<<"inputs: “<< m <<“ “<<n<<“endl;

}void swap(int * i, int * j){

int temp = *i; //moves contents in i to local variable temp*i = *j; //moves contents in j to address pointed to by i*j = temp; //moves value temp to address pointed to by j

}

This function is a mutator; it changes the original value

inputs 5 10outputs 10 5

C++ style program to swap values//swap3.cc#define <iostream>using namespace std;int main() {

int m = 5, n =10;

cout<<"inputs: “<< m <<“ “<<n<<“endl;swap(m, n);cout<<"inputs: “<< m <<“ “<<n<<“endl;

}void swap(int &i, int &j){

int temp = i; //moves contents in i to local variable tempi = j; //moves contents in j to address pointed to by ij = temp; //moves value temp to address pointed to by j

}

This function is a mutator; it changes the original valueThe & lets the compiler know to use the addresses rather than the values

inputs 5 10outputs 10 5

References are also useful because they prevent the copying of the function parameters.

Arrays

Variable designators are pointers that refer to the address of the lowest lot

//Declares/Allocates 6 consecutive integers //init depends upon scope

int pos[6];

//Declares/Allocates 6 consecutive integers and initializes them with given values

int pos={0,2,4,6,8,10};

pos[0]=16;

cout<<pos; //prints 1320cout<<pos[0]; //prints 16cout<<*pos; //print 16*pos++;cout<<*pos; //prints 2

pos variable is a pointer 1320

Arrays//arrays1.cc#include <iostream>using namespace std;

int main() { int intArray1[6]; int intArray2[]={2,4,6,8,10};

cout<<"pointer to Array 1 "<<intArray1<<endl;

for (int i=0;i<6;i++) cout <<"pos "<< i << " int intArray1 is "<< intArray1[i] <<endl;

cout<<endl; cout<<"pointer to Array 2 "<<intArray2<<endl<<endl;

//cout<<"Print iterated intArray2 pointer "<<--intArray2; //intArray2++;

int *iterator = intArray2; cout<<"value of iterator (ptr to Array 2) "<<iterator<<endl;

for (int i=0;i<6;i++){ cout <<"pos "<< i << " int intArray2 is "<< *iterator <<endl; cout <<"iterator value "<< i << " int intArray2 is "<< iterator++ <<endl; }}

OUTPUTpointer to Array 1 0xffbff158pos 0 int intArray1 is -4198016pos 1 int intArray1 is 67896pos 2 int intArray1 is -12775520pos 3 int intArray1 is 0pos 4 int intArray1 is 0pos 5 int intArray1 is 0

pointer to Array 2 0xffbff140

value of iterator (ptr to Array 2) 0xffbff140pos 0 int intArray2 is 2iterator value 0 int intArray2 is 0xffbff140pos 1 int intArray2 is 4iterator value 1 int intArray2 is 0xffbff144pos 2 int intArray2 is 6iterator value 2 int intArray2 is 0xffbff148pos 3 int intArray2 is 8iterator value 3 int intArray2 is 0xffbff14cpos 4 int intArray2 is 10iterator value 4 int intArray2 is 0xffbff150pos 5 int intArray2 is 0iterator value 5 int intArray2 is 0xffbff154

Print uninitialized array

Can’t increment/change the declared array valueYou can create a 2nd pointer that points at the beginning of the array

Pointer arithmetic (++,--,+,-)Iterates over array just as indexes can

Array Allocation, Declaration, and Initialization

//ADI array pointer and AD(I) array of 10 intsint array2[10];

//Arrays can only be allocated in global/local //scope is if the size is knownint array1[]; //why not allocate the array?

**Sometimes we do not know the size of array we will need

Dynamic allocation

• Many array sizes cannot be anticipated at compile time• Unknown array sizes have to be allocated at runtime• Space is allocated from heap (free store) which is

separate from the storage for other variables• Allocated space can be used globally *if* there is a

global pointer to it• Allocated space with a local pointer will cause the

pointer to be deallocated when it is out of scope *but* the space it pointed to still exists, called memory leak

• Once allocated, the space stays allocated until it is explicitly freed

Variable ADIWhere and how matters

integerList

counti

2,4,6,8,9,10,….

0,0,0,0

L2

int L2[4]; //reference ADI; array-ADI in data

You writing a program where you will generate a random number of integers and store them in an array

//dynamic1.cc#include <iostream>#include <ctime>#include <cstdlib>using namespace std;//reference-ADI; array-nothingint * integerList; //don’t know the random number of integers; can’t use []

int main() {srand(time(NULL));int count=rand()%10;

integerList = new int[count]; //dynamic allocation of space-”operator new”

for (int i=0;i<count;i++) integerList[i]=rand();

cout<<“Number list of ”<<count<<endl;for (int i=0;i<count;i++) cout <<”Value at “<< i << ” is “<< integerList[i]<<endl;return 0;

}

int *

Object-oriented Programming

• Data and functions are linked together• Attributes are the data fields in a class• Methods can be functions that act on the data

fields• Methods can be functions that return data state

to the caller• Classes are a type definition – No memory is allocated in a class definition

• Classes usually follow natural relationships and actions

Smart Car

Vehicle

AttributesdoorCount=2isDriverDoorClosed=T MethodsgetDoorCount(){

return doorCount;}isDriverDoorClosed() {

return isDriverDoorClosed;}openDriverDoor() {

driverDoorIsClosed=F;}

Contain both static

and dynamic state

Accessor-return state;read only

Mutator-changes state

Class definition

class Vehicle {

public:getDoorCount();

isDriverDoorClosed();openDriverDoor();

private:doorCount=2;

isDriverDoorClosed=T;}

Upcoming preparation• Class definitions• class scope• classes vs object• declaration, allocation and initialization• constructors and destructors

I will talk about syntax and semantics, not object oriented analysis and design (determining appropriate classes, methods, attributes and interactions)

Resources• Bjarne Stroustrup, Grady Booch• Object-Oriented Analysis and Design with Applications• Design patterns: Elements of Reusable Code

Recap

• Using references to allow functions to mutate values

• Reference types contain native types and other reference types

• Array declaration, allocation and initialization• Array variable holds address of first element• Dynamic array allocation of native type arrays• Pointer arithmetic

• I will not be here (either Dr Hong or a graduate student will be here)

• Thursday will be an in class assignment• 3 programs to be due at the end of class• Review the techniques and concepts so that

you can complete the assignment in 50 minutes