Dynamically Allocated Arrays

December 4, 2013

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Read Chapter 8,pp. 351-357

Intro to Pointers

A pointer is a variable that holds an address Declaration uses *

int* p; int i;

& is “Address of” operator. Gets the address of a variable.

p = &i;

Intro to Pointers

* is used to dereference a pointer:

*p = 15; Above: what p points to is assigned the value, 15.

pointers.cpp

Arrays Are Pointers

In a program, an array, a, and a pointer, p, are the same kind of pointer.

int* p; //p is an int pointer int a[10];

They both point at an int.

Arrays Are Pointers

p can get the address that a is pointing at.

p = a;

It is illegal to change a.

a = p;

Once p is assigned the value of a it can be used like an array variable.

arrayPtr.cpp

Dynamically Allocated Arrays

Sometimes the amount of memory needed by an array can vary greatly. e.g. Number of campers in Ponderosa park in January

vs. August.

To save memory, use dynamic array.

Dynamically Allocated Arrays

Declare using a pointer Can return to memory Can vary size

//allocate array with 20 intsint *pt = new int[20];

Initialize Array

double *buff = new double[10];

for (int i=0; i<10; i++){*buff = 100.0;buff++;

}

Initialize Array Alternative

double *buff = new double[10];

for (int i=0; i<10; i++){buff[i] = 100.0;

}

Release memory

delete []pt;delete []buff;

If an array is no longer needed this can free up memory for other programs.

Another Example int* p = new int[10];

for (int i = 0; i< 10; i++){ p[i] = i * i;}

cout << *p << " " << *(p+1) << " " << p[2] << endl;

NULL

Can assign a pointer to a NULL value, which is pointing to nothing.

Memory Leak

void myfunction( ){ int *pt;pt = new int[100];..//no delete

}

//in main:while(cond.) myfunction();

Destructors

Return data memory When object goes out of scope.

Look at messageDest.cpp

Copying Objects with Dynamic Arrays(Skip)

Assignment Operator copies objects message2 = message; Copies a pointer, not the array

Example in message.cpp Messy error.

Copying Objects with Dynamic Arrays(Skip)

Deep Copying Create a new array and copy contents to the

array messageFixed.cpp

copyFrom function fixes

Overload ==(Skip)

To make sure no problems happen, overload ==

Do deep copy when an assignment is done.

Copy Constructor(Skip)

Copying of objects occur: Passing a copy of an argument using pass-by-

value. Returning an object from a function:return msg1;

By default these are shallow copying. Better to provide a copy constructor that does

a deep copy. messageCopyConstr.cpp

Copy Constructor

Odd things can happen without copy constructor.

If two objects point to same array, something done to one object effects the other. Like the problem with message