Post on 04-Jan-2016
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