Dynamically Allocated Arrays December 4, 2013

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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 ints int *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