Lecture 14: Dynamic Allocation and Midterm Review

Where we left off

Time t;
Time *pt = &t;

t.hour = 5;
t.minute = 0;
cout << pt->hour << ':' 
     << pt->minute << endl;

Today’s topics

• Dynamic memory allocation
• Midterm review exercise

Textbook Sections 9.2, 6.1

The heap and the stack

• There are two accessible areas of memory for a program:
  • The stack is used for local variables and function calls
  • The heap (or “freestore”) is used for dynamic memory allocation

The new operator

To create a variable on the heap, use the new operator:

int x = 0; // x is a named memory location on the stack
int *ptr; // memory for pointer is on the stack
ptr = new int; // what it points at is on the heap

• By using new, we tell C++ that we want the memory to be allocated on the heap
• The only way to access the value at ptr is through the pointer
• What if we do the following?

  ptr = &x;
  

• If you lose the address of the pointer, your integer is lost and gone forever!

new structures

• Creating an int on the heap is a bit silly, they don’t take up much space anyway
• More useful for a struct:

  Applicant *a = new Applicant; // Allocates all 19 fields on the heap
  

• Recall the pointer + struct syntax:

  strcpy(a->name, "Aaron Grimm");
  cout << a->name << endl;
  

Every new needs a delete

• After allocating space on the heap (for an Applicant or an int or anything else), you should free the memory using delete when you’re done with it
• This prevents memory leaks
• Syntax:

  delete a;
  

  where delete is an operator and the operand is the pointer variable name

Summary of new and delete

Risks:

• Memory leaks - forgetting to delete a pointer
• Dangling pointers - deleteing a pointer and then trying to use it
• Double delete - deleteing a pointer twice
• deleteing a pointer that was not created with new

Allocating variable sized arrays

• To create a variable sized array, we need to use new:

  int n;
  cin >> n;
  int *arr = new int[n];
  

• This allocates contiguous memory on the heap for n integers
• We can then use the array the way we normally would:

  arr[0] = 5;
  a_func_that_uses_an_array(arr, n);
  

• deleteing an array needs a bit of extra syntax:

  delete [] arr;
  

Static vs dynamically allocated arrays

Midterm review exercise

Pub trivia style! Answers are now posted .

• Groups of 3-4
• I’ll read questions out loud, you have 2 minutes per question to discuss and write down your answers.
• Do not shout out answers - write them down and we’ll peer mark at the end.

int x = 5;
int *p1;
int *p2 = &x;

int nums[8], n;
cin >> n;
for (int i = 0; i < n; i++) {
    cin >> nums[i];
}