more on dynamic memory allocation
DESCRIPTION
More on Dynamic Memory Allocation. Seokhee Jeon Department of Computer Engineering Kyung Hee University. Illustrations, examples, and text in the lecture note courtesy of Prof. David Bernstein, https://users.cs.jmu.edu/bernstdh/web/common/lectures/slides_cpp_dynamic-memory.php. Reminders. - PowerPoint PPT PresentationTRANSCRIPT
More on Dynamic Memory AllocationMore on Dynamic Memory Allocation
Seokhee Jeon
Department of Computer Engineering
Kyung Hee University
•1
Illustrations, examples, and text in the lecture note courtesy of Prof. David Bernstein, https://users.cs.jmu.edu/bernstdh/web/common/lectures/slides_cpp_dynamic-memory.php
RemindersReminders
• Dynamically allocated memory is kept on the memory heap
• Dynamically allocated memory can't have a "name" it must be referred by pointers
• Declarations are used to statically allocate memory, the new operator is used to dynamically allocate memory
Pointing to Memory Allocated at Run Pointing to Memory Allocated at Run TimeTime
Returning Memory to the HeapReturning Memory to the Heap
• How Big is the Heap?
– It can only contain as much physical memory as you have installed or as much virtual memory as your operating system can make available (if it supports virtual memory)
• Running Out of Memory:
– Most applications request memory from the heap when they are running
– It is possible to run out of memory (you may even have gotten a message like "Running Low On Virtual Memory")
– So, it is important to return memory to the heap when you no longer need it
Returning Memory to the HeapReturning Memory to the Heap
• Dangling Pointers:
– The delete operator does not delete the pointer, it takes the memory being pointed to and returns it to the heap
– It does not even change the contents of the pointer
– Since the memory being pointed to is no longer available (and may even be given to another application), such a pointer is said to be dangling
Returning Memory to the HeapReturning Memory to the Heap
• Remember:– Return memory to the heap before undangling the
pointer• What's Wrong with the Following:
– ptr = NULL;– delete ptr;
Returning Memory to the HeapReturning Memory to the Heap
• What About Arrays?– You want to return all of the memory to the heap– So, a different form of the delete operator is
needed– Also, the memory allocator must keep track of the
size of the array
Memory LeakMemory Leak
• Memory leaks when it is allocated from the heap using the new operator but not returned to the heap using the delete operator
Linked Lists in Linked Lists in CC and and C++C++
CS-2303System Programming Concepts
(Slides include materials from The C Programming Language, 2nd edition, by Kernighan and Ritchie and from C: How to Program, 5th and 6th editions, by Deitel and Deitel)
•17
DefinitionsDefinitions
• Linked List• A data structure in which each element is
dynamically allocated and in which elements point to each other to define a linear relationship
• Singly- or doubly-linked• Stack, queue, circular list
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Note: elements are usually the
same type (but not always).
Linked ListLinked List
struct listItem {type payload;struct listItem *next;
};
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payload
next
payload
next
payload
next
payload
next
Note: payload may be
multiple members.
Linked List Linked List (continued)(continued)
• Items of list are usually same type• Generally obtained from new operator
• Each item points to next item• Last item points to null• Need “head” to point to first item!
• “Payload” of item may be almost anything• A single member or multiple members• Any type of object whose size is known at compile time• Including struct, union, char * or other pointers• Also arrays of fixed size at compile time (see p. 214)
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Usage of Linked ListsUsage of Linked Lists
• Not massive amounts of data• Linear search is okay
• Sorting not necessary• or sometimes not possible
• Need to add and delete data “on the fly”• Even from middle of list
• Items often need to be added to or deleted from the “ends”
21
Linked List Linked List (continued)(continued)
struct listItem {type payload;struct listItem *next;
};struct listItem *head;
22
payload
nextpayload
nextpayload
next
payload
next
Adding an Item to a ListAdding an Item to a List
struct listItem *p, *q;• Add an item pointed to by q after item pointed to by p
– Neither p nor q is NULL
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payload
nextpayload
nextpayload
next
payload
next
payload
next
Adding an Item to a ListAdding an Item to a List
listItem *addAfter(listItem *p, listItem *q){q -> next = p -> next;p -> next = q;return p;
}
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payload
nextpayload
nextpayload
next
payload
next
payload
next
Adding an Item to a ListAdding an Item to a List
listItem *addAfter(listItem *p, listItem *q){q -> next = p -> next;p -> next = q;return p;
}
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payload
nextpayload
nextpayload
next
payload
next
payload
next
Adding an Item to a ListAdding an Item to a List
listItem *addAfter(listItem *p, listItem *q){q -> next = p -> next;p -> next = q;return p;
}
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payload
nextpayload
nextpayload
next
payload
next
payload
next
Question: What to do if we cannotguarantee that p and q are non-NULL?
Adding an Item to a List Adding an Item to a List (continued)(continued)
listItem *addAfter(listItem *p, listItem *q){if (p && q) {
q -> next = p -> next;p -> next = q;
}return p;
}
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payload
nextpayload
nextpayload
next
payload
next
payload
next
Note test for non-null p and q
What about Adding an ItemWhat about Adding an Itembeforebefore another Item? another Item?
struct listItem *p;• Add an item before item pointed to by p (p != NULL)
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payload
nextpayload
nextpayload
next
payload
next
payload
next
What about Adding an ItemWhat about Adding an Itembeforebefore another Item? another Item?
• Answer:–– Need to search list from beginning to find previous
item– Add new item after previous item
• This is needed in PA#3– Insert item after earlier event times and before
later ones– Need to search the list
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Doubly-Linked ListDoubly-Linked List
struct listItem {type payload;listItem *prev;listItem *next;
};struct listItem *head, *tail;
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prev next
payload
prev next
payloadprev next
payload
prev next
payload
In-class exercise
:– how to
add a new item q after a lis
t
item p
Other Kinds of List StructuresOther Kinds of List Structures
• Queue — FIFO (First In, First Out)
• Items added at end• Items removed from beginning
• Stack — LIFO (Last In, First Out)
• Items added at beginning, removed from beginning
• Circular list• Last item points to first item• Head may point to first or last item• Items added to end, removed from beginning 31
Circular ListCircular List
listItem *addAfter (listItem *p, listItem *tail){if (p && tail) {
p -> next = tail -> next;tail = p;
} else if (p) {tail p -> next = p;
}return tail;
}32
payload
nextpayload
nextpayload
next
payload
next
struct listItem *tail;
Optional:–struct listItem *head;