Memory Management

CS101 2012.1

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Variable storage thus far Never used global variables All variables allocated inside functions,

passed by value or reference to other functions

In case of non-collection variables, the storage was always taken from the stack

In case of collection variables, storage was taken from the stack and the heap, but we did not need to understand how, or interfere

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Want a variable to outlive its scope Same customer can have a checking

account and a fixed deposit Should not make copies of customer record Supposing address or phone changes

Customer makeCustomer(…) { … }Vs.Customer *makeCustomer(…) { … }

For simplicity let’s start with int variables

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new and delete

int *pi = new int;

*pi = 5;

cout << pi << ' ' << *pi + 1 << endl;

delete pi;

pi points to an integer

Allocates four bytes fom heap

The value is always accessed

as *pi

Returns four allocated bytes to system: do this

exactly once!

The value is always accessed

as *pi

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Pointer syntaxTypeName *pointerVariableName; TypeName is int, float, double etc. or the

name of a class like vector<int> etc. * is the “pointer access” or “pointer

dereference” operator To read or write the storage holding the

value of type TypeName, use *pointerVariableNamein either the lhs or rhs

To read or write the pointer itself use pointerVariableName without a star

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Allocating arrays in the heapint nn = 5;

int *pi = new int[nn];for (int ix=0; ix < nn; ++ix) { cout << pi[ix] << endl;

cout << *(pi+ix) << endl;

}delete [] pi;

Type remains same

Allocates in heap space for nn ints

Returns space back to heap (which remembers size but won’t tell you)

Looks like array access

Contents of cell ix integers after address pi

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The null pointer Address 0 is not a legal address to read/write Programs reliably fail if they attempt to do so Therefore, int *pa = 0; is a safe

initializer until memory is allocated (or after it is freed)

But 0 is of type int, while pa is of type int * (pointer to int)

Therefore a type cast is requiredint *pa = (int*) 0;

Can do this will any type in place of int

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Null pointer for classesclass Customer {public: static Customer * const null;};

Customer * const Customer::null = (Customer*) 0;

Can now use Customer::null anywhere Cleaner than (Customer*) 0 everywhere

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Our own vector class: specificationclass Vector {public: Vector(); Vector(const Vector& other); ~Vector(); void insert(int p, float v); float remove(int p); float get(int p) const; int size() const;}

Promises that these methods will not modify the

Vector in any way

Default constructor: empty vector

Copy constructor

Destructor

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Vector implementation Three private fields Native array pa of floats allocated on heap int cap recording the number of floats that

can fit in the array (capacity) int siz recording the number of floats

currently in the array (from position 0 onward)

Invariant: siz cap Run out of space: allocate larger array, copy siz<< cap: allocate smaller array, copy

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insertvoid insert(int p, float v) { if (siz + 1 <= cap) { // enough space } else { float *nPa = new float[siz+1]; int wx = 0, rx = 0; while (rx < p) { nPa[wx++] = pa[rx++]; } nPa[wx++] = v; while (rx < siz) { nPa[wx++] = pa[rx++]; } if (cap > 0) { delete [] pa; } cap = siz = siz + 1; pa = nPa; }}

pa

p

nPa v

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Our own queue class: specification

class Queue { // of ints, saypublic: bool isEmpty() const; int removeFirst(); void pushLast(int val); void print() const;private: QueueElement *first, *last;};

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Queue implementation

struct QueueElement { int value; QueueElement *next;

QueueElement(int v) : value(v), next(null) { }}

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Printing a queue

QueueElement *pqe = first;while (pqe != null) { cout << pqe->value << endl; pqe = pqe->next;}

value next value next

pqe

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Constructor and destructor

Queue::Queue() { first=last=null; }

Queue::~Queue() { while (first != null) { QueueElement *pQe = first; first = first->next; delete pQe; }}

first next

pQe

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pushLastvoid Queue::pushLast(int val) { QueueElement *pQe = new QueueElement(val); if (first == null) { first = pQe; }

if (last != null){ last->next=pQe; }

last = pQe;}

last

val

pQe

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removeFirstint Queue::removeFirst() { int ans = first->value; QueueElement *pQe = first; first = first->next; delete pQe; return ans;}

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Binary search tree A binary tree is either empty Or it has a root node With two children left and right Each of which is a binary tree Suppose each node contains an int key Assume no duplicate keys for starters The tree is a search tree if

• All keys in left are smaller than the root• All keys in right are larger than the root

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Search tree from main()TreeNode *root = null;for (;;) { int key; cin >> key; if (root == TreeNode::null) { root = new TreeNode(key); } else { root->insert(key); }}

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Definition of the tree nodestruct TreeNode { const int key; TreeNode *left, *right; TreeNode(int _key) : key(_key) { left = right = null; } void insert(int nKey) { // while satisfying property }};

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insert, first cutif (nKey < key) { if (left == null) { left = new TreeNode(nKey); } else { left->insert(nKey); }}else { // repeat above, right instead of left}

Tedious, can be avoided if you have a good understanding of * and &

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void insert(int nKey)

TreeNode*& child = nKey < key? left : right;if (child == null) { child = new TreeNode(nKey);}else { child->insert(nKey);}

child is a reference to a pointer to a TreeNode

Recursive call from child node

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How to print the keys in ordervoid print() { if (left != null) left->print(); cout << key << ' '; if (right != null) right->print();}

main() { if (root != null) root->print(); cout << endl;}

1

2

3

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Adding a parent pointer Walking down from parent to child is easy Sometimes handy to be able to walk up from child

to parent as well, e.g., to find siblings root->up is null struct TreeNode { const int key; TreeNode *left, *right, *up; TreeNode(int _key, TreeNode *_up) : key(_key), up(_up) {

left = right = null; } void insert(int nKey) { … }};

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Constant pointer vs. pointer to constant Suppose we want the up pointer to be

constant This is declared asTreeNode * const up;

Content of TreeNode can change A pointer to a constant TreeNode would beconst TreeNode * up;

Not the same! Can also haveconst TreeNode * const up;

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Pointers vs. references

int a = 3, b = 5;int *px = &a;px = &b;// above statements do not// change values in cells// called a and bint &rx = a;rx = b;// this results in a = b = 5

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Multiple pointers to one record Generally necessary in applications E.g., fixed deposit accounts and checking

accounts pointing to shared customer record Care is needed in destructors Deleting a checking account should not

destroy a customer record Should deleting the last account of a

customer delete the customer record? Clear “home” collection whose destruction

deletes customer record