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    Pointers and DynamicMemory

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    Memory Management

    Memory managementArrays

    Dynamic memoryDynamic arrays and pointersBuilding a vector class

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    Memory Management

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    Memory Requirements

    codestorage

    datastorage

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    Memory Management

    When a program runs memory hasto be allocated for its processes and

    its variablesThat is, working memory, or RAM

    There needs to be a system foreciently allocating such memory We will !ust consider how memory is

    allocated to program data "variables#

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    RAM

    We can consider Random AccessMemory "RAM# as a long se$uence of

    bytes %tarting with &

    'nding with the amount of main memory"()#

    RAM is addressable and supportsrandom accessThat is, we can go to byte *,++,-)*

    without having to visit all the preceding

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    RAM Illustrated

    & ) * + . / - 0

    0 )&-+-.)1)/ )&-+-.)1)- )&-+-.)1)1 )&-+-.)1)2 )&-+-.)1*& )&-+-.)1*) )&-+-.)1** )&-+-.)1*+

    3) 4B 5 ),&-+,-.),1*. bytes

    6onsider a computer with )4B3of RAM

    This is a fairly abstractillustration "so ignores a

    variety of issues#

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    Goals

    As much as possible we would like tosatisfy three goals when allocating

    memoryTime eciencyThat is, we should be able to $uickly 7nd

    sucient memory to store a variable%pace eciency We don8t want to waste memory

    9ow overhead

    We don8t want a large amount of

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    Stack Memory

    There is a very simple way oforgani:ing memory that meets these

    goals Assign memory locations to variables in

    se$uence With no gaps

    ;nce a variable is no longer re$uiredrelease the memory, allowing it to beover(written

    We will call this organi:ation the

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    Stack Memory SimpleExample

    9et8s look at a simple e/-=

    ?otice that this e/-

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    Stack and Functions

    9et8s look at a more comple).)= double s$Cr 5s$uare"radius#= return s$Cr 3 pi=

    double s$uare"double

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    Anoter Example

    int main"# int r 5 += double area 5circleArea"r#=

    double circleArea"doubleradius# double pi 5 +>).)= double s$Cr 5s$uare"radius#= return s$Cr 3 pi=

    double s$uare"double

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    Stack Memory

    n stack memory variables for afunction are allocated the ne

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    Re!erence Parameters

    %ome functions have referenceparameters

    There are two common reasons fordoing this'ciency int sum(const vector & v)

    %ums the contents of the vectorAnd because we want the changes

    made to the variable to persist after

    the function

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    Stack Pro"lems

    There is one big issue with stackmemory

    Because memory is allocated inse$uence it is not possible to change thebyte si:e of a variable

    %trings and vectors fre$uently

    change si:e t is more correct to say that a string

    variable may refer to strings of diFerent

    si:es

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    #anging a $ector%s Si&e

    int main"# vector GintHvec"+#= vI&J 5 )= vI)J 5 *= vI*J 5 += double vid 5*>+2-=

    insertCnesi:e"# L)#=

    main memory

    vec vid3

    ) * + *>+2-

    3very importantdouble

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    #anging a $ector%s Si&e

    int main"# vector GintHvec"+#= vI&J 5 )= vI)J 5 *= vI*J 5 += double vid 5*>+2-=

    insertCnesi:e"# L)#=

    main memory

    vec v3vid3

    * +

    3v is a referenceto vec, itactually contains the byte

    address of vec

    *>+2-

    3very importantdouble

    +n

    oi

    .)

    This is a problem,we8ve !ust corruptedthe 7rst . bytes ofvid

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    'ypes (! Memory

    t turns out you can divide memoryinto three broad categories

    %tatic Automatic

    DynamicThese types of memory are typically

    used for diFerent sorts of programdata Although 6LL gives programmers

    choice over which type of memory to

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    datastoragestatic

    Storage

    codestorage

    automatic

    dynamic

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    Static Storage

    %tatically stored variables last for thelifetime of a program

    The number of static variables doesnot change as a program runs %o no special system is re$uired to

    maintain them%tatic storage is used for global

    constants And other things 0

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    Automatic Storage

    Eunction variables and parametersuse automatic storage by default

    Automatic variables are typicallymanaged on a stack as we havebeen discussingThe stack is an allocated area of

    memory that grows and shrinks asfunctions are called

    Memory is allocated se$uentially

    "without gaps#

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    Functions and Memory

    A variable de7ned in a function blockonly persists for the lifetime of that

    function call nless it is declared as static

    6onsider what memory might beallocated when a function is running Memory re$uired for the functionNs data

    and only re$uired during the functioncall

    Memory that is to persist beyond the8

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    ArraysA Digression Erom Memory

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    Arrays

    Before looking at dynamic memorylet8s look at a basic data structure O

    the array Arrays are used to store multiple values

    of the same type, much as vectors do

    '

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    )at Is An Array*

    An array variable is a collection ofother variablesQou can think of an array as something

    that contains variablesThis is important because an integer

    array is notan integer, it is a collectionof integers

    The items stored in an array"elements# are stored se$uentially inmain memory

    ust like variables on the stack

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    Declaring Arrays

    An array is declared with a type, and[]s to indicate that the variable is an

    arrayThe type declares the type of the array

    elements

    intmyArray[10]

    type of thedata stored in

    the array

    brackets declarethat the variable is

    an array

    si:e of thearray

    name of thearray

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    Array Indexing

    The elements of the array areaccessed using an inde

    This does nothappen with

    other pass(by(value

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    )at%s an Array*

    To understand what is going on inthe previous e

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    More A"out Arrays

    An array is a se$uence of bytes inmain memory reserved for the array

    contents e>g> int arr[10]; Reserves .& contiguous bytes

    An intis . bytes, . 3 )& 5 .&

    'ach element can be referenced usinginde

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    More A"out Indexing

    6onsider this assignment statementV arr[] = 63;

    To 7nd this array element 9ook up the address stored in arr Multiply type si:e ". for an int# by the

    indee>

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    'e si&eo! (perator

    The sizeofoperator allows us to 7ndout the si:e of a type or variable

    cout GG si:eof"int#= cout GG si:eof"YcN#= ZZsi:e of a char

    cout GG si:eof"6ylinder#= ZZthe si:e of a6ylinder

    sizeofreturns a number of bytes

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    Re!erence (perator

    The K operator is also used to createreference parameters

    Where a variable is passed to a functionby reference, rather than by value

    Reference parameters are notpointersA reference parameter is given the

    address of the argument passed tothe function And thereafter behaves like a normal

    variable

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    (4erloaded (perators

    6LL tends to re(use operators The meaning can be determined by the conte

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    Pointers andAssignment

    int = 16;inty = 77;int5'1 = :; //assi.n '1 the address o(

    int5'6 = :y;//assi.n '6 the address o( y

    p)

    p*

    )

    *--

    arr"i-e;int5 arr = newint[arr"i-e];

    The variable is apointer, used tostore an address

    6reates thisarray in dynamic

    memoryData created in dynamicmemory persists until theapplication is terminated,regardless of which

    function it was created in

    +sing D namic Arra s

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    +sing Dynamic Arrays

    ;nce a dynamic array has beencreated it can be used like any otherarray ndividual array elements are accessed

    using an indeoperator can be used to referto an ob!ect8s methods using apointer "tudent5 ' = new "tudent)9101;

    '>addFrade)3, 3C33;The >operator is onlyused with

    pointers t should not be used with regular ob!ect

    variables

    se dot notation for such variables

    Deletion

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    Deletion

    %tack "non dynamic# memory isreleased automatically When a function 7nishes e

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    Deleting Memory

    The keyword delete is used to freedynamically allocated memory

    When deleting a dynamic arraydelete must be followed by IJs

    Memory space that has beendeallocated cannot be freed again

    Attempting to do this results in an error

    delete (perator

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    delete(perator

    The delete operator is used to freememory that has been dynamicallyallocated int 5 ' = new int;

    5' = 63;

    CCC

    delete ';'ach new should have a matching

    delete

    Although often not in the same function

    6e5 and Delete

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    6e5 and Delete

    int' arr = )*++;

    int s, = 0;

    cout -- lease enter si,e;

    cin s,;

    arr = ne int[s,];int' ("int = ne int12;

    delete ("int;

    delete[] arr;

    arr = )*++;delete[] arr;

    delete ("int;

    frees up . bytes

    frees up the .& bytesallocated for the dynamic

    arraysafe since arris ?99, but

    pointlessresults in an

    indicates that arrdoes not point to

    anythingallocates space for an int

    array of si:e sz, andreturns its address which is

    then assigned to arr

    3uilding a Simple

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    3uilding a Simple$ector #lassBringing t All Toegether 0

    3uilding a $ector #lass

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    3uilding a $ector #lass

    We will write a class that usespointers and dynamic arrays tobehave much like a vector

    To keep things simple we will !ust mplement a vector of doubles rather

    than a template for any type

    mplement pushCback and si:e methods %tart by allowing access to the vector

    elements using get"inde


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