l2: pointerssfdv4001.wdfiles.com/local--files/lectures/l2_pointers.pdf · c[0] int 8 3 c[1] int 12...

L2: Pointers

Pointers.

Dynamic Memory Management.

SFDV4001 / L2 - Pointers / 1

Pointers

Assign a pointer a type.int *ptr;

Assign a pointer the address of a variable. int a = 0; ptr = &a;

Use a pointer to assign a value (dereferencing).*ptr = 9; // the value of a is now = 9.

Pointer operators are *, &, >


1: void foo(int v) 2: {3: v = 2000;4: }5:6: void bar(int &v)7: {8: v = 3000;9: }10:11: void zee(int *v)12: {13: *v = 4000;14: foo(*v);15: }

17: int main() {18: int a=1;19: int b=2;20: int c[3]={3,4,5};21: int *d = &a;22: int &e = a;23: 24: a = b+c[0];25: b = a;26: c[1] = *d;27: *d = 7;28: e = 8;29: 30: foo(a);31: bar(b);32: zee(d);33: }

Pointers


Execution point: Line 23

Variable Name Variable Type

a int 0 1b int 4 2

c[0] int 8 3c[1] int 12 4c[2] int 16 5d pointer to int 20 0e pointer to int 24 0

Stack Location in

Memory

Stack Contents



a int 0 5b int 4 2


Stack Location in

Memory

Stack Contents

Execution point: Line 24: a = b+c[0]



a int 0 5b int 4 5


Stack Location in

Memory

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Execution point: Line 25: b = a;



a int 0 5b int 4 5


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Memory

Stack Contents

Execution point: Line 26: c[1] = *d;



a int 0 7b int 4 5


Stack Location in

Memory

Stack Contents

Execution point: Line 27: *d = 7;



a int 0 8b int 4 5


Stack Location in

Memory

Stack Contents

Execution point: Line 28: e = 8;



a int 0 8b int 4 5

c[0] int 8 3c[1] int 12 5c[2] int 16 5d pointer to int 20 0e pointer to int 24 0v int 28 8

Stack Location in

Memory

Stack Contents

Execution point: Line 30: foo(a); > Line 2



a int 0 8b int 4 5

c[0] int 8 3c[1] int 12 5c[2] int 16 5d pointer to int 20 0e pointer to int 24 0v int 28 2000

Stack Location in

Memory

Stack Contents

Execution point: Line 3: v = 2000;



a int 0 8b int 4 5

c[0] int 8 3c[1] int 12 5c[2] int 16 5d pointer to int 20 0e pointer to int 24 0v pointer to int 28 4

Stack Location in

Memory

Stack Contents

Execution point: Line 31: bar(b); > Line 7



a int 0 8b int 4 3000


Stack Location in

Memory

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a int 0 8b int 4 3000


Stack Location in

Memory

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Execution point: Line 32: zee(d); > Line 12



a int 0 4000b int 4 3000


Stack Location in

Memory

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Execution point: Line 13: *v = 4000;



a int 0 4000b int 4 3000

c[0] int 8 3c[1] int 12 5c[2] int 16 5d pointer to int 20 0e pointer to int 24 0v pointer to int 28 0v int 32 4000

Stack Location in

Memory

Stack Contents

Execution point: Line 14: foo(*v); > Line 2



a int 0 4000b int 4 3000

c[0] int 8 3c[1] int 12 5c[2] int 16 5d pointer to int 20 0e pointer to int 24 0v pointer to int 28 0v int 32 2000

Stack Location in

Memory

Stack Contents




a int 0 4000b int 4 3000


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Execution point: Line 15: };



a int 0 4000b int 4 3000


Stack Location in

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Execution point: Line 33: }


Passing Argument

struct IntArray {

int contents[500];

};

int sum(IntArray a); //pass struct by value.

int sum(IntArray *a); //pass struct by ref. with pointer

int sum(IntArray &a); //pass struct by reference

int sum(const IntArray &a); //pass const struct. by reference


C++ has both statically allocated and dynamically allocated memory (like C, unlike Java).But you should never have to use malloc/free in a C++ program.Always use new and delete.

• Is not garbage collected unless 3rd party libraries are used.

Dynamic Memory Management


12:13: a = new int;14: b = new int;15: a = b;16: delete a;17: delete b;18: }

1 : int main() {2 : int *a = new int;3 : *a = 3;4 : delete a;5 : 6 : a = new int;7 : *a = 4;8 : int *b;9 : b = a;10: delete a;11: delete b;

Dynamic Memory Management



a ptr to int 0 undefined

Type Heap Location Heap Contents

Stack Location in Memory

Stack Contents

Execution point: Line 1: int main() { WRONG ?



a ptr to int 0 2000

Type Heap Location Heap ContentsInt 2000 Undefined


Stack Contents

Execution point: Line 2: int *a = new int;



a ptr to int 0 2000

Type Heap Location Heap ContentsInt 2000 3


Stack Contents

Execution point: Line 3: *a = 3;



a ptr to int 0 2000



Stack Contents

Execution point: Line 4: delete a;



a ptr to int 0 3000

Type Heap Location Heap ContentsInt 3000 Undefined


Stack Contents

Execution point: Line 6: a = new int;


Execution point: Line 7: *a = 4;


a ptr to int 0 3000



Stack Contents


Execution point: Line 8: int *b;


a ptr to int 0 3000b ptr to int 4 undefined



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Execution point: Line 9: b = a;


a ptr to int 0 3000b ptr to int 4 3000



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Stack Contents


Execution point: Line 11: delete b;





Stack Contents


Execution point: Line 13: a = new int;



Type Heap Location Heap Contentsint 3456 undefined


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Execution point: Line 14: b = new int;



Type Heap Location Heap Contentsint 3456 undefinedint 5678 undefined


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Execution point: Line 15: a = b;



Type Heap Location Heap Contentsint 3456 undefinedint 5678 undefined


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Stack Contents


Execution point: Line 17: delete b;





Stack Contents


l2: pointerssfdv4001.wdfiles.com/local--files/lectures/l2_pointers.pdf · c[0] int 8 3 c[1] int 12...

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