Pointers - MST · Like other variables or constants, pointers must be declared General pointer...

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Dereferenceassignment

Issues?

Pointers to pointers

Careful cancellation

Pointers andarrays

What is an array?

Array and pointerindexing

Pointerarithmetic

Type sizing

Operatorprecedence

Pointers

Comp Sci 1575 Data Structures

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Pointers

“Writing in C or C++ is like running a chain saw with all thesafety guards removed,” Bob Gray.

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Pointers

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Outline

1 Data Structures?

2 Definitions

3 AddressesMemory

4 PointersDeclarationInitializationDereference

Dereference assignmentUsesIssues?Pointers to pointersCareful cancellation

5 Pointers and arraysWhat is an array?Array and pointer indexing

6 Pointer arithmeticType sizingOperator precedence

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Data Structures yet?

• We are finishing C++ first, and won’t start the actualData Structures material for a while.

• Reserve judgment about how fun you consider the materialto be until then!

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Outline

1 Data Structures?

2 Definitions

3 AddressesMemory

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Definition of a pointer

• A pointer is a variable whose value is the address ofanother variable.

• What is an address?

• How do you get the memory address of a variable?

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Outline

1 Data Structures?

2 Definitions

3 AddressesMemory

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Address of an int?

How do you get the memory address of an int for example?

i n t genePos = 4 3 5 ;cout << &genePos << e n d l ; // 0 x7 f f cb158c144

• & is the “address of” operator

• What is that weird number?

• How is memory structured?

• It is different than, but similar to the reference typespecified with & (more to come later)

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Outline

1 Data Structures?

2 Definitions

3 AddressesMemory

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Memory as a tape

i n t genePos = 4 3 5 ;cout << &genePos << e n d l ; // 0 x7 f f cb158c144

Name of variable Storage address Value

0x7ffcb158c140

genePos 0x7ffcb158c144 435

0x7ffcb158c148

0x7ffcb158c14c

0x7ffcb158c150

0x7ffcb158c154

• Variable name is an alias for address itself,which is accessible via the & operator

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Outline

1 Data Structures?

2 Definitions

3 AddressesMemory

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

What is a pointer?

i n t genePos = 4 3 5 ;i n t ∗p1 = &genePos ;cout << p1 << e n d l ; // 0 x7 f f cb158c144

0x7ffcb158c140

0x7ffcb158c148

0x7ffcb158c14c

p1 0x7ffcb158c150 0x7ffcb158c144

0x7ffcb158c154

• p1 is a pointer – a variable whose value is the address ofanother variable.

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Kahoot

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Outline

1 Data Structures?

2 Definitions

3 AddressesMemory

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Pointer declaration (type specific)

• Like other variables or constants, pointers must bedeclared

• General pointer variable declaration is:type ∗ pointerName = &varNamewhere type is the pointer’s base type

• Pointers have a type (of the thing they address) restriction(e.g., type is “pointer to an int” or “pointer to a double”)

• Can cast between pointer types, e.g., static cast, butshould not generally to non-pointer types. This won’tusually be needed.

Declaring types of pointers:

i n t ∗numberObject ; // p o i n t e r to an i n tchar ∗ c h a r a c t e r O b j e c t ; // p o i n t e r to a cha rdouble ∗ d e c i m a l O b j e c t ; // p o i n t e r to a doub l e

DataStructures?

Definitions

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Memory

Pointers

Declaration

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Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Pointer declaration

i n t ∗p1 , ∗p2 ; // both p1 and p2 a r e p o i n t e r si n t ∗p1 , p2 ; // p2 i s not a p o i n t e r !

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Outline

1 Data Structures?

2 Definitions

3 AddressesMemory

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Pointer initialization variations

i n t genePos ; // what i s the v a l u e o f genePos ?i n t ∗p1 = &genePos ;

i n t genePos ;i n t ∗p1 ; // what does p1 po i n t to ?p1 = &genePos ;

i n t genePos ;i n t ∗p1 = &genePos ;i n t ∗p2 = p1 ;

// To s p e c i f y no t a r g e ti n t genePos ;i n t ∗p1 = n u l l p t r ; // or =NULL , or =0 ( o l d C++)// Use n u l l p t r f o r t h i s c l a s s !

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Outline

1 Data Structures?

2 Definitions

3 AddressesMemory

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Contents of object being pointed to

i n t genePos = 4 3 5 ;i n t ∗p1 = &genePos ;cout << ∗p1 << e n d l ; // outpu t s : 435i n t x = ∗p1 ;cout << x << e n d l ; // outpu t s : 435

0x7ffcb158c140

0x7ffcb158c148

0x7ffcb158c14c

0x7ffcb158c154

• Contents of operator also known as derefernece operator, *

• This is not the same as the * used during initialization;the * on lines 2 and 3 are different

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Assignment via dereferenced pointer

i n t genePos = 4 3 5 ;i n t ∗p1 = &genePos ;cout << ∗p1 << e n d l ; // outpu t s : 435∗p1 = 2 4 8 ;cout << ∗p1 << e n d l ; // outpu t s : 248

0x7ffcb158c140

genePos 0x7ffcb158c144 435 changed to 248

0x7ffcb158c148

0x7ffcb158c14c

0x7ffcb158c154

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Outline

1 Data Structures?

2 Definitions

3 AddressesMemory

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Why are pointers useful?

• Used for new memory during execution,e.g., dynamic memory

• Which is useful for what?User-defined quantities or sized of objects

• Can refer/pass large data structures without copying,for efficiency

• Can specify relationships among data,e.g., linked lists, trees, graphs, coming up

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Outline

1 Data Structures?

2 Definitions

3 AddressesMemory

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Pointers are dangerous

With great power comes great responsibility! You can breakmore fundamental things with pointers... Get ready forsegfaults.

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Outline

1 Data Structures?

2 Definitions

3 AddressesMemory

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Address of address of?

i n t genePos = 4 3 5 ;i n t ∗p1 = &genePos ;i n t ∗∗metaP = &p1 ;cout << metaP << e n d l ; // ??cout << ∗metaP << e n d l ; // ??cout << ∗∗metaP << e n d l ; // ??

0x7ffcb158c140

0x7ffcb158c148

0x7ffcb158c14c

metaP 0x7ffcb158c154 0x7ffcb158c150

Remember, ** for declaring and dereferencing are different,as are the two *

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Outline

1 Data Structures?

2 Definitions

3 AddressesMemory

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

What about these statements?

cout << &∗p1 << e n d l ; // ??cout << ∗&p1 << e n d l ; // ??cout << &∗&∗p1 << e n d l ; // ??cout << ∗&∗&p1 << e n d l ; // ??

0x7ffcb158c140

0x7ffcb158c148

0x7ffcb158c14c

metaP 0x7ffcb158c154 0x7ffcb158c150

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Outline

1 Data Structures?

2 Definitions

3 AddressesMemory

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Outline

1 Data Structures?

2 Definitions

3 AddressesMemory

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Kahoot

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Pointers and arrays

What is an array really?

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Pointers and arrays

What is an array really?

i n t a [ 6 ] = {1 , 7 , 3 , 4 , 2 , 8} ;cout << a [ 2 ] << e n d l ; // ou tpu t s : 3cout << a << e n d l ; // ??cout << ∗a << e n d l ; // ??

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Pointers and arrays

• Arrays are like pointers, but const, addressing the firstelement of the array• Below, mypointer can be assigned a different address, but

myarray can’t.

i n t myarray [ 2 0 ] ;cout << myarray << e n d l ; // 0 x7 f f cb158c140

i n t ∗m y p o i n t e r ;

// Va l id , why no & ope r a t o r b e f o r e myarray ?// R e c a l l p a s s i n g a r r a y s by r e f e r e n c e ?m y p o i n t e r = myarray ;cout << m y p o i n t e r << e n d l ; // 0 x7 f f cb158c140

// I n v a l i d , why?myarray = m y p o i n t e r ;

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Outline

1 Data Structures?

2 Definitions

3 AddressesMemory

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Pointers and arrays

• The following have the same result:

i n t a [ 6 ] = {1 , 7 , 3 , 4 , 2 , 8} ;

a [ 5 ] = 0 ; // a [ o f f s e t o f 5 ] = 0cout << a [ 5 ] << e n d l ; // ou tpu t s : 0cout << ∗( a + 5) << e n d l ; // ou tpu t s : 0

∗( a+5) = 1 ; // a [ o f f s e t o f 5 ] = 1cout << a [ 5 ] << e n d l ; // ou tpu t s : 1cout << ∗( a + 5) << e n d l ; // ou tpu t s : 1

Why does adding 5 to array a work?

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Outline

1 Data Structures?

2 Definitions

3 AddressesMemory

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Pointer arithmetic

i n t a [ 6 ] = {1 , 7 , 3 , 4 , 2 , 8} ;i n t ∗pa = a ;cout << pa + 2 << e n d l ; // 0 x7 f f cb158c148cout << ∗( pa + 2) << e n d l ; // 3cout << pa++ << e n d l ; // 0 x7 f f cb158c144cout << ∗pa << e n d l ; // 7

a[0] or *a 0x7ffcb158c140 1

a[1] or *(a+1) 0x7ffcb158c144 7

a[2] or *(a+2) 0x7ffcb158c148 3

a[3] or *(a+3) 0x7ffcb158c14c 4

a[4] or *(a+4) 0x7ffcb158c150 2

a[5] or *(a+5) 0x7ffcb158c154 8

a 0x... 0x7ffcb158c140

pa 0x... 0x7ffcb158c140

• Why increments of 4?

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Pointer and array arithmetic

i n t ∗pa = a ;cout << pa + 2 << e n d l ; // 0 x7 f f cb158c148cout << a + 2 << e n d l ; // 0 x7 f f cb158c148

cout << ∗( pa+2) << e n d l ; // 3cout << ∗( a+2) << e n d l ; // 3

cout << pa [ 2 ] << e n d l ; // 3cout << a [ 2 ] << e n d l ; // 3

cout << pa++ << e n d l ; // 0 x7 f f cb158c144// cout << a++ << end l ; // not v a l i d , a r r a y con s t

cout << ∗pa << e n d l ; // 7cout << ∗a << e n d l ; // 1

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Kahoot

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Each type is a different size

Use sizeof(p) without the ’*’ operator to determine thememory utilized on your system for types like int, which aredifferent per system.

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Messy pointer arithmetic

Postfix operators (- - , ++), have higher precedence thanprefix operators (dereference *).

i n t genePosArray [ 3 ] = {4 3 5 , 1 2 3 , 9 8 7} ;i n t ∗p = genePosArray ;cout << p << e n d l ; // 0 x7 f f e35b36ee0cout << ∗( p++) << p << e n d l ; // 435 0 x7 f f e35b36ee4p = genePosArray ;cout << ∗p++ << p << e n d l ; // 435 0 x7 f f e35b36ee4p = genePosArray ;cout << ∗(++p ) << p << e n d l ; // 123 0 x7 f f e35b36ee4p = genePosArray ;cout << ∗++p << p << e n d l ; // 123 0 x7 f f e35b36ee4p = genePosArray ;cout << ++(∗p ) << p << e n d l ; // 436 x7 f f e35b36ee0p = genePosArray ;cout << ++∗p << p << e n d l ; // 437 x7 f f e35b36ee0p = genePosArray ;cout << (∗p)++ << p << e n d l ; // 437 x7 f f e35b36ee0

DataStructures?

Definitions

Addresses

Memory

Pointers

Declaration

Initialization

Dereference

Issues?

Pointers andarrays

What is an array?

Pointerarithmetic

Type sizing

Operatorprecedence

Next time

Dynamic memory (heap, stack, garbage collection, danglingpointers), pointers to classes and structs, const pointers, arraysof pointers, void pointers, pointers to functions, returningpointers from functions

Pointers - MST · Like other variables or constants, pointers must be declared General pointer...

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