©Zachary Wartell, UNCC9/28/2006 11:30 AM 1 Thinking in C++ Revision: 10/16/2007 8:26:20 PM...

©Zachary Wartell, UNCC 9/28/2006 11:30 AM 1

Thinking in C++

Revision: 10/16/2007 8:26:20 PMCopyright Professor Zachary Wartell, University of North Carolina

All Rights Reserved


C++ Systems Programming Language

●Systems Programming: operating systems, database systems, device drivers, compilers, byte-code interpreters, high-level graphics libraries (‘game engines’)

-performance matters: -generate native machine language instructions-may hand optimize with assembly lang.

-memory management approaches:-each type of above system is best served by diff. memory

management approach-single system may need multiple approaches

●Applications Programming: spreadsheet, word processor, WWW stuff, game scripts

-performance: easy of programming versus run-time efficiency-memory management: may accept standard garbage collection

●C = ‘platform independent assembly language’, C++ = C +OOP

-for systems programming, for applications programming (?)


C++ & System Programming Conceptual Stumbling Blocks

• file organization – separate code implementation and class/type definitions

• variables – do variables name an object or a reference to an object?

• pointers – addresses, address arithmetic, arrays, address for function parameter, function pointer

• explicit dynamic memory allocation and deallocation• constructors/destructors• C++ reference• C++ polymorphism• memory errors – C++ says “Yes, Sir!”


library code

header files

C++ Files

• source code files (.cpp, etc.) – function and member function implementation and global variable definitions

• header files (.h or no extension) – data types, function prototypes, global variable declarations used by multiple source code files

COMPILER

LINKER

object code

executable code

#include

source code header files

#include

library code

header files

std C++ library

other library(s)


library code

header files

Example

COMPILER

LINKER

object code

executable code

#include

source code header files

#include

library code

header files

std C++ library

other library(s)

List.cppmain.cpp

List.h

main.obj,List.obj

List.exe

iostream

cppstdlib.li

b


Example

main.cpp#include "List.h"#include <iostream>…int main (int argc, char** argv) { … }

List.cpp#include "List.h"

// constructorList::List() {…}

// destructorList::~List() {…}

// print listList::print () {…}

List.h

class List {

… List(); ~List(); void print();

… ListNode* head; ListNode* tail; };


Java vs C++ Variables

• variables of built-in type versus class type• process address space• level of indirection

– allocation and operator= – operator==– pass-by value


C++ and Java Variables• C++

– class variables and built-in type variables are placeholders for the memory address of the class or built-in objects

• Java:– built-in variable is placeholder for memory address of built-in

object– but class variable is placeholder for memory address of

reference to class object

Javaclass Vehicle {…}

…int a,b;Vehicle c,d;a = 10;c = new Vehicle;d = c;d.wheelCount = 4;

C++class Vehicle {…}

…int a,b;Vehicle c,d;a = 10;c.wheelCount = 4;d.wheelCount = 6;


Review: Process Address Space

Code

StaticData

Stack

0x00000000

0xfffffffeHeap

0xffffffff

….

user spaceos space

unallocated

Stack Frame

Arguments

IPFP

Local Variables


Level of Indirection – instance allocation and =

0x000100100x000100140x00010018

C++int a;double f;Vehicle c,d;a = 10;c.wheelCount = 4;d = c; // gets copy of cd.wheelCount = 6;

af

c

d

0x0001001c

…

0x00010034

…

Javaint a;double f;Vehicle c,d;a = 10;c = new Vehicle;d = c; // d is synonym of cd.wheelCount = 4;

0x000100100x000100140x00010018

afc 0x0001001cd 0x00010020

0x0ffff5020

0x0ffff50200x0ffff5020

MyClass object?

Heap Stack

Stack


Level of Indirection – == C++int a,b; Vehicle c,d;…if (a==b) {…}…if (c==d) {…}

c

d

0x0001001c

…

0x00010034

…

Javaint a,b; Vehicle c,d;…if (a==b) {…}…if (c==d) {…}…if (c.equals(d)) {…}

0x000100100x000100140x00010018

ab

c 0x0001001cd 0x00010020

0x0ffff5020


MyClass object?

Heap Stack

Stack

defined in Vehicle class

e

0x000100100x00010014

abe 0x00010018


Level of Indirection – pass by value

0x000100100x000100140x00010018

C++…void foo(Vehicle v, int i) {v.wheelCount=0;i=10;}…int a,b; Vehicle c,d;…foo(c,a)

af

c

d

0x0001001c

…

0x00010034

…

Java…void foo(Vehicle v, int i) {v.wheelCount=0;i=10;}…int a,b; Vehicle c,d;…q.foo(c,a)

0x000100100x000100140x00010018

afc 0x0001001cd 0x00010020

0x0ffff5020


Vehicle object?

Heap Stack

Stack


Pointers

• What are they?• Pointer arithmetic• Dynamic allocation: new, delete• Relation to C++ arrays (non-dynamic allocation)• Dynamic allocation - arrays: new[], delete[]• Pointers and pass-by-value• NULL


Pointer Type• stores an address of another object• syntax

– declaration typename *variableName;

– address-of operator: &– pointer dereference operator:

*varName(*varName).memberName varName->memberName

C++int a; int* pi;Vehicle c; Vehicle* pv;pi = &a;*pi = 10;pv = &c;pv->move(NORTH,5)c.move(NORTH,10)

0x000100100x000100140x00010018

api

c

pv

…

0x00010020

…

Stack

0x00010010

0x00010018


Pointer Type - = and ==int a,b; int *pi,*pj;Vehicle c,d; Vehicle *pv1,*pv2;pj = pi = &a;pv1 = &c;pv2 = &d;

if (pj == pi) {…}…if (pv1 == pv2) {…}…if (*pj == *pi) {…}…if (*pv1 == *pv2) {…}

0x10

0x18

a

c

pi

Stack

0x100x14b

0x1cpj0x20

d 0x38

pv2pv1 0x50

0x54

0x10

0x200x38


pv2

Dynamic allocation/deallocation: new and delete C++int* pi;Vehicle *pv1,*pv2;pi = new int;pv1 = new Vehicle;pv2 = new Vehicle;*pi = 10;pv1->move(NORTH,10);pv2->move(EAST,15);…delete pv1;…delete pv2;…delete pi;

0x000100100x000100140x00010018

pipv1

Stack …..

0xffff5020

0xffff5020

Heap

?

0xffff5030

?

Vehicle object

0xffff5030

…..

…..

?

0xffff5060

Vehicle object

0xffff5040


C++ Pointers – Address is accessible

• In other languagespointer is an opaque type. In C++, access as numeric value is possible.

C++int a,b;int* pi;pi = &a;*pi = 10;cout << (unsigned int)pi << endl;

C++int* pi;pi = reinterpret_cast<int*>(0x0000ffff);*pi = 10;


C++ - Address Arithmetic

• add an integer to apointer to get newpointer

• subtract pointer from pointer to get integer

• Pointer arithmetic takes into account size of pointed to type!

C++char a[10]="hello";char *p1, *p2;for (p1 = p2 = &a[0]; *p2 != '\0‘; p1 += 1, p2 += 2) cout << *p1 << ‘ ‘ << *p2 << endl;

C++char a[10]="hello";char *p1, *p2;p1 = &a[0]; p2 = &a[9];cout << (int)(p2-p1) << endl;


…int two[2],ten[10];int *pi; *(ten+9) = 9;// same as ten[9] = 9;pi = ten; // cast int[X] to int*,OK!*pi = 0;for(i=1;i<=8;i++)

ten[i]=i;for(pi=&ten[1]; pi != &ten[9]; pi++)

(*pi)++;// two = pi; casting int* to

int[X] illegal

C++ Arrays• C++ arrays and pointers are closely related

– a typename[X] can be type cast to a typename*

0x100x14

a

pi

ten

0x3C

…

two0x44 0x14

Stack …..


Dynamic arrays: new[] and delete[] C++int four[4];int* pi;pi = new int [7];…delete[] pi;

0x10four

pi

Stack …..

Heap

?

0xffff5030

int[7]

…..

…..

0x20 0xffff5030


NULL

• How to indicate that a pointer points to nothing?

C++int a;int* pi=NULL;*pi = 10; // run-time error…pi = &a;*pi = 10; // OK…pi = new int;*pi = 10; // OK…delete pi;pi=NULL; // good practice!


Pointers + pass-by-value = output parameter

• Function parameters are pass by value – to change the actual parameter, pass pointer to target variable and in function dereference pointer to change target variable

C/C++void upperCase(char* pc) {

if (isalpha(*pc))*pc = *pc - 'a' + 'A';

} …char a[10]="Hello"; int i;for (i=0;a[i]!='\0';i++)

upperCase(&a[i]);

C++ programmersdon’t use pointers

for output arguments.

See C++ Reference (#27)


Linked List Example

class List {class Node { char text[10];

Node* next;void printAll(void);};

List(); Node* head; void print(void); };

List::List () {head=NULL;}void List::print() { if (head != NULL) head->printAll();}void List::Node::printAll(void)

{cout << text << endl;if (next != NULL) next->printAll();}

List.h

List.cpp


Destructor

class List {class Node { Node* next;

}; ~List(); Node* head;};

List::~List () { Node* n; for (n=head;n!=NULL;n=n->next)

delete n; }

List.h

List.cpp

• destructor – (~classname()) automatically called when object is deallocated (either automatically or via delete)


Function Pointer

• Function Pointer - holds address of function’s first machine instruction– rettype (*pointerVarName)(type,type,...);– dereferencing syntax: same as a function call– address arithmetic not relevant

void doItTenTimes(int (*funcPtr)(int), int ten[10]) { for (int i=0;i<10;i++)

ten[i] = funcPtr(ten[i]) }int plusone(int i) { return i+1;}int minusone(int i) { return i-1;}…{

int ten[10];doItTenTimes(plusone,ten);doItTenTimes(minusone,ten);

}


Function Pointer example

void List::Node::traverseAll(NodeFunction nf) { nf (text); if (next!=NULL) next->traverseAll(nf); }void SayHello (char text[]){cout << "Hi" << text …;}void SayGoodbye (char text[]) {cout << “Bye" << text …;}…{

List list;… make a list …list.traverse(SayHello);list.traverse(SayGoodbye);

}

class List { typedef void (*NodeFunction) (char text[]); class Node { char text[10];

Node* next;void traverseAll (NodeFunction nf);};

void traverse(NodeFunction nf); };

List.h

List.cpp


C++ Reference – typename &• Not present in C; similar to pointer but

– no dereference operator needed (nor possible) – reference can only be initialized to reference another

variable; it can’t be assigned a new referent later on– use reference function parameter instead of pointer for

output function parameter and for polymorphism• denoted by: typename & variable;

void upperCase(char& pc) {if (isalpha(pc))

pc = pc - 'a' + 'A';}

…char a[10]="Hello";for (i=0;a[i]!='\0';i++)

upperCase(a[i]);

compare to slide #22


C++ polymorphism – (1)

• method invocation – only polymorphic on pointers or referencesclass Shape { virtual void draw(); };

class Circle { void draw(); };

Shape s1,*s2;Circle c1;c1.draw(); // Circle::draws1.draw(); // Shape::draw

if (makeCircle) s2 = new Circle();else s2 = new Shape();// Circle::draw or Shape::draws2->draw();


C++ polymorphism – (2)

• method invocation – only polymorphic on pointers or referencesvoid foo (Shape s1, Shape& s2) { s1.draw(); // Shape::draw // Circle::draw or Shape::draw s2.draw(); }

class Shape { virtual void draw(); };



C++ polymorphism - virtual

• class method is only polymorphic if the base class declares them as virtual; otherwise overriding a method in derived class will not be polymorphic

class Shape { virtual void draw(); };

class Shape { void draw(); };



if (makeCircle) s2 = new Circle();else s2 = new Shape();// always Shape::draws2->draw();

if (makeCircle) s2 = new Circle();else s2 = new Shape();// Shape::draw || Circle::draws2->draw();


How pointer bugs manifest themselves - ‘Memory Errors’

• Memory Leaks• Memory Access Errors - C/C++ won’t protect

programmer from stomping on random memory locations! (C++ - "Yes, Sir!")– Invalid Array Index– Bad Pointer I– Bad Pointer II– Bad Pointer III– Bad Pointer IV– …


Memory Leaks• memory leak – program does not execute

a delete for each new

int four[4];int* pi;pi = new int [7];…// forgot delete[] pi;return;

0x10four

pi

Stack …..

Heap

?

0xffff5030

int[7]

…..

…..

0x20 0xffff5030


Memory Error: Invalid Array Index

C++…int two[2],ten[10];int *pi; *(ten+9) = 9; // same as ten[9] = 9;pi = ten;*pi = 0;for(i=1;i<=15;i++)



int[X] illegal

0x100x14

a

pi

ten

0x3C

…

two0x44 0x14

Stack …..


Memory Error: Bad Pointer I – overwrite local variable

C++…int two[2],ten[10];int *pi; *(ten+9) = 9; // same as ten[9] = 9;pi = ten;*pi = 0;for(i=1;i<=9;i++)



int[X] illegal

0x100x14

a

pi

ten

0x3C

…

two0x44 0x14

Stack …..


Memory Error: Bad Pointer II – overwrite stack frame

0x100x14

a

pi

ten

0x3C

…

two0x44 0x14

Stack

What if call frame puts IP and FP here?

IPFP

C++…int two[2],ten[10];int *pi; *(ten+9) = 9; // same as ten[9] = 9;pi = ten;*pi = 0;for(i=9;i>=-10;i--)



int[X] illegal


pv2

Memory Error: Bad Pointer III – overwrite reallocated heap memory

C++int* pi;Vehicle *pv1,*pv2;pi = new int;pv1 = new Vehicle;*pi = 10;…delete pv1;…pv2 = new Vehicle;…// YIKES!pv1->wheelCount = 4;

0x000100100x000100140x00010018

pipv1

Stack …..

0xffff5020

0xffff5020

Heap

?

0xffff5030

?

Vehicle object

0xffff5030

…..

…..

0xffff5030


pv2

Memory Error: Bad Pointer IV – overwrite internal heap data structures (Super YIKES!)

C++int* pi;Vehicle *pv1,*pv2;pi = new int;pv1 = new Vehicle;*pi = 10;…delete pv1;…delete pi;…pi = new int [30];…// YIKES!pv1->wheelCount = 4;

0x000100100x000100140x00010018

pipv1

Stack …..

0xffff5020

0xffff5020

Heap

?

0xffff5030

?

Vehicle object

0xffff5030

…..

…..

0xffff5030


In general with memory errors ….

• careless coding can change memorylocations (and hencevariables) you didn’twant to change

Errors can show upmuch later…

C++char a[10]="Hello"; // missing '!‘int i=5;char b[10]="Goodbye!";char *pa=&a[0];while(*pa != '!')

{upperCase(pa);pa++;} … do other stuff …i += 5;cout << i << endl;


Revisions

• Revision 1.0:• Revision 1.1: spelling and syntax typos• Revision 1.2: added diagram that I drew by hand in class, slide 5• …way to tedious to keep this up … see SVN logs…

©Zachary Wartell, UNCC9/28/2006 11:30 AM 1 Thinking in C++ Revision: 10/16/2007 8:26:20 PM...

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