lec week2 function pointers

7/29/2019 Lec Week2 Function Pointers

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System Programming Lec-2

Dr. Abdul Basit SiddiquiAssistant Professor

FUIEMSSystem Programming

1


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Quiz # 1


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What is difference between call by valueand call by reference? Ellaborate youranswer with the help of an example.

Write a function which receives an arrayand prints all its values. Use pointers in

your code.


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Creating, Compiling andRunning Program


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Creating the program

Use any text editor

Save file with extension .c

Compilation

Compile using gcc compiler

gcc program.c

gcc program.c -o program Running the program

./a.out

./program


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Command Line Arguments


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./myprg hello world

int main( int argc, char* argv[] )

{

for(int i=0; i


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Compiling A Multi-Source "C"Program


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Single Command lineSuppose source files are

main.c, a.c, b.c

gcc main.c a.c b.c -o program Multiple Command lines

gcc -c main.c

gcc -c a.c

gcc -c b.c

gcc main.o a.o b.o -o program


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Lecture References


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The Function Pointer Tutorial by LarsHaendel

CALLBACKS IN C++ by Rich Hickey


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Today We Will Cover:


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What are function pointers? Why do we need them?

How to declare function pointers? How to declare function pointers for C++

functions? What are callback functions? How to implement CallBacks using

function pointers?


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What is a Function Pointer?


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What is a pointer? Variable containing address of another

variable (Pointing another variable)

What is a function pointer?Not different from simple pointerVariable containing the address of a

functionFunctions also have addresses like memory

variables


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Function Pointers


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Function pointers are means to addanother level of indirection

Function pointers give the power tomodify the behavior of a single piece ofcode by dynamically plugging in differentfunctions


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Identical Functions


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Functions Having the same

Return type

Parameters listare called identical functions

A function pointer can hold the address

of identical functions


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Function Pointers


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Suppose a and b are two identicalfunctions

fptr is a function pointer that can hold the

address of a or b. How? fptr is declared in a way that it shares thesame return type and parameter list as a andb

If fptr contains the address of a thenfunction a will be called and if fptrcontains the address of b then functionb will be called


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Important Note


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A function pointer always points to afunction with a specific signature! Thusall functions you want to use with the

same function pointer, must have thesame parameters and return-type!


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Declaring Function Pointers


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Function pointer declaration is verysimilar to a function declaration

Consider following function pointer

declaration int (*ptrtofunction)(int,int,char);

ptrtofunction is a function pointer whichcan hold address of functions returning

int type variable and having threearguments of type int, int and char.


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Assigning a Function to a

Function Pointer


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Assignment can be done in one offollowing two waysptrtofunction = &function1; //like memory

variablesptrtofunction = function1;

&operator is optional in front offunction name


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Calling a Function Via FunctionPointer


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Function can be called via functionpointer

int x=ptrtofunction(5,4,c);int x=(*ptrtofunction)(5,4,c);


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Simple Example


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int add(int a,int b) { return a+b; }

int sub(int a,int b) { return a-b; }

void main(void)

{int (*p2fun)(int,int);

p2fun=&add;

cout


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Declaring Function Pointers forMember Functions


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Declaring function pointers for ClassMember functions is different

Why?

Because of Hidden Pointer this


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Member function Example


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class MyClass{

public: void print(void) { //do printing }};

void main(void){

void (MyClass::*pt2member)(void);pt2member=MyClass::print;

//alternativelypt2member=&MyClass::print can be usedMyClass obj;(obj.*pt2member)(); //calling memberfunction }


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Comparing FunctionPointers


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== operator can be used to comparefunctions with each other or withfunctions

if (ptrtofunction==&add)printf(pointer points to add);

if (ptrfunction1 == ptrfunction2)

printf(boht pointers point to samefunction);

if (ptr2member==&MyClass::print)

printf(pointer pointing to memberfunction


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Avoiding ComplicatedSyntax


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Function pointer declaration lookssomewhat complicated.

Standard remedy for this is to create a

new function pointer type by using thetypedefoperator

typedef void (*FuncType)(void);

FuncType p;

p = a;

(*p)(); // p() is also allowed.


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Passing Function Pointer as an

Argument


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int add(int a,int b) {return a+b; }int sub(int a,int b) {return a-b; }int calculate(int a, int b,int (*ptr2fun)(int,int))

{return (*ptr2fun)(a,b);

}void main()

{cout


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Returning a Function Pointer


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Assume same add,sub and caluclate functions as given in the lastexample

int (*GetPtr(char op))(int,int){

if(op==+) return &add;

if(op==-) retrun sub;}void Return_A_Function_Pointer()

{cout


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Easy way of Returning aFunction Pointer


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typedef int (*funtype)(int,int);

funtype GetPtr(char op)

{

if(op==+) return &add;

if(op==-) retrun sub;

}


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Using Array of FunctionPointers


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typedef int (*ptr2fun)(int,int);

ptr2fun FuncArray[2];

FuncArray[0]=&add;

FuncArray[1]=sub;

cout


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Callback Functions (Navedefinition)


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I have a function, if you call my functiongiving your function as an argument, myfunction will execute your function.

If you give as an argument the samefunction you are in then what happens?


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Callback Functions


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A callback function is one that is notinvoked explicitly by the programmer;rather the responsibility for its invocation

is delegated to another function thatreceives the callback function's address.


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Example: qsort


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void qsort(void *base, size_t num_elements,size_t element_size, int (* cmpFunc)(voidconst *, void const *))

{

/* sort algorithm - note: item1 and item2 arevoid-pointers */

int bigger=cmpFunc(item1, item2); // makecallback

/* use the result */

}


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Compare Function


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int CmpFunc(const void* _a, const void* _b)

{

// youve got to explicitly cast to the correct

typeconst float* a = (const float*) _a;

const float* b = (const float*) _b;

if(*a > *b) return 1; else

if(*a == *b) return 0;

else return -1;

}


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Reading Assignment


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The Function Pointer Tutorial by LarsHaendel

Dietle and Dietle Ch-7

lec week2 function pointers

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