Function pointers — declaration, calling, use in callbacks

5.1.13 · Coding › C Programming

WHAT exactly is a function pointer?

Ek function ka naam uske address mein decay ho jaata hai. Toh int add(int,int) ke liye:

add → woh function
&add → uska address (same value, type int(*)(int,int))
add → zyaadatar expressions mein woh address mein bhi decay ho jaata hai

HOW to declare one (derive the syntax from scratch)

Ek normal declaration se shuru karo aur use step by step transform karo.

HOW to assign and call

int add(int a, int b) { return a + b; }
 
int (*p)(int, int);   // declare
p = add;              // assign (add decays to &add); p = &add; bhi kaam karta hai
int r1 = p(3, 4);     // call via pointer  -> 7
int r2 = (*p)(3, 4);  // explicit deref, bhi 7 (identical)

Callbacks — the killer use

Ek callback = tum ek function pointer doosre function ko dete ho, aur woh tumhe wapas call karta hai.

// qsort signature (from <stdlib.h>):
void qsort(void *base, size_t n, size_t size,
           int (*cmp)(const void *, const void *));

Worked example Sorting ints ascending with qsort

int cmp_asc(const void *a, const void *b) {
    int x = *(const int *)a;   // Cast kyun? qsort void* pass karta hai; hum jaante hain ye ints hain
    int y = *(const int *)b;
    return (x > y) - (x < y);  // Yeh kyun? overflow risk ke bina -1/0/+1 return karta hai x-y ke
}
int arr[] = {3,1,2};
qsort(arr, 3, sizeof(int), cmp_asc);  // arr -> {1,2,3}

Yeh step kyun (trick (x>y)-(x<y))? qsort ko sirf sign chahiye: negative agar a<b, 0 agar equal, positive agar a>b. Large/negative ints ke liye x - y overflow ho sakta hai; comparison form hamesha safe hai.

Worked example Descending — just swap

int cmp_desc(const void *a, const void *b) {
    int x = *(const int*)a, y = *(const int*)b;
    return (y > x) - (y < x);   // Kyun? order reverse karne ke liye x,y ke roles flip karo
}

Same qsort call, alag behaviour — sirf pass ki gayi function badli. Yahi pura point hai.

Dual coding: picture it

Figure — Function pointers — declaration, calling, use in callbacks

Arrays of function pointers (dispatch table)

Worked example Replace a

switch with a table

int add(int a,int b){return a+b;}
int sub(int a,int b){return a-b;}
int mul(int a,int b){return a*b;}
 
int (*ops[3])(int,int) = { add, sub, mul };  // 3 fn pointers ka array
int r = ops[1](10, 4);  // -> 6, sub ko call karta hai

Yeh powerful kyun hai: if/else chains ki jagah, tum behaviour mein index karte ho. ops[opcode](x,y) — clean, extensible, O(1) dispatch. Yahi tarah interpreters aur virtual machines kaam karte hain.

typedef to stay sane

Worked example

typedef int (*BinOp)(int, int);   // BinOp = "int(int,int) ka pointer"
BinOp p = add;                    // Kyun? ab ek normal variable jaisa padhta hai
void run(BinOp f) { f(1,2); }     // Kyun? clean parameter type

typedef kyun? Raw syntax int (*ops[3])(int,int) padhna mushkil hai aur galat karna aasaan. typedef type ko ek baar naam deta hai; uske baad sab kuch ordinary code jaisa padhta hai.

Common mistakes (Steel-man each)

Forecast-then-Verify

Recall Answer padhne se pehle outputs predict karo

int (*ops[2])(int,int) = {add, sub}; diya hai add/sub upar ki tarah:

ops[0](5,2) → ?
ops[1](5,2) → ?
(*ops[1])(5,2) → ?

Answers: 1) 7 2) 3 3) 3 (explicit deref ≡ plain call).

Recall Feynman: 12-saal ke bacche ko samjhao

Ek TV remote imagine karo. Remote mein buttons hain, aur har button kuch karne ke liye wired hai (volume badhao, channel badlo). Ek function pointer ek aisa button hai jiska wiring tum rewire kar sakte ho. Tum keh sakte ho "jab main YEH button dabaaun, WOH action chalao." Ek callback tab hota hai jab tum TV ko ek button dete ho aur kehte ho "jab kuch ho to ise dabao." Toh tum kisi ko ek button (ek function) dete ho, aur woh tumhare liye sahi waqt par use dabata hai. Tum action decide karte ho; woh timing decide karte hain.

Active-recall flashcards

Ek function pointer kya store karta hai?

Ek function ka address (memory mein uske code ki shuruat).

Ek pointer p declare karo jo do ints lene wale aur int return karne wale function ki taraf point kare.

int (*p)(int, int);

int (*p)(int,int) mein parentheses kyun zaroori hain?

Unke bina int *p(int,int) ek int* return karne wala function declare karta hai; parens * ko pehle p se bind karte hain.

Function pointer p ko args 3,4 ke saath call karne ke do equivalent tarike?

p(3,4) aur (*p)(3,4).

Callback kya hota hai?

Ek function pointer jo tum doosre function ko pass karte ho taaki woh sahi waqt par tumhara code call kar sake.

qsort mein comparator parameter ka type kya hai?

int (*)(const void *, const void *).

Comparator mein x-y ki jagah (x>y)-(x<y) kyun use karte hain?

Yeh integer overflow ke risk ke bina sahi sign return karta hai.

3 function pointers int(int,int) ka array kaise likhte hain?

int (*ops[3])(int, int);

pointer to int(int,int) ko BinOp naam dene wala typedef do.

typedef int (*BinOp)(int, int);

Uninitialized function pointer ka khatre kya hai?

Use call karna garbage memory mein jump karta hai → undefined behaviour / crash; NULL se initialize karo aur check karo.

Kya add (ek function name) &add ke barabar hai?

Haan — ek function name uske address mein decay ho jaata hai; dono ka type int(*)(int,int) hai.

Connections

Pointers in C — data pointers vs code pointers
qsort aur C Standard Library — real-world callback
Arrays in C — function pointers ke arrays as dispatch tables
typedef — complex declarations ko tame karna
void pointers — callbacks mein generic args
Polymorphism — OOP languages is idea ko kaise implement karti hain (vtables = function pointers ke arrays)