noexcept specifier — Hinglish notes

5.2.30 · Coding › C++ Programming

YEH HAI KYA?

void a() noexcept;          // promises: never throws
void b() noexcept(true);    // same thing
void c() noexcept(false);   // may throw (this is also the default)
void d();                   // may throw (default)

YEH EXIST KYUN KARTA HAI?

Purana C++98 throw() wahi kaam karta tha lekin clunky tha aur ab deprecated/removed hai. noexcept ne ise replace kiya.

YEH BEHAVE KAISE KARTA HAI? (consequences derive karo)

Hum rules memorize nahi karte — hum unhe derive karte hain ek single contract se: "koi bhi exception escape nahi ho sakti."

Terminate kyun, rethrow kyun nahi? Kyunki poora point yahi tha ki compiler is function ke liye exception-handling tables skip kar sake. Literally koi unwinding info use karne ke liye ho hi nahi sakti — toh ek hi safe action hai: sab kuch rok do.

Yeh hume conditional noexcept likhne deta hai:

template<class T>
void swap(T& a, T& b) noexcept( noexcept(T(std::move(a))) &&
                                noexcept(a = std::move(b)) );

Yahan outer noexcept(...) specifier hai; inner noexcept(...) woh operator hai jo bool compute karta hai. "Main noexcept hoon exactly tab jab T ko move karna throw nahi kar sakta."

Worked examples

Worked example 1 — Operator se query karna

int safe() noexcept { return 42; }
int risky() { throw 1; }
 
constexpr bool s = noexcept(safe());   // true
constexpr bool r = noexcept(risky());  // false

s == true kyun? safe() ko noexcept declare kiya gaya hai, toh compiler promise par trust karta hai. r == false kyun? risky() ke paas koi noexcept promise nahi hai (default may-throw), toh operator assume karta hai ki yeh throw kar sakta hai.

Worked example 2 — Promise todna = death

void liar() noexcept { throw std::runtime_error("oops"); }
int main() { liar(); }   // -> std::terminate() -> abort

Yeh step kyun? liar ne kasam khayi thi ki throw nahi karega, phir throw kiya. Runtime cleanup path par trust nahi kar sakta, toh terminate ho jaata hai. Woh catch jo tumne kahin aur likha hai kabhi run nahi hota.

Worked example 3 — Vector kyun care karta hai (bada payoff)

struct Fast { Fast(Fast&&) noexcept; };  // move can't throw
struct Slow { Slow(Slow&&); };           // move MIGHT throw
 
std::vector<Fast> vf;  // grows by MOVING elements  (cheap)
std::vector<Slow> vs;  // grows by COPYING elements (safe but costly)

Yeh step kyun? Jab vector reallocate karta hai, use N existing elements ko naye storage mein move karna padta hai. Agar beech mein ek move throw kare, toh aadhe elements naye buffer mein hain, aadhe lost — corrupted state. Strong guarantee maintain karne ke liye, vector tabhi move karne ki himmat karta hai jab move noexcept ho; warna woh copy karta hai (originals safe rehte hain agar ek copy throw kare).

Yahi exactly woh kaam hai jo std::move_if_noexcept internally karta hai.

Worked example 4 — Destructors by default noexcept hote hain

struct S { ~S() { /* if this throws -> terminate */ } };

Kyun? C++11 se, destructors implicitly noexcept hain. Agar tumhara destructor throw kare, tum terminate se takraate ho. (Rationale: kisi aur exception se stack unwinding ke dauran throw karna undefined chaos hai, toh language ise ban karti hai.)

Common mistakes

Recall Feynman: ek 12-saal ke bachche ko samjhao

Socho tum apne teacher ko bolte ho: "MAIN PROMISE KARTA HOON ki is kamre ko cross karte waqt paani ka glass nahi giraunga." Agar tum promise nibhao, teacher relax karta hai aur har jagah towels aur mops nahi bikhata — time aur effort bachaata hai. Lekin agar tumne GIRADIYA promise karne ke baad? Tumhe sirf daanta nahi jaata — poori class turant ghar bhej di jaati hai, koi sawaal nahi. noexcept woh promise hai exception "spillage" ke baare mein. Nibhao aur program lean chalega; todo aur program turant band ho jaata hai.

Recall flashcards

noexcept specifier kya promise karta hai?

Ki function kisi bhi exception ko apne se bahar (propagate) nahi hone dega.

Agar ek noexcept function exception escape hone de toh kya hota hai?

std::terminate() turant call hota hai; program kill ho jaata hai (koi guaranteed unwinding nahi).

noexcept kis cheez ka shorthand hai?

noexcept(true) ka.

Ek ordinary function ki default exception specification kya hai?

noexcept(false) — yeh throw kar sakta hai.

noexcept(expr) operator kya return karta hai?

Ek compile-time bool: true agar expr guaranteed non-throwing ho, warna false. expr unevaluated hota hai.

std::vector kyun care karta hai ki kisi type ka move constructor noexcept hai ya nahi?

Agar move noexcept hai toh reallocation par elements move karta hai (fast); warna strong exception guarantee preserve karne ke liye copy karta hai (slow).

noexcept specifier aur noexcept operator mein kya fark hai?

Specifier (signature ke baad) ek promise karta hai; operator noexcept(x) bool evaluate karta hai. Same keyword, do roles.

C++11 se destructors kya by default noexcept hain?

Haan — implicitly noexcept; unse throw karna terminate call karta hai.

Kya ek noexcept function ke andar exception throw aur catch karna contract violate karta hai?

Nahi — sirf woh exception jo function se escape kare woh terminate trigger karti hai.

noexcept ne kis purani cheez ko replace kiya?

C++98 dynamic exception specification throw() ko (ab deprecated/removed).

T ke move ke basis par conditional noexcept likho.

void f() noexcept(noexcept(T(std::move(t))));

Connections

Exception handling in C++ — try/catch/throw mechanics jis par yeh build karta hai
Move semantics — kyun noexcept move ctors fast reallocation unlock karte hain
std::vector — growth ke dauran move_if_noexcept use karta hai
Strong exception guarantee — woh safety level jise noexcept maintain karne mein help karta hai
Destructors — C++11 se implicitly noexcept
std::terminate — jo broken promise par fire hota hai
constexpr — noexcept-operator aur constexpr dono compile-time evaluated hain