cpp core · high

Exceptions & Stack Unwinding

A throw unwinds the stack to the nearest matching catch, running destructors (RAII) on the way; no handler -> std::terminate.

The code

struct Guard {
    ~Guard();
}; // RAII cleanup in its destructor

void h() {
    Guard x;
    throw std::runtime_error("boom");
}
void g() {
    Guard x;
    h();
} // no catch -> unwinds
void f() {
    try {
        g();
    } catch (const std::exception& e) { /* handled here */
    }
}
int main() {
    f();
}

What this lesson walks through

1. 01The call stack: main -> f -> g -> h
2. 02h() throws
3. 03Unwind h: destructors run
4. 04Unwind g: destructors run
5. 05f's catch matches -> stop
6. 06No handler anywhere -> std::terminate
7. 07Stack unwinding in one line

main calls f, f calls g, g calls h. Only f wraps its call in a try/catch. Each frame holds local objects (a Guard) whose destructors must run.