panic!
- Rust course
- Beginner
- Lesson 107/115
- 4 min
Learn unrecoverable errors with panic!.
panic!
panic! reports that the program has entered an unrecoverable state: the message is printed, the stack is unwound and the thread ends. Some operations panic on their own:
let v = vec![1, 2, 3];
// v[10]; // index out of bounds
// let x: u8 = 255; x + 1; // overflow in a debug build
panic!("yapılandırma bozuk: {}", "port eksik");RUST_BACKTRACE=1 cargo run shows the call chain at the moment of the panic.
unwind and abort
By default a panic unwinds the stack and runs each value's drop. For a smaller binary, terminating directly can be chosen:
# Cargo.toml
[profile.release]
panic = "abort"unreachable!(), todo!() and assert!(kosul) are also macros that panic depending on a condition; assert_eq!(a, b) is common in tests.
Practice Precondition
Write fn ikiye_katla(n: i32) -> i32: if n < 0, panic!("negatif değer: {}", n), otherwise return n * 2. Print the result of ikiye_katla(5).
Starter code
fn main() {
// buraya yaz
}Sample solution
fn ikiye_katla(n: i32) -> i32 {
if n < 0 {
panic!("negatif değer: {}", n);
}
n * 2
}
fn main() {
println!("{}", ikiye_katla(5));
}Expected output
10
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which is a suitable case for a panic?
- The user entering a wrong password
- The network connection dropping
- A file not being found
- A logically impossible situation actually happening Correct answer
Explanation: Expected errors caused by the user or the environment should be handled with Result; panic is for bugs.
2 Conditional panic
____(n >= 0, "n negatif olamaz: {}", n);assert!(n >= 0, "n negatif olamaz: {}", n);Explanation: assert! panics with the given message when the condition is false.
3 Macro and where to use it
| Term | Meaning |
|---|---|
panic!(..) |
Unrecoverable error |
assert!(..) |
Condition check |
todo!() |
Not written yet |
unreachable!() |
An arm that should never run |