# Drop, panic, and abort --- What happens in detail when values drop? --- ## Drop-Order Rust generally guarantees drop order ([RFC1857](https://github.com/rust-lang/rfcs/issues/1857)) --- ## Drop-Order - Values are dropped at the end of their scope - The order is *the reverse introduction order* - Unbound values drop immediately - Structure fields are dropped *first to last* --- ## Destructors Sometimes, certain actions must be taken before deallocation. For this, the `Drop` trait can be implemented. --- ```rust ignore [] struct LevelDB { handle: *mut leveldb_database_t } impl Drop for LevelDB { fn drop(&mut self) { unsafe { leveldb_close(self.handle) }; } } ``` --- ## Warning! Destructors cannot return errors. --- ## Also possible Explicit destruction of a value through a consuming function. This cannot be statically enforced currently. Implementing a `Drop`-bomb (a failing destructor) can make sure this error is caught early. --- ## Panics Rust also has another error mechanism: `panic!` ```rust should_panic [] fn main() { panicking_function(); } fn panicking_function() { panic!("gosh, don't call me!"); } ``` --- In case of a panic, the following happens: * The current thread immediately halts * The stack is unwound * All affected values are dropped and their destructors run --- Panics are implementation-wise similar to C++-Exceptions, but should only be used for fatal errors. They cannot be (normally) caught. The affected thread dies. --- ## Catching Panics Panicking across FFI-boundaries is undefined behaviour. In these cases, panics _must_ be caught. For cases like this, there are [std::panic::catch-unwind](https://doc.rust-lang.org/std/panic/fn.catch_unwind.html) and [std::panic::resume-unwind](https://doc.rust-lang.org/std/panic/fn.resume_unwind.html). --- ## Hooks [std::panic::set_hook](https://doc.rust-lang.org/std/panic/fn.set_hook.html) allows setting a global handler that is run *before* the unwinding happens. --- In general, `Result` is always the right way to propagate errors if they are to be handled. --- ## Abort In some environments, unwinding on `panic!` is not very meaningful. For those cases, `rustc` and `cargo` have a switch that immediately aborts the program on panic. The panic hook is executed. --- ## Double-panics Panicking while a panic is being handled - for example in a destructor - invokes undefined behaviour. For that reason, the program will immediately abort.
