68 lines
26 KiB
Plaintext
68 lines
26 KiB
Plaintext
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rust
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has_fieldsÁµ` ã¹q·Ù›ß¨ > ÿ' Û �( ã ”& ” ö ¸ š ±ž ï ª% ö ”&Í ±&• þ&N 9í À Í0 × Ü0 ` î \ò0 • î – ¼d�1 ÃÛ r,ª1 é! DerefMutÁD±1 ö! µ Å1 “
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Ú,±< ü ò… ütE B A scope guard will run a given closure when it goes out of scope,Á üº$ ! even if the code between panics.Á üß$ ! (as long as panic doesn't abort)Á „ ú tˆ # ExamplesÁ — ú ”› ## Hello WorldÁ ® ú ü²@ = This example creates a scope guard with an example function:Á ó ú <÷ ```Á äÿ extern crate scopeguard;Á œ ú d fn f() {Á ü0 - let _guard = scopeguard::guard((), |_| {Á üÞ* ' println!("Hello Scope Exit!");Á \‰ });Á • ú ü™! // rest of the code here.Á » ú ü¿O L // Here, at the end of `_guard`'s scope, the guard's closure is called.Á ü�M J // It is also called if we exit this scope through unwinding instead.Á ,Ý }Á Œã # fn main() {Á lõ
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# f();Á <ƒ # }Á <‹ ÿ, “ ú |— ## `defer!`Á § ú ü«< 9 Use the `defer` macro to run an operation at scope exit,Á üè? < either regular scope exit or during unwinding from a panic.Á ¨ ú <¬ ÿ, ü´0 - #[macro_use(defer)] extern crate scopeguard;Á å ú Äé use std::cell::Cell;Á ‚ ú |† fn main() {Á ü–Q N // use a cell to observe drops during and after the scope guard is activeÁ üè( % let drop_counter = Cell::new(0);Á L‘ {Á ü› H E // Create a scope guard using `defer!` for the current scopeÁ ¤ä defer! {Á üù 9 6 drop_counter.set(1 + drop_counter.get());Á l³
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: 7 // Do regular operations here in the meantime.Á € ú ü„ 9 6 // Just before scope exit: it hasn't run yet.Á ü¾ . + assert_eq!(drop_counter.get(), 0);Á í ú üñ K H // The following scope end is where the defer closure is calledÁ L½ }Á üÇ * ' assert_eq!(drop_counter.get(), 1);Á ,ò Ê0 <ø ÿ, €
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ú ì„
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## Scope Guard with ValueÁ ¢
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ú ü¦
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J G If the scope guard closure needs to access an outer value that is alsoÁ üñ
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P M mutated outside of the scope guard, then you may want to use the scope guardÁ üÂN K with a value. The guard works like a smart pointer, so the inner value canÁ ü‘5 2 be accessed by reference or by mutable reference.Á Ç ú üË ### 1. The guard owns a fileÁ ì ú üðO L In this example, the scope guard owns a file and ensures pending writes areÁ ÌÀ synced at scope exit.Á Ú ú <Þ ÿ, äæ �- ƒ ú œ‡ use std::fs::*;Á ü› use std::io::{self, Write};Á ü»9 6 # // Mock file so that we don't actually write a fileÁ ´õ # struct MockFile;Á ¬Œ # impl MockFile {Á ü¢B ? # fn create(_s: &str) -> io::Result<Self> { Ok(MockFile) }Á üåE B # fn write_all(&self, _b: &[u8]) -> io::Result<()> { Ok(()) }Á ü«9 6 # fn sync_all(&self) -> io::Result<()> { Ok(()) }Á <å ˆ1 üí! # use self::MockFile as File;Á � ú ü“% " fn try_main() -> io::Result<()> {Á ü¹- * let f = File::create("newfile.txt")?;Á üç1 . let mut file = scopeguard::guard(f, |f| {Á ü™6 3 // ensure we flush file at return or panicÁ üÐ! let _ = f.sync_all();Á \ò Ã. üþ9 6 // Access the file through the scope guard itselfÁ ü¸0 - file.write_all(b"test me\n").map(|_| ())Á ,é Ê0 ï ú |ó î3 äƒ try_main().unwrap();Á , Ê0 ¦ ú <ª ÿ, ² ú ü¶8 5 ### 2. The guard restores an invariant on scope exitÁ ï ú <ó ÿ, äû �- ˜ ú üœ use std::mem::ManuallyDrop;Á Œ¼ use std::ptr;Á Î ú üÒD A // This function, just for this example, takes the first elementÁ ü—A > // and inserts it into the assumed sorted tail of the vector.Á 4Ù //Á üàK H // For optimization purposes we temporarily violate an invariant of theÁ ü¬- * // Vec, that it owns all of its elements.Á 4Ú ÁI üáJ G // The safe approach is to use swap, which means two writes to memory,Á ü¬R O // the optimization is to use a “holeâ€� which uses only one write of memoryÁ üÿ" // for each position it moves.Á 4¢ ÁI ü© > ; // We *must* use a scope guard to run this code safely. WeÁ üè M J // are running arbitrary user code (comparison operators) that may panic.Á ü¶ H E // The scope guard ensures we restore the invariant after successfulÁ üÿ + ( // exit or during unwinding from panic.Á ü« . + fn insertion_sort_first<T>(v: &mut Vec<T>)Á ÜÚ where T: PartialOrdÁ ,ö {Á üü struct Hole<'a, T: 'a> {Á ô� v: &'a mut Vec<T>,Á ̼ index: usize,Á üÖ # value: ManuallyDrop<T>,Á Lú ¥9 „ ú „ˆ
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unsafe {Á ü™ H E // Create a moved-from location in the vector, a “holeâ€�.Á üâ ) & let value = ptr::read(&v[0]);Á üŒ!T Q let mut hole = Hole { v: v, index: 0, value: ManuallyDrop::new(value) };Á á! ú üå!. + // Use a scope guard with a value.Á ü”"G D // At scope exit, plug the hole so that the vector is fullyÁ üÜ"! // initialized again.Á üþ"U R // The scope guard owns the hole, but we can access it through the guard.Á üÔ#A > let mut hole_guard = scopeguard::guard(hole, |hole| {Á ü–$S P // plug the hole in the vector with the value that was // taken outÁ üê$' $ let index =
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Ú
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^ĸ0 nD×0 €Äå0 “´ƒ1 ¹DŸ1 ÌÌççDÀ1 ú”‰2 üË1= : Controls in which cases the associated code should be runÁ D“2 Îì ü‰2« Öh Öh Äš3 ü¢2= : Return `true` if the guard’s associated code should runÁ üä21 . (in the context where this method is called).ÁT�3
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