Crate scopeguard

A scope guard will run a given closure when it goes out of scope, even if the code between panics. (as long as panic doesn't abort)

Examples

defer!

Use the defer macro to run an operation at scope exit, either regular scope exit or during unwinding from a panic.

#[macro_use(defer)] extern crate scopeguard;

use std::cell::Cell;

fn main() {
    // use a cell to observe drops during and after the scope guard is active
    let drop_counter = Cell::new(0);
    {
        // Create a scope guard using `defer!` for the current scope
        defer! {{
            drop_counter.set(1 + drop_counter.get());
        }};

        // Do regular operations here in the meantime.

        // Just before scope exit: it hasn't run yet.
        assert_eq!(drop_counter.get(), 0);

        // The following scope end is where the defer closure is called
    }
    assert_eq!(drop_counter.get(), 1);
}

Scope Guard with Value

If the scope guard closure needs to access an outer value that is also mutated outside of the scope guard, then you may want to use the scope guard with a value. The guard works like a smart pointer, so the inner value can be accessed by reference or by mutable reference.

1. The guard owns a file

In this example, the scope guard owns a file and ensures pending writes are synced at scope exit.

extern crate scopeguard;
 
use std::fs::File;
use std::io::{self, Write};
 
fn try_main() -> io::Result<()> {
    let f = File::create("newfile.txt")?;
    let mut file = scopeguard::guard(f, |f| {
        // ensure we flush file at return or panic
        let _ = f.sync_all();
    });
    // Access the file through the scope guard itself
    file.write(b"test me\n").map(|_| ())
}

fn main() {
    try_main().unwrap();
}

2. The guard restores an invariant on scope exit

extern crate scopeguard;

use std::mem::ManuallyDrop;
use std::ptr;

// This function, just for this example, takes the first element
// and inserts it into the assumed sorted tail of the vector.
//
// For optimization purposes we temporarily violate an invariant of the
// Vec, that it owns all of its elements.
// 
// The safe approach is to use swap, which means two writes to memory,
// the optimization is to use a “hole” which uses only one write of memory
// for each position it moves.
//
// We *must* use a scope guard to run this code safely. We
// are running arbitrary user code (comparison operators) that may panic.
// The scope guard ensures we restore the invariant after successful
// exit or during unwinding from panic.
fn insertion_sort_first<T>(v: &mut Vec<T>)
    where T: PartialOrd
{
    struct Hole<'a, T: 'a> {
        v: &'a mut Vec<T>,
        index: usize,
        value: ManuallyDrop<T>,
    }

    unsafe {
        // Create a moved-from location in the vector, a “hole”.
        let value = ptr::read(&v[0]);
        let mut hole = Hole { v: v, index: 0, value: ManuallyDrop::new(value) };

        // Use a scope guard with a value.
        // At scope exit, plug the hole so that the vector is fully
        // initialized again.
        // The scope guard owns the hole, but we can access it through the guard.
        let mut hole_guard = scopeguard::guard(hole, |hole| {
            // plug the hole in the vector with the value that was // taken out
            let index = hole.index;
            ptr::copy_nonoverlapping(&*hole.value, &mut hole.v[index], 1);
        });

        // run algorithm that moves the hole in the vector here
        // move the hole until it's in a sorted position
        for i in 1..hole_guard.v.len() {
            if *hole_guard.value >= hole_guard.v[i] {
                // move the element back and the hole forward
                let index = hole_guard.index;
                ptr::copy_nonoverlapping(&hole_guard.v[index + 1], &mut hole_guard.v[index], 1);
                hole_guard.index += 1;
            } else {
                break;
            }
        }

        // When the scope exits here, the Vec becomes whole again!
    }
}

fn main() {
    let string = String::from;
    let mut data = vec![string("c"), string("a"), string("b"), string("d")];
    insertion_sort_first(&mut data);
    assert_eq!(data, vec!["a", "b", "c", "d"]);
}

Crate features:

• use_std
  • Enabled by default. Enables the OnUnwind strategy.
  • Disable to use no_std.

Macros

defer	Macro to create a ScopeGuard (always run).
defer_on_unwind	Macro to create a ScopeGuard (run on unwinding from panic).

Structs

ScopeGuard

ScopeGuard is a scope guard that may own a protected value.

Enums

Always

Always run on scope exit.

Traits

Strategy

Functions

guard

Create a new ScopeGuard owning v and with deferred closure dropfn.