func atomic_get_and_inc() -> Int {
sem.wait()
defer {
value += 1
sem.signal()
}
return value
} struct PrintOnDrop;
impl Drop for PrintOnDrop {
fn drop(&mut self) {
println!("dropped");
}
}
fn main() {
let p = PrintOnDrop;
return println!("returning");
}
It gets even better in swift, because you can put the return statement in the defer, creating a sort of named return value:
func getInt() -> Int {
let i: Int // declared but not
// defined yet!
defer { return i }
// all code paths must define i
// exactly once, or it’s a compiler
// error
if foo() {
i = 0
} else {
i = 1
}
doOtherStuff()
}The magical thing I was misremembering is that you can reference a not-yet-defined value in a defer, so long as all code paths define it once:
fn callFoo() -> FooResult {
let fooParam: Int // declared, not defined yet
defer {
// fooParam must get defined by the end of the function
foo(fooParam)
otherStuffAfterFoo() // …
}
// all code paths must assign fooParam
if cond {
fooParam = 0
} else {
fooParam = 1
return // early return!
}
doOtherStuff()
}