Defer
A defer statement defers the execution of a block of statements until the surrounding function
returns.
import os
fn read_log() {
mut ok := false
mut f := os.open('log.txt') or { panic(err) }
defer {
f.close()
}
// ...
if !ok {
// defer statement will be called here, the file will be closed
return
}
// ...
// defer statement will be called here, the file will be closed
}
If the function returns a value the defer block is executed after the return expression is
evaluated:
import os
enum State {
normal
write_log
return_error
}
fn write_log(s State) !int {
mut f := os.create('log.txt')!
defer {
f.close()
}
if s == .write_log {
// `f.close()` will be called after `f.write()` has been
// executed, but before `write_log()` finally returns the
// number of bytes written to `main()`
return f.writeln('This is a log file')
} else if s == .return_error {
// the file will be closed after the `error()` function
// has returned – so the error message will still report
// it as open
return error('nothing written; file open: ${f.is_opened}')
}
// the file will be closed here, too
return 0
}
fn main() {
n := write_log(.return_error) or { panic('Error: ${err}') }
println('${n} bytes written')
}
Unlike Go, in V
deferwill not be called if there is a panic.