The program below returns
<nil>
hello
I expected both to return "hello" but it's not the case. I've found that the behaviour is given as example in the language spec.
For instance, if the deferred function is a function literal and the surrounding function has named result parameters that are in scope within the literal, the deferred function may access and modify the result parameters before they are returned.
My question is: why does defer works differently with variables defined in the surrounding function than with named results? Isn't it just a closure executed before the surrounding function returns?
package main
import (
"errors"
"fmt"
)
func main() {
fmt.Println(up())
fmt.Println(up2())
}
func up() error {
var err error
defer func() {
err = errors.New("hello")
}()
return err
}
func up2() (err error) {
defer func() {
err = errors.New("hello")
}()
return err
}
func up3() error {
var err error
fn := func() {
err = errors.New("hello")
}
fn()
return err
}
The spec says:
A "defer" statement invokes a function whose execution is deferred to the moment the surrounding function returns, either because the surrounding function executed a return statement, reached the end of its function body, or because the corresponding goroutine is panicking.
A key point is that the deferred function is executed after the return statement is executed.
The spec also says:
A "return" statement in a function F terminates the execution of F, and optionally provides one or more result values. Any functions deferred by F are executed before F returns to its caller.
The function up returns nil because the deferred function sets err after the return statement provides the function result.
The function up2 overwrites the result value set by the return statement.