What is the use of defer
in Go? The language documentation says it is executed when the surrounding function returns. Why not just put the code at end of given function?
在Go中使用defer
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- dongren5293 2017-12-02 12:44关注
We usually use
defer
to close or deallocate resources.A surrounding function executes all deferred function calls before it returns, even if it panics. If you just place a function call at the end of a surrounding function, it is skipped when panic happens.
Moreover a deferred function call can handle panic by calling the
recover
built-in function. This cannot be done by an ordinary function call at the end of a function.Each deferred call is put on stack, and executed in reverse order when the surrounding function ends. The reversed order helps deallocate resources correctly.
The
defer
statement must be reached for a function to be called.You can think of it as another way to implement
try-catch-finally
blocks.Closing like
try-finally
:func main() { f, err := os.Create("file") if err != nil { panic("cannot create file") } defer f.Close() // no matter what happens here file will be closed // for sake of simplicity I skip checking close result fmt.Fprintf(f,"hello") }
Closing and panic handling like
try-catch-finally
func main() { defer func() { msg := recover() fmt.Println(msg) }() f, err := os.Create(".") // . is a current directory if err != nil { panic("cannot create file") } defer f.Close() // no matter what happens here file will be closed // for sake of simplicity I skip checking close result fmt.Fprintf(f,"hello") }
The benefit over try-catch-finally is that there is no nesting of blocks and variable scopes. This simplifies the structure of the surrounding function.
Just like finally blocks, deferred function calls can also modify the return value if they can reach the returned data.
func yes() (text string) { defer func() { text = "no" }() return "yes" } func main() { fmt.Println(yes()) }
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