go by example defer statement explained
Aug 02, 2025 am 06:26 AM
defer is used to perform specified operations before the function returns, such as cleaning resources; parameters are evaluated immediately when defer, and the functions are executed in the order of last-in-first-out (LIFO); 1. Multiple defers are executed in reverse order of declarations; 2. Commonly used for secure cleaning such as file closing; 3. The named return value can be modified; 4. It will be executed even if panic occurs, suitable for recovery; 5. Avoid abuse of defers in loops to prevent resource leakage; correct use can improve code security and readability.
In Go, the defer statement is a useful feature that allows you to schedule a function call to run just before the surrounding function returns. It's commonly used for cleanup actions like closing files, unlocking mutexes, or logging exit conditions.
Let's break it down with clear examples.
What defer does
When you use defer , the function call is pushed onto a stack and executed in last-in, first-out (LIFO) order when the function exits.
func main() {
defer fmt.Println("World")
fmt.Println("Hello")
}Output:
Hello World
Even though defer fmt.Println("World") appears first, it runs after the fmt.Println("Hello") because defer delays execution until the function returns.
Defer with function calls and arguments
One key thing: arguments to a deferred function are evaluated immediately , but the function itself runs later.
func main() {
i := 1
defer fmt.Println("Deferred:", i) // i is 1 now
i = 2
fmt.Println("Immediate:", i) // prints 2
}Output:
Immediate: 2 Deferred: 1
So:
-
iis evaluated whendeferis encountered (value is 1). - The
fmt.Printlnruns later, but with the value captured at defer time.
Multiple defers: LIFO order
Deferred functions are executed in reverse order.
func main() {
defer fmt.Println("First deferred")
defer fmt.Println("Second deferred")
defer fmt.Println("Third deferred")
fmt.Println("Normal execution")
}Output:
Normal execution Third deferred Second deferred First deferred
This stack-like behavior is useful for nested cleanup operations.
Practical use: File handling
A common use case is closing files safely.
func readFile() {
file, err := os.Open("data.txt")
if err != nil {
log.Fatal(err)
}
defer file.Close() // Guaranteed to run before function exits
// Use the file...
scanner := bufio.NewScanner(file)
for scanner.Scan() {
fmt.Println(scanner.Text())
}
} Even if an error occurs or the function returns early, file.Close() will be called thanks to defer .
Defer and return values
defer can even modify named return values:
func counter() (i int) {
defer func() {
i // increments the named return value
}()
return 5 // i become 5, then defer adds 1 → returns 6
} func main() {
fmt.Println(counter()) // Output: 6
} This works because i is a named return value, and the deferred closure captures it by reference.
You can't do this with regular return statements unless the return value is named.
Common gotchas
Don't defer in loops unnecessarily
for _, file := range files { f, _ := os.Open(file) defer f.Close() // Bad: all Close() calls happen at end, may exceed file limit }Better: handle closing inside the loop or use a helper function.
Defer runs even if panic occurs
This makesdeferperfect for recovery:defer func() { if r := recover(); r != nil { fmt.Println("Recovered from panic:", r) } }()
Summary
-
deferschedules a function call to run before function returns. - Arguments are evaluated immediately; function runs later.
- Multiple defers execute in reverse order (LIFO).
- Great for cleanup: file closing, unlock, recover.
- Can modify named return values.
Used wisely,
defermakes Go code cleaner and safer.Basically, if you need something done at the end — use
defer.The above is the detailed content of go by example defer statement explained. For more information, please follow other related articles on the PHP Chinese website!
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