Overview

defer function will be executed even if panic happens in a program. In fact, defer function is the only function that is called after the panic. When the panic is raised in a function then the execution of that function is stopped and any deferred function will be executed. In fact deferred function in all the function calls in the stack will also be executed until all the functions have returned. At that time the program will exit and it will print the panic message

So if a defer function is present then it will be executed and the control will be returned back to the caller function which will again execute its defer function if present and the chain goes on until the program exists.

Example

Let’s see an example

package main

import "fmt"

func main() {
    defer fmt.Println("Defer in main")
    panic("Panic with Defer")
    fmt.Println("After painc in f2")
}

Output

Defer in main
panic: Panic Create

goroutine 1 [running]:
main.main()
        /Users/slohia/go/src/github.com/golang-examples/articles/tutorial/panicRecover/deferWithPanic/main.go:7 +0x95
exit status 2

In the above program we have a defer function first and then we manually create the panic. As you can see from the output that defer function got executed as below line is printed in the output

Defer in main

Using defer to recover from a panic

Go provides a built-in function recover for recovering from a panic. Below is the signature of this function

func recover() interface{}

defer function is the only function that is called after the panic. So it makes sense to put the recover function in the defer function only. If the recover  function is not within the defer function then it will not stop panic.

Let’s see an example of recover

package main

import "fmt"

func main() {

	a := []string{"a", "b"}
	checkAndPrint(a, 2)
	fmt.Println("Exiting normally")
}

func checkAndPrint(a []string, index int) {
	defer handleOutOfBounds()
	if index > (len(a) - 1) {
		panic("Out of bound access for slice")
	}
	fmt.Println(a[index])
}

func handleOutOfBounds() {
	if r := recover(); r != nil {
		fmt.Println("Recovering from panic:", r)
	}
}

Output

Recovering from panic: Out of bound access for slice
Exiting normally

In the above program we have a function checkAndPrint which checks and prints slice element at an index passed in the argument. If the index passed is greater than the length of the array then the program panics.  We have added a defer function named handleOutIfBounds as well at the start of the function checkAndPrint. This function contains the call to recover function as below

if r := recover(); r != nil {
    fmt.Println("Recovering from panic:", r)
}

The recover function will catch the panic and we can also print the message from the panic. 

Recovering from panic: Out of bound access for slice

After the recover function the program continues and the control returns to the called function which is  main here. That is why we get output as

Exiting normally