An interview question combining defer and function in golang

What does the following code output ?

package main

import "fmt"

//  What does the following code output ?
func calc(index string, a, b int) int {
    
	ret := a + b
	fmt.Printf("%v,%v,%v,%v\n", index, a, b, ret)
	return ret
}

func main() {
    
	a := 1
	b := 2
	defer calc("1", a, calc("10", a, b))
	a = 0
	defer calc("2", a, calc("20", a, b))
	b = 1
}

Please think about it. , The answer is below
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
answer

10,1,2,3
20,0,2,2
2,0,2,2
1,1,3,4

Why? ？
because defer Will first calculate the value of ordinary functions , And then pass it in as a parameter defer function

We will solve this problem , Make a little change , as follows ,

package main

import "fmt"

//  What does the following code output ?
func calc(index string, a, b *int) *int {
    
	ret := *a + *b
	fmt.Printf("%v,%v,%v,%v\n", index, *a, *b, ret)
	return &ret
}

func main() {
    
	a := 1
	b := 2
	defer calc("1", &a, calc("10", &a, &b))
	a = 0
	defer calc("2", &a, calc("20", &a, &b))
	b = 1
}

Please think about it. , The answer is below
,
,
,
,
,
,
,
,
,
,
,
,
,
,
,
answer

10,1,2,3
20,0,2,2
2,0,2,2
1,0,3,3

Why? ？ Compared with the above question , This question is changed to pointer as defer The parameters of the function , therefore , Unlike value passing

---

Let's see , The following questions

What is the output of the following code ？ Why? ？

package main

import "fmt"

func main() {
    

	{
    
		defer func() {
    
			fmt.Println("A")
		}()
	}
	fmt.Println("B")
}

,
,
,
Output

B
A

What is the output of the following code ？ Why? ？

func main() {
    

	func(){
    
		defer func() {
    
			fmt.Println("A")
		}()
	}()
	fmt.Println("B")
}

,
,
,
Output

A
B

defer Function is to register with the current function , The scope is the current function . The first 1 An example is easy to understand .

The first 2 An example , Because it's a closure ,“ The current function ” It's a closure , When the closure is complete ,defer The registered function will be called . therefore , First, the output A