golang_test

package main

// o O
func f1() {
	defer println("f1-start")
	f2()
	defer println("f1-end")
}
func f2() {
	defer println("f2-start")
	f3()
	defer println("f2-end")
}
func f3() {
	defer println("f3-start")
	//panic(0)
	defer println("f3-end")
}

func main() {
	f1()
}

　

f3-end
f3-start
f2-end
f2-start
f1-end
f1-start

package main

// o O
func f1() {
	defer println("f1-start")
	f2()
	defer println("f1-end")
}
func f2() {
	defer println("f2-start")
	f3()
	defer println("f2-end")
}
func f3() {
	defer println("f3-start")
	panic(0)
	defer println("f3-end")
}

func main() {
	f1()
}

　　

defer 执行原理

The Evaluation Moment of Deferred Function Values

The called function (value) in a deferred function call is evaluated when the call is pushed into the deferred call stack of the current goroutine. For example, the following program will print false.

package main

import "fmt"

func main() {
	var f = func () {
		fmt.Println(false)
	}
	defer f()
	f = func () {
		fmt.Println(true)
	}
}

GOROOT=C:Go #gosetup
GOPATH=D:myGOPATH #gosetup
C:Goingo.exe build -o D:mytmpOLDc\___go_build_run_go.exe D:/myGPrject/lChan/run.go #gosetup
D:mytmpOLDc\___go_build_run_go.exe #gosetup
f3-start
f2-start
f1-start
panic: 0

goroutine 1 [running]:
main.f3()
	D:/myGPrject/lChan/run.go:16 +0x71
main.f2()
	D:/myGPrject/lChan/run.go:11 +0x5e
main.f1()
	D:/myGPrject/lChan/run.go:6 +0x5e
main.main()
	D:/myGPrject/lChan/run.go:21 +0x27

Process finished with exit code 2

　　

package main

// o O

func main() {
	var m map[string]int
	m["a"] = 1
	if v := m["b"]; v != nil {
		println(v)
	}
}

　　

# command-line-arguments
lMap un.go:8:20: cannot convert nil to type int

package main

// o O

func main() {
	var m map[string]int
	m["a"] = 1
	//if v := m["b"]; v != nil {
	//	println(v)
	//}
}

　

panic: assignment to entry in nil map

goroutine 1 [running]:
main.main()
D:/myGPrject/lMap/run.go:7 +0x52

package main

// o O

func main() {
	m := make(map[string]int)
	m["a"] = 1
	println(m["a"])
	println(m["b"]) // 键不存在，返回零值  int 的零值是0

	n := make(map[string]string)
	n["a"] = "stra"
	println(n["a"])
	println(n["b"]) //  string 的零值是空

}

　　

package main

// o O

func main() {
	m := make(map[string]int)
	m["a"] = 1
	if v := m["b"]; v != 0 {
		println(v)
	}
}

　　

package main

// o O
const N = 3

func main() {
	m := make(map[int]*int)
	for i := 0; i < N; i++ {
		m[i] = &i
	}
	for _, v := range m {
		print(*v)
	}
}

　　

// Unmarshal parses the JSON-encoded data and stores the result
// in the value pointed to by v. If v is nil or not a pointer,
// Unmarshal returns an InvalidUnmarshalError.

json  编码解码

package main

import (
	"encoding/json"
	"fmt"
)

// o O
type Result struct {
	status int
}

func main() {
	data := []byte(`{"status":200}`)
	result := Result{}
	if err := json.Unmarshal(data, result); err != nil {
		fmt.Println("error", err)
		return
	}
	fmt.Println("result=%+v", result)
}

　　

error json: Unmarshal(non-pointer main.Result)

借助map

package main

import (
	"encoding/json"
	"fmt"
)

// o O
func main() {
	data := []byte(`{"status":200}`)
	result := make(map[string]int)
	if err := json.Unmarshal(data, &result); err != nil {
		fmt.Println("error", err)
		return
	}
	fmt.Printf("result=%d", result["status"])
}

　　

 https://go101.org/article/defer-more.html

More about Deferred Function Calls

package main

import "fmt"

func main() {
	var f = func () {
		fmt.Println(false)
	}
	defer f()
	f = func () {
		fmt.Println(true)
	}
}

　　

package main

type T int

func (t T) M(n int) T {
  print(n)
  return t
}

func main() {
	var t T
	// "t.M(1)" is the receiver argument of the method
	// call ".M(2)", so it is evaluated before the
	// ".M(2)" call is pushed into deferred call stack.
	defer t.M(1).M(2)
	t.M(3).M(4)
}

　　

package main

import "fmt"

func main() {
	defer func() {
		fmt.Println(recover())
	}()
	defer panic(3)
	defer panic(2)
	defer panic(1)
	panic(0)
}