go - When should I initialize Golang variables -

there kind of variables in golang:

1. global variable: var int
2. local variable: func hello() { var int }
3. return variable: func hello() (a int) {}

golang auto-init variables,
don't know when , why? confused me.

example:

type user struct {   name string `json:"name"`   age  int    `json:"age"` }  func foo(bts []byte) {   var user   err := json.unmarshal(bts, &a)  // it's ok }  func bar(bts []byte) (a *user) {   err := json.unmarshal(bts, a) // crash } 

which 1 should have initialize before use?

golang init variables (not sometimes, not some):

in code:

func bar(bts []byte) (a *user) {   err := json.unmarshal(bts, a) // crash } 

you passed nil pointer need value pointed a , not nil pointer:
may create value store address of value inside a:

---

when use var *user or func bar(bts []byte) (a *user):
a pointer user type, , initialized it's 0 value, nil,
see (the go playground):

package main  import "fmt"  func main() {     var *user     fmt.printf("%#v\n\n", a)     }  type user struct {     name string `json:"name"`     age  int    `json:"age"` } 

output:

(*main.user)(nil) 

---

and may use a = &user{} initialize it, working code (the go playground):

package main  import (     "encoding/json"     "fmt" )  func foo(bts []byte) (*user, error) {     var user     err := json.unmarshal(bts, &a) // it's ok     return &a, err }  func bar(bts []byte) (a *user, err error) {     = &user{}     err = json.unmarshal(bts, a) // it's ok     return } func main() {     str := `{ "name": "alex", "age": 3 }`     u, err := foo([]byte(str))     if err != nil {         panic(err)     }     fmt.printf("%#v\n\n", u) // &main.user{name:"alex", age:3}      u, err = bar([]byte(str))     if err != nil {         panic(err)     }     fmt.printf("%#v\n\n", u) // &main.user{name:"alex", age:3}  }  type user struct {     name string `json:"name"`     age  int    `json:"age"` } 

output:

&main.user{name:"alex", age:3}  &main.user{name:"alex", age:3} 

---

variable declarations:

a variable declaration creates 1 or more variables, binds corresponding identifiers them, and gives each type , initial value.

the initial value (zero value):

when storage allocated variable, either through declaration or call of new, or when new value created, either through composite literal or call of make, , no explicit initialization provided, variable or value given default value. each element of such variable or value set 0 value type: false booleans, 0 integers, 0.0 floats, "" strings, and nil pointers, functions, interfaces, slices, channels, , maps. initialization done recursively, instance each element of array of structs have fields zeroed if no value specified.

and see func unmarshal(data []byte, v interface{}) error docs:

unmarshal parses json-encoded data , stores result in value pointed v.