Struct's zero value

Question

Here is sample code:

package main

import (
    "fmt"
)

type A struct {
    Name string
}

func (this *A) demo(tag string) {
    fmt.Printf("%#v\n", this)
    fmt.Println(tag)
}

func main() {
    var ele A
    ele.demo("ele are called")

    ele2 := A{}
    ele2.demo("ele2 are called")
}

Run results:

&main.A{Name:""}
ele are called
&main.A{Name:""}
ele2 are called

It looks like those are the same about var ele A and ele2 := A{}

So, the struct's Zero value is not nil, but a struct that all of the property are initialized Zero value. Is the guess right?

If the guess is right, then the nature of var ele A and ele2 := A{} are the same right?

Community · Accepted Answer · 2020-06-20 09:12:55Z

80

Why guess (correctly) when there's some documentation ?

When storage is allocated for a variable, either through a declaration or a call of new, or when a new value is created, either through a composite literal or a call of make, and no explicit initialization is provided, the variable or value is given a default value.

Each element of such a variable or value is set to the zero value for its type:

false for booleans,

0 for integers,

0.0 for floats,

"" for strings,

and nil for pointers, functions, interfaces, slices, channels, and maps.

This initialization is done recursively, so for instance each element of an array of structs will have its fields zeroed if no value is specified.

Note that there's no way to set a struct value to nil (but you could set the value of a pointer to a struct to nil).

edited Jun 20, 2020 at 9:12

CommunityBot

11 silver badge

answered Feb 20, 2015 at 9:42

Denys Séguret

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1 Comment

m0r0n Over a year ago

@user98761 actually it does, though not very explicit and isolated but instead in the context of array initialization. But you can also infer this behavior of struct initialization with a bit of attention from the subsequently given examples.

informatik01 · Accepted Answer · 2024-12-17 04:21:50Z

18

I don't believe the top-voted answer is clearly worded to answer the question, so here is a more clear explanation:

The elements of an array or struct will have its fields zeroed if no value is specified. This initialization is done recursively:
type T struct {
    n int
    f float64
    next *T
}
fmt.Println([2]T{}) // [{0 0 <nil>} {0 0 <nil>}]

Source

edited Dec 17, 2024 at 4:21

informatik01

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answered Aug 12, 2020 at 2:44

Josh Diehl

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Comments

score 2 · Accepted Answer · 2020-07-05 12:58:05Z

Demonstrating @Denys Séguret's first-rate answer. Each element of such a variable or value is set to the zero value for its type (https://golang.org/ref/spec#The_zero_value):

package main

import "fmt"

func main() {
    // false for booleans,
    var bl bool // false
    //0 for numeric types,
    var in int // 0
    // "" for strings,
    var st string // ""
    // and nil for pointers, functions, interfaces, channels,
    var pi *int    // <nil>
    var ps *string // <nil>
    var fu func()      // <nil> Go vet error. https://stackoverflow.com/a/56663166/12817546
    var ir interface{} // <nil>
    var ch chan string // <nil>
    fmt.Println(bl, in, st, pi, ps, fu, ir, ch)
    // slices, and maps.
    var sl []int          // true
    var mp map[int]string // true
    var pm *map[int]string // <nil>
    fmt.Printf("%v %v %v\n", sl == nil, mp == nil, pm)
}

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