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Using NEW, instead of MAKE, to create slice

 3 years ago
source link: https://dev.to/freakynit/using-new-instead-of-make-to-create-slice-17bl
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Nitin Bansal

Posted on Nov 28

Using NEW, instead of MAKE, to create slice

#go

As we know, slices in golang are reference types, and hence, need make to initialize them.

func main() {
    a := make([]int32, 0)
    a = append(a, 10, 20, 30)
    fmt.Println(len(a), cap(a), reflect.ValueOf(a).Kind())
}
>>> 3 4 slice
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But, can we use new to make slices? It's meant to be used for value types only. So how can it work here??

Let's see

func main() {
    a := new([]int32)
}
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Does this↑ work? Yes. It does. So what is the difference?

The difference is that this gives you a pointer, as is the case anytime you use new. Thus to actually use it, you have to dereference it everytime.

Example:

func main() {
    a := new([]int32)
    *a = append(*a, 10, 20, 30)
    fmt.Println(len(*a), cap(*a), reflect.ValueOf(*a).Kind())
}
>>> 3 4 slice
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Another simple change you can do is to dereference it immediately after creation. This will save you the effort of dereferencing it every other time.

func main() {
    a := *new([]int32)
    a = append(a, 10, 20, 30)
    fmt.Println(len(a), cap(a), reflect.ValueOf(a).Kind())
}
>>> 3 4 slice
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There's another important concept:

Every time you append to a slice, it gives you a new slice with a different address (though it points to same underlying array)

Let's see this with same example:

func main() {
    a := make([]int32, 0)
    fmt.Printf("%p\n", a)

    a = append(a, 10, 20, 30)
    fmt.Printf("%p\n", a)

    a = append(a, 10, 20, 30)
    fmt.Printf("%p\n", a)

    fmt.Println(len(a), cap(a), reflect.ValueOf(a).Kind())
}
>>> 0x116bea0
>>> 0xc0000220e0
>>> 0xc00001c100
>>> 6 8 slice
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You see, all 3 addresses are different.

But, can we keep the addresses same even after append? Well yes!! It's here your NEW approach comes into place. See this:

func main() {
    a := new([]int32)
    fmt.Printf("%p\n", a)

    *a = append(*a, 10, 20, 30)
    fmt.Printf("%p\n", a)

    *a = append(*a, 10, 20, 30)
    fmt.Printf("%p\n", a)

    fmt.Println(len(*a), cap(*a), reflect.ValueOf(*a).Kind())
}
>>> 0xc00011c030
>>> 0xc00011c030
>>> 0xc00011c030
>>> 6 8 slice
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You see? All 3 addresses remain same!!!

That's all folks for now?


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