CODEWITHSUNDEEP
pointersgoslice
Mrj
Mrj

Reputation: 69

What's the difference between []struct{} and []*struct{}?

What's the difference about below ?

type Demo struct {s string}

func getDemo1()([]*Demo)  // 1

func getDemo2()([]Demo)  // 2

Is there any memory difference between getDemo1 and getDemo2?

Upvotes: 1

Views: 973

Answers (1)

RayfenWindspear
RayfenWindspear

Reputation: 6274

I'm going to answer this, despite my better judgement to just send OP to the tour and documentation/specification. Mostly because of this:

Is there any memory difference between getDemo1 and getDemo2?

The answer to this specific question depends on how you utilize the slice. Go is Pass by value, so passing struct values around copies them. For instance, consider the following example.

https://play.golang.org/p/VzjYXwUy0EI

d1 := getDemo1()
d2 := getDemo2()
for _, v := range d1 {
    // v is of type *Demo, so this modifies the value in the slice
    v.s = "same"
}
fmt.Println(d1)

for _, v := range d2 {
    // v is of type Demo, and is a COPY of the struct in the slice, so the original is not modified
    v.s = "same"
}

So as to the memory question, obviously using *Demo, which returns a copy of the pointer in the range (effectively a uint64) as opposed to returning a copy of a Demo (the entire struct and all it's fields) would use less memory. BUT, you can still index directly to the array to avoid copies, except when you pass individual items in the slice around.

That said, passing the slice itself around, the two types have no difference in overhead. A slice is an abstraction of an array, and the slice itself that gets passed around is merely a slice header, which would be the same memory footprint regardless what type the slice contains.

BTW, the paradigm for modifying the values in the case of []Demo is:

for i, _ := range d2 {
    d2[i].s = "same"
}

Upvotes: 1

Related Questions