Concisely deep copy a slice?

Question

In Go, what's a concise/well-performing way to deep copy a slice? I need to copy the slice to a new backing array, because the other array is owned by something else and may be modified after the copy.

I'm currently doing it like this:

copy := append([]T{}, orig...)

where T is the element type of orig.

Answer 1

Not sure which solution is fastest without a benchmark, but an alternative is using the built in copy:

cpy := make([]T, len(orig))
copy(cpy, orig)

From the documentation:

func copy(dst, src []Type) int

The copy built-in function copies elements from a source slice into a destination slice. (As a special case, it also will copy bytes from a string to a slice of bytes.) The source and destination may overlap. Copy returns the number of elements copied, which will be the minimum of len(src) and len(dst).

Note

The solution will copy all the values in the slice. If the slice contains pointers or structs with pointer fields, these pointer values will still point to the same values as the orig slice.

Benchmark

Benchmarking the two options, you can see they have very similar performance.

BenchmarkCopy     100000         24724 ns/op
BenchmarkAppend   100000         24967 ns/op
ok      benchmark   5.478s

This is the benchmark code:

package main

import "testing"

var result []T

const size = 10000

type T int

func BenchmarkCopy(b *testing.B) {
    orig := make([]T, size)

    for n := 0; n < b.N; n++ {
        cpy := make([]T, len(orig))
        copy(cpy, orig)
        orig = cpy
    }
    result = orig
}

func BenchmarkAppend(b *testing.B) {
    orig := make([]T, size)

    for n := 0; n < b.N; n++ {
        cpy := append([]T{}, orig...)
        orig = cpy
    }
    result = orig
}

I am not sure when/if the zero-fill is performed. But if you look at the assembly, in the append version you will have:

CALL    ,runtime.growslice(SB)

while the copy will call:

CALL    ,runtime.makeslice(SB)

and I would guess that both of these calls performs the zero-fill.

Answer 2

slicecopy := append([]T(nil), slice...)

For example,

package main

import "fmt"

func main() {
    type T int
    slice := make([]T, 8)
    for i := range slice {
        slice[i] = T(i)
    }
    fmt.Println(len(slice), cap(slice), &slice[0], slice)
    slicecopy := append([]T(nil), slice...)
    fmt.Println(len(slicecopy), cap(slicecopy), &slicecopy[0], slicecopy)
}

Output:

8 8 0x10322160 [0 1 2 3 4 5 6 7]
8 8 0x103221a0 [0 1 2 3 4 5 6 7]

References:

Arrays, slices (and strings): The mechanics of 'append'

// Make a copy of a slice (of int).
slice3 := append([]int(nil), slice...)
fmt.Println("Copy a slice:", slice3)

---

Benchmarks:

package main

import "testing"

var result []T

const size = 1000

type T int

func BenchmarkCopy(b *testing.B) {
    orig := make([]T, size)
    for n := 0; n < b.N; n++ {
        cpy := make([]T, len(orig))
        copy(cpy, orig)
        orig = cpy
    }
    result = orig
}

func BenchmarkAppend(b *testing.B) {
    orig := make([]T, size)
    for n := 0; n < b.N; n++ {
        cpy := append([]T{}, orig...)
        orig = cpy
    }
    result = orig
}

func BenchmarkAppendPreCapped(b *testing.B) {
    orig := make([]T, size)
    for n := 0; n < b.N; n++ {
        cpy := append(make([]T, 0, len(orig)), orig...)
        orig = cpy
    }
    result = orig
}

func BenchmarkAppendNil(b *testing.B) {
    orig := make([]T, size)
    for n := 0; n < b.N; n++ {
        cpy := append([]T(nil), orig...)
        orig = cpy
    }
    result = orig
}

func main() {}

Output:

$ go version
go version devel +ffe33f1f1f17 Tue Nov 25 15:41:33 2014 +1100 linux/amd64
$ go test -v -bench=.
testing: warning: no tests to run
PASS
BenchmarkCopy                   200000        9983 ns/op
BenchmarkAppend                 200000       10004 ns/op
BenchmarkAppendPreCapped        200000       10077 ns/op
BenchmarkAppendNil              200000        9960 ns/op
ok      so/test 8.412s
$ go test -v -bench=.
testing: warning: no tests to run
PASS
BenchmarkCopy                   200000       10000 ns/op
BenchmarkAppend                 200000       10112 ns/op
BenchmarkAppendPreCapped        200000        9892 ns/op
BenchmarkAppendNil              200000       10005 ns/op
ok      so/test 8.422s
$ go test -v -bench=.
testing: warning: no tests to run
PASS
BenchmarkCopy                   200000        9967 ns/op
BenchmarkAppend                 200000        9898 ns/op
BenchmarkAppendPreCapped        200000       10123 ns/op
BenchmarkAppendNil              200000       10022 ns/op
ok      so/test 8.424s
$