Copying Slices

copy() makes a real, separate duplicate of a slice's contents, so changing the copy never touches the original.

In this page:

Using copy()
copy() Is Limited by the Shorter Slice
Independence After Copying

Using copy()

The built-in copy function copies elements from a source slice into a destination slice, and unlike plain assignment, the two slices end up backed by completely separate memory afterward.

Example: Using copy()

markup

package main

import "fmt"

func main() {
	src := []int{1, 2, 3}
	dst := make([]int, 3)
	n := copy(dst, src)
	fmt.Println("copied:", n, "dst:", dst)
}

copy() Is Limited by the Shorter Slice

copy only transfers as many elements as fit in the smaller of the two slices -- if the destination is shorter than the source, the extra source elements are simply left out, with no error raised.

Note: Make sure the destination slice is at least as long as the source before copying, using make(len(src)) if needed.

Example: copy() Is Limited by the Shorter Slice

markup

package main

import "fmt"

func main() {
	src := []int{1, 2, 3, 4, 5}
	dst := make([]int, 2)
	n := copy(dst, src)
	fmt.Println("copied:", n, "dst:", dst)
}

Independence After Copying

Once copy() has run, modifying the destination slice never affects the source, and vice versa -- they are backed by entirely separate underlying arrays, which is the whole point of using copy instead of assignment.

Example: Independence After Copying

markup

package main

import "fmt"

func main() {
	original := []int{1, 2, 3}
	duplicate := make([]int, len(original))
	copy(duplicate, original)
	duplicate[0] = 999
	fmt.Println("original:", original)
	fmt.Println("duplicate:", duplicate)
}

Common Mistakes

Assigning one slice variable to another ('b := a') expecting an independent copy, when it actually just shares the same underlying array.
Calling copy(dst, src) with dst having fewer elements than src, and not realizing copy silently only copies up to len(dst).
Forgetting copy() returns the number of elements actually copied, which is useful for verifying a full copy happened.

Chapter Summary

copy(dst, src) copies elements from src into dst, up to the length of the shorter one.
copy() is the way to create a truly independent slice, unlike plain assignment.
The number of elements copied is limited by whichever slice (dst or src) is shorter.
copy() returns an int: how many elements were actually copied.

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Copying Slices

Using copy()

copy() Is Limited by the Shorter Slice

Independence After Copying

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