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To correctly copy slices in Go, you must create a new underlying array instead of directly assigning values; 1. Use make and copy functions: dst := make([]T, len(src)); copy(dst, src); 2. Use append and nil slices: dst := append([]T(nil), src...); both methods can achieve element-level copying, avoid sharing the underlying array, and ensure that modifications do not affect each other. Direct assignment of dst = src will cause both to refer to the same array and are not real copying.
In Go, copying a slice isn't always as straightforward as assigning one variable to another—because slices are reference types. If you just assign a slice to a new variable, both will refer to the same underlying array. So, to properly copy a slice , you need to create a new slice and copy the elements into it. Here's how to do it correctly.
? Use the copy() built-in function
The idiomatic and safest way to copy a slice in Go is using the built-in copy() function:
src := []int{1, 2, 3}
dst := make([]int, len(src))
copy(dst, src) Now dst is a separate slice with the same values as src , but modifications to one won't affect the other.
You can also combine it into one line:
dst := make([]int, len(src)) copy(dst, src)
Or even more concisely:
dst := append([]int(nil), src...)
This works because append will allocate new space when adding elements to a nil slice.
?? Common ways to copy a slice
Here are a few standard approaches:
Using
makecopy(recommended for clarity):dst := make([]T, len(src)) copy(dst, src)
Using
appendwith a nil slice:dst := append([]T(nil), src...)
Using
copyinto a slice of the same length: Make sure the destination has enough space—copyonly copies up to the minimum of the two lengths.?? This is a common mistake:
dst := []int{} // len = 0 copy(dst, src) // copyies 0 elements!So ensure
dsthas sufficient length or capacity.
? Important notes
copy()returns the number of elements copied (which ismin(len(src), len(dst))), but you usually don't need this value.If you want to preserve capacity (eg, for future appends), use:
dst := make([]int, len(src), cap(src)) copy(dst, src)
For slices of pointser or complex structs , this method still works, but remember: it's a shallow copy . The pointed-to data isn't duplicated.
Example:
type Person struct{ Name string } src := []*Person{{Name: "Alice"}} dst := make([]*Person, len(src)) copy(dst, src)Now
dst[0]andsrc[0]point to the samePersonstruct. Changingdst[0].Nameaffects the same object.To deep copy, you'd need to manually clone each element.
? What doesn't work
dst := src // Just copies the slice header—both point to same data! dst[0] = 99 // This modifies src[0] too!
This is not a copy—it's just another reference to the same underlying array.
? Summary: Best practice
To properly copy a slice:
dst := make([]T, len(src)) copy(dst, src)
Or, if you prefer brevity and don't need to pre-allocate:
dst := append([]T(nil), src...)
Both are safe, efficient, and commonly used in Go code.
Basically, just avoid direct assignment and make sure you're copying elements into a new backing array.
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