Golang copy function

last modified May 8, 2025

This tutorial explains how to use the copy built-in function in Go. We'll cover slice copying basics with practical examples of efficient data copying.

The copy function copies elements from a source slice into a destination slice. It returns the number of elements copied, which is the minimum of the source and destination lengths.

In Go, copy is used to efficiently transfer data between slices or arrays. It handles overlapping slices correctly and is more efficient than manual looping for copying data.

Basic slice copying example

The simplest use of copy transfers data between two slices. This example demonstrates basic slice copying.
Note: The destination slice must have sufficient capacity.

basic_copy.go

package main

import "fmt"

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

The copy function copies all elements from src to dst. The returned value n equals the length of src since dst has the same length.

Partial slice copying

We can copy only a portion of a slice by using slice expressions. This example shows partial copying between slices.

partial_copy.go

package main

import "fmt"

func main() {
    src := []string{"apple", "banana", "cherry", "date", "elderberry"}
    dst := make([]string, 3)
    
    n := copy(dst, src[1:4])
    
    fmt.Println("Copied", n, "elements")
    fmt.Println("Source:", src)
    fmt.Println("Destination:", dst)
}

The code copies elements 1 through 3 from src to dst. Only 3 elements are copied because dst has length 3.

Copying between different types

The copy function can work between different types if they have identical underlying types. This example demonstrates copying between byte and uint8 slices.

type_copy.go

package main

import "fmt"

func main() {
    src := []byte{'G', 'o', 'l', 'a', 'n', 'g'}
    dst := make([]uint8, len(src))
    
    n := copy(dst, src)
    
    fmt.Println("Copied", n, "elements")
    fmt.Printf("Source: %v\n", src)
    fmt.Printf("Destination: %v\n", dst)
}

Byte and uint8 are identical types in Go, so copy works between them. The function copies all elements successfully.

Copying with overlapping slices

The copy function handles overlapping slices correctly. This example shows how copy can be used to shift elements within a single slice.

overlap_copy.go

package main

import "fmt"

func main() {
    data := []int{1, 2, 3, 4, 5, 6, 7, 8}
    
    // Shift elements left by 2 positions
    n := copy(data, data[2:])
    
    // Truncate the slice
    data = data[:n]
    
    fmt.Println("Copied", n, "elements")
    fmt.Println("Result:", data)
}

The copy operation shifts elements left within the same slice. The final slice is truncated to remove the leftover elements at the end.

Copying from arrays to slices

The copy function can copy data from arrays to slices. This example demonstrates copying from a fixed-size array to a slice.

array_copy.go

package main

import "fmt"

func main() {
    src := [5]int{10, 20, 30, 40, 50}
    dst := make([]int, 3)
    
    n := copy(dst, src[:])
    
    fmt.Println("Copied", n, "elements")
    fmt.Println("Source array:", src)
    fmt.Println("Destination slice:", dst)
}

The array is converted to a slice using the [:] operator before copying. Only 3 elements are copied because dst has length 3.

Source

Go language specification

This tutorial covered the copy function in Go with practical examples of efficient data copying between slices and arrays.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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