Go - make Keyword (New Slice, Map)

Make. In the Go language, the slice, map and channel types are created with "make," not "new." This function creates a new instance of a slice, map or channel.

new

The arguments to make indicate what type we are creating, the initial element count, and (optionally) a capacity of the underlying buffer. With a capacity, we can avoid further allocations.

Example. This program uses the make() built-in in several different ways. And it uses an alternative syntax to create empty slices and maps, without calling make.

Part 1 We use make to create a slice of a certain number of elements. With 0, the slice is empty. With 5, the slice has 5 ints of value 0.

Slice

Part 2 With make() calls to create slices, we can use an optional third argument as a hint of what underlying buffer size is desired (a capacity).

Part 3 For slices, it is often better just to use an empty initialization instead of a call to make().

Part 4 Maps can be created too with make. The optional second argument is a capacity hint.

map

Part 5 As with slices, maps often are created without make—an empty initialization can be used.

Part 6 We use make to create a channel with an underlying buffer size of 10 elements. The buffer size is separate from the channel's length.

chan

package main import "fmt" func main() { // Part 1: use make to create an empty slice, and a 5-element slice of all zero values. slice1 := make([]int, 0) slice1 = append(slice1, 10) fmt.Println("make:", slice1) slice2 := make([]int, 5) fmt.Println("make:", slice2) // Part 2: use make to create a zero-element string slice of capacity 20. slice3 := make([]string, 0, 20) slice3 = append(slice3, "cat", "frog", "bird") fmt.Println("make:", slice3) // Part 3: a slice can be created without make. slice4 := []string{} slice4 = append(slice4, "xyz") fmt.Println("[]string:", slice4) // Part 4: use make to create a map with capacity 10. map1 := make(map[string]int, 10) map1["abc"] = 1 fmt.Println("make:", map1) // Part 5: a map can be created without make, but it will not have a custom capacity. map2 := map[string]int{} map2["cat"] = 500 fmt.Println("map[string]int:", map2) // Part 6: use make to create a channel of 10 elements. channel1 := make(chan int, 10) channel1 <- 1 channel1 <- 10 fmt.Println("Channel:", len(channel1)) }

make: [10] make: [0 0 0 0 0] make: [cat frog bird] []string: [xyz] make: map[abc:1] map[string]int: map[cat:500] Channel: 2

Summary. Make is essentially a new() method for slices, maps and channels in the Go language. It is unclear why it is not called "new," but make fills this important initialization need.

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This page was last updated on Jan 24, 2025 (edit link).

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