The make() Function

The built-in make function initializes the runtime structures for slices, maps and channels, returning a value of the requested type.

Use make([]int, 2, 5) for a slice with length 2 and capacity 5. Use make(map[string]int) for an empty writable map. For a buffered channel, use a form such as make(chan int, 2).

Unlike new(T), make(T, ...) returns a T, not a *T. It does not apply to ordinary scalars or structs. Channels are explained in the concurrency group; the example here uses just a slice and a map.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func main() {
	numbers := make([]int, 2, 5)
	numbers[0], numbers[1] = 4, 8
	counts := make(map[string]int)
	counts["apples"] = 3
	fmt.Println("Slice:", numbers)
	fmt.Println("Length/capacity:", len(numbers), cap(numbers))
	fmt.Println("Apples:", counts["apples"])
}

Output

Slice: [4 8]
Length/capacity: 2 5
Apples: 3

make creates a slice with two accessible zero-valued slots and a capacity of five. Setting the first two elements produces [4 8]. It also creates a map that accepts the apples entry.

• make([]int, 2, 5) exposes indexes 0 and 1, not indexes through 4.
• make returns a slice, map or channel value, not a pointer to one.

Make a slice with length 1 and capacity 3. Append a number and print its new length and capacity.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced The make() Function in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.