Understanding nil

nil is the zero value of several reference-capable types, including pointers, slices, maps, functions, channels and interfaces.

A nil pointer has no target; dereferencing it panics. A nil slice has length and capacity zero and may be appended to. A nil map permits reads and deletion but assigning a key panics. The operation matters more than the word nil itself.

An interface value is nil only when it has neither a dynamic type nor a dynamic value. An interface holding a typed nil pointer is therefore not equal to nil; we revisit this subtlety in the interfaces and errors lessons.

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() {
	var ptr *int
	var items []int
	var ages map[string]int
	fmt.Println("Pointer nil:", ptr == nil)
	fmt.Println("Slice nil:", items == nil)
	items = append(items, 7)
	fmt.Println("Appended slice:", items)
	fmt.Println("Missing map key:", ages["unknown"])
}

Output

Pointer nil: true
Slice nil: true
Appended slice: [7]
Missing map key: 0

All three variables initially have nil values. Appending to the nil slice creates usable storage and yields [7]; looking up an absent key in the nil map safely returns int’s zero value, 0.

• Do not dereference a nil pointer.
• Do not insert into a nil map before initializing it with make or a literal.

Create a nil slice of strings, append two names, and confirm its length. Compare this with a nil map before trying to write into it.

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

You have practiced Understanding nil 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.