The Difference Between make and new

In Golang, there are two keywords, make and new, which are often confused when first learning Go. Here is a quick note on them.

new

new can be used to allocate memory for a type, and it returns its memory address. Therefore, we typically use a pointer variable to hold the value allocated by new. It is particularly important to note that new automatically initializes the type with its zero value—meaning strings will be "", numbers will be 0, and channels, funcs, maps, slices, etc., will be nil.

Because of this behavior, performing the following operation on a map will result in panic: assignment to entry in nil map:

func main() {
	people := new(map[string]string)
	p := *people
	p["name"] = "Kalan" // panic: assignment to entry in nil map
}

This happens because the initialized map is a nil map, unlike other primitive types that have ready-to-use default values.

When initializing a struct, you can also directly use & to represent the pointer to its address. The two approaches below have the same effect:

type Person struct {
  Name string
  Age  int
}

func main() {
    p := &Person{}
    p := new(Person)
}

The advantage of using &Person{} is that you can also set custom values for the fields as needed, whereas new fills all fields with their zero values.

make

Unlike new, make is used to initialize specific built-in types such as channels, maps, and slices. Another critical point is that make does not return a pointer. If you need a pointer, you should consider using something like new to initialize the type instead.

func main() {
    receiver := make(chan string) // 初始化 channel，但不回傳指標
    person := make(map[string]string)
    people := make([]string, 100) // 初始化長度為 100 的字串陣列
}

To emphasize once again: make does not return a pointer! The following code will throw an error:

func main() {
	person := make(map[string]string)
	fmt.Println(*person) // invalid indirect of person
}

Conclusion

make and new are easy to confuse when starting out with Golang. Sometimes, when a function requires a pointer parameter, you might accidentally pass the result of make. Knowing the difference between the two is essential in such scenarios.

Further Reading

• https://golang.org/doc/effective_go.html#allocation_make
• https://golang.org/doc/effective_go.html#allocation_new