Make Maps and Slices in Golang - A Guide to Initialization

There are quite a few ways to create new maps and slices in Go, for example, they can both be initialized using the make() function, the new() function, as literals, or by using the var keyword. With so many choices, which option is best? Or perhaps better asked, which one is best in your situation? Let’s take a look.

Slices

var varStyle []string

literalStyle := []string{}

newStyle := new([]string)

makeStyle := make([]string, 0)

var varStyle []string is the idiomatic way to declare an empty slice. The slice is actually nil, which means it will be null when marshalled to JSON and will succeed nil checks.

literalStyle := []string{} should probably only be used when the literal is going to start with values in it, as in literalStyle := []string{"cat", "dog", etc}. Otherwise prefer make()

newStyle := new([]string) returns a pointer to the slice. Same as ptrStyle := &[]string{}. Only use if you want a pointer.

makeStyle := make([]string, 0) is the same as the literal style, but is preferred for idiomatic reasons when the slice doesn’t need non-zero starting values. Make() allows the slice to be initialized with a starting length and capacity, which can have good performance implications in some circumstances:

makeStyle := make([]string, len, cap)

Maps

var varStyle map[int]int

literalStyle = map[string]int{}

newStyle := new(map[string]int)

makeStyle := make(map[string]int)

var varStyle map[int]int creates a nil map. Writing (but not reading interestingly enough) will cause a panic. You probably don’t want a nil map.

literalStyle := map[string]int{} using the literal syntax is just fine, though idiomatically it’s probably best to use a make function. Developers are more used to seeing a make function and make offer some additional features.

newStyle := new(map[string]int) creates a pointer to a nil map… very often not what you want.

makeStyle := make(map[string]int) This is probably what you want! If you know your space requirements you can optimize for allocation by passing in a size:

// Give me a map with room for 10 items before needing to allocate more space
makeStyle := make(map[string]int, 10)

Check out our How To: Global Constant Maps and Slices article if you want to learn more about the proper use of maps and slices in Go.