Go Structs: What's the Difference Between `struct{}` and `struct{}{}?`

structs in Go: struct{} and struct{}{} Demystified

In Go, struct is a powerful construct used to define data types. However, there are two special cases of structs worth examining: struct{} and struct{}.

struct{}: The Zero-Element Struct

struct{} is a struct type with no named elements. It serves a few useful purposes:

• Zero-sized: A struct{} value does not occupy any memory, making it particularly useful for space-efficient scenarios.

Usage:

var emptyStruct struct{}

struct{}: Empty Composite Literal

struct{}{} is a composite literal that constructs an empty struct value. Composite literals allow for concisely initializing values of specific types. For struct{}, the syntax is simple:

var emptyStructValue = struct{}{}

Usage in a Set Implementation

One practical application of the empty struct is creating a set-like data structure using a Go map. Since a map only allows unique keys, we can use struct{} as the value type to create a set of unique elements without storing any values:

type Set map[string]struct{}

func main() {
    set := make(Set)
    set["foo"] = struct{}{}
    set["bar"] = struct{}{}

    _, ok := set["foo"]
    println("Is \"foo\" in the set?", ok)
}

Conclusion

struct{} and struct{}{} offer unique features that extend the utility of Go's struct system. The zero-element struct provides memory efficiency, while the empty composite literal assists in creating empty struct values or constructing set-like data structures.

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