What are the Uses and Properties of Empty Structs (`struct{}` and `struct{}{}`) in Go?

Understanding the Empty Struct in Go: struct{} and struct{}{}

In Go, the struct keyword defines a structured data type composed of named elements. However, struct{} and struct{}{}, often encountered in idiomatic Go code, deserve specific attention.

struct{}: An Empty Struct

The struct{} is a special case representing an empty struct with no elements. It has the unique property of being zero-sized, meaning no memory is allocated to a value of this type.

struct{}{} as a Composite Literal

In contrast, struct{}{}, written similarly to int{5} or []string{"Go", "Python"}, is a composite literal that explicitly constructs a value of type struct{}. This literal has no fields since struct{} has no defined elements.

Applications of Empty Structs

Empty structs can find various uses in Go code:

• As a "placeholder" in map values: When using a map to store a set (collection of unique elements), setting the value type to struct{} ensures that only keys (elements) are stored, without wasting space on associated values.
• As a field within a struct: An empty struct can provide additional padding within a larger struct, ensuring certain alignment requirements for specific architectures.

Example: Creating a Set Using an Empty Struct

Here's an example of using a map with an empty struct value type to create a set:

package main

import "fmt"

type Set map[string]struct{}

func main() {
    s := make(Set)
    s["red"] = struct{}{}
    s["blue"] = struct{}{}

    fmt.Println(s)  // Prints: map[blue:{} red:{}]
}

Note that this approach only checks for the existence of keys, not their associated values.

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