What is the usage of empty structure in go language?

Usage: 1. When using map as a Set, you can define the value type as an empty structure, which is only used as a placeholder; the channel does not need to send data, it is only used to notify sub-coroutines to perform tasks, etc. When , you can use an empty structure as a placeholder; 3. When the structure only contains methods and does not contain any fields, you can declare an empty structure to save resources.

The operating environment of this tutorial: Windows 10 system, GO 1.11.2, Dell G3 computer.

1. Does an empty structure take up space?

In Go language, we can use unsafe.Sizeof to calculate the number of bytes occupied by a data type instance.

package main

import (
	"fmt"
	"unsafe"
)

func main() {
	fmt.Println(unsafe.Sizeof(struct{}{}))
}

Running the above example will output:

$ go run main.go
0

In other words, the empty structure struct{} instance does not occupy any memory space.

2. The role of empty structures

Because empty structures do not occupy memory space, they are widely used as placeholders in various scenarios. One is to save resources, and the other is that the empty structure itself has strong semantics, that is, no value is needed here, it is just a placeholder.

2.1 Implementing Set

The Go language standard library does not provide an implementation of Set, and map is usually used instead. In fact, for collections, only the keys of the map are needed, not the values. Even if the value is set to bool type, it will occupy 1 more byte. If there are one million pieces of data in the map, 1MB of space will be wasted.

Therefore, when using map as a set, you can define the value type as an empty structure and use it only as a placeholder.

type Set map[string]struct{}

func (s Set) Has(key string) bool {
	_, ok := s[key]
	return ok
}

func (s Set) Add(key string) {
	s[key] = struct{}{}
}

func (s Set) Delete(key string) {
	delete(s, key)
}

func main() {
	s := make(Set)
	s.Add("Tom")
	s.Add("Sam")
	fmt.Println(s.Has("Tom"))
	fmt.Println(s.Has("Jack"))
}

2.2 Channel that does not send data

func worker(ch chan struct{}) {
	<-ch
	fmt.Println("do something")
	close(ch)
}

func main() {
	ch := make(chan struct{})
	go worker(ch)
	ch <- struct{}{}
}

Sometimes when using a channel, you do not need to send any data, it is only used to notify the sub-coroutine (goroutine) Execute tasks, or just control coroutine concurrency. In this case, using an empty structure as a placeholder is very suitable.

2.3 Structure containing only methods

type Door struct{}

func (d Door) Open() {
	fmt.Println("Open the door")
}

func (d Door) Close() {
	fmt.Println("Close the door")
}

In some scenarios, the structure only contains methods and does not contain any fields. For example, Door in the above example. In this case, Door can actually be replaced by any data structure. For example:

type Door int
type Door bool

Whether it is int or bool, extra memory will be wasted. Therefore, in this case, it is most appropriate to declare an empty structure.

Recommended learning: Golang tutorial

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