Nil Map Golang, A map is an unordered collection of key-value pairs. panic: assignment to entry in nil map| Instead, we can use make to initialize a map of the specified type. non-nillable and nillable types Types can be either nillable or non-nillable in go. depends on the case. var foo *int) it's a nil-pointer. The zero value of a map is nil. Nil is "typed" in that it requires some kind of type to go along with it. nil is a predeclared identifier in Go that represents zero values for pointers, interfaces, channels, maps, slices and function types. 1 ok only tells if the key is in the map, whether the value associated with it is nil (or in general the zero value of he value type) or not is another thing. Like a nil int pointer, or nil string pointer. Trying to add an entry will panic though. Why does "print" of a nil map output "map []" instead of <nil> ? Many new Go programmers with experiences of some other popular languages may view nil as the counterpart of null (or NULL) in other languages. Println("The value of m is nil") } } The result is like this： The value of s is nil The value of m is nil When a slice's value is nil, you could also do operations on it Technically this is the 2-value form of the type assertion, not the map lookup. You create a new map with a make statement or a map literal. CODE EXAMPLE You have to initialize a map using the make function (or a map literal) before you can add any elements. Map types are reference types, like pointers or slices, and so the value of m above is nil; it doesn't point to an initialized map. Like many languages, Go provides a built-in data type for situations where you want to associate one …. In terms of abstract data types, they represent an array and an associative arr I am learning to use the map of maps. Click here to learn about The Supreme Guide to Golang Maps. edit: i forgot how maps work in go XD, but i'd still return the map but my goto is: return the map! What is a map in Golang and how does it work? Are Go maps thread-safe? Are maps pointers? How do you check if a map is empty? Can maps be nil? How do you clear a map? How do you copy a map? How do you convert a map to JSON or YAML? Go teacher and expert John Arundel answers your questions on Go maps. Maps and slices have to be allocated elsewhere in order to be able to grow arbitrarily, while channels need that to be accessible from two different places. Sep 11, 2025 · Learn how to handle GoLang map of nil value with best practices, preventing common errors and improving code reliability in your Go applications. otherwhise its going to be the map since that saves my code of some potential traps where i expect a map, but got a nil nontheless. Go provides a built-in map type that implements a hash table. the same map. Copy has any way to allocate memory (a new map) and provide it to the caller who passed nil, because maps. 1. A map is a inbuilt data type in Go which is used to store key-value pairs. Sep 30, 2024 · Map in golang basically acts as a hash table and the internal data structures needs to be initialized first before we can store the key-value pair. The map data structure is used for fast lookups, retrieval, and deletion of data based on keys. Find out how Go (golang) maps work and when to use them for fast in-memory storage, with a set of practical examples. Adding any key value pair to that map will cause a panic. My method have pointer reciever to use the structs directly instead of making copies. Use the `make ()` function to create a new map. if the map could become huge, then i would return a pointer and nil. The nil interface value from the map is passed to the assertion which can then assert the interface to the desired zero value. This will prevent you from accidentally overwriting a valid map with a nil map. The keys are unique within a map while the values may not be. Println("The value of s is nil") } if m == nil { fmt. インターフェースを受け取り、構造体を返す 関数はインターフェースパラメータを受け取り、具体的な型を返すべきです。 // Good: Accepts interface, returns concrete type func ProcessData(r io. There are no "empty" values. package main import ( &quot;fmt&quot; ) func main() { var data = map[string]map[string]map[st 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. Create a proper distinction between the declaration and initialization of maps or any similar data type in golang. Dec 26, 2022 · In today's article, we will discuss how to detect nil value in Go. Maps, slices and channels are essentially fancy pointers. See this example: The map data structure is used for fast lookups, retrieval, and deletion of data based on keys. Mastering Maps in Go: A Comprehensive Guide Slices are useful for organizing sequential data. Map in programming is a compelling data structure. g. But when I try to create a new key when it doesn't exists, I get a runtime error "panic: assignment to entry in nil map". Master GoLang JSON encoding with omitempty, handling missing and optional fields for robust API responses and efficient data transfer. Avoid common mistakes and learn best practices. I tried: var m map [string]string m ["time"] = "asdf" and get error: assignment to entry in nil map Maps are unordered collection of key/value pairs, implemented as hash tables in other languages like java and dictionary in python. The non-nillable types can never be nil In golang, if you create a struct with a map in it, which is uninitialized, printing the map yields a non <nil> string. This is partly right, but there are many differences between nil in Go and null (or NULL) in other languages. Go Gotcha: Nil Maps Common beginner mistakes around declaring, initializing, and allocating map types. Same for maps/slices/channels. Let’s look at various ways to initialize a map: package main import "fmt" func main() { m1 := map[string]string{} // initializes map m2 := make(map[string]string) // also initializes map var m3 map[string]string See go. A common beginner mistake in Go is forgetting to allocate the space for a map. 认识 nil map nil map 是 Go 语言中表示空 map 的一种特殊值。 与其他值不同，nil map 并不指向任何实际的存储空间，而是代表一个不存在的 map。 通常，我们使用 make (map [key-type]value-type) 来创建空 map，但 nil map 则是一种更简洁的方式，可以直接赋值 var m map [string]int。 Posted by u/vinay737 - No votes and 3 comments However, you cannot store nil in an untyped variable, like thing := nil. I don't think maps. A nil map is equivalent to an empty map except that no elements may be added. If the length of the map is 0, then it is nil. In this post I will try to describe how nil and zero-values are used in golang. Clone. Many hash table implementations exist with varying properties,but in general they offer fast lookups, adds, and deletes. When we declare a map as nil (var m map[KeyType]ValueType), it is uninitialized and not yet ready to store key-value pairs. Copy doesn't return a collection like append or maps. With so many choices, which option is best? Or perhaps better asked, which one is best in your situation? Let’s take a look. I’ve been a bit confused for a while over when a map in Golang gets created with a value of nil (its zero value) and when it does not, so I’m writing this to help me remember. ☛ What is a map? A map is like an array except, instead of an integer index The initial capacity does not bound its size: maps grow to accommodate the number of items stored in them, with the exception of nil maps. Maps A map maps keys to values. nil maps just cant be used to add any new elements. Learn essential Golang map handling techniques to prevent assignment panics, master safe map initialization, and avoid common runtime errors in your Go programming projects. But then, it's kind of typed It's definitely confusing. The language spec says: A nil map is equivalent to an empty map except that no elements may be added. ReadAll(r) Learn how to build a golang tree with B-Tree structure and efficient traversal methods using simple, clear code for beginners and developers. Map is defined in the spec as being a reference type (i. Sep 21, 2023 · golang maps don't panic, when someone reads using a [] operator, even on a nil map. I try to get myself familiarise with interface {} in Go. In this post, I will cover, what are maps, how to create maps, and especially how to properly declare and initialize them. In the driver file I try to call a method from and item type from the items map of the Cashier type. You can read from a nil map, but cannot write to it. The make function returns a map of the given type, initialized and ready for use. " Wrong. Learn how to work with maps in Go. It maps keys to values. If you declare a pointer (e. In the following example, there are three nested maps. May 3, 2025 · By having an in-depth understanding of the mechanism of nil, developers can write more robust Go code more accurately and avoid runtime errors caused by improper handling of nil. nil is a predefined identifier in Go that represents zero values of many types. Use the `delete ()` function to remove entries from a map. The error "assignment to entry in nil map" often occurs in the Go programming language when you try to insert or modify an element in a map that hasn’t been initialized. Golang Bad: Discover why Go's design falls short, exploring its limitations and potential drawbacks for developers and programmers. We explain how it works for pointers, slices, interfaces, and more. One of the most useful data structures in computer science is the hash table. To figure out what a map is, I recommend reading the following article: Inside the Map… Nillability and zero-values in go November 06, 2018 Beeing a long time java-developer, I am obsessed with null-checking and handling null values. GOLANG The anatomy of maps in Go A map is a composite data type that can hold data represented by key:value pairs. The remaining of this article will list all kinds of facts and details related to nil. In some aspects, it behaves like an empty map: you can check its size with len(), loop over it (won't execute loop body), delete an entry (won't do anything), print it (will print map[]), etc. Includes examples of creating, accessing, and modifying maps. In golang, the story is somewhat different. you can assign maps to multiple variables but they would share the underlying stora Understand nil values in Go. We’re then free to set and retrieve key:value pairs in the map as usual. B) Nested Maps If you are trying to use a map within another map, for example when building JSON-like data, things can become more complicated, but the same principles remain in that make is required to initialize Testing golang&#39;s map behavior when the map is nil. Reader) (*Result, error) { data, err := io. But if the map is storing pointers, then that means the storage for the pointer will persist, but the pointed-to value will be GC'd provided nothing is pointing to it. Learn golang dictionary basics with clear examples and simple explanations to boost your coding skills and build efficient, readable Go programs. dev/doc/effective_go#allocation_new The zero value of a map is equal to nil. It is one of the most used data structures in computer science. nil is usually mistaken as null (or NULL) in other languages, but they are different. counts map [string] int // nil map will panic } 3. A map can also be declared with var keyword, but it creates a nil map as default zero value of map is nil. A nil map behaves like an empty map when reading, but attempts to write to a nil map will cause a runtime panic; don't do that. How come I can range over a slice, map etc if they are `nil`, or check their len safely, but accessing a field of a `nil` ref will cause a runtime `npd` error? Found this surprising, but I believe there is some consistency here. e. I am new to golang and I am trying to create a map of type map [string]interface {}. The value returned is the zero value. If the map is storing a value type (like int), the storage for that int is inside the bucket, so it will also never be cleaned up unless you nil out the map. Our guide provides detailed explanations and examples. A nil map cannot be used to store key-value pairs, while an empty map has been initialized but lacks elements. They both reference (or container) types. For keys on a nil map to be assignable you would need an additional level of indirection, so that all the other copies of the As far as I understand, types slice and map are similar in many ways in Go. Learn golang reflect with simple examples and clear explanations for beginners to master runtime reflection in Go. This guide delves deep into the world of Golang Dynamic Informers, a sophisticated feature within client-go that empowers developers to monitor and react to changes in multiple Kubernetes resource types dynamically. A practical use of a map is for storing the currency codes and the corresponding currency names mapassign1: runtime·panicstring("assignment to entry in nil map"); I attempt to make an array of Maps, with each Map containing two indicies, a "Id" and a "Investor". To initialize a map, use the built in make function: A nil map and a map with no key-value pairs (an empty map) are distinct. Master Golang list initialization and operations with our comprehensive tutorial, covering data types, append, and more, for efficient coding. "In golang, the value of a map is empty value when the key has not been set. CODE EXAMPLE Go maps are implemented by hash tables and offer efficient add, get and delete operations. What is a map in Golang? A map or a hashmap in golang is a basic data type that allows us to store key-value Declaring golang maps using the var keyword, Initializing map using map literal, Initializing golang map using the make() function, Adding Key value pair in a Golang map, Accessing values in a Golang map, Changing the value of a map in Go, Deleting element from a Go map, Checking if a key exists in a map Looping in Go maps, Looping through ordered map Understand what is nil in GoLang and how to use it effectively in your code. A nil map has no keys, nor can keys be added. nil being the zero value of pointers and interfaces, uninitialized pointers and interfaces will be nil. You cannot just have a nil pointer to some unknown type without it being an interface. This will ensure that the map is initialized and that you can safely assign values to it. Slice and map are all reference types in Go, and their default values are nil: package main import "fmt" func main() { var ( s []int m map[int]bool ) if s == nil { fmt. Jul 20, 2025 · Learn why golang nil maps cause runtime panics, how to check if maps are nil vs empty, and production-ready patterns to prevent server crashes in Go. 6xxs, xnh9, 73nu, badd, rcnm, mjc5r, jswd5, em2pk, bzy0wq, eh7p,