Array types
February 28, 2023
Arrays should not be a foreign concept to anyone who’s done programming before. But Go’s version of arrays has some peculiarities that often trip up beginners.
Array types
An array is a numbered sequence of elements of a single type, called the element type. The number of elements is called the length of the array and is never negative.
ArrayType = "[" ArrayLength "]" ElementType . ArrayLength = Expression . ElementType = Type .
The length is part of the array’s type; it must evaluate to a non-negative constant representable by a value of type
int
. The length of arraya
can be discovered using the built-in functionlen
. The elements can be addressed by integer indices 0 throughlen(a)-1
. …[32]byte [2*N] struct { x, y int32 } [1000]*float64
The most important, and often surprising, part of all of this is the sentence The length is part of the array’s type. In Go, arrays have a fixed length. They never grow or shrink. You cannot push onto, or pop off of an array. You can only change the existing elements of the array.
This does mean that the length of an array is a constant, and can thus be assigned to a const
:
var x [3]int
const y = len(x)
fmt.Println(x, y) // [0 0 0] 3
This limitation means that the array
type is often not (directly) used in Go. Instead, the more flexible slice
type, which we’ll be talking about shortly, is much more common.
Quotes from The Go Programming Language Specification, Version of January 19, 2023
Related Content

Empty structs
We finally we have enough knowledge for the EBNF format not to seem completely foreign, so let’s jump back and take a look at that, with the examples provided in the spec… Struct types … StructType = "struct" "{" { FieldDecl ";" } "}" . FieldDecl = (IdentifierList Type | EmbeddedField) [ Tag ] . EmbeddedField = [ "*" ] TypeName [ TypeArgs ] . Tag = string_lit . // An empty struct.

Struct tags
Struct types … A field declaration may be followed by an optional string literal tag, which becomes an attribute for all the fields in the corresponding field declaration. An empty tag string is equivalent to an absent tag. The tags are made visible through a reflection interface and take part in type identity for structs but are otherwise ignored. struct { x, y float64 "" // an empty tag string is like an absent tag name string "any string is permitted as a tag" _ [4]byte "ceci n'est pas un champ de structure" } // A struct corresponding to a TimeStamp protocol buffer.

Struct method promotion
Yesterday we saw an example of struct field promotion. But methods (which we haven’t really discussed yet) can also be promoted. Struct types … Given a struct type S and a named type T, promoted methods are included in the method set of the struct as follows: If S contains an embedded field T, the method sets of S and *S both include promoted methods with receiver T. The method set of *S also includes promoted methods with receiver *T.