Variables and struct fields
February 7, 2023
Structured variables of array, slice, and struct types have elements and fields that may be addressed individually. Each such element acts like a variable.
This is pretty straight forward. In most contexts, the elements of a struct, array, or slice, can be treated as variables. They can be assigned, or assigned to, using the same syntax.
A few examples to illustrate:
type Person struct {
ID int
Name string
Age *int
}
var p Person // p is an instance of the Person struct, with each field initially
// set to its zero value.
p.ID = 123 // Assign value to p.ID
fmt.Println(p.Name) // Print the zero value (the empty string) of p.Name
fmt.Println(p.Age) // Panic. Because p.Age is a pointer to int, and not
// initalized, as with new(), dereferencing it is not allowed.
var age = int(43)
p.Age = &age // Now p.Age properly points to an int, which contains the value 43
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.