Go Structs

External

• https://go.dev/ref/spec#Struct_types

Internal

• Go Language

Overview

Structs are user-defined composite types, grouping together instances of arbitrary types, into one object.

A struct declaration consists in a series of fields, where each fields has an arbitrary name and a type. A struct may also contain other types, named embedded types and also methods that operate on the structure's fields. The fields usually represent a has-a relationship, consistent with the struct's composite type nature.

Structs can represent data values that could have either a primitive or non-primitive value. "Primitive" structs imply creating a new instance every time an existing value is mutated. In this case, values themselves, rather than pointers, are used to share values of those structs. An example is the Time structure in the time package. However, in most cases, structs exhibit a non-primitive structure: adding or removing something from the value mutates the value, does not create a new value.

Printing Structs

The entire struct, including field names and values can be printed with the %+v format specifier of fmt.Printf(). For more details see:

Printing to stdout and stderr

Declaration

The struct type definition is introduced by the type keyword, to indicate that this is a user-defined type, followed by the new type name and the keyword struct, followed by a curly-brackets-enclosed enumeration of fields. Each field is defined by an arbitrary name and a type. The type can be a pre-declared type or a user-defined type, such as another struct or an interface.

Struct types can be declared at package level or inside a function.

type Item struct {
    color string
    size int
}

Fields with the same types can be collapsed:

type SomeStruct struct {
  ...
   i, j, k int
  ...
}

Once the type has been defined in a package or a function, variables of that type can be declared using the long declaration:

var i Item

If no explicit initialization follows, the struct variables declared as above have all their fields initialized with zero values.

Short declaration can also be used, as shown in the Initialization section, below.

Literal

Struct literals can have all field initialization values on the same line, or on different lines:

i := Item{color: "blue", size: 5}

i := Item {
  color: "blue",
  size: 5, // mandatory comma
}

There is "short" literal where the name of the fields are omitted, provided that the order in which they are declared is maintained:

i := Item {"blue", 5}

Embedded Literals

When a field of a struct is another struct, embedded literals can be used in initialization:

i := Item {
  color: "blue",
  packaging: {
    material: "plastic",
    protectionLevel: 10,
  }
  size: 5,
}

Initialization

Initialize a struct variable with an empty struct using the new() built-in function and the short variable declaration:

i := new(Item)

This results in an "empty" structure, with all fields initialized to zero. Note new() returns a pointer to the structure and not the structure itself.

A struct literal can also be used:

i := Item{color: "blue", size: 5}

Operators

The Dot Operator

Individual fields in a struct can be read and modified using the "dot notation", the . operator.

var i Item
i.color = "blue"
i.size = 2

fmt.Printf("color %s, size %d\b", i.color, i.size)

Note that the . operator works with a regular struct variable as well as with a pointer to the struct. The compiler knows how to compensate for both of these cases:

i := new(Item)
fmt.Printf("i: %p\n", i)    // i is a pointer
i.color = "blue"            // the dot operator is applied to a pointer
i2 := Item{"red", 1}
fmt.Printf("i2: %+v\n", i2) // i2 is a struct
i2.color = "yellow"         // the dot operator is applied to a struct

. used for field access.

ms := myStruct{1, "s"}
msPtr := new(myStruct)

// valid:
ms.i

// valid:
msPtr.i

Even if the enclosing struct type is exported by a package, not all fields are exported automatically, only those whose first character is an upper case letter. This behavior makes possible to have "private" fields in a public structure, when the structure is used outside its package.

TO DISTRIBUTE

Field Tags and Attributes

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. The tags are made visible through a reflection interface and take part in type identity for structs but are otherwise ignored.

Example:

type A struct {
    name string `json:"name"`
}

In the example above, tags have been included to provide metadata the JSON decoding function needs to parse JSON content. Each tag maps a field name in the struct type to a filed name in the JSON document.

Embedded Types

An embedded type is initialized by declaring a name of a struct inside other struct - without associating it with a field. An embedded type is sometimes referred to as anonymous field. It models an is-a relationship. Example:

type A struct {
    f1 int
}

type B struct {
    A
    f2 int
}

Methods and Embedded Types

A pointer receiver method associated with the embedded type also works with the embedding type. For example, if there's a:

func (a *A) m1(...) {
    ...
}

m1() also works directly with B instances as such:

bPtr := new(B)
...
bPtr.m1(...)

In the code sequence above, the invocations bPtr.m1(...) and bPtr.A.m1(...) are equivalent.

Empty Structs

An empty struct allocates zero bytes when values of this type are created. They are useful when a type (but not state) is needed. Example:

// we declared an empty struct that defines the type "A"
type A struct{}