Go Structs

Internal

• The Type System

Overview

A struct is a user-defined type that contains named typed fields (and possibly other type names, see embedded types below). structs may also declare methods that operate on that data. The fields usually represent a has-a relationship, struct allows creating composite types. structs can also have methods associated with them.

Questions

• Are all users can define (in terms of types) structs, or there are other user-defined types?
• What is the difference between the methods that have the type as receiver and the function declared as fields by the type?

Declaration

The struct type definition is introduced by the type keyword, to indicated that this is a user-defined type, followed by the type name and the keyword struct. Each field has a name and a type. The type can be a built-in type or a user-defined type, such as another struct or an interface.

The structs can be declare at package level or inside a function.

type myStruct struct {
    i int
    s string
}

Fields with the same types can be collapsed:

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

Filed Tags

See

Filed Tag and Attributes

Initialization

Once the type has been defined, variables of that type can be declared using the long declaration or short declaration syntax, as shown below. More details about variable declaration can be found here:

Long Variable Declaration

var ms myStruct

If no explicit initialization follows, all the struct's fields are initialized with their zero value.

Short Variable Declaration

The short variable declaration uses the short variable declaration operator :=

Literal

Struct literal initialization comes in two forms.

1. All the values are specified on the same line:

ms := myStruct{i: 5, s: "something"}

or

2. Values are specified on separate lines:

ms := myStruct{
    i: 5, 
    s: "something",
}

Note the trailing comma when using the multi-line literal.

There is a shorter struct literal where the name of the fields are omitted, provided that the order is maintained:

ms := myStruct{5, "something"}

new()

new() initialization:

msPtr := new(myStruct)

Note new() returns a pointer to the structure and not the structure itself.

Fields

Fields can be access with the . operator.

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

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.

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{}