Need For Speed

In Go, a struct is a sequence of named elements called fields, each field has a name and type. The name of a field must be unique within the struct. Structs can be compared with classes in the Object-Oriented Programming paradigm.

Defining a struct

You create a new struct by using the type and struct keywords, and explicitly define the name and type of the fields. For example, here we define Shape struct, that holds the name of a shape and its size:

type Shape struct {
    name string
    size int
}

Field names in structs follow the Go convention - fields whose name starts with a lower case letter are only visible to code in the same package, whereas those whose name starts with an upper case letter are visible in other packages.

Creating instances of a struct

Once you have defined the struct, you need to create a new instance defining the fields using their field name in any order:

s := Shape {
    name: "Square",
    size: 25,
}

To read or modify instance fields, use the . notation:

// Update the name and the size
s.name = "Circle"
s.size = 35
fmt.Printf("name: %s size: %d\n", s.name, s.size)
// Output: name: Circle size: 35

Fields that don't have an initial value assigned, will have their zero value. For example:

s := Shape{name: "Circle"}
fmt.Printf("name: %s size: %d\n", s.name, s.size)
// Output: name: Circle size: 0

You can create an instance of a struct without using the field names, as long as you define the fields in order:

s := Shape {
	"Oval",
	20,
}

However, this syntax is considered brittle code since it can break when a field is added, especially when the new field is of a different type. In the following example we add an extra field to Shape:

type Shape struct {
	name        string
	description string // new field 'description' added
	size        int
}

s := Shape{
    "Circle",
    20,
}
// Since the second field of the struct is now a string and not an int,
// the compiler will throw an error when compiling the program:
// Output: cannot use 20 (type untyped int) as type string in field value
// Output: too few values in Shape{...}

"New" functions

Sometimes it can be nice to have functions that help us create struct instances. By convention, these functions are usually called New or have their names starting with New, but since they are just regular functions, you can give them any name you want. They might remind you of constructors in other languages, but in Go they are just regular functions.

In the following example, one of these New functions is used to create a new instance of Shape and automatically set a default value for the size of the shape:

func NewShape(name string) Shape {
	return Shape{
		name: name,
		size: 100, //default-value for size is 100
	}
}

NewShape("Triangle")
// => Shape { name: "Triangle", size: 100 }

Using New functions can have the following advantages:

• validation of the given values
• handling of default-values
• since New functions are often declared in the same package of the structs they initialize, they can initialize even private fields of the struct