Structs

Published Jun 13, 2023Updated Jun 20, 2023
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A struct is a user-defined type that combines several fields of different data types, normally related, forming a collection.

It works as a container, holding multiple fields of data, such as integers, strings, booleans, or even other structs, making it easy to access and store all the related data without cluttering up the code with multiple variables.

Syntax

To declare a struct in Go, use the type and struct keywords. Inside the curly braces data fields are declared as needed with their corresponding data type.

type <struct_name> struct {
    <field_one> <data-type>
    <field_two> <data-type>
    ...
}

Example

type Rectangle struct {
width float64
height float64
}

In this example, Rectangle is the name of the struct, and width and height are the fields contained in it. The fields have a corresponding data type, float64, to be able to store decimal numbers.

In this particular example, both members have the same data type, but it’s not mandatory.

Setting Values

Using Dot Notation

In Go, there are a few ways to set values on a struct. One of them is setting each value individually by using dot notation.

type Car struct {
brand string
year int
mileage float64
}
func main(){
myCar := Car{}
myCar.brand = "Ford"
myCar.year = 2023
myCar.mileage = 15000
}

In the example above, a Car struct is defined. In the main function, the values are set using the dot notation. This method is used to access and modify the fields of a struct. To set the value of a field, use the . operator followed by the name of the field to modify, and then assign the desired value.

This way of setting values is very intuitive and convenient. However, for more complex initializations, it is best to consider other options.

Using Default Values

An alternative method is to use default values rather than explicitly setting each one to its value every time a new instance of that struct is created.

To use this method, a constructor function is defined. This will create a new instance of the struct and will set its default values if none are provided. These default values will be replaced when a new value is set to the field.

func NewCar(brand string, year int, mileage float64) *Car {
return &Car{brand, year, mileage}
}
func main(){
car1 := NewCar("Toyota", 2023, 0)
fmt.Printf("Brand: %s\n", car1.brand) // Output: Brand: Toyota
fmt.Printf("Year: %d\n", car1.year) // Output: Year: 2023
fmt.Printf("Mileage: %f\n", car1.mileage) // Output: Mileage: 0
}

Note: Constructor functions such as NewCar, in the code above, return struct pointers. The * operator is used in the function statement to declare the return value as a pointer type, and the & address operator is designating the returned Car struct as a pointer, a reference to a memory location.

There’s another way to set default values.

type Car struct {
brand string `default: "Tesla"`
year int `default: 2022`
mileage float64 `default: 1520.20`
}

If a default value is not specified for a field, when initializing it, it will be assigned a zero value (e.g. 0 for integers, false for bools).

Initializing Structs

There are two ways to initialize a struct in Go: the var keyword or a literal value.

Using the var Keyword

type Car struct {
brand string
year int
mileage float64
}
func main() {
var grandmaCar Car
grandmaCar.brand = "Toyota"
grandmaCar.year = 1993
grandmaCar.mileage = 251200.84
}

This approach is useful when initializing the fields individually.

Using a Literal Value

func main() {
dadsCar := Car{
brand: "Range Rover",
year: 2022,
mileage: 10005.25,
}
}

This way of initialization is handy to set all the field values at once.

Adding a Method

Methods are added outside the struct. For instance, a receiver function is needed which will have a reference to the struct, to allow it to operate on the structs’ data. The receiver, the struct itself, is a parameter that provides access to its own fields.

In the codebyte example below, the function Drive() takes the Car struct as the receiver, in the main function the method is invoked using dot notation printing to the console I am driving my car.

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