LESSON 281 OF 671 · FUNCTIONS & CLASSES

Constructors in Go

Lesson

TL ; DR:

A constructor is a special function that runs when you create an object. It's used to set up the object's initial state.

In Go, the convention is to write a NewXxx function that takes parameters and returns an initialized struct:

type Dog struct {
    name string
    age  int
}

func NewDog(name string, age int) Dog {
    return Dog{name: name, age: age}
}

Then create objects with arguments: rex := NewDog("Rex", 4) — the constructor function fills in the fields for you.

Full lesson:

In the previous lesson you set member variables one at a time after creating the object:

var dog Dog
dog.name = "Rex"
dog.age = 4

That works, but it's verbose and easy to forget. Constructor functions let you initialize the fields the moment the object is created.

Defining a constructor

A Go constructor is a regular function with two distinguishing features:

By convention, its name starts with New followed by the type name (NewDog for a Dog struct).
It returns the struct type — Go constructor functions return an initialized value.

func NewDog(name string, age int) Dog {
    return Dog{name: name, age: age}
}

Struct literal initialization

Inside the constructor, Go uses a struct literal Dog{name: name, age: age} to build the value. The left-hand name is the field name, and the right-hand name is the parameter — there is no ambiguity because Go uses named fields rather than a this pointer.

Calling the constructor

rex := NewDog("Rex", 4)
luna := NewDog("Luna", 7)

By the time these lines finish, rex.name is already "Rex", rex.age is 4, and the same for Luna.

Your task

The Dog struct already has the name and age fields. Add a NewDog constructor function that takes a name string and an age int and returns an initialized Dog. Then in mainFunction, create two dogs:

rex := NewDog("Rex", 4)
luna := NewDog("Luna", 7)

Expected output:

Rex is 4
Luna is 7

STEP 1 OF 2

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