Constructor Functions

Part of the Object Oriented Programming section of Coddy's GO journey — lesson 10 of 107.

Go doesn't have constructors like other languages. Instead, the convention is to create a function named NewTypeName that returns an initialized struct, often as a pointer.

Basic constructor function

type Person struct {
    Name string
    Age  int
}

func NewPerson(name string, age int) *Person {
    return &Person{
        Name: name,
        Age:  age,
    }
}

Constructor with default values

type Config struct {
    Host    string
    Port    int
    Timeout int
}

func NewConfig(host string) *Config {
    return &Config{
        Host:    host,
        Port:    8080,    // Default value
        Timeout: 30,      // Default value
    }
}

Constructor with validation

func NewPerson(name string, age int) *Person {
    if name == "" {
        name = "Unknown"
    }
    if age < 0 {
        age = 0
    }
    return &Person{
        Name: name,
        Age:  age,
    }
}

Using constructor functions

func main() {
    p1 := NewPerson("Alice", 25)
    fmt.Println(p1.Name) // Alice

    p2 := NewPerson("", -5)
    fmt.Println(p2.Name) // Unknown
    fmt.Println(p2.Age)  // 0
}

Constructor functions return a pointer (*StructName) using &. This avoids copying the struct and lets methods with pointer receivers work directly. Validation and default values go inside the constructor.

Challenge

Medium

Create a constructor function NewBankAccount for the BankAccount struct with validation:

Takes name (string) and balance (float64) as parameters
If name is empty, default to "Unnamed"
If balance is negative, default to 0
Set Owner to "Default Owner" always
Return a pointer to the new struct

Cheat sheet

Go uses constructor functions instead of constructors. The convention is to name them NewTypeName and return a pointer to the initialized struct.

Basic constructor function:

type Person struct {
    Name string
    Age  int
}

func NewPerson(name string, age int) *Person {
    return &Person{
        Name: name,
        Age:  age,
    }
}

Constructor with default values:

func NewConfig(host string) *Config {
    return &Config{
        Host:    host,
        Port:    8080,    // Default value
        Timeout: 30,      // Default value
    }
}

Constructor with validation:

func NewPerson(name string, age int) *Person {
    if name == "" {
        name = "Unknown"
    }
    if age < 0 {
        age = 0
    }
    return &Person{
        Name: name,
        Age:  age,
    }
}

Constructor functions return a pointer (*StructName) using & to avoid copying the struct and allow pointer receiver methods to work directly.

Try it yourself

package main

import "fmt"

func main() {
    var name string
    var balance float64
    fmt.Scan(&name, &balance)

    account := NewBankAccount(name, balance)

    fmt.Printf("Account: %s\n", account.Name)
    fmt.Printf("Balance: %.2f\n", account.Balance)
    fmt.Printf("Owner: %s\n", account.Owner)
}