The new() Function

Part of the Object Oriented Programming section of Coddy's GO journey — lesson 22 of 107.

Go provides a built-in function called new() that allocates memory for a type and returns a pointer to it. The allocated memory is initialized to the type's zero value.

p := new(int)      // allocates memory for an int
fmt.Println(*p)    // 0 (zero value)
fmt.Println(p)     // 0xc000018030 (memory address)

The new() function takes a type as its argument and returns a pointer to a newly allocated zero value of that type. This is particularly useful when you need a pointer but don't have an existing variable to reference.

With structs, new() creates a pointer to a struct with all fields set to their zero values.

type Person struct {
    Name string
    Age  int
}

p := new(Person)
fmt.Println(p.Name)  // "" (empty string)
fmt.Println(p.Age)   // 0

You might wonder when to use new() versus the address operator &. Both approaches below are equivalent:

// Using new()
p1 := new(Person)

// Using address operator with literal
p2 := &Person{}

The &Person{} syntax is more common in practice because it allows you to initialize fields at the same time. Use new() when you specifically want a zero-valued instance and prefer explicit memory allocation semantics.

Challenge

Easy

Let's build a configuration manager that demonstrates how new() allocates memory and initializes structs to their zero values. You'll see how this approach differs from using the address operator with a struct literal.

You'll organize your code across two files:

config.go: Define a ServerConfig struct with Host (string), Port (int), MaxConnections (int), and DebugMode (bool) fields. Create two functions:
- NewServerConfig that uses new() to allocate a ServerConfig and returns the pointer directly (with all zero values)
- ConfigureServer that takes a *ServerConfig pointer along with a host string and port integer, then sets those fields on the config through the pointer
Also add a Display method on *ServerConfig that returns a formatted string showing all configuration values.
main.go: Read a host and port from input. First, create a config using NewServerConfig() and display it to show the zero values. Then use ConfigureServer to set the host and port, and display the config again to show the updated values.

The following inputs will be provided:

Line 1: Host name (string)
Line 2: Port number (integer)

Your Display method should return a string in this exact format:

Host: [Host]
Port: [Port]
MaxConnections: [MaxConnections]
DebugMode: [DebugMode]

Print the display output twice—once before configuring (showing zero values) and once after configuring (showing the updated host and port).

For example, given localhost and 8080, your output should be:

Host: 
Port: 0
MaxConnections: 0
DebugMode: false
Host: localhost
Port: 8080
MaxConnections: 0
DebugMode: false

Notice how the first display shows all zero values (empty string, 0, 0, false) because new() initializes everything to zero values. After calling ConfigureServer, only the host and port change while the other fields remain at their zero values.

Cheat sheet

The new() function allocates memory for a type and returns a pointer to it, initialized to the type's zero value:

p := new(int)      // allocates memory for an int
fmt.Println(*p)    // 0 (zero value)
fmt.Println(p)     // 0xc000018030 (memory address)

With structs, new() creates a pointer to a struct with all fields set to their zero values:

type Person struct {
    Name string
    Age  int
}

p := new(Person)
fmt.Println(p.Name)  // "" (empty string)
fmt.Println(p.Age)   // 0

Using new() versus the address operator & - both are equivalent:

// Using new()
p1 := new(Person)

// Using address operator with literal
p2 := &Person{}

The &Person{} syntax is more common because it allows field initialization at creation time. Use new() when you want a zero-valued instance with explicit memory allocation semantics.

Try it yourself

package main

import (
	"fmt"
)

func main() {
	// Read input
	var host string
	var port int
	fmt.Scanln(&host)
	fmt.Scanln(&port)

	// TODO: Create a config using NewServerConfig() and display it
	// This will show the zero values

	// TODO: Use ConfigureServer to set the host and port

	// TODO: Display the config again to show updated values
}