Tell me about the Singleton design pattern.

The Singleton design pattern guarantees that a class has only one instance and provides a global point of access to it. In Go, it is usually implemented using the sync package (sync.Once) to ensure thread safety.

Pros:

• Controlled access to the single instance.
• Helps save resources by avoiding the creation of multiple identical objects.

Cons:

• Can make testing difficult due to global state.
• Violates the single responsibility principle, as the class manages its own creation.

Implementation in Go using sync.Once:

package singleton

import (
	"fmt"
	"sync"
)

// singleton represents the single instance of the object.
type singleton struct {
	value string
}

// once ensures the code runs only once.
var once sync.Once

// instance holds the single instance of singleton.
var instance *singleton

// GetInstance returns the single instance of singleton.
func GetInstance() *singleton {
	once.Do(func() {
		instance = &singleton{value: "I am the only instance!"}
		fmt.Println("Created the singleton instance.")
	})
	return instance
}

// SomeMethod is an example method of singleton.
func (s *singleton) SomeMethod() {
	fmt.Println("Called the singleton method:", s.value)
}