Singleton in go

Question

How does one implement the Singleton design pattern in the go programming language?

Answer 1

Setting aside the argument of whether or not implementing the singleton pattern is a good idea, here's a possible implementation:

package singleton

type single struct {
        O interface{};
}

var instantiated *single = nil

func New() *single {
        if instantiated == nil {
                instantiated = new(single);
        }
        return instantiated;
}

single and instantiated are private, but New() is public. Thus, you can't directly instantiate single without going through New(), and it tracks the number of instantiations with the private boolean instantiated. Adjust the definition of single to taste.

However, as several others have noted, this is not thread-safe, unless you're only initializing your singleton in init(). A better approach would be to leverage sync.Once to do the hard work for you:

package singleton

import "sync"

type single struct {
        O interface{};
}

var instantiated *single
var once sync.Once

func New() *single {
        once.Do(func() {
                instantiated = &single{}
        })
        return instantiated
}

See also, hasan j's suggestion of just thinking of a package as a singleton. And finally, do consider what others are suggesting: that singletons are often an indicator of a problematic implementation.