Questions tagged [singleton]
Singleton is a Gang of Four's creational design pattern that ensures that exactly one application-wide instance of a particular class exists. Use this tag for questions about the singleton design pattern. Do not use this tag for questions that include the singleton design pattern but are not about it.
singleton
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What are drawbacks or disadvantages of singleton pattern? [closed]
The singleton pattern is a fully paid up member of the GoF's patterns book, but it lately seems rather orphaned by the developer world. I still use quite a lot of singletons, especially for factory ...
2059
votes
42
answers
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views
Difference between static class and singleton pattern?
What real (i.e. practical) difference exists between a static class and a singleton pattern?
Both can be invoked without instantiation, both provide only one "Instance" and neither of them is thread-...
- 51.5k
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votes
42
answers
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What is the best way of implementing singleton in Python
This question is not for the discussion of whether or not the singleton design pattern is desirable, is an anti-pattern, or for any religious wars, but to discuss how this pattern is best implemented ...
- 21.3k
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answers
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897
votes
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answers
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How do you implement the Singleton design pattern?
Recently I've bumped into a realization/implementation of the Singleton design pattern for C++. It has looked like this (I have adopted it from the real-life example):
// a lot of methods are omitted ...
- 41k
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What is an efficient way to implement a singleton pattern in Java? [closed]
What is an efficient way to implement a singleton design pattern in Java?
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17
answers
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How to declare global variables in Android?
I am creating an application which requires login. I created the main and the login activity.
In the main activity onCreate method I added the following condition:
public void onCreate(Bundle ...
- 66.3k
596
votes
30
answers
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views
Using a dispatch_once singleton model in Swift
I'm trying to work out an appropriate singleton model for usage in Swift. So far, I've been able to get a non-thread safe model working as:
class var sharedInstance: TPScopeManager {
get {
...
- 41.1k
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votes
24
answers
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On design patterns: When should I use the singleton?
The glorified global variable - becomes a gloried global class. Some say breaking object-oriented design.
Give me scenarios, other than the good old logger where it makes sense to use the singleton.
- 10.4k
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votes
21
answers
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Is there a simple, elegant way to define singletons? [duplicate]
There seem to be many ways to define singletons in Python. Is there a consensus opinion on Stack Overflow?
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votes
31
answers
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views
How do you build a Singleton in Dart?
The singleton pattern ensures only one instance of a class is ever created. How do I build this in Dart?
- 116k
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46
answers
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views
Simplest/cleanest way to implement a singleton in JavaScript
What is the simplest/cleanest way to implement the singleton pattern in JavaScript?
- 86.4k
378
votes
10
answers
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Singletons vs. Application Context in Android?
Recalling this post enumerating several problems of using singletons
and having seen several examples of Android applications using singleton pattern, I wonder if it's a good idea to use Singletons ...
- 7,281
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votes
10
answers
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Create singleton using GCD's dispatch_once in Objective-C
If you can target iOS 4.0 or above
Using GCD, is it the best way to create singleton in Objective-C (thread safe)?
+ (instancetype)sharedInstance
{
static dispatch_once_t once;
static id ...
- 10.3k
333
votes
26
answers
185k
views
What should my Objective-C singleton look like? [closed]
My singleton accessor method is usually some variant of:
static MyClass *gInstance = NULL;
+ (MyClass *)instance
{
@synchronized(self)
{
if (gInstance == NULL)
gInstance =...
- 12k
321
votes
24
answers
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views
Singleton: How should it be used
Edit:
From another question I provided an answer that has links to a lot of questions/answers about singletons: More info about singletons here:
So I have read the thread Singletons: good design ...
313
votes
2
answers
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Why does Apple recommend to use dispatch_once for implementing the singleton pattern under ARC?
What's the exact reason for using dispatch_once in the shared instance accessor of a singleton under ARC?
+ (MyClass *)sharedInstance
{
// Static local predicate must be initialized to 0
...
- 40.3k
292
votes
5
answers
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views
How to create module-wide variables in Python? [duplicate]
Is there a way to set up a global variable inside of a module? When I tried to do it the most obvious way as appears below, the Python interpreter said the variable __DBNAME__ did not exist.
...
...
- 10.1k
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votes
11
answers
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Is the C# static constructor thread safe?
In other words, is this Singleton implementation thread safe:
public class Singleton
{
private static Singleton instance;
private Singleton() { }
static Singleton()
{
...
- 32.7k
225
votes
6
answers
157k
views
Implementing Singleton with an Enum (in Java)
I have read that it is possible to implement Singleton in Java using an Enum such as:
public enum MySingleton {
INSTANCE;
}
But, how does the above work? Specifically, an Object has to be ...
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votes
2
answers
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Singleton by Jon Skeet clarification
public sealed class Singleton
{
Singleton() {}
public static Singleton Instance
{
get
{
return Nested.instance;
}
}
class Nested
{
...
- 2,263
217
votes
26
answers
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views
How to define Singleton in TypeScript
What is the best and most convenient way to implement a Singleton pattern for a class in TypeScript? (Both with and without lazy initialisation).
- 17.6k
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votes
22
answers
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views
Creating the Singleton design pattern in PHP5
How would one create a Singleton class using PHP5 classes?
- 94.4k
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15
answers
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178
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answers
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How do I implement an Objective-C singleton that is compatible with ARC?
How do I convert (or create) a singleton class that compiles and behaves correctly when using automatic reference counting (ARC) in Xcode 4.2?
- 3,706
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votes
21
answers
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Singleton with Arguments in Java
I was reading the Singleton article on Wikipedia and I came across this example:
public class Singleton {
// Private constructor prevents instantiation from other classes
private Singleton() {...
Scott
147
votes
10
answers
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views
Java serialization: readObject() vs. readResolve()
The book Effective Java and other sources provide a pretty good explanation on how and when to use the readObject() method when working with serializable Java classes. The readResolve() method, on the ...
- 2,806
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votes
11
answers
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views
Is there a use-case for singletons with database access in PHP?
I access my MySQL database via PDO. I'm setting up access to the database, and my first attempt was to use the following:
The first thing I thought of is global:
$db = new PDO('mysql:host=127.0.0.1;...
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135
votes
8
answers
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views
Java Singleton and Synchronization
Please clarify my queries regarding Singleton and Multithreading:
What is the best way to implement Singleton in Java, in a multithreaded
environment?
What happens when multiple threads try to access ...
- 2,286
123
votes
12
answers
56k
views
What's Alternative to Singleton
We have a class that holds configuration information for the application. It used to be a singleton. After some architectural review, we were told to remove the singleton. We did see some benefits of ...
- 1,233
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votes
8
answers
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views
Why use a singleton instead of static methods?
I have never found good answers to these simple questions about helper/utility classes:
Why would I create a singleton (stateless) instead of using static methods?
Why would an object instance be ...
- 91k
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votes
10
answers
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views
Singleton pattern in nodejs - is it needed?
I recently came across this article on how to write a singleton in Node.js. I know the documentation of require states that:
Modules are cached after the first time they are loaded. Multiple calls ...
- 57.5k
112
votes
11
answers
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views
Singleton design pattern vs Singleton beans in Spring container
As we all know we have beans as singleton by default in Spring container and if we have a web application based on Spring framework then in that case do we really need to implement Singleton design ...
- 1,121
110
votes
11
answers
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Thread Safe C# Singleton Pattern
I have some questions regarding the the singleton pattern as documented here:
http://msdn.microsoft.com/en-us/library/ff650316.aspx
The following code is an extract from the article:
using System;
...
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votes
7
answers
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views
Is there an "Empty List" singleton in C#?
In C# I use LINQ and IEnumerable a good bit. And all is well-and-good (or at least mostly so).
However, in many cases I find myself that I need an empty IEnumerable<X> as a default. That is, I ...
user166390
103
votes
5
answers
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views
ASP.NET Core initialize singleton after configuring DI
So let's say I have a singleton class instance that I register in the DI like this:
services.AddSingleton<IFoo, Foo>();
And let's say the Foo class has a number of other dependencies (mostly ...
- 9,005
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votes
8
answers
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views
Why is volatile used in double checked locking
From Head First design patterns book, the singleton pattern with double checked locking has been implemented as below:
public class Singleton {
private volatile static Singleton instance;
...
- 2,647
98
votes
3
answers
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views
Javascript: best Singleton pattern [duplicate]
Possible Duplicate:
Simplest/Cleanest way to implement singleton in JavaScript?
I'm using this pattern for singletons, in the example the singleton is PlanetEarth:
var NAMESPACE = function () {
...
- 3,264
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votes
13
answers
39k
views
How to constrain a table to contain a single row?
I want to store a single row in a configuration table for my application. I would like to enforce that this table can contain only one row.
What is the simplest way to enforce the single row ...
- 9,834
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votes
16
answers
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views
Are there any viable alternatives to the GOF Singleton Pattern?
Let's face it. The Singleton Pattern is highly controversial topic with hordes programmers on both sides of the fence. There are those who feel like the Singleton is nothing more then a glorified ...
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votes
8
answers
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views
The need for volatile modifier in double checked locking in .NET
Multiple texts say that when implementing double-checked locking in .NET the field you are locking on should have volatile modifier applied. But why exactly? Considering the following example:
public ...
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93
votes
5
answers
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views
What exactly is the singleton class in ruby?
Is the singleton class in Ruby a class in and of itself? Is it the reason why all objects belong to "class?" The concept is fuzzy, but I believe it has something to do with why I can define a class ...
- 1,646
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votes
6
answers
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views
Why is the Borg pattern better than the Singleton pattern in Python
Why is the Borg pattern better than the Singleton pattern?
I ask because I don't see them resulting in anything different.
Borg:
class Borg:
__shared_state = {}
# init internal state variables ...
- 48.8k
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votes
0
answers
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views
Python and the Singleton Pattern [duplicate]
What is the best way to implement the singleton pattern in Python? It seems impossible to declare the constructor private or protected as is normally done with the Singleton pattern...
- 51.9k
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votes
8
answers
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views
Non-Singleton Services in AngularJS
AngularJS clearly states in its documentation that Services are Singletons:
AngularJS services are singletons
Counterintuitively, module.factory also returns a Singleton instance.
Given that there ...
- 43.3k
89
votes
4
answers
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views
Singleton with properties in Swift 3
In Apple's Using Swift with Cocoa and Objective-C document (updated for Swift 3) they give the following example of the Singleton pattern:
class Singleton {
static let sharedInstance: Singleton = ...
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Is it a good practice to have logger as a singleton?
I had a habit to pass logger to constructor, like:
public class OrderService : IOrderService {
public OrderService(ILogger logger) {
}
}
But that is quite annoying, so I've used it a ...
- 8,510
88
votes
4
answers
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views
What is the best approach for using an Enum as a singleton in Java?
Building on what has been written in SO question Best Singleton Implementation In Java - namely about using an enum to create a singleton - what are the differences/pros/cons between (constructor ...
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votes
9
answers
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views
How do you implement a singleton efficiently and thread-safely? [duplicate]
The usual pattern for a singleton class is something like
static Foo &getInst()
{
static Foo *inst = NULL;
if(inst == NULL)
inst = new Foo(...);
return *inst;
}
However, it's my ...
- 5,539
82
votes
2
answers
52k
views
Using Singleton design pattern for SQLiteDatabase
I'm rather newbie on Android, and I'm working on a simple application to get some basic experience. My app is pretty simple and consists among other things of a broadcast receiver and some activities. ...
- 1,526