Questions tagged [singleton]
The singleton is a design pattern aiming to ensure that only a single instance of a class can be created and used.
145 questions
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Are there any good and modern approaches, to have variables that feel global (no need to pass them around explicitly), yet are local to an instance?
I'm developing a library in JavaScript (TypeScript, actually) which is split into several modules. It's meant to run both on the web and in non-web environments like Node.js.
The library is meant to ...
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Global Variables State Management
Background:
I am working in a Java environment using Spring Boot, where I often encounter scenarios where global variable state management is critical, especially within singleton services. I have ...
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Are there any benefits to delegation over subclassing/inheritance in the case of a singleton?
Example:
The MacOS API known as Cocoa uses Delegation to specify various behaviors within the app. A Delegate is a separate object that implements a set of methods that are called by an original ...
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Is it considered a code smell to not delete static pointers of a singleton?
I have a singleton that needs to be initialized at the start of the program and it will live on until the end of the program. As is usual in this pattern, a function initializing the singleton creates ...
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Is utilizing a singleton for a cache an antipattern?
I'm currently writing an MVC application and I need a class that can:
A: get, add and remove data(specifically a TreeSet of sorted strings that I want stored in memory, but I doubt the data itself is ...
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Should I use a singleton to represent the application as a whole?
I make sure when designing software/firmware to make heavy use of dependency injection so that different pieces of the application are not directly coupled to one another. This also allows me to (...
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Storing multiple instances on a Singleton?
RefactoringGuru's example Singleton in Python has an _instances dictionary field
class Singleton(type):
_instances = {}
def __call__(cls, *args, **kwargs):
if cls not in cls....
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Using class attributes as globals in Python - is there a catch?
I have found myself in the habit of using code like this.
class glb:
"just for holding globals"
args = None # from argparse
conf = None # from configparser
def main():
......
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Should I design a factory that returns a singleton?
I design a common API for selected printers of different brands in Java. Each printer uses a different underlying SDK with different functions, but any hardware my code runs on will have only one ...
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Would Injecting dependencies in C# as default parameters be a bad practice?
Given the (old) debate over whether Singletons are overused/abused/are worth it - would it be a bad idea to inject the dependencies as default parameters? In this way, we could get rid of defining ...
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How do Singletons differ from Static variables?
Although I do find some (apparently old) posts on the topic on the web, I could not find one here at SE. Thought of raising this here to see if what I read is accurate/is all there is to it.
So ...
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Singleton as Interface for testability via dependancy injection
It can be found in many advices on topic that having Singletons is an anti-pattern. Especially for cases of testability. Can someone please advice/critique on this way (please see code below) of ...
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How should I provide access to "global" objects down to other objects which are contained in a central "application" object
Note This is a bit lengthy to have give a better understanding of the situation and to get some context. You might spot other architectural flaws (it's from an ancient application). I appreciate any ...
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Dependency injection in .NET Core 3.x Worker Services: Why so hard to consume Transient services now?
I have a Repository service that should be Transient. It is used in many applications.
I have a new application that's a Console App, and current guidance suggests implementing my business logic in ...
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Best way to access (grand grand) parent element in gui
I'm creating a multi window gui program, in c++ with Qt Widgets. I do have many custom gui elements, which usually are c++ classes inherited from QWidget or other Qt elements. When foo is the main ...
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Is it right way to make Window to Singleton and call Visibility and Shutdown() methods directly in WPF?
There is a parent window that is the basis of the program, and there are several sub windows under it. Editor is one of the sub window and I'm making it. The code is as follows:
// This is Code-Behind
...
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Can I use singleton in a Flyweight factory and still be a valid GoF implementation?
For teaching purposes, I am trying to create a "something useful" example of Flyweight pattern using PHP.
My idea was load some "intrinsic" data from a csv (link) to a pool and ...
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What's wrong with using a Singleton?
I'm working on a Python application in which there are two Singleton classes: App and Configuration.
The former seems straight forward, only ever instantiate one App instance; the latter seems ...
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Alternatives to service locator with opaque pointer in C
I have a project in which one module keeps the state of the target device (things like current command level, but mostly status registers caches).
I'm aware that having a global public variable (...
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Better way than singleton pattern to ensure single instance of class
When this question has been asked before on StackOverflow in 2011 and 2015, all answers as of now suggest to use a Singleton.
But that’s not right.
Singletons are defined by the Gang of Four to ...
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What kind of factory method abstract singleton design pattern is this?
I made a code that seems to mix Singleton design pattern, and Fatory method. But my factory method is in an abstract class inherited by my Singleton ... what the hell have I created ? Does it have a ...
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Convert singleton to single instance with concrete Go example
I have been using singletons in the past.
However, reading articles like Singletons are Pathological Liars have led me to explore alternatives to singletons.
There are a few posts discussion ...
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How to separate data and logic to achieve Single Responsibility Principle in C# way? [duplicate]
I'm still learning C# and best practices around it.
Consider this block of code
public class Counter {
private int _value = 0;
public void SetValue(int x) {
_value = x;
// ...
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What form of single instance class for a cache under Android
The app is to communicate with a bluetooth device with assorted data (config, measurements, service logs, status telemetry) - and it should cache obtained data so the user flipping between activities ...
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What is the difference between singleton pattern using inner static "Holder" class and "Instance" variable
I have encountered two patterns to make a singleton class
Holder class
public class Singleton {
private static final class Holder {
private static final Singleton INSTANCE = new Singleton()...
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Is singleton use acceptable for static single responsibility?
I have been reading into possible design patterns and have found the use of singletons always referred to as an anti-pattern.
I am currently using a singleton for the sole purpose of gathering ...
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Why do I define my Queries, Data, and Mutation as Singleton when using GraphQL in .NET Core?
Why do I define my Queries, Data, and Mutation as Singleton when using GraphQL in .NET Core?
From the doc's dependency injection page:
public void ConfigureServices(IServiceCollection services)
{
...
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Is there a name for a "multi-instance singleton pattern" -- returning a reference for a same-valued object?
I am tasked to write an assembler in C, which lends itself sufficiently well in certain design-pattern scenarios, and in a parsing phase, after tokenization, I need to "fill-in" certain information ...
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Singleton's applicability clarification
According to the GOF Design patterns' book, singleton pattern should be used when:
there must be exactly one instance of a class, and it must be
accessible to clients from a well-known access ...
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Can I add other methods to the classic Singleton Implementation?
I've seen Singleton design pattern represented like below. Always with just getInstance() method.
My doubt is: once I get the instance of the object I need, if I would like to modify it don't I need ...
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Is this Singleton-like design pattern a feasible framework to build on?
I am posting this question here after it having been determined to be "off-topic" for stackoverflow, and "too hypothetical" for codereview.
I am experimenting with different singleton-style design ...
user313371
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Is it a good idea to use Singleton pattern for an API manager?
So basically I am writing a wrapper for a REST API in objective-c so that our customer can easily use them in their iOS development.
I am trying to find a good design pattern for this purpose, it ...
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IDisposable without finalizer in a Singleton scenario
Is it a right expectation that if a C# class deals with unmanaged resources and implements IDisposable, then it also should implement some kind of finalizer logic?
We have a vendor-supplied utility ...
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C# best/cleanest way to do singletons?
I'm working on a small project and I've decided to move over from DI to singleton pattern. Although I know 2 ways to do singleton.
The first one is where every non-model class is a singleton. This ...
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Handling a limited resource in rest WEB API 2
I have developed a restful WEB API 2 (C#, .NET 4.7) for an internal integration project. In my API's backend, I need to use a vendor's dll in order to process the provided input.
The vendor's dll is ...
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Singleton for Java Functional Interface
I know there has been 1 million and 1 discussions about Singletons on SO and here and I have had to clean up my fair share of terrible singletons in our code base; one of the reasons I am gun-shy here....
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Is it worthy subclassing a Singleton class?
When do we need to subclass a Singleton class or in other words, is it good to subclassing a Singleton?
If its generally allowed what would be the pros and cons and how to handle or get instance from ...
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Should I use "dynamic" singleton design?
We've created a simple 2D game, it has fields and elements on it, the player can move the elements. The game has two views: a graphical (with images), and a textual (printed to the console). But only ...
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Singleton pattern without static
I heard that static (in the Java sense, basically a static method is called on the class itself and not on an instance) is not True OOP. However, how would the Singleton pattern be implemented in such ...
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Best Approach to have a shared view
I have a website where I want to have a partial view on the page that contains a Singleton service.
I am trying to do this with Razor Pages, and have tried Partial Views and ViewComponents, but ...
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Can the port of a UML component only be a singleton?
I am working on a software assignment where the design is component based. The components have ports which provide interfaces.
My professor argues that the Port class which is exposed by each ...
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Improve design with singleton pattern
I want to create a media management tool, for which I try to draw an appropriate UML class diagram. The tool shall hold a collection of different media types (movies, music, etc.) which inherit from a ...
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Overhead of using big singleton with all files includes
I have several subsystem managers for various uses, for example:
AudioManager
CollisionManager
InputManager
etc.
At first I wanted them to be all singletons, however now want to make the architecture ...
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Singleton with dependency injection ,is that a good practice in some cases?
It's a good idea to have a Singleton implementation with dependency injection?
I have some classes that performs some heavy tasks on instantiation (related to database loading).
I would like to use ...
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Base global system for two customizable subcomponents
The following explanation is rather detailed, but I think if I simplified the problem at hand no proper answer could be given.
I'm working on a scripting system for a game (for end users rather than ...
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React Native - Is using a singleton the best alternative to DI?
I've been reading a lot about the singleton pattern and how it's "bad" because it makes the classes using it hard to test so it should be avoided. I've read some articles explaining how the singleton ...
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Trying to understand singleton objects in Swift [duplicate]
In Swift, if I have a class with only type properties and methods (everything declared static) would that be considered a singleton object? If not, why?
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If a class has no instance specific data is it okay to make it a singleton?
Let's say I have the following Java code:
public class ObjectConsumer implements Consumer<Object> {
@Override
public void accept(Object o) {
System.out.println("Accepted " + o);
...
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How should I represent an object whose instances share the same set of member function identifiers, but those identifiers specify different behaviors?
I'm attempting to develop an open-source Python module for modeling task networks for discrete event simulation. The most fundamental component is the task object, which includes various data such as ...
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Creating a singleton to access static data
I have some properties in a property file that I referenced in a singleton so that I can access them easily from different parts of my application. For each property, I put the properties along with ...
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