Chapter 7.1: Singleton Pattern

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はじめに

The singleton pattern guarantees that a class has exactly one instance, accessible globally. In DayZ modding it is the most common architectural pattern --- virtually every manager, cache, registry, and subsystem uses it. COT, VPP, Expansion, Dabs Framework, and MyMod all rely on singletons to coordinate state across the engine's script layers.

This chapter covers the canonical implementation, lifecycle management, when the pattern is appropriate, and where it goes wrong.

The Canonical Implementation

The standard DayZ singleton follows a simple formula: a private static ref field, a static GetInstance() accessor, and a static DestroyInstance() for cleanup.

class LootManager
{
    // The single instance. 'ref' keeps it alive; 'private' prevents external tampering.
    private static ref LootManager s_Instance;

    // Private data owned by the singleton
    protected ref map<string, int> m_SpawnCounts;

    // Constructor — called exactly once
    void LootManager()
    {
        m_SpawnCounts = new map<string, int>();
    }

    // Destructor — called when s_Instance is set to null
    void ~LootManager()
    {
        m_SpawnCounts = null;
    }

    // Lazy accessor: creates on first call
    static LootManager GetInstance()
    {
        if (!s_Instance)
        {
            s_Instance = new LootManager();
        }
        return s_Instance;
    }

    // Explicit teardown
    static void DestroyInstance()
    {
        s_Instance = null;
    }

    // --- Public API ---

    void RecordSpawn(string className)
    {
        int count = 0;
        m_SpawnCounts.Find(className, count);
        m_SpawnCounts.Set(className, count + 1);
    }

    int GetSpawnCount(string className)
    {
        int count = 0;
        m_SpawnCounts.Find(className, count);
        return count;
    }
};

Why `private static ref`?

Keyword	Purpose
`private`	Prevents other classes from setting `s_Instance` to null or replacing it
`static`	Shared across all code --- no instance needed to access it
`ref`	Strong reference --- keeps the object alive as long as `s_Instance` is non-null

Without ref, the instance would be a weak reference and could be garbage-collected while still in use.

Lazy vs Eager Initialization

Lazy Initialization (Recommended Default)

The GetInstance() method creates the instance on first access. This is the approach used by most DayZ mods.

static LootManager GetInstance()
{
    if (!s_Instance)
    {
        s_Instance = new LootManager();
    }
    return s_Instance;
}

Advantages:

No work done until actually needed
No dependency on initialization order between mods
Safe if the singleton is optional (some server configurations may never call it)

Disadvantage:

First caller pays the construction cost (usually negligible)

Eager Initialization

Some singletons are created explicitly during mission startup, typically from MissionServer.OnInit() or a module's OnMissionStart().

// In your modded MissionServer.OnInit():
void OnInit()
{
    super.OnInit();
    LootManager.Create();  // Eager: constructed now, not on first use
}

// In LootManager:
static void Create()
{
    if (!s_Instance)
    {
        s_Instance = new LootManager();
    }
}

When to prefer eager:

The singleton loads data from disk (configs, JSON files) and you want load errors to surface at startup
The singleton registers RPC handlers that must be in place before any client connects
Initialization order matters and you need to control it explicitly

Lifecycle Management

The most common source of singleton bugs in DayZ is failing to clean up on mission end. DayZ servers can restart missions without restarting the process, which means static fields survive across mission restarts. If you do not null out s_Instance in OnMissionFinish, you carry stale references, dead objects, and orphaned callbacks into the next mission.

The Lifecycle Contract

Server Process Start
  └─ MissionServer.OnInit()
       └─ Create singletons (eager) or let them self-create (lazy)
  └─ MissionServer.OnMissionStart()
       └─ Singletons begin operation
  └─ ... server runs ...
  └─ MissionServer.OnMissionFinish()
       └─ DestroyInstance() on every singleton
       └─ All static refs set to null
  └─ (Mission may restart)
       └─ Fresh singletons created again

Cleanup Pattern

Always pair your singleton with a DestroyInstance() method and call it during shutdown:

class VehicleRegistry
{
    private static ref VehicleRegistry s_Instance;
    protected ref array<ref VehicleData> m_Vehicles;

    static VehicleRegistry GetInstance()
    {
        if (!s_Instance) s_Instance = new VehicleRegistry();
        return s_Instance;
    }

    static void DestroyInstance()
    {
        s_Instance = null;  // Drops the ref, destructor runs
    }

    void ~VehicleRegistry()
    {
        if (m_Vehicles) m_Vehicles.Clear();
        m_Vehicles = null;
    }
};

// In your modded MissionServer:
modded class MissionServer
{
    override void OnMissionFinish()
    {
        VehicleRegistry.DestroyInstance();
        super.OnMissionFinish();
    }
};

MyMod Centralized Shutdown

MyFramework consolidates all singleton cleanup into MyFramework.ShutdownAll(), which is called from the modded MissionServer.OnMissionFinish(). This prevents the common mistake of forgetting one singleton:

// Conceptual pattern (simplified from MyFramework):
static void ShutdownAll()
{
    MyRPC.Cleanup();
    MyEventBus.Cleanup();
    MyModuleManager.Cleanup();
    MyConfigManager.DestroyInstance();
    MyPermissions.DestroyInstance();
}

使い分け Singletons

Good Candidates

Use Case	Why Singleton Works
Manager classes (LootManager, VehicleManager)	Exactly one coordinator for a domain
Caches (CfgVehicles cache, icon cache)	Single source of truth avoids redundant computation
Registries (RPC handler registry, module registry)	Central lookup must be globally accessible
Config holders (server settings, permissions)	One config per mod, loaded once from disk
RPC dispatchers	Single entry point for all incoming RPCs

Poor Candidates

Use Case	Why Not
Per-player data	One instance per player, not one global instance
Temporary computations	Create, use, discard --- no global state needed
UI views / dialogs	Multiple can coexist; use the view stack instead
Entity components	Attached to individual objects, not global

実践的な例

COT (Community Online Tools)

COT uses a module-based singleton pattern through the CF framework. Each tool is a JMModuleBase singleton registered at startup:

// COT pattern: CF auto-instantiates modules declared in config.cpp
class JM_COT_ESP : JMModuleBase
{
    // CF manages the singleton lifecycle
    // Access via: JM_COT_ESP.Cast(GetModuleManager().GetModule(JM_COT_ESP));
}

VPP Admin Tools

VPP uses explicit GetInstance() on manager classes:

// VPP pattern (simplified)
class VPPATBanManager
{
    private static ref VPPATBanManager m_Instance;

    static VPPATBanManager GetInstance()
    {
        if (!m_Instance)
            m_Instance = new VPPATBanManager();
        return m_Instance;
    }
}

Expansion

Expansion declares singletons for each subsystem and hooks into the mission lifecycle for cleanup:

// Expansion pattern (simplified)
class ExpansionMarketModule : CF_ModuleWorld
{
    // CF_ModuleWorld is itself a singleton managed by the CF module system
    // ExpansionMarketModule.Cast(CF_ModuleCoreManager.Get(ExpansionMarketModule));
}

Thread Safety Considerations

Enforce Script is single-threaded. All script execution happens on the main thread within the Enfusion engine's game loop. This means:

There are no race conditions between concurrent threads
You do not need mutexes, locks, or atomic operations
GetInstance() with lazy initialization is always safe

However, re-entrancy can still cause problems. If GetInstance() triggers code that calls GetInstance() again during construction, you can get a partially-initialized singleton:

// DANGEROUS: re-entrant singleton construction
class BadManager
{
    private static ref BadManager s_Instance;

    void BadManager()
    {
        // This calls GetInstance() during construction!
        OtherSystem.Register(BadManager.GetInstance());
    }

    static BadManager GetInstance()
    {
        if (!s_Instance)
        {
            // s_Instance is still null here during construction
            s_Instance = new BadManager();
        }
        return s_Instance;
    }
};

The fix is to assign s_Instance before running any initialization that might re-enter:

static BadManager GetInstance()
{
    if (!s_Instance)
    {
        s_Instance = new BadManager();  // Assign first
        s_Instance.Initialize();         // Then run initialization that may call GetInstance()
    }
    return s_Instance;
}

Or better yet, avoid circular initialization entirely.

アンチパターン

1. Global Mutable State Without Encapsulation

The singleton pattern gives you global access. That does not mean the data should be globally writable.

// BAD: Public fields invite uncontrolled mutation
class GameState
{
    private static ref GameState s_Instance;
    int PlayerCount;         // Anyone can write this
    bool ServerLocked;       // Anyone can write this
    string CurrentWeather;   // Anyone can write this

    static GameState GetInstance() { ... }
};

// Any code can do:
GameState.GetInstance().PlayerCount = -999;  // Chaos

// GOOD: Controlled access through methods
class GameState
{
    private static ref GameState s_Instance;
    protected int m_PlayerCount;
    protected bool m_ServerLocked;

    int GetPlayerCount() { return m_PlayerCount; }

    void IncrementPlayerCount()
    {
        m_PlayerCount++;
    }

    static GameState GetInstance() { ... }
};

2. Missing DestroyInstance

If you forget cleanup, the singleton persists across mission restarts with stale data:

// BAD: No cleanup path
class ZombieTracker
{
    private static ref ZombieTracker s_Instance;
    ref array<Object> m_TrackedZombies;  // These objects get deleted on mission end!

    static ZombieTracker GetInstance() { ... }
    // No DestroyInstance() — m_TrackedZombies now holds dead references
};

3. Singletons That Own Everything

When a singleton accumulates too many responsibilities, it becomes a "God object" that is impossible to reason about:

// BAD: One singleton doing everything
class ServerManager
{
    // Manages loot AND vehicles AND weather AND spawns AND bans AND...
    ref array<Object> m_Loot;
    ref array<Object> m_Vehicles;
    ref WeatherData m_Weather;
    ref array<string> m_BannedPlayers;

    void SpawnLoot() { ... }
    void DespawnVehicle() { ... }
    void SetWeather() { ... }
    void BanPlayer() { ... }
    // 2000 lines later...
};

Split into focused singletons: LootManager, VehicleManager, WeatherManager, BanManager. Each one is small, testable, and has a clear domain.

4. Accessing Singletons in Constructors of Other Singletons

This creates hidden initialization-order dependencies:

// BAD: Constructor depends on another singleton
class ModuleA
{
    void ModuleA()
    {
        // What if ModuleB hasn't been created yet?
        ModuleB.GetInstance().Register(this);
    }
};

Defer cross-singleton registration to OnInit() or OnMissionStart(), where initialization order is controlled.

Alternative: Static-Only Classes

Some "singletons" do not need an instance at all. If the class holds no instance state and only has static methods and static fields, skip the GetInstance() ceremony entirely:

// No instance needed — all static
class MyLog
{
    private static FileHandle s_LogFile;
    private static int s_LogLevel;

    static void Info(string tag, string msg)
    {
        WriteLog("INFO", tag, msg);
    }

    static void Error(string tag, string msg)
    {
        WriteLog("ERROR", tag, msg);
    }

    static void Cleanup()
    {
        if (s_LogFile) CloseFile(s_LogFile);
        s_LogFile = null;
    }

    private static void WriteLog(string level, string tag, string msg)
    {
        // ...
    }
};

This is the approach used by MyLog, MyRPC, MyEventBus, and MyModuleManager in MyFramework. It is simpler, avoids the GetInstance() null-check overhead, and makes the intent clear: there is no instance, only shared state.

Use a static-only class when:

All methods are stateless or operate on static fields
There is no meaningful constructor/destructor logic
You never need to pass the "instance" as a parameter

Use a true singleton when:

The class has instance state that benefits from encapsulation (protected fields)
You need polymorphism (a base class with overridden methods)
The object needs to be passed to other systems by reference

Checklist

Before shipping a singleton, verify:

[ ] s_Instance is declared private static ref
[ ] GetInstance() handles the null case (lazy init) or you have an explicit Create() call
[ ] DestroyInstance() exists and sets s_Instance = null
[ ] DestroyInstance() is called from OnMissionFinish() or a centralized shutdown method
[ ] The destructor cleans up owned collections (.Clear(), set to null)
[ ] No public fields --- all mutation goes through methods
[ ] The constructor does not call GetInstance() on other singletons (defer to OnInit())

ホーム | Singleton Pattern | 次： Module Systems >>

Chapter 7.1: Singleton Pattern ​

はじめに ​

目次 ​

The Canonical Implementation ​

Why private static ref? ​

Lazy vs Eager Initialization ​

Lazy Initialization (Recommended Default) ​

Eager Initialization ​

Lifecycle Management ​

The Lifecycle Contract ​

Cleanup Pattern ​

MyMod Centralized Shutdown ​

使い分け Singletons ​

Good Candidates ​

Poor Candidates ​

実践的な例 ​

COT (Community Online Tools) ​

VPP Admin Tools ​

Expansion ​

Thread Safety Considerations ​

アンチパターン ​

1. Global Mutable State Without Encapsulation ​

2. Missing DestroyInstance ​

3. Singletons That Own Everything ​

4. Accessing Singletons in Constructors of Other Singletons ​

Alternative: Static-Only Classes ​

Checklist ​