juce_Singleton.h

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 /*
  ==============================================================================
 
   This file is part of the JUCE library.
   Copyright (c) 2022 - Raw Material Software Limited
 
   JUCE is an open source library subject to commercial or open-source
   licensing.
 
   The code included in this file is provided under the terms of the ISC license
   http://www.isc.org/downloads/software-support-policy/isc-license. Permission
   To use, copy, modify, and/or distribute this software for any purpose with or
   without fee is hereby granted provided that the above copyright notice and
   this permission notice appear in all copies.
 
   JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
   DISCLAIMED.
 
  ==============================================================================
*/
 
namespace juce
{
 
//==============================================================================
template <typename Type, typename MutexType, bool onlyCreateOncePerRun>
struct SingletonHolder  : private MutexType // (inherited so we can use the empty-base-class optimisation)
{
    SingletonHolder() = default;
 
    ~SingletonHolder()
    {
        /* The static singleton holder is being deleted before the object that it holds
           has been deleted. This could mean that you've forgotten to call clearSingletonInstance()
           in the class's destructor, or have failed to delete it before your app shuts down.
           If you're having trouble cleaning up your singletons, perhaps consider using the
           SharedResourcePointer class instead.
        */
        jassert (instance.load() == nullptr);
    }
 
    Type* get()
    {
        if (auto* ptr = instance.load())
            return ptr;
 
        typename MutexType::ScopedLockType sl (*this);
 
        if (auto* ptr = instance.load())
            return ptr;
 
        auto once = onlyCreateOncePerRun; // (local copy avoids VS compiler warning about this being constant)
 
        if (once)
        {
            static bool createdOnceAlready = false;
 
            if (createdOnceAlready)
            {
                // This means that the doNotRecreateAfterDeletion flag was set
                // and you tried to create the singleton more than once.
                jassertfalse;
                return nullptr;
            }
 
            createdOnceAlready = true;
        }
 
        static bool alreadyInside = false;
 
        if (alreadyInside)
        {
            // This means that your object's constructor has done something which has
            // ended up causing a recursive loop of singleton creation.
            jassertfalse;
            return nullptr;
        }
 
        const ScopedValueSetter<bool> scope (alreadyInside, true);
        return getWithoutChecking();
    }
 
    Type* getWithoutChecking()
    {
        if (auto* p = instance.load())
            return p;
 
        auto* newObject = new Type(); // (create into a local so that instance is still null during construction)
        instance.store (newObject);
        return newObject;
    }
 
    void deleteInstance()
    {
        typename MutexType::ScopedLockType sl (*this);
        delete instance.exchange (nullptr);
    }
 
    void clear (Type* expectedObject) noexcept
    {
        instance.compare_exchange_strong (expectedObject, nullptr);
    }
 
    // This must be atomic, otherwise a late call to get() may attempt to read instance while it is
    // being modified by the very first call to get().
    std::atomic<Type*> instance { nullptr };
};
 
 
//==============================================================================
#define JUCE_DECLARE_SINGLETON(Classname, doNotRecreateAfterDeletion) \
\
    static juce::SingletonHolder<Classname, juce::CriticalSection, doNotRecreateAfterDeletion> singletonHolder; \
    friend juce::SingletonHolder<Classname, juce::CriticalSection, doNotRecreateAfterDeletion>; \
\
    static Classname* JUCE_CALLTYPE getInstance()                           { return singletonHolder.get(); } \
    static Classname* JUCE_CALLTYPE getInstanceWithoutCreating() noexcept   { return singletonHolder.instance; } \
    static void JUCE_CALLTYPE deleteInstance() noexcept                     { singletonHolder.deleteInstance(); } \
    void clearSingletonInstance() noexcept                                  { singletonHolder.clear (this); }
 
 
//==============================================================================
#define JUCE_IMPLEMENT_SINGLETON(Classname) \
\
    decltype (Classname::singletonHolder) Classname::singletonHolder;
 
 
//==============================================================================
#define JUCE_DECLARE_SINGLETON_SINGLETHREADED(Classname, doNotRecreateAfterDeletion) \
\
    static juce::SingletonHolder<Classname, juce::DummyCriticalSection, doNotRecreateAfterDeletion> singletonHolder; \
    friend decltype (singletonHolder); \
\
    static Classname* JUCE_CALLTYPE getInstance()                           { return singletonHolder.get(); } \
    static Classname* JUCE_CALLTYPE getInstanceWithoutCreating() noexcept   { return singletonHolder.instance; } \
    static void JUCE_CALLTYPE deleteInstance() noexcept                     { singletonHolder.deleteInstance(); } \
    void clearSingletonInstance() noexcept                                  { singletonHolder.clear (this); }
 
 
//==============================================================================
#define JUCE_DECLARE_SINGLETON_SINGLETHREADED_MINIMAL(Classname) \
\
    static juce::SingletonHolder<Classname, juce::DummyCriticalSection, false> singletonHolder; \
    friend decltype (singletonHolder); \
\
    static Classname* JUCE_CALLTYPE getInstance()                           { return singletonHolder.getWithoutChecking(); } \
    static Classname* JUCE_CALLTYPE getInstanceWithoutCreating() noexcept   { return singletonHolder.instance; } \
    static void JUCE_CALLTYPE deleteInstance() noexcept                     { singletonHolder.deleteInstance(); } \
    void clearSingletonInstance() noexcept                                  { singletonHolder.clear (this); }
 
 
//==============================================================================
#ifndef DOXYGEN
 // These are ancient macros, and have now been updated with new names to match the JUCE style guide,
 // so please update your code to use the newer versions!
 #define juce_DeclareSingleton(Classname, doNotRecreate)                JUCE_DECLARE_SINGLETON(Classname, doNotRecreate)
 #define juce_DeclareSingleton_SingleThreaded(Classname, doNotRecreate) JUCE_DECLARE_SINGLETON_SINGLETHREADED(Classname, doNotRecreate)
 #define juce_DeclareSingleton_SingleThreaded_Minimal(Classname)        JUCE_DECLARE_SINGLETON_SINGLETHREADED_MINIMAL(Classname)
 #define juce_ImplementSingleton(Classname)                             JUCE_IMPLEMENT_SINGLETON(Classname)
 #define juce_ImplementSingleton_SingleThreaded(Classname)              JUCE_IMPLEMENT_SINGLETON(Classname)
#endif
 
} // namespace juce