JUCE/modules/juce_core/threads/juce_ThreadLocalValue.h

/*
  ==============================================================================

   This file is part of the JUCE library.
   Copyright (c) 2020 - 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
{

//==============================================================================
/**
    Provides cross-platform support for thread-local objects.

    This class holds an internal list of objects of the templated type, keeping
    an instance for each thread that requests one. The first time a thread attempts
    to access its value, an object is created and added to the list for that thread.

    Typically, you'll probably want to create a static instance of a ThreadLocalValue
    object, or hold one within a singleton.

    The templated class for your value must be a primitive type, or a simple POD struct.

    When a thread no longer needs to use its value, it can call releaseCurrentThreadStorage()
    to allow the storage to be re-used by another thread. If a thread exits without calling
    this method, the object storage will be left allocated until the ThreadLocalValue object
    is deleted.

    @tags{Core}
*/
template <typename Type>
class ThreadLocalValue
{
public:
    /** */
    ThreadLocalValue() = default;

    /** Destructor.
        When this object is deleted, all the value objects for all threads will be deleted.
    */
    ~ThreadLocalValue()
    {
        for (auto* o = first.get(); o != nullptr;)
        {
            auto* next = o->next;
            delete o;
            o = next;
        }
    }

    /** Returns a reference to this thread's instance of the value.
        Note that the first time a thread tries to access the value, an instance of the
        value object will be created - so if your value's class has a non-trivial
        constructor, be aware that this method could invoke it.
    */
    Type& operator*() const noexcept                        { return get(); }

    /** Returns a pointer to this thread's instance of the value.
        Note that the first time a thread tries to access the value, an instance of the
        value object will be created - so if your value's class has a non-trivial
        constructor, be aware that this method could invoke it.
    */
    operator Type*() const noexcept                         { return &get(); }

    /** Accesses a method or field of the value object.
        Note that the first time a thread tries to access the value, an instance of the
        value object will be created - so if your value's class has a non-trivial
        constructor, be aware that this method could invoke it.
    */
    Type* operator->() const noexcept                       { return &get(); }

    /** Assigns a new value to the thread-local object. */
    ThreadLocalValue& operator= (const Type& newValue)      { get() = newValue; return *this; }

    /** Returns a reference to this thread's instance of the value.
        Note that the first time a thread tries to access the value, an instance of the
        value object will be created - so if your value's class has a non-trivial
        constructor, be aware that this method could invoke it.
    */
    Type& get() const noexcept
    {
        auto threadId = Thread::getCurrentThreadId();
        ObjectHolder* o = nullptr;

        for (o = first.get(); o != nullptr; o = o->next)
            if (o->threadId.get() == threadId)
                return o->object;

        for (o = first.get(); o != nullptr; o = o->next)
            if (o->threadId.compareAndSetBool (threadId, nullptr))
                break;

        if (o != nullptr)
            o->object = Type();
        else
            for (o = new ObjectHolder (threadId, first.get());
                 ! first.compareAndSetBool (o, o->next);
                 o->next = first.get());

        return o->object;
    }

    /** Called by a thread before it terminates, to allow this class to release
        any storage associated with the thread.
    */
    void releaseCurrentThreadStorage()
    {
        auto threadId = Thread::getCurrentThreadId();

        for (auto* o = first.get(); o != nullptr; o = o->next)
            if (o->threadId.compareAndSetBool (nullptr, threadId))
                return;
    }

private:
    //==============================================================================
    struct ObjectHolder
    {
        ObjectHolder (Thread::ThreadID idToUse, ObjectHolder* n) : threadId (idToUse), next (n), object() {}

        Atomic<Thread::ThreadID> threadId;
        ObjectHolder* next;
        Type object;

        JUCE_DECLARE_NON_COPYABLE (ObjectHolder)
    };

    mutable Atomic<ObjectHolder*> first;

    JUCE_DECLARE_NON_COPYABLE (ThreadLocalValue)
};

} // namespace juce