JUCE/modules/juce_core/threads/juce_ThreadLocalValue.h

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

   This file is part of the JUCE library.
   Copyright (c) 2016 - ROLI Ltd.

   Permission is granted to use this software 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.

   THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH REGARD
   TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
   FITNESS. IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT,
   OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF
   USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
   TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE
   OF THIS SOFTWARE.

   -----------------------------------------------------------------------------

   To release a closed-source product which uses other parts of JUCE not
   licensed under the ISC terms, commercial licenses are available: visit
   www.juce.com for more information.

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

#pragma once

// (NB: on win32, native thread-locals aren't possible in a dynamically loaded DLL in XP).
#if ! ((JUCE_MSVC && (JUCE_64BIT || ! defined (JucePlugin_PluginCode))) \
       || (JUCE_MAC && JUCE_CLANG && defined (MAC_OS_X_VERSION_10_7) \
             && MAC_OS_X_VERSION_MIN_REQUIRED >= MAC_OS_X_VERSION_10_7))
 #define JUCE_NO_COMPILER_THREAD_LOCAL 1
#endif

//==============================================================================
/**
    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.
*/
template <typename Type>
class ThreadLocalValue
{
public:
    /** */
    ThreadLocalValue() noexcept
    {
    }

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

    /** 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
    {
       #if JUCE_NO_COMPILER_THREAD_LOCAL
        const Thread::ThreadID threadId = Thread::getCurrentThreadId();

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

        for (ObjectHolder* o = first.get(); o != nullptr; o = o->next)
        {
            if (o->threadId == nullptr)
            {
                {
                    SpinLock::ScopedLockType sl (lock);

                    if (o->threadId != nullptr)
                        continue;

                    o->threadId = threadId;
                }

                o->object = Type();
                return o->object;
            }
        }

        ObjectHolder* const newObject = new ObjectHolder (threadId);

        do
        {
            newObject->next = first.get();
        }
        while (! first.compareAndSetBool (newObject, newObject->next));

        return newObject->object;
       #elif JUCE_MAC
        static __thread Type object;
        return object;
       #elif JUCE_MSVC
        static __declspec(thread) Type object;
        return object;
       #endif
    }

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

        for (ObjectHolder* o = first.get(); o != nullptr; o = o->next)
        {
            if (o->threadId == threadId)
            {
                SpinLock::ScopedLockType sl (lock);
                o->threadId = nullptr;
            }
        }
       #endif
    }

private:
    //==============================================================================
   #if JUCE_NO_COMPILER_THREAD_LOCAL
    struct ObjectHolder
    {
        ObjectHolder (const Thread::ThreadID& tid)
            : threadId (tid), next (nullptr), object()
        {}

        Thread::ThreadID threadId;
        ObjectHolder* next;
        Type object;

        JUCE_DECLARE_NON_COPYABLE (ObjectHolder)
    };

    mutable Atomic<ObjectHolder*> first;
    SpinLock lock;
   #endif

    JUCE_DECLARE_NON_COPYABLE (ThreadLocalValue)
};