Cardinal/src/extra/SharedResourcePointer.hpp

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

   This file is part of the Water library.
   Copyright (c) 2016 ROLI Ltd.
   Copyright (C) 2017-2022 Filipe Coelho <falktx@falktx.com>

   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.

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

#ifndef WATER_SHAREDRESOURCEPOINTER_HPP_INCLUDED
#define WATER_SHAREDRESOURCEPOINTER_HPP_INCLUDED

#include "ReferenceCountedObject.hpp"
#include "extra/Mutex.hpp"
#include "extra/ScopedPointer.hpp"

START_NAMESPACE_DISTRHO

//==============================================================================
/**
    A smart-pointer that automatically creates and manages the lifetime of a
    shared static instance of a class.

    The SharedObjectType template type indicates the class to use for the shared
    object - the only requirements on this class are that it must have a public
    default constructor and destructor.

    The SharedResourcePointer offers a pattern that differs from using a singleton or
    static instance of an object, because it uses reference-counting to make sure that
    the underlying shared object is automatically created/destroyed according to the
    number of SharedResourcePointer objects that exist. When the last one is deleted,
    the underlying object is also immediately destroyed. This allows you to use scoping
    to manage the lifetime of a shared resource.

    Note: the construction/deletion of the shared object must not involve any
    code that makes recursive calls to a SharedResourcePointer, or you'll cause
    a deadlock.

    Example:
    @code
    // An example of a class that contains the shared data you want to use.
    struct MySharedData
    {
        // There's no need to ever create an instance of this class directly yourself,
        // but it does need a public constructor that does the initialisation.
        MySharedData()
        {
            sharedStuff = generateHeavyweightStuff();
        }

        Array<SomeKindOfData> sharedStuff;
    };

    struct DataUserClass
    {
        DataUserClass()
        {
            // Multiple instances of the DataUserClass will all have the same
            // shared common instance of MySharedData referenced by their sharedData
            // member variables.
            useSharedStuff (sharedData->sharedStuff);
        }

        // By keeping this pointer as a member variable, the shared resource
        // is guaranteed to be available for as long as the DataUserClass object.
        SharedResourcePointer<MySharedData> sharedData;
    };

    @endcode
 */
template <typename SharedObjectType>
class SharedResourcePointer
{
public:
    /** Creates an instance of the shared object.
        If other SharedResourcePointer objects for this type already exist, then
        this one will simply point to the same shared object that they are already
        using. Otherwise, if this is the first SharedResourcePointer to be created,
        then a shared object will be created automatically.
    */
    SharedResourcePointer()
      : sharedObject(nullptr)
    {
        initialise();
    }

    template<class T>
    SharedResourcePointer(const T* const variant)
      : sharedObject(nullptr)
    {
        initialise_variant<T>(variant);
    }

    template<class T1, class T2>
    SharedResourcePointer(const T1* const v1, const T2* const v2)
      : sharedObject(nullptr)
    {
        initialise_variant2<T1, T2>(v1, v2);
    }

    SharedResourcePointer (const SharedResourcePointer&)
      : sharedObject(nullptr)
    {
        initialise();
    }

    /** Destructor.
        If no other SharedResourcePointer objects exist, this will also delete
        the shared object to which it refers.
    */
    ~SharedResourcePointer()
    {
        SharedObjectHolder& holder = getSharedObjectHolder();
        const MutexLocker cml (holder.lock);

        if (--(holder.refCount) == 0)
            holder.sharedInstance = nullptr;
    }

    /** Returns the shared object. */
    operator SharedObjectType*() const noexcept         { return sharedObject; }

    /** Returns the shared object. */
    SharedObjectType& get() const noexcept              { return *sharedObject; }

    /** Returns the object that this pointer references.
        The pointer returned may be a nullptr, of course.
    */
    SharedObjectType& getObject() const noexcept        { return *sharedObject; }
    SharedObjectType* getPointer() const noexcept       { return sharedObject; }

    SharedObjectType* operator->() const noexcept       { return sharedObject; }

private:
    struct SharedObjectHolder  : public ReferenceCountedObject
    {
        Mutex lock;
        ScopedPointer<SharedObjectType> sharedInstance;
        int refCount;
    };

    static SharedObjectHolder& getSharedObjectHolder() noexcept
    {
        static void* holder [(sizeof (SharedObjectHolder) + sizeof(void*) - 1) / sizeof(void*)] = { nullptr };
        return *reinterpret_cast<SharedObjectHolder*> (holder);
    }

    SharedObjectType* sharedObject;

    void initialise()
    {
        SharedObjectHolder& holder = getSharedObjectHolder();
        const MutexLocker cml (holder.lock);

        if (++(holder.refCount) == 1)
            holder.sharedInstance = new SharedObjectType();

        sharedObject = holder.sharedInstance;
    }

    template<class T>
    void initialise_variant(const T* const variant)
    {
        SharedObjectHolder& holder = getSharedObjectHolder();
        const MutexLocker cml (holder.lock);

        if (++(holder.refCount) == 1)
            holder.sharedInstance = new SharedObjectType(variant);

        sharedObject = holder.sharedInstance;
    }

    template<class T1, class T2>
    void initialise_variant2(const T1* const v1, const T2* const v2)
    {
        SharedObjectHolder& holder = getSharedObjectHolder();
        const MutexLocker cml (holder.lock);

        if (++(holder.refCount) == 1)
            holder.sharedInstance = new SharedObjectType(v1, v2);

        sharedObject = holder.sharedInstance;
    }

    // There's no need to assign to a SharedResourcePointer because every
    // instance of the class is exactly the same!
    SharedResourcePointer& operator= (const SharedResourcePointer&) = delete;
};

}

#endif // WATER_SHAREDRESOURCEPOINTER_HPP_INCLUDED