JUCE/modules/juce_dsp/containers/juce_FixedSizeFunction.h

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

   This file is part of the JUCE 6 technical preview.
   Copyright (c) 2020 - Raw Material Software Limited

   You may use this code under the terms of the GPL v3
   (see www.gnu.org/licenses).

   For this technical preview, this file is not subject to commercial licensing.

   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
{
namespace dsp
{

namespace detail
{
    template <typename Ret, typename... Args>
    struct Vtable
    {
        using Storage = void*;

        using Move  = void (*) (Storage, Storage);
        using Call  = Ret  (*) (Storage, Args...);
        using Clear = void (*) (Storage);

        constexpr Vtable (Move moveIn, Call callIn, Clear clearIn) noexcept
                : move (moveIn), call (callIn), clear (clearIn) {}

        Move  move  = nullptr;
        Call  call  = nullptr;
        Clear clear = nullptr;
    };

    template <typename Fn>
    void move (void* from, void* to)
    {
        new (to) Fn (std::move (*reinterpret_cast<Fn*> (from)));
    }

    template <typename Fn, typename Ret, typename... Args>
    typename std::enable_if<std::is_same<Ret, void>::value, Ret>::type call (void* s, Args... args)
    {
        (*reinterpret_cast<Fn*> (s)) (args...);
    }

    template <typename Fn, typename Ret, typename... Args>
    typename std::enable_if<! std::is_same<Ret, void>::value, Ret>::type call (void* s, Args... args)
    {
        return (*reinterpret_cast<Fn*> (s)) (std::forward<Args> (args)...);
    }

    template <typename Fn>
    void clear (void* s)
    {
        auto& fn = *reinterpret_cast<Fn*> (s);
        fn.~Fn();
        // I know this looks insane, for some reason MSVC 14 sometimes thinks fn is unreferenced
        juce::ignoreUnused (fn);
    }

    template <typename Fn, typename Ret, typename... Args>
    constexpr Vtable<Ret, Args...> makeVtable()
    {
        return { move <Fn>, call <Fn, Ret, Args...>, clear<Fn> };
    }
} // namespace detail

template <size_t len, typename T>
class FixedSizeFunction;

/**
    A type similar to `std::function` that holds a callable object.

    Unlike `std::function`, the callable object will always be stored in
    a buffer of size `len` that is internal to the FixedSizeFunction instance.
    This in turn means that creating a FixedSizeFunction instance will never allocate,
    making FixedSizeFunctions suitable for use in realtime contexts.
*/
template <size_t len, typename Ret, typename... Args>
class FixedSizeFunction<len, Ret (Args...)>
{
private:
    using Storage = typename std::aligned_storage<len>::type;

    template <typename Item>
    using Decay = typename std::decay<Item>::type;

    template <typename Item, typename Fn = Decay<Item>>
    using IntIfValidConversion = typename std::enable_if<sizeof (Fn) <= len
                                                             && alignof (Fn) <= alignof (Storage)
                                                             && ! std::is_same<FixedSizeFunction, Fn>::value,
                                                         int>::type;

public:
    /** Create an empty function. */
    FixedSizeFunction() noexcept = default;

    /** Create an empty function. */
    FixedSizeFunction (std::nullptr_t) noexcept
        : FixedSizeFunction() {}

    FixedSizeFunction (const FixedSizeFunction&) = delete;

    /** Forwards the passed Callable into the internal storage buffer. */
    template <typename Callable,
              typename Fn = Decay<Callable>,
              IntIfValidConversion<Callable> = 0>
    FixedSizeFunction (Callable&& callable)
    {
        static_assert (sizeof (Fn) <= len,
                       "The requested function cannot fit in this FixedSizeFunction");
        static_assert (alignof (Fn) <= alignof (Storage),
                       "FixedSizeFunction cannot accommodate the requested alignment requirements");

        static constexpr auto vtableForCallable = detail::makeVtable<Fn, Ret, Args...>();
        vtable = &vtableForCallable;

        auto* ptr = new (&storage) Fn (std::forward<Callable> (callable));
        jassert ((void*) ptr == (void*) &storage);
        juce::ignoreUnused (ptr);
    }

    /** Move constructor. */
    FixedSizeFunction (FixedSizeFunction&& other) noexcept
        : vtable (other.vtable)
    {
        move (std::move (other));
    }

    /** Converting constructor from smaller FixedSizeFunctions. */
    template <size_t otherLen, typename std::enable_if<(otherLen < len), int>::type = 0>
    FixedSizeFunction (FixedSizeFunction<otherLen, Ret (Args...)>&& other) noexcept
        : vtable (other.vtable)
    {
        move (std::move (other));
    }

    /** Nulls this instance. */
    FixedSizeFunction& operator= (std::nullptr_t) noexcept
    {
        return *this = FixedSizeFunction();
    }

    FixedSizeFunction& operator= (const FixedSizeFunction&) = delete;

    /** Assigns a new callable to this instance. */
    template <typename Callable, IntIfValidConversion<Callable> = 0>
    FixedSizeFunction& operator= (Callable&& callable)
    {
        return *this = FixedSizeFunction (std::forward<Callable> (callable));
    }

    /** Move assignment from smaller FixedSizeFunctions. */
    template <size_t otherLen, typename std::enable_if<(otherLen < len), int>::type = 0>
    FixedSizeFunction& operator= (FixedSizeFunction<otherLen, Ret (Args...)>&& other) noexcept
    {
        return *this = FixedSizeFunction (std::move (other));
    }

    /** Move assignment operator. */
    FixedSizeFunction& operator= (FixedSizeFunction&& other) noexcept
    {
        clear();
        vtable = other.vtable;
        move (std::move (other));
        return *this;
    }

    /** Destructor. */
    ~FixedSizeFunction() noexcept { clear(); }

    /** If this instance is currently storing a callable object, calls that object,
        otherwise throws `std::bad_function_call`.
    */
    Ret operator() (Args... args) const
    {
        if (vtable != nullptr)
            return vtable->call (&storage, std::forward<Args> (args)...);

        throw std::bad_function_call();
    }

    /** Returns true if this instance currently holds a callable. */
    explicit operator bool() const noexcept { return vtable != nullptr; }

private:
    template <size_t, typename>
    friend class FixedSizeFunction;

    void clear() noexcept
    {
        if (vtable != nullptr)
            vtable->clear (&storage);
    }

    template <size_t otherLen, typename T>
    void move (FixedSizeFunction<otherLen, T>&& other) noexcept
    {
        if (vtable != nullptr)
            vtable->move (&other.storage, &storage);
    }

    const detail::Vtable<Ret, Args...>* vtable = nullptr;
    mutable Storage storage;
};

template <size_t len, typename T>
bool operator!= (const FixedSizeFunction<len, T>& fn, std::nullptr_t) { return bool (fn); }

template <size_t len, typename T>
bool operator!= (std::nullptr_t, const FixedSizeFunction<len, T>& fn) { return bool (fn); }

template <size_t len, typename T>
bool operator== (const FixedSizeFunction<len, T>& fn, std::nullptr_t) { return ! (fn != nullptr); }

template <size_t len, typename T>
bool operator== (std::nullptr_t, const FixedSizeFunction<len, T>& fn) { return ! (fn != nullptr); }


}
}