IntrusivePointer.h

//------------------------------------------------------------------------------
/*
    This file is part of rippled: https://github.com/ripple/rippled
    Copyright (c) 2023 Ripple Labs Inc.
 
    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 THE AUTHOR DISCLAIMS ALL WARRANTIES
    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR 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 RIPPLE_BASICS_INTRUSIVEPOINTER_H_INCLUDED
#define RIPPLE_BASICS_INTRUSIVEPOINTER_H_INCLUDED
 
#include <concepts>
#include <cstdint>
#include <type_traits>
#include <utility>
 
namespace ripple {
 
//------------------------------------------------------------------------------
 
struct StaticCastTagSharedIntrusive
{
};
 
struct DynamicCastTagSharedIntrusive
{
};
 
struct SharedIntrusiveAdoptIncrementStrongTag
{
};
 
struct SharedIntrusiveAdoptNoIncrementTag
{
};
 
//------------------------------------------------------------------------------
//
 
template <class T>
concept CAdoptTag = std::is_same_v<T, SharedIntrusiveAdoptIncrementStrongTag> ||
    std::is_same_v<T, SharedIntrusiveAdoptNoIncrementTag>;
 
//------------------------------------------------------------------------------
 
template <class T>
class SharedIntrusive
{
public:
    SharedIntrusive() = default;
 
    template <CAdoptTag TAdoptTag>
    SharedIntrusive(T* p, TAdoptTag) noexcept;
 
    SharedIntrusive(SharedIntrusive const& rhs);
 
    template <class TT>
    // TODO: convertible_to isn't quite right. That include a static castable.
    // Find the right concept.
        requires std::convertible_to<TT*, T*>
    SharedIntrusive(SharedIntrusive<TT> const& rhs);
 
    SharedIntrusive(SharedIntrusive&& rhs);
 
    template <class TT>
        requires std::convertible_to<TT*, T*>
    SharedIntrusive(SharedIntrusive<TT>&& rhs);
 
    SharedIntrusive&
    operator=(SharedIntrusive const& rhs);
 
    bool
    operator!=(std::nullptr_t) const;
 
    bool
    operator==(std::nullptr_t) const;
 
    template <class TT>
        requires std::convertible_to<TT*, T*>
    SharedIntrusive&
    operator=(SharedIntrusive<TT> const& rhs);
 
    SharedIntrusive&
    operator=(SharedIntrusive&& rhs);
 
    template <class TT>
        requires std::convertible_to<TT*, T*>
    SharedIntrusive&
    operator=(SharedIntrusive<TT>&& rhs);
 
    template <CAdoptTag TAdoptTag = SharedIntrusiveAdoptIncrementStrongTag>
    void
    adopt(T* p);
 
    ~SharedIntrusive();
 
    template <class TT>
    SharedIntrusive(
        StaticCastTagSharedIntrusive,
        SharedIntrusive<TT> const& rhs);
 
    template <class TT>
    SharedIntrusive(StaticCastTagSharedIntrusive, SharedIntrusive<TT>&& rhs);
 
    template <class TT>
    SharedIntrusive(
        DynamicCastTagSharedIntrusive,
        SharedIntrusive<TT> const& rhs);
 
    template <class TT>
    SharedIntrusive(DynamicCastTagSharedIntrusive, SharedIntrusive<TT>&& rhs);
 
    T&
    operator*() const noexcept;
 
    T*
    operator->() const noexcept;
 
    explicit
    operator bool() const noexcept;
 
    void
    reset();
 
    T*
    get() const;
 
    std::size_t
    use_count() const;
 
    template <class TT, class... Args>
    friend SharedIntrusive<TT>
    make_SharedIntrusive(Args&&... args);
 
    template <class TT>
    friend class SharedIntrusive;
 
    template <class TT>
    friend class SharedWeakUnion;
 
    template <class TT>
    friend class WeakIntrusive;
 
private:
    T*
    unsafeGetRawPtr() const;
 
    void
    unsafeReleaseAndStore(T* next);
 
    void
    unsafeSetRawPtr(T* p);
 
    T*
    unsafeExchange(T* p);
 
    T* ptr_{nullptr};
};
 
//------------------------------------------------------------------------------
 
template <class T>
class WeakIntrusive
{
public:
    WeakIntrusive() = default;
 
    WeakIntrusive(WeakIntrusive const& rhs);
 
    WeakIntrusive(WeakIntrusive&& rhs);
 
    WeakIntrusive(SharedIntrusive<T> const& rhs);
 
    // There is no move constructor from a strong intrusive ptr because
    // moving would be move expensive than copying in this case (the strong
    // ref would need to be decremented)
    WeakIntrusive(SharedIntrusive<T> const&& rhs) = delete;
 
    // Since there are no current use cases for copy assignment in
    // WeakIntrusive, we delete this operator to simplify the implementation. If
    // a need arises in the future, we can reintroduce it with proper
    // consideration."
    WeakIntrusive&
    operator=(WeakIntrusive const&) = delete;
 
    template <class TT>
        requires std::convertible_to<TT*, T*>
    WeakIntrusive&
    operator=(SharedIntrusive<TT> const& rhs);
 
    void
    adopt(T* ptr);
 
    ~WeakIntrusive();
 
    SharedIntrusive<T>
    lock() const;
 
    bool
    expired() const;
 
    void
    reset();
 
private:
    T* ptr_ = nullptr;
 
    void
    unsafeReleaseNoStore();
};
 
//------------------------------------------------------------------------------
 
template <class T>
class SharedWeakUnion
{
    // Tagged pointer. Low bit determines if this is a strong or a weak
    // pointer. The low bit must be masked to zero when converting back to a
    // pointer. If the low bit is '1', this is a weak pointer.
    static_assert(
        alignof(T) >= 2,
        "Bad alignment: Combo pointer requires low bit to be zero");
 
public:
    SharedWeakUnion() = default;
 
    SharedWeakUnion(SharedWeakUnion const& rhs);
 
    template <class TT>
        requires std::convertible_to<TT*, T*>
    SharedWeakUnion(SharedIntrusive<TT> const& rhs);
 
    SharedWeakUnion(SharedWeakUnion&& rhs);
 
    template <class TT>
        requires std::convertible_to<TT*, T*>
    SharedWeakUnion(SharedIntrusive<TT>&& rhs);
 
    SharedWeakUnion&
    operator=(SharedWeakUnion const& rhs);
 
    template <class TT>
        requires std::convertible_to<TT*, T*>
    SharedWeakUnion&
    operator=(SharedIntrusive<TT> const& rhs);
 
    template <class TT>
        requires std::convertible_to<TT*, T*>
    SharedWeakUnion&
    operator=(SharedIntrusive<TT>&& rhs);
 
    ~SharedWeakUnion();
 
    SharedIntrusive<T>
    getStrong() const;
 
    explicit
    operator bool() const noexcept;
 
    void
    reset();
 
    T*
    get() const;
 
    std::size_t
    use_count() const;
 
    bool
    expired() const;
 
    SharedIntrusive<T>
    lock() const;
 
    bool
    isStrong() const;
 
    bool
    isWeak() const;
 
    bool
    convertToStrong();
 
    bool
    convertToWeak();
 
private:
    // Tagged pointer. Low bit determines if this is a strong or a weak
    // pointer. The low bit must be masked to zero when converting back to a
    // pointer. If the low bit is '1', this is a weak pointer.
    std::uintptr_t tp_{0};
    static constexpr std::uintptr_t tagMask = 1;
    static constexpr std::uintptr_t ptrMask = ~tagMask;
 
private:
    T*
    unsafeGetRawPtr() const;
 
    enum class RefStrength { strong, weak };
    void
    unsafeSetRawPtr(T* p, RefStrength rs);
 
    void unsafeSetRawPtr(std::nullptr_t);
 
    void
    unsafeReleaseNoStore();
};
 
//------------------------------------------------------------------------------
 
template <class TT, class... Args>
SharedIntrusive<TT>
make_SharedIntrusive(Args&&... args)
{
    auto p = new TT(std::forward<Args>(args)...);
 
    static_assert(
        noexcept(SharedIntrusive<TT>(
            std::declval<TT*>(),
            std::declval<SharedIntrusiveAdoptNoIncrementTag>())),
        "SharedIntrusive constructor should not throw or this can leak "
        "memory");
 
    return SharedIntrusive<TT>(p, SharedIntrusiveAdoptNoIncrementTag{});
}
 
//------------------------------------------------------------------------------
 
namespace intr_ptr {
template <class T>
using SharedPtr = SharedIntrusive<T>;
 
template <class T>
using WeakPtr = WeakIntrusive<T>;
 
template <class T>
using SharedWeakUnionPtr = SharedWeakUnion<T>;
 
template <class T, class... A>
SharedPtr<T>
make_shared(A&&... args)
{
    return make_SharedIntrusive<T>(std::forward<A>(args)...);
}
 
template <class T, class TT>
SharedPtr<T>
static_pointer_cast(TT const& v)
{
    return SharedPtr<T>(StaticCastTagSharedIntrusive{}, v);
}
 
template <class T, class TT>
SharedPtr<T>
dynamic_pointer_cast(TT const& v)
{
    return SharedPtr<T>(DynamicCastTagSharedIntrusive{}, v);
}
}  // namespace intr_ptr
}  // namespace ripple
#endif