scylladb/core/shared_ptr.hh

/*
 * This file is open source software, licensed to you under the terms
 * of the Apache License, Version 2.0 (the "License").  See the NOTICE file
 * distributed with this work for additional information regarding copyright
 * ownership.  You may not use this file except in compliance with the License.
 *
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
/*
 * Copyright (C) 2014 Cloudius Systems, Ltd.
 */

#ifndef SHARED_PTR_HH_
#define SHARED_PTR_HH_

#include <utility>
#include <type_traits>
#include <functional>
#include <iostream>

// This header defines two shared pointer facilities, lw_shared_ptr<> and
// shared_ptr<>, both modeled after std::shared_ptr<>.
//
// Unlike std::shared_ptr<>, neither of these implementations are thread
// safe, and two pointers sharing the same object must not be used in
// different threads.
//
// lw_shared_ptr<> is the more lightweight variant, with a lw_shared_ptr<>
// occupying just one machine word, and adding just one word to the shared
// object.  However, it does not support polymorphism.
//
// shared_ptr<> is more expensive, with a pointer occupying two machine
// words, and with two words of overhead in the shared object.  In return,
// it does support polymorphism.
//
// Both variants support shared_from_this() via enable_shared_from_this<>
// and lw_enable_shared_from_this<>().
//

template <typename T>
class lw_shared_ptr;

template <typename T>
class shared_ptr;

template <typename T>
class enable_lw_shared_from_this;

template <typename T>
class enable_shared_from_this;

template <typename T, typename... A>
lw_shared_ptr<T> make_lw_shared(A&&... a);

template <typename T>
lw_shared_ptr<T> make_lw_shared(T&& a);

template <typename T>
lw_shared_ptr<T> make_lw_shared(T& a);

template <typename T, typename... A>
shared_ptr<T> make_shared(A&&... a);

template <typename T>
shared_ptr<T> make_shared(T&& a);

template <typename T, typename U>
shared_ptr<T> static_pointer_cast(const shared_ptr<U>& p);

template <typename T, typename U>
shared_ptr<T> dynamic_pointer_cast(const shared_ptr<U>& p);

template <typename T, typename U>
shared_ptr<T> const_pointer_cast(const shared_ptr<U>& p);

// We want to support two use cases for shared_ptr<T>:
//
//   1. T is any type (primitive or class type)
//
//   2. T is a class type that inherits from enable_shared_from_this<T>.
//
// In the first case, we must wrap T in an object containing the counter,
// since T may be a primitive type and cannot be a base class.
//
// In the second case, we want T to reach the counter through its
// enable_shared_from_this<> base class, so that we can implement
// shared_from_this().
//
// To implement those two conflicting requirements (T alongside its counter;
// T inherits from an object containing the counter) we use std::conditional<>
// and some accessor functions to select between two implementations.


// CRTP from this to enable shared_from_this:
template <typename T>
class enable_lw_shared_from_this {
    long _count = 0;
    using ctor = T;
    T* to_value() { return static_cast<T*>(this); }
    T* to_internal_object() { return static_cast<T*>(this); }
protected:
    enable_lw_shared_from_this& operator=(const enable_lw_shared_from_this&) { return *this; }
    enable_lw_shared_from_this& operator=(enable_lw_shared_from_this&&) { return *this; }
public:
    lw_shared_ptr<T> shared_from_this();
    template <typename X>
    friend class lw_shared_ptr;
};

template <typename T>
struct shared_ptr_no_esft {
    long _count = 0;
    T _value;
    using ctor = shared_ptr_no_esft;

    T* to_value() { return &_value; }
    shared_ptr_no_esft* to_internal_object() { return this; }
    shared_ptr_no_esft() = default;
    shared_ptr_no_esft(const T& x) : _value(x) {}
    shared_ptr_no_esft(T&& x) : _value(std::move(x)) {}
    template <typename... A>
    shared_ptr_no_esft(A&&... a) : _value(std::forward<A>(a)...) {}
    template <typename X>
    friend class lw_shared_ptr;
};

template <typename T>
using shared_ptr_impl
    = std::conditional_t<
        std::is_base_of<enable_lw_shared_from_this<T>, T>::value,
        enable_lw_shared_from_this<T>,
        shared_ptr_no_esft<T>
      >;

template <typename T>
class lw_shared_ptr {
    mutable shared_ptr_impl<T>* _p = nullptr;
private:
    lw_shared_ptr(shared_ptr_impl<T>* p) noexcept : _p(p) {
        if (_p) {
            ++_p->_count;
        }
    }
    template <typename... A>
    static lw_shared_ptr make(A&&... a) noexcept {
        return lw_shared_ptr(new typename shared_ptr_impl<T>::ctor(std::forward<A>(a)...));
    }
public:
    using element_type = T;

    lw_shared_ptr() noexcept = default;
    lw_shared_ptr(std::nullptr_t) noexcept : lw_shared_ptr() {}
    lw_shared_ptr(const lw_shared_ptr& x) noexcept : _p(x._p) {
        if (_p) {
            ++_p->_count;
        }
    }
    lw_shared_ptr(lw_shared_ptr&& x) noexcept  : _p(x._p) {
        x._p = nullptr;
    }
    ~lw_shared_ptr() {
        if (_p && !--_p->_count) {
            delete _p->to_internal_object();
        }
    }
    lw_shared_ptr& operator=(const lw_shared_ptr& x) noexcept {
        if (_p != x._p) {
            this->~lw_shared_ptr();
            new (this) lw_shared_ptr(x);
        }
        return *this;
    }
    lw_shared_ptr& operator=(lw_shared_ptr&& x) noexcept {
        if (_p != x._p) {
            this->~lw_shared_ptr();
            new (this) lw_shared_ptr(std::move(x));
        }
        return *this;
    }
    lw_shared_ptr& operator=(std::nullptr_t) noexcept {
        return *this = lw_shared_ptr();
    }
    lw_shared_ptr& operator=(T&& x) noexcept {
        this->~lw_shared_ptr();
        new (this) lw_shared_ptr(make_lw_shared<T>(std::move(x)));
        return *this;
    }

    T& operator*() const noexcept { return *_p->to_value(); }
    T* operator->() const noexcept { return _p->to_value(); }
    T* get() const noexcept { return _p->to_value(); }

    long int use_count() noexcept {
        if (_p) {
            return _p->_count;
        } else {
            return 0;
        }
    }

    operator lw_shared_ptr<const T>() const noexcept {
        return lw_shared_ptr<const T>(_p);
    }

    explicit operator bool() const noexcept {
        return _p;
    }

    bool owned() const noexcept {
        return _p->_count == 1;
    }

    template <typename X, typename... A>
    friend lw_shared_ptr<X> make_lw_shared(A&&...);

    template <typename U>
    friend lw_shared_ptr<U> make_lw_shared(U&&);

    template <typename U>
    friend lw_shared_ptr<U> make_lw_shared(U&);

    template <typename U>
    friend class enable_lw_shared_from_this;
};

template <typename T, typename... A>
inline
lw_shared_ptr<T> make_lw_shared(A&&... a) {
    return lw_shared_ptr<T>::make(std::forward<A>(a)...);
}

template <typename T>
inline
lw_shared_ptr<T> make_lw_shared(T&& a) {
    return lw_shared_ptr<T>::make(std::move(a));
}

template <typename T>
inline
lw_shared_ptr<T> make_lw_shared(T& a) {
    return lw_shared_ptr<T>::make(a);
}

template <typename T>
inline
lw_shared_ptr<T>
enable_lw_shared_from_this<T>::shared_from_this() {
    return lw_shared_ptr<T>(this);
}

template <typename T>
static inline
std::ostream& operator<<(std::ostream& out, const lw_shared_ptr<T>& p) {
    if (!p) {
        return out << "null";
    }
    return out << *p;
}

// Polymorphic shared pointer class

struct shared_ptr_count_base {
    // destructor is responsible for fully-typed deletion
    virtual ~shared_ptr_count_base() {}
    long count = 0;
};

template <typename T>
struct shared_ptr_count_for : shared_ptr_count_base {
    T data;
    template <typename... A>
    shared_ptr_count_for(A&&... a) : data(std::forward<A>(a)...) {}
};

template <typename T>
class enable_shared_from_this : private shared_ptr_count_base {
public:
    shared_ptr<T> shared_from_this();
    shared_ptr<const T> shared_from_this() const;

    template <typename U>
    friend class shared_ptr;

    template <typename U, bool esft>
    friend struct shared_ptr_make_helper;
};

template <typename T>
class shared_ptr {
    mutable shared_ptr_count_base* _b = nullptr;
    mutable T* _p = nullptr;
private:
    explicit shared_ptr(shared_ptr_count_for<T>* b) noexcept : _b(b), _p(&b->data) {
        ++_b->count;
    }
    shared_ptr(shared_ptr_count_base* b, T* p) noexcept : _b(b), _p(p) {
        if (_b) {
            ++_b->count;
        }
    }
    explicit shared_ptr(enable_shared_from_this<std::remove_const_t<T>>* p) noexcept : _b(p), _p(static_cast<T*>(p)) {
        if (_b) {
            ++_b->count;
        }
    }
public:
    using element_type = T;

    shared_ptr() noexcept = default;
    shared_ptr(std::nullptr_t) noexcept : shared_ptr() {}
    shared_ptr(const shared_ptr& x) noexcept
            : _b(x._b)
            , _p(x._p) {
        if (_b) {
            ++_b->count;
        }
    }
    shared_ptr(shared_ptr&& x) noexcept
            : _b(x._b)
            , _p(x._p) {
        x._b = nullptr;
        x._p = nullptr;
    }
    template <typename U, typename = std::enable_if_t<std::is_base_of<T, U>::value>>
    shared_ptr(const shared_ptr<U>& x) noexcept
            : _b(x._b)
            , _p(x._p) {
        if (_b) {
            ++_b->count;
        }
    }
    template <typename U, typename = std::enable_if_t<std::is_base_of<T, U>::value>>
    shared_ptr(shared_ptr<U>&& x) noexcept
            : _b(x._b)
            , _p(x._p) {
        x._b = nullptr;
        x._p = nullptr;
    }
    ~shared_ptr() {
        if (_b && !--_b->count) {
            delete _b;
        }
    }
    shared_ptr& operator=(const shared_ptr& x) noexcept {
        if (this != &x) {
            this->~shared_ptr();
            new (this) shared_ptr(x);
        }
        return *this;
    }
    shared_ptr& operator=(shared_ptr&& x) noexcept {
        if (this != &x) {
            this->~shared_ptr();
            new (this) shared_ptr(std::move(x));
        }
        return *this;
    }
    shared_ptr& operator=(std::nullptr_t) noexcept {
        return *this = shared_ptr();
    }
    template <typename U, typename = std::enable_if_t<std::is_base_of<T, U>::value>>
    shared_ptr& operator=(const shared_ptr<U>& x) noexcept {
        if (*this != x) {
            this->~shared_ptr();
            new (this) shared_ptr(x);
        }
        return *this;
    }
    template <typename U, typename = std::enable_if_t<std::is_base_of<T, U>::value>>
    shared_ptr& operator=(shared_ptr<U>&& x) noexcept {
        if (*this != x) {
            this->~shared_ptr();
            new (this) shared_ptr(std::move(x));
        }
        return *this;
    }
    explicit operator bool() const noexcept {
        return _p;
    }
    T& operator*() const noexcept {
        return *_p;
    }
    T* operator->() const noexcept {
        return _p;
    }
    T* get() const noexcept {
        return _p;
    }

    template <bool esft>
    struct make_helper;

    template <typename U, typename... A>
    friend shared_ptr<U> make_shared(A&&... a);

    template <typename U>
    friend shared_ptr<U> make_shared(U&& a);

    template <typename V, typename U>
    friend shared_ptr<V> static_pointer_cast(const shared_ptr<U>& p);

    template <typename V, typename U>
    friend shared_ptr<V> dynamic_pointer_cast(const shared_ptr<U>& p);

    template <typename V, typename U>
    friend shared_ptr<V> const_pointer_cast(const shared_ptr<U>& p);

    template <bool esft, typename... A>
    static shared_ptr make(A&&... a);

    template <typename U>
    friend class enable_shared_from_this;

    template <typename U, bool esft>
    friend struct shared_ptr_make_helper;

    template <typename U>
    friend class shared_ptr;
};

template <typename U, bool esft>
struct shared_ptr_make_helper;

template <typename T>
struct shared_ptr_make_helper<T, false> {
    template <typename... A>
    static shared_ptr<T> make(A&&... a) {
        return shared_ptr<T>(new shared_ptr_count_for<T>(std::forward<A>(a)...));
    }
};

template <typename T>
struct shared_ptr_make_helper<T, true> {
    template <typename... A>
    static shared_ptr<T> make(A&&... a) {
        auto p = new T(std::forward<A>(a)...);
        return shared_ptr<T>(p, p);
    }
};

template <typename T, typename... A>
inline
shared_ptr<T>
make_shared(A&&... a) {
    using helper = shared_ptr_make_helper<T, std::is_base_of<shared_ptr_count_base, T>::value>;
    return helper::make(std::forward<A>(a)...);
}

template <typename T>
inline
shared_ptr<T>
make_shared(T&& a) {
    using helper = shared_ptr_make_helper<T, std::is_base_of<shared_ptr_count_base, T>::value>;
    return helper::make(std::forward<T>(a));
}

template <typename T, typename U>
inline
shared_ptr<T>
static_pointer_cast(const shared_ptr<U>& p) {
    return shared_ptr<T>(p._b, static_cast<T*>(p._p));
}

template <typename T, typename U>
inline
shared_ptr<T>
dynamic_pointer_cast(const shared_ptr<U>& p) {
    auto q = dynamic_cast<T*>(p._p);
    return shared_ptr<T>(q ? p._b : nullptr, q);
}

template <typename T, typename U>
inline
shared_ptr<T>
const_pointer_cast(const shared_ptr<U>& p) {
    return shared_ptr<T>(p._b, const_cast<T*>(p._p));
}

template <typename T>
inline
shared_ptr<T>
enable_shared_from_this<T>::shared_from_this() {
    auto unconst = reinterpret_cast<enable_shared_from_this<std::remove_const_t<T>>*>(this);
    return shared_ptr<T>(unconst);
}

template <typename T>
inline
shared_ptr<const T>
enable_shared_from_this<T>::shared_from_this() const {
    auto esft = const_cast<enable_shared_from_this*>(this);
    auto unconst = reinterpret_cast<enable_shared_from_this<std::remove_const_t<T>>*>(esft);
    return shared_ptr<const T>(unconst);
}

template <typename T, typename U>
inline
bool
operator==(const shared_ptr<T>& x, const shared_ptr<U>& y) {
    return x.get() == y.get();
}

template <typename T>
inline
bool
operator==(const shared_ptr<T>& x, std::nullptr_t) {
    return x.get() == nullptr;
}

template <typename T>
inline
bool
operator==(std::nullptr_t, const shared_ptr<T>& y) {
    return nullptr == y.get();
}

template <typename T, typename U>
inline
bool
operator!=(const shared_ptr<T>& x, const shared_ptr<U>& y) {
    return x.get() != y.get();
}

template <typename T>
inline
bool
operator!=(const shared_ptr<T>& x, std::nullptr_t) {
    return x.get() != nullptr;
}

template <typename T>
inline
bool
operator!=(std::nullptr_t, const shared_ptr<T>& y) {
    return nullptr != y.get();
}

template <typename T, typename U>
inline
bool
operator<(const shared_ptr<T>& x, const shared_ptr<U>& y) {
    return x.get() < y.get();
}

template <typename T>
inline
bool
operator<(const shared_ptr<T>& x, std::nullptr_t) {
    return x.get() < nullptr;
}

template <typename T>
inline
bool
operator<(std::nullptr_t, const shared_ptr<T>& y) {
    return nullptr < y.get();
}

template <typename T, typename U>
inline
bool
operator<=(const shared_ptr<T>& x, const shared_ptr<U>& y) {
    return x.get() <= y.get();
}

template <typename T>
inline
bool
operator<=(const shared_ptr<T>& x, std::nullptr_t) {
    return x.get() <= nullptr;
}

template <typename T>
inline
bool
operator<=(std::nullptr_t, const shared_ptr<T>& y) {
    return nullptr <= y.get();
}

template <typename T, typename U>
inline
bool
operator>(const shared_ptr<T>& x, const shared_ptr<U>& y) {
    return x.get() > y.get();
}

template <typename T>
inline
bool
operator>(const shared_ptr<T>& x, std::nullptr_t) {
    return x.get() > nullptr;
}

template <typename T>
inline
bool
operator>(std::nullptr_t, const shared_ptr<T>& y) {
    return nullptr > y.get();
}

template <typename T, typename U>
inline
bool
operator>=(const shared_ptr<T>& x, const shared_ptr<U>& y) {
    return x.get() >= y.get();
}

template <typename T>
inline
bool
operator>=(const shared_ptr<T>& x, std::nullptr_t) {
    return x.get() >= nullptr;
}

template <typename T>
inline
bool
operator>=(std::nullptr_t, const shared_ptr<T>& y) {
    return nullptr >= y.get();
}

template <typename T>
static inline
std::ostream& operator<<(std::ostream& out, const shared_ptr<T>& p) {
    if (!p) {
        return out << "null";
    }
    return out << *p;
}

template<typename T>
struct shared_ptr_equal_by_value {
    bool operator()(const shared_ptr<T>& i1, const shared_ptr<T>& i2) const {
        if (bool(i1) ^ bool(i2)) {
            return false;
        }
        return !i1 || *i1 == *i2;
    }
};

template<typename T>
struct shared_ptr_value_hash {
    size_t operator()(const shared_ptr<T>& p) const {
        if (p) {
            return std::hash<T>()(*p);
        }
        return 0;
    }
};

namespace std {

template <typename T>
struct hash<lw_shared_ptr<T>> : private hash<T*> {
    size_t operator()(const lw_shared_ptr<T>& p) const {
        return hash<T*>::operator()(p.get());
    }
};

template <typename T>
struct hash<::shared_ptr<T>> : private hash<T*> {
    size_t operator()(const ::shared_ptr<T>& p) const {
        return hash<T*>::operator()(p.get());
    }
};

}

#endif /* SHARED_PTR_HH_ */