ClickHouse/src/Common/COW.h

#pragma once

#include <boost/smart_ptr/intrusive_ptr.hpp>
#include <boost/smart_ptr/intrusive_ref_counter.hpp>
#include <initializer_list>


/** Copy-on-write shared ptr.
  * Allows to work with shared immutable objects and sometimes unshare and mutate you own unique copy.
  *
  * Usage:

    class Column : public COW<Column>
    {
    private:
        friend class COW<Column>;

        /// Leave all constructors in private section. They will be available through 'create' method.
        Column();

        /// Provide 'clone' method. It can be virtual if you want polymorphic behaviour.
        virtual Column * clone() const;
    public:
        /// Correctly use const qualifiers in your interface.

        virtual ~Column() {}
    };

  * It will provide 'create' and 'mutate' methods.
  * And 'Ptr' and 'MutablePtr' types.
  * Ptr is refcounted pointer to immutable object.
  * MutablePtr is refcounted noncopyable pointer to mutable object.
  * MutablePtr can be assigned to Ptr through move assignment.
  *
  * 'create' method creates MutablePtr: you cannot share mutable objects.
  * To share, move-assign to immutable pointer.
  * 'mutate' method allows to create mutable noncopyable object from immutable object:
  *   either by cloning or by using directly, if it is not shared.
  * These methods are thread-safe.
  *
  * Example:
  *
    /// Creating and assigning to immutable ptr.
    Column::Ptr x = Column::create(1);
    /// Sharing single immutable object in two ptrs.
    Column::Ptr y = x;

    /// Now x and y are shared.

    /// Change value of x.
    {
        /// Creating mutable ptr. It can clone an object under the hood if it was shared.
        Column::MutablePtr mutate_x = IColumn::mutate(std::move(x));
        /// Using non-const methods of an object.
        mutate_x->set(2);
        /// Assigning pointer 'x' to mutated object.
        x = std::move(mutate_x);
    }

    /// Now x and y are unshared and have different values.

  * Note. You may have heard that COW is bad practice.
  * Actually it is, if your values are small or if copying is done implicitly.
  * This is the case for string implementations.
  *
  * In contrast, COW is intended for the cases when you need to share states of large objects,
  * (when you usually will use std::shared_ptr) but you also want precise control over modification
  * of this shared state.
  *
  * Caveats:
  * - after a call to 'mutate' method, you can still have a reference to immutable ptr somewhere.
  * - as 'mutable_ptr' should be unique, it's refcount is redundant - probably it would be better
  *   to use std::unique_ptr for it somehow.
  */
template <typename Derived>
class COW : public boost::intrusive_ref_counter<Derived>
{
private:
    Derived * derived() { return static_cast<Derived *>(this); }
    const Derived * derived() const { return static_cast<const Derived *>(this); }

protected:
    template <typename T>
    class mutable_ptr : public boost::intrusive_ptr<T>
    {
    private:
        using Base = boost::intrusive_ptr<T>;

        template <typename> friend class COW;
        template <typename, typename> friend class COWHelper;

        explicit mutable_ptr(T * ptr) : Base(ptr) {}

    public:
        /// Copy: not possible.
        mutable_ptr(const mutable_ptr &) = delete;

        /// Move: ok.
        mutable_ptr(mutable_ptr &&) = default;
        mutable_ptr & operator=(mutable_ptr &&) = default;

        /// Initializing from temporary of compatible type.
        template <typename U>
        mutable_ptr(mutable_ptr<U> && other) : Base(std::move(other)) {}

        mutable_ptr() = default;

        mutable_ptr(std::nullptr_t) {}
    };

public:
    using MutablePtr = mutable_ptr<Derived>;

protected:
    template <typename T>
    class immutable_ptr : public boost::intrusive_ptr<const T>
    {
    private:
        using Base = boost::intrusive_ptr<const T>;

        template <typename> friend class COW;
        template <typename, typename> friend class COWHelper;

        explicit immutable_ptr(const T * ptr) : Base(ptr) {}

    public:
        /// Copy from immutable ptr: ok.
        immutable_ptr(const immutable_ptr &) = default;
        immutable_ptr & operator=(const immutable_ptr &) = default;

        template <typename U>
        immutable_ptr(const immutable_ptr<U> & other) : Base(other) {}

        /// Move: ok.
        immutable_ptr(immutable_ptr &&) = default;
        immutable_ptr & operator=(immutable_ptr &&) = default;

        /// Initializing from temporary of compatible type.
        template <typename U>
        immutable_ptr(immutable_ptr<U> && other) : Base(std::move(other)) {}

        /// Move from mutable ptr: ok.
        template <typename U>
        immutable_ptr(mutable_ptr<U> && other) : Base(std::move(other)) {}

        /// Copy from mutable ptr: not possible.
        template <typename U>
        immutable_ptr(const mutable_ptr<U> &) = delete;

        immutable_ptr() = default;

        immutable_ptr(std::nullptr_t) {}
    };

public:
    using Ptr = immutable_ptr<Derived>;

    template <typename... Args>
    static MutablePtr create(Args &&... args) { return MutablePtr(new Derived(std::forward<Args>(args)...)); }

    template <typename T>
    static MutablePtr create(std::initializer_list<T> && arg) { return create(std::forward<std::initializer_list<T>>(arg)); }

public:
    Ptr getPtr() const { return static_cast<Ptr>(derived()); }
    MutablePtr getPtr() { return static_cast<MutablePtr>(derived()); }

protected:
    MutablePtr shallowMutate() const
    {
        if (this->use_count() > 1)
            return derived()->clone();
        else
            return assumeMutable();
    }

public:
    static MutablePtr mutate(Ptr ptr)
    {
        return ptr.shallowMutate();
    }

    MutablePtr assumeMutable() const
    {
        return const_cast<COW*>(this)->getPtr();
    }

    Derived & assumeMutableRef() const
    {
        return const_cast<Derived &>(*derived());
    }

protected:
    /// It works as immutable_ptr if it is const and as mutable_ptr if it is non const.
    template <typename T>
    class chameleon_ptr
    {
    private:
        immutable_ptr<T> value;

    public:
        template <typename... Args>
        chameleon_ptr(Args &&... args) : value(std::forward<Args>(args)...) {}

        template <typename U>
        chameleon_ptr(std::initializer_list<U> && arg) : value(std::forward<std::initializer_list<U>>(arg)) {}

        const T * get() const { return value.get(); }
        T * get() { return &value->assumeMutableRef(); }

        const T * operator->() const { return get(); }
        T * operator->() { return get(); }

        const T & operator*() const { return *value; }
        T & operator*() { return value->assumeMutableRef(); }

        operator const immutable_ptr<T> & () const { return value; }
        operator immutable_ptr<T> & () { return value; }

        operator bool() const { return value != nullptr; }
        bool operator! () const { return value == nullptr; }

        bool operator== (const chameleon_ptr & rhs) const { return value == rhs.value; }
        bool operator!= (const chameleon_ptr & rhs) const { return value != rhs.value; }
    };

public:
    /** Use this type in class members for compositions.
      *
      * NOTE:
      * For classes with WrappedPtr members,
      * you must reimplement 'mutate' method, so it will call 'mutate' of all subobjects (do deep mutate).
      * It will guarantee, that mutable object have all subobjects unshared.
      *
      * NOTE:
      * If you override 'mutate' method in inherited classes, don't forget to make it virtual in base class or to make it call a virtual method.
      * (COW itself doesn't force any methods to be virtual).
      *
      * See example in "cow_compositions.cpp".
      */
    using WrappedPtr = chameleon_ptr<Derived>;
};


/** Helper class to support inheritance.
  * Example:
  *
  * class IColumn : public COW<IColumn>
  * {
  *     friend class COW<IColumn>;
  *     virtual MutablePtr clone() const = 0;
  *     virtual ~IColumn() {}
  * };
  *
  * class ConcreteColumn : public COWHelper<IColumn, ConcreteColumn>
  * {
  *     friend class COWHelper<IColumn, ConcreteColumn>;
  * };
  *
  * Here is complete inheritance diagram:
  *
  * ConcreteColumn
  *  COWHelper<IColumn, ConcreteColumn>
  *   IColumn
  *    CowPtr<IColumn>
  *     boost::intrusive_ref_counter<IColumn>
  *
  * See example in "cow_columns.cpp".
  */
template <typename Base, typename Derived>
class COWHelper : public Base
{
private:
    Derived * derived() { return static_cast<Derived *>(this); }
    const Derived * derived() const { return static_cast<const Derived *>(this); }

public:
    using Ptr = typename Base::template immutable_ptr<Derived>;
    using MutablePtr = typename Base::template mutable_ptr<Derived>;

    template <typename... Args>
    static MutablePtr create(Args &&... args) { return MutablePtr(new Derived(std::forward<Args>(args)...)); }

    template <typename T>
    static MutablePtr create(std::initializer_list<T> && arg) { return MutablePtr(new Derived(std::forward<std::initializer_list<T>>(arg))); }

    typename Base::MutablePtr clone() const override { return typename Base::MutablePtr(new Derived(*derived())); }

protected:
    MutablePtr shallowMutate() const { return MutablePtr(static_cast<Derived *>(Base::shallowMutate().get())); }
};