cow_wrapper.hxx

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#ifndef INCLUDED_O3TL_COW_WRAPPER_HXX
#define INCLUDED_O3TL_COW_WRAPPER_HXX

#include <memory>
#include <osl/interlck.h>

#include <utility>
#include <cstddef>

namespace o3tl
{
    struct UnsafeRefCountingPolicy
    {
        typedef std::size_t ref_count_t;
        static void incrementCount( ref_count_t& rCount ) { ++rCount; }
        static bool decrementCount( ref_count_t& rCount ) { return --rCount != 0; }
    };

    struct ThreadSafeRefCountingPolicy
    {
        typedef oslInterlockedCount ref_count_t;
        static void incrementCount( ref_count_t& rCount ) { osl_atomic_increment(&rCount); }
        static bool decrementCount( ref_count_t& rCount )
        {
            if( rCount == 1 ) // caller is already the only/last reference
                return false;
            else
                return osl_atomic_decrement(&rCount) != 0;
        }
    };

    template<typename T, class MTPolicy=UnsafeRefCountingPolicy> class cow_wrapper
    {
        struct impl_t
        {
            impl_t(const impl_t&) = delete;
            impl_t& operator=(const impl_t&) = delete;

            impl_t() :
                m_value(),
                m_ref_count(1)
            {
            }

            explicit impl_t( const T& v ) :
                m_value(v),
                m_ref_count(1)
            {
            }

            explicit impl_t( T&& v ) :
                m_value(std::move(v)),
                m_ref_count(1)
            {
            }

            T                              m_value;
            typename MTPolicy::ref_count_t m_ref_count;
        };

        void release()
        {
            if( m_pimpl && !MTPolicy::decrementCount(m_pimpl->m_ref_count) )
            {
                delete m_pimpl;
                m_pimpl = nullptr;
            }
        }

    public:
        typedef T        value_type;
        typedef T*       pointer;
        typedef const T* const_pointer;
        typedef MTPolicy mt_policy;

        cow_wrapper() :
            m_pimpl( new impl_t() )
        {
        }

        explicit cow_wrapper( const value_type& r ) :
            m_pimpl( new impl_t(r) )
        {
        }

        explicit cow_wrapper( value_type&& r ) :
            m_pimpl( new impl_t(std::move(r)) )
        {
        }

        explicit cow_wrapper( const cow_wrapper& rSrc ) : // nothrow
            m_pimpl( rSrc.m_pimpl )
        {
            MTPolicy::incrementCount( m_pimpl->m_ref_count );
        }

        explicit cow_wrapper( cow_wrapper&& rSrc ) noexcept :
            m_pimpl( rSrc.m_pimpl )
        {
            rSrc.m_pimpl = nullptr;
        }

        ~cow_wrapper() // nothrow, if ~T does not throw
        {
            release();
        }

        cow_wrapper& operator=( const cow_wrapper& rSrc ) // nothrow
        {
            // this already guards against self-assignment
            MTPolicy::incrementCount( rSrc.m_pimpl->m_ref_count );

            release();
            m_pimpl = rSrc.m_pimpl;

            return *this;
        }

        cow_wrapper& operator=(cow_wrapper&& rSrc) noexcept
        {
            // self-movement guts ourself, see also 17.6.4.9
            release();
            m_pimpl = rSrc.m_pimpl;

            rSrc.m_pimpl = nullptr;

            return *this;
        }

        value_type& make_unique()
        {
            if( m_pimpl->m_ref_count > 1 )
            {
                impl_t* pimpl = new impl_t(m_pimpl->m_value);
                release();
                m_pimpl = pimpl;
            }

            return m_pimpl->m_value;
        }

        bool is_unique() const // nothrow
        {
            return !m_pimpl || m_pimpl->m_ref_count == 1;
        }

        typename MTPolicy::ref_count_t use_count() const // nothrow
        {
            return m_pimpl ? m_pimpl->m_ref_count : 0;
        }

        void swap(cow_wrapper& r) // never throws
        {
            std::swap(m_pimpl, r.m_pimpl);
        }

        pointer           operator->()       { return &make_unique(); }
        value_type&       operator*()        { return make_unique(); }
        const_pointer     operator->() const { return &m_pimpl->m_value; }
        const value_type& operator*()  const { return m_pimpl->m_value; }

        pointer           get()       { return &make_unique(); }
        const_pointer     get() const { return &m_pimpl->m_value; }

        bool              same_object( const cow_wrapper& rOther ) const
        {
            return rOther.m_pimpl == m_pimpl;
        }

    private:
        impl_t* m_pimpl;
    };


    template<class T, class P> inline bool operator==( const cow_wrapper<T,P>& a,
                                                       const cow_wrapper<T,P>& b )
    {
        return a.same_object(b) || *a == *b;
    }

    template<class T, class P> inline bool operator!=( const cow_wrapper<T,P>& a,
                                                       const cow_wrapper<T,P>& b )
    {
        return !a.same_object(b) && *a != *b;
    }

    template<class A, class B, class P> inline bool operator<( const cow_wrapper<A,P>& a,
                                                               const cow_wrapper<B,P>& b )
    {
        return *a < *b;
    }

    template<class T, class P> inline void swap( cow_wrapper<T,P>& a,
                                                 cow_wrapper<T,P>& b )
    {
        a.swap(b);
    }

}

#endif /* INCLUDED_O3TL_COW_WRAPPER_HXX */

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