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Diffstat (limited to 'include/o3tl/cow_wrapper.hxx')
| -rw-r--r-- | include/o3tl/cow_wrapper.hxx | 387 |
1 files changed, 387 insertions, 0 deletions
diff --git a/include/o3tl/cow_wrapper.hxx b/include/o3tl/cow_wrapper.hxx new file mode 100644 index 0000000000..cc823f60ec --- /dev/null +++ b/include/o3tl/cow_wrapper.hxx @@ -0,0 +1,387 @@ +/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */ +/* + * This file is part of the LibreOffice project. + * + * This Source Code Form is subject to the terms of the Mozilla Public + * License, v. 2.0. If a copy of the MPL was not distributed with this + * file, You can obtain one at http://mozilla.org/MPL/2.0/. + * + * This file incorporates work covered by the following license notice: + * + * Licensed to the Apache Software Foundation (ASF) under one or more + * contributor license agreements. See the NOTICE file distributed + * with this work for additional information regarding copyright + * ownership. The ASF licenses this file to you under the Apache + * License, Version 2.0 (the "License"); 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 . + */ + +#ifndef INCLUDED_O3TL_COW_WRAPPER_HXX +#define INCLUDED_O3TL_COW_WRAPPER_HXX + +#include <osl/interlck.h> + +#include <optional> +#include <cstddef> + +namespace o3tl +{ + /** Thread-unsafe refcounting + + This is the default locking policy for cow_wrapper. No + locking/guarding against concurrent access is performed + whatsoever. + */ + 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; } + }; + + /** Thread-safe refcounting + + Use this to have the cow_wrapper refcounting mechanisms employ + the thread-safe oslInterlockedCount . + */ + 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 ) + { + return osl_atomic_decrement(&rCount) != 0; + } + }; + + /** Copy-on-write wrapper. + + This template provides copy-on-write semantics for the wrapped + type: when copying, the operation is performed shallow, + i.e. different cow_wrapper objects share the same underlying + instance. Only when accessing the underlying object via + non-const methods, a unique copy is provided. + + The type parameter <code>T</code> must satisfy the following + requirements: it must be default-constructible, copyable (it + need not be assignable), and be of non-reference type. Note + that, despite the fact that this template provides access to + the wrapped type via pointer-like methods + (<code>operator->()</code> and <code>operator*()</code>), it does + <em>not</em> work like e.g. the std smart pointer wrappers + (shared_ptr, unique_ptr, etc.). Internally, the cow_wrapper + holds a by-value instance of the wrapped object. This is to + avoid one additional heap allocation, and providing access via + <code>operator->()</code>/<code>operator*()</code> is because + <code>operator.()</code> cannot be overridden. + + Regarding thread safety: this wrapper is <em>not</em> + thread-safe per se, because cow_wrapper has no way of + synchronizing the potentially many different cow_wrapper + instances, that reference a single shared value_type + instance. That said, when passing + <code>ThreadSafeRefCountingPolicy</code> as the + <code>MTPolicy</code> parameter, accessing a thread-safe + pointee through multiple cow_wrapper instances might be + thread-safe, if the individual pointee methods are + thread-safe, <em>including</em> pointee's copy + constructor. Any wrapped object that needs external + synchronisation (e.g. via an external mutex, which arbitrates + access to object methods, and can be held across multiple + object method calls) cannot easily be dealt with in a + thread-safe way, because, as noted, objects are shared behind + the client's back. + + @attention if one wants to use the pimpl idiom together with + cow_wrapper (i.e. put an opaque type into the cow_wrapper), + then <em>all<em> methods in the surrounding class needs to be + non-inline (<em>including</em> destructor, copy constructor + and assignment operator). + + @example + <pre> +class cow_wrapper_client_impl; + +class cow_wrapper_client +{ +public: + cow_wrapper_client(); + cow_wrapper_client( const cow_wrapper_client& ); + cow_wrapper_client( cow_wrapper_client&& ); + ~cow_wrapper_client(); + + cow_wrapper_client& operator=( const cow_wrapper_client& ); + cow_wrapper_client& operator=( cow_wrapper_client&& ); + + void modify( int nVal ); + int queryUnmodified() const; + +private: + o3tl::cow_wrapper< cow_wrapper_client_impl > maImpl; +}; + </pre> + and the implementation file would look like this: + <pre> +class cow_wrapper_client_impl +{ +public: + void setValue( int nVal ) { mnValue = nVal; } + int getValue() const { return mnValue; } + +private: + int mnValue; +} + +cow_wrapper_client::cow_wrapper_client() : + maImpl() +{ +} +cow_wrapper_client::cow_wrapper_client( const cow_wrapper_client& rSrc ) : + maImpl( rSrc.maImpl ) +{ +} +cow_wrapper_client::cow_wrapper_client( cow_wrapper_client& rSrc ) : + maImpl( std::move( rSrc.maImpl ) ) +{ +} +cow_wrapper_client::~cow_wrapper_client() +{ +} +cow_wrapper_client& cow_wrapper_client::operator=( const cow_wrapper_client& rSrc ) +{ + maImpl = rSrc.maImpl; + return *this; +} +cow_wrapper_client& cow_wrapper_client::operator=( cow_wrapper_client&& rSrc ) +{ + maImpl = std::move( rSrc.maImpl ); + return *this; +} +void cow_wrapper_client::modify( int nVal ) +{ + maImpl->setValue( nVal ); +} +int cow_wrapper_client::queryUnmodified() const +{ + return maImpl->getValue(); +} + </pre> + */ + template<typename T, class MTPolicy=UnsafeRefCountingPolicy> class cow_wrapper + { + /** shared value object - gets cloned before cow_wrapper hands + out a non-const reference to it + */ + 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; + + /** Default-construct wrapped type instance + */ + cow_wrapper() : + m_pimpl( new impl_t() ) + { + } + + /** Copy-construct wrapped type instance from given object + */ + explicit cow_wrapper( const value_type& r ) : + m_pimpl( new impl_t(r) ) + { + } + + /** Move-construct wrapped type instance from given object + */ + explicit cow_wrapper( value_type&& r ) : + m_pimpl( new impl_t(std::move(r)) ) + { + } + + /** Shallow-copy given cow_wrapper + */ + explicit cow_wrapper( const cow_wrapper& rSrc ) : // nothrow + m_pimpl( rSrc.m_pimpl ) + { + MTPolicy::incrementCount( m_pimpl->m_ref_count ); + } + + /** Move-construct and steal rSrc shared resource + */ + explicit cow_wrapper( cow_wrapper&& rSrc ) noexcept : + m_pimpl( rSrc.m_pimpl ) + { + rSrc.m_pimpl = nullptr; + } + + // Only intended to be used by std::optional specialisations + explicit cow_wrapper( std::nullopt_t ) noexcept : + m_pimpl( nullptr ) + { + } + + // Only intended to be used by std::optional specialisations + explicit cow_wrapper( const cow_wrapper& rSrc, std::nullopt_t ) : // nothrow + m_pimpl( rSrc.m_pimpl ) + { + if (m_pimpl) + MTPolicy::incrementCount( m_pimpl->m_ref_count ); + } + + ~cow_wrapper() // nothrow, if ~T does not throw + { + release(); + } + + /// now sharing rSrc cow_wrapper instance with us + 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; + } + + /// stealing rSrc's resource + 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; + } + + /// unshare with any other cow_wrapper instance + 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; + } + + /// true, if not shared with any other cow_wrapper instance + bool is_unique() const // nothrow + { + return !m_pimpl || m_pimpl->m_ref_count == 1; + } + + /// return number of shared instances (1 for unique object) + 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; } + + /// true, if both cow_wrapper internally share the same object + bool same_object( const cow_wrapper& rOther ) const + { + return rOther.m_pimpl == m_pimpl; + } + + // Only intended to be used by std::optional specialisations + bool empty() const { return m_pimpl == nullptr; } + // Only intended to be used by std::optional specialisations + void set_empty() + { + if (m_pimpl) + { + release(); + m_pimpl = nullptr; + } + } + + 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 */ + +/* vim:set shiftwidth=4 softtabstop=4 expandtab: */ |