LibreOffice Module o3tl (master)  1
/home/tdf/lode/jenkins/workspace/lo_tb_master_linux_dbg/include/o3tl/cow_wrapper.hxx

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 T 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 (operator->() and operator*()), it does not 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 operator->()/operator*() is because operator.() cannot be overridden.

Regarding thread safety: this wrapper is not 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 ThreadSafeRefCountingPolicy as the MTPolicy parameter, accessing a thread-safe pointee through multiple cow_wrapper instances might be thread-safe, if the individual pointee methods are thread-safe, including 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 all methods in the surrounding class needs to be non-inline (including destructor, copy constructor and assignment operator).
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;
};
        
        and the implementation file would look like this:
        

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();
}
        

/* -*- 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 <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() :
{
}
explicit impl_t( const T& v ) :
m_value(v),
{
}
explicit impl_t( T&& v ) :
m_value(std::move(v)),
{
}
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;
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
{
}
cow_wrapper& operator=( const cow_wrapper& rSrc ) // nothrow
{
// this already guards against self-assignment
MTPolicy::incrementCount( rSrc.m_pimpl->m_ref_count );
m_pimpl = rSrc.m_pimpl;
return *this;
}
cow_wrapper& operator=(cow_wrapper&& rSrc) noexcept
{
// self-movement guts ourself, see also 17.6.4.9
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);
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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