How can boost::serialization be used with std::shared_ptr from C++11?

Question

I know that there is a Boost module for serialization of boost::shared_ptr, but I cannot find anything for std::shared_ptr.

Also, I don\'t know how to implement it e

Answer 1

Recent versions of Boost Serialization include support for all standard library smart pointers.

Answer 2

This is improvement of denim's solution, which supports loading shared_ptr which points to the same memory, but with different types. This problem can appear when archive contains at the same time shared_ptr and shared_ptr which are pointing to the same object, where A is inherited from B.

namespace boost {
namespace serialization {

    template<class Archive, class Type>
    void save(Archive & archive, const std::shared_ptr<Type> & value, const unsigned int /*version*/)
    {
        Type *data = value.get();
        archive << data;
    }

    static std::map<void*, std::weak_ptr<void>> hash;

    template<class Archive, class Type>
    void load(Archive & archive, std::shared_ptr<Type> & value, const unsigned int /*version*/)
    {
        Type *data;
        archive >> data;

        if (hash[data].expired())
        {
            std::shared_ptr<void> ptr(data);
            value = static_pointer_cast<Type>(ptr);
            hash[data] = ptr;
        }
        else value = static_pointer_cast<Type>(hash[data].lock());
    }

    template<class Archive, class Type>
    inline void serialize(Archive & archive, std::shared_ptr<Type> & value, const unsigned int version)
    {
        split_free(archive, value, version);
    }

}}

As a weakness of this realization - one massive map.

Answer 3

You haven't said what "doesn't work" means; it doesn't compile? It doesn't load/store the value properly? It doesn't..what?

There are two problems I can identify here, one may be part of your intentional design though.

The first, you have not made a correct pointer in the load procedure. Let's break it down:

inline void serialize(Archive & ar, std::shared_ptr<T> &t, const unsigned int version) {
    if (1) { //unimportant
        T* r;
        ar >> r;
        t = r;
    }
}

When you make an object of std::shared_ptr, you are instantiating a class template to provide pointer-like capability (as you know). If you made with an int, it will work as an int pointer. However, simply passing the type as T does NOT mean a pointer created of that type will automatically use that template; indeed, you're creating a bare pointer with T* r. It may as well be int *r. You then fail to initialize it with new; r could be pointing anywhere. If it were intialized properly with a new, you MAY get correct reference counting for creation/deletion of that object; this is one area where std::shared_ptr doesn't seem worth the effort to me. I think the assignment from a bare pointer counts as the second reference, not the first, but I may be wrong? Anyhow, that's not the problem. You're probably corrupting the heap; a compiler should spit out a warning about using an uninitialized pointer, it's a wonder it hasn't. I hope you don't have warnings turned off.

If I remember correctly, that declaration of r needs to be replaced with:

std::shared_ptr<T> r = new std::shared_ptr<T>;

Although it may be

std::shared_ptr<T> r = new std::shared_ptr<T>(r());

I haven't used shared_ptr for a while.

TR1, by the way, has been out for at least 2 years. It is based off of boost's shared_ptr. I don't know why you're using Boost 1.46, but I think that it was out by the time shared_ptr became part of the standard? So it should be compatible...?

Anyhow, the second potential error comes with

t = r;

I'm assuming - incorrectly? - that you WISH to decrement the reference count to t by reassigning it (and possibly destroying the object t points to). If you meant to copy it, you would of course use:

*t = *r;

and make sure your copy constructor works properly.

Answer 4

This is the result of rolling your own based on the boost shared pointer header e.g. based on <boost/serialization/shared_ptr.hpp>.

Just copy & paste below into a header file and include it:

#ifndef BOOST_SERIALIZATION_STD_SHARED_PTR_HPP
#define BOOST_SERIALIZATION_STD_SHARED_PTR_HPP

// MS compatible compilers support #pragma once
#if defined(_MSC_VER) && (_MSC_VER >= 1020)
# pragma once
#endif

/////////1/////////2/////////3/////////4/////////5/////////6/////////7/////////8
// shared_ptr.hpp: serialization for boost shared pointer

// (C) Copyright 2004 Robert Ramey and Martin Ecker
// Use, modification and distribution is subject to the Boost Software
// License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)

//  See http://www.boost.org for updates, documentation, and revision history.

#include <cstddef> // NULL

#include <boost/config.hpp>
#include <boost/mpl/integral_c.hpp>
#include <boost/mpl/integral_c_tag.hpp>

#include <boost/detail/workaround.hpp>
#include <memory>

#include <boost/serialization/split_free.hpp>
#include <boost/serialization/nvp.hpp>
#include <boost/serialization/version.hpp>
#include <boost/serialization/tracking.hpp>

/////////1/////////2/////////3/////////4/////////5/////////6/////////7/////////8
// shared_ptr serialization traits
// version 1 to distinguish from boost 1.32 version. Note: we can only do this
// for a template when the compiler supports partial template specialization

#ifndef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION
    namespace boost {
    namespace serialization{
        template<class T>
        struct version< ::std::shared_ptr< T > > {
            typedef mpl::integral_c_tag tag;
            #if BOOST_WORKAROUND(__MWERKS__, BOOST_TESTED_AT(0x3206))
            typedef BOOST_DEDUCED_TYPENAME mpl::int_<1> type;
            #else
            typedef mpl::int_<1> type;
            #endif
            #if BOOST_WORKAROUND(__BORLANDC__, BOOST_TESTED_AT(0x570))
            BOOST_STATIC_CONSTANT(int, value = 1);
            #else
            BOOST_STATIC_CONSTANT(int, value = type::value);
            #endif
        };
        // don't track shared pointers
        template<class T>
        struct tracking_level< ::std::shared_ptr< T > > {
            typedef mpl::integral_c_tag tag;
            #if BOOST_WORKAROUND(__MWERKS__, BOOST_TESTED_AT(0x3206))
            typedef BOOST_DEDUCED_TYPENAME mpl::int_< ::boost::serialization::track_never> type;
            #else
            typedef mpl::int_< ::boost::serialization::track_never> type;
            #endif
            #if BOOST_WORKAROUND(__BORLANDC__, BOOST_TESTED_AT(0x570))
            BOOST_STATIC_CONSTANT(int, value = ::boost::serialization::track_never);
            #else
            BOOST_STATIC_CONSTANT(int, value = type::value);
            #endif
        };
    }}
    #define BOOST_SERIALIZATION_SHARED_PTR(T)
#else
    // define macro to let users of these compilers do this
    #define BOOST_SERIALIZATION_SHARED_PTR(T)                         \
    BOOST_CLASS_VERSION(                                              \
        ::std::shared_ptr< T >,                                     \
        1                                                             \
    )                                                                 \
    BOOST_CLASS_TRACKING(                                             \
        ::std::shared_ptr< T >,                                     \
        ::boost::serialization::track_never                           \
    )                                                                 \
    /**/
#endif

namespace boost {
namespace serialization{

#ifndef BOOST_SERIALIZATION_SHARED_PTR_HPP
struct null_deleter {
    void operator()(void const *) const {}
};
#endif

/////////1/////////2/////////3/////////4/////////5/////////6/////////7/////////8
// serialization for shared_ptr

template<class Archive, class T>
inline void save(
    Archive & ar,
    const std::shared_ptr< T > &t,
    const unsigned int /* file_version */
){
    // The most common cause of trapping here would be serializing
    // something like shared_ptr<int>.  This occurs because int
    // is never tracked by default.  Wrap int in a trackable type
    BOOST_STATIC_ASSERT((tracking_level< T >::value != track_never));
    const T * t_ptr = t.get();
    ar << boost::serialization::make_nvp("px", t_ptr);
}

template<class Archive, class T>
inline void load(
    Archive & ar,
    std::shared_ptr< T > &t,
    const unsigned int /*file_version*/
){
    // The most common cause of trapping here would be serializing
    // something like shared_ptr<int>.  This occurs because int
    // is never tracked by default.  Wrap int in a trackable type
    BOOST_STATIC_ASSERT((tracking_level< T >::value != track_never));
    T* r;
    ar >> boost::serialization::make_nvp("px", r);
    ar.reset(t,r);
}

template<class Archive, class T>
inline void serialize(
    Archive & ar,
    std::shared_ptr< T > &t,
    const unsigned int file_version
){
    // correct shared_ptr serialization depends upon object tracking
    // being used.
    BOOST_STATIC_ASSERT(
        boost::serialization::tracking_level< T >::value
        != boost::serialization::track_never
    );
    boost::serialization::split_free(ar, t, file_version);
}

} // namespace serialization
} // namespace boost

#endif // BOOST_SERIALIZATION_STD_SHARED_PTR_HPP

You can view the differences to <boost/serialization/shared_ptr.hpp> here

Basically,

• renamed include guard
• changed boost::shared_ptr to std::shared_ptr
• included <memory> instead of <boost/shared_ptr.hpp>
• protected null_deleter from redefinition in case you also use boost::shared_ptr
• deleted BOOST_SERIALIZATION_SHARED_PTR_132_HPP - whatever that is about?

So far, this seems to be working just fine.

Answer 5

Serialisation is provided by boost and not by the standard library and although shared_ptr is included in the standard it is part of TR1 (technical report 1).

TR1 as of now does not have serialization. So I would recommend that you use boost's shared pointer.

Answer 6

I finally found a solution on how to serialize the std::shared_ptr using boost serialization. All you need is the following piece of code (explanation follows):

#include <boost/serialization/split_free.hpp>
#include <boost/unordered_map.hpp>

//---/ Wrapper for std::shared_ptr<> /------------------------------------------

namespace boost { namespace serialization {

template<class Archive, class Type>
void save(Archive & archive, const std::shared_ptr<Type> & value, const unsigned int /*version*/)
{
    Type *data = value.get();
    archive << data;
}

template<class Archive, class Type>
void load(Archive & archive, std::shared_ptr<Type> & value, const unsigned int /*version*/)
{
    Type *data;
    archive >> data;

    typedef std::weak_ptr<Type> WeakPtr;
    static boost::unordered_map<void*, WeakPtr> hash;

    if (hash[data].expired())
    {
         value = std::shared_ptr<Type>(data);
         hash[data] = value;
    }
    else value = hash[data].lock();
}

template<class Archive, class Type>
inline void serialize(Archive & archive, std::shared_ptr<Type> & value, const unsigned int version)
{
    split_free(archive, value, version);
}

}}

This code simply serializes the object managed by the std::shared_ptr in the function save(). If multiple std::shared_ptr instances point to same object boost serialization will take automatically care to store it only once. The magic happens in load() where boost serialization returns a raw pointer to the object (data). This raw pointer is looked up in a hash that holds a weak_ptr for each raw pointer. In case that the weak_ptr in the hash is expired we can safely create a new shared_ptr instance, let it manage the raw pointer and store a weak_ptr in the hash. In case that the weak_ptr is not expired we simply lock it to return a shared_ptr. This way the reference counting is correct.