boost serialization

Archive

An archive is a sequence of bytes that represented serialized C++ objects. Objects can be added to an archive to serialize them and then later loaded from the archive.

1. boost::archive::text_iarchive

#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <iostream>
#include <fstream>

using namespace boost::archive;

void save()
{
  std::ofstream file("archive.txt");
  text_oarchive oa{file};
  int i = ;
  oa << i;
}

void load()
{
  std::ifstream file("archive.txt");
  text_iarchive ia{file};
  int i = ;
  ia >> i;
  std::cout << i << std::endl;
}

int main()
{
  save();
  load();
  return ;
}

The class boost::archive::text_oarchive serializes data as a text stream, and the class boost::archive::text_iarchive restores data from such a text stream. Constructors of archives expect an input or output stream as a parameter. The stream is used to serialize or restore data.

2. serializing with a stringstream

#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <iostream>
#include <sstream>

using namespace boost::archive;

std::stringstream ss;

void save()
{
  text_oarchive oa{ss};
  int i = ;
  oa << i;
}

void load()
{
  text_iarchive ia{ss};
  int i = ;
  ia >> i;
  std::cout << i << std::endl;
}

int main()
{
  save();
  load();
  return ;
}

output: 1

3. user-defined types with a member function

#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <iostream>
#include <sstream>

using namespace boost::archive;

std::stringstream ss;

class animal
{
public:
  animal() = default;
  animal(int legs) : legs_(legs) {}
  int legs() const { return legs_; }

private:
  friend class boost::serialization::access;

  template <typename Archive>
  void serialize(Archive &ar, const unsigned int version) { ar & legs_; }

  int legs_;
};

void save()
{
  text_oarchive oa(ss);
  animal a{};
  oa << a;
}

void load()
{
  text_iarchive ia(ss);
  animal a;
  ia >> a;
  std::cout << a.legs() << std::endl;
}

int main()
{
  save();
  load();
  return ;
}

In order to serialize objects of user-defined types, you must define the member function serialize(). This function is called when the object is serialized to or restored from a byte stream.

serialize() is automatically called any time an object is serialized or restored.

4. serializing with a free-standing function and serializing strings

#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <boost/serialization/string.hpp>
#include <iostream>
#include <sstream>
#include <string>
#include <utility>

using namespace boost::archive;

std::stringstream ss;

class animal
{
public:
  animal() = default;
  animal(int legs, std::string name) :
    legs_{legs}, name_{std::move(name)} {}
  int legs() const { return legs_; }
  const std::string &name() const { return name_; }

private:
  friend class boost::serialization::access;

  template <typename Archive>
  friend void serialize(Archive &ar, animal &a, const unsigned int version);

  int legs_;
  std::string name_;
};

template <typename Archive>
void serialize(Archive &ar, animal &a, const unsigned int version)
{
  ar & a.legs_;
  ar & a.name_;
}

void save()
{
  text_oarchive oa{ss};
  animal a{, "cat"};
  oa << a;
}

void load()
{
  text_iarchive ia{ss};
  animal a;
  ia >> a;
  std::cout << a.legs() << std::endl;
  std::cout << a.name() << std::endl;
}

int main()
{
  save();
  load();
  return ;
}

a member variable of type std::string, in order to serialize this member variable. the header file boost/serialization/string.hpp must be included to provide the appropriate free-standing function serialize().

Pointers and References

1. serializing pointers

#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <iostream>
#include <sstream>

std::stringstream ss;

class animal
{
public:
  animal() = default;
  animal(int legs) : legs_{legs} {}
  int legs() const { return legs_; }

private:
  friend class boost::serialization::access;

  template <typename Archive>
  void serialize(Archive &ar, const unsigned int version) { ar & legs_; }

  int legs_;
};

void save()
{
  boost::archive::text_oarchive oa(ss);
  animal *a = new animal();
  oa << a;
  std::cout << std::hex << a << std::endl;
  delete a;
}

void load()
{
  boost::archive::text_iarchive ia(ss);
  animal *a;
  ia >> a;
  std::cout << std::hex << a << std::endl;
  std::cout << std::dec << a->legs() << std::endl;
  delete a;
}

int main()
{
  save();
  load();
  return ;
}

Boost.Serialization automatically serializes the object referenced by a and not the address of the object.

If the archive is restored, a will not necessarily contain the same address. A new object is created and its address is assigned to a instead. Boost.Serialization only guarantees that the object is the same as the one serialized, not that its address is the same.

2 serializing smart pointers

#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <boost/serialization/scoped_ptr.hpp>
#include <boost/scoped_ptr.hpp>
#include <iostream>
#include <sstream>

using namespace boost::archive;

std::stringstream ss;

class animal
{
public:
  animal() = default;
  animal(int legs) : legs_(legs) {}
  int legs() const { return legs_; }

private:
  friend class boost::serialization::access;

  template <typename Archive>
  void serialize(Archive &ar, const unsigned int version) { ar & legs_; }

  int legs_;
};

void save()
{
  text_oarchive oa(ss);
  boost::scoped_ptr<animal> a(new animal(4));
  oa << a;
}

void load()
{
  text_iarchive ia(ss);
  boost::scoped_ptr<animal> a;
  ia >> a;
  std::cout << a->legs() << std::endl;
}

int main()
{
  save();
  load();
  return ;
}

uses the smart pointer boost::scoped_ptr to manage a dynamically allocated object of type animal. Include the header file boost/serialization/scoped_ptr.hpp to serialize such a pointer. To serialize a smart pointer of type boost::shared_ptr, use the header file boost/serialization/shared_ptr.hpp.

3. serializing references

#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <iostream>
#include <sstream>

using namespace boost::archive;

std::stringstream ss;

class animal
{
public:
  animal() = default;
  animal(int legs) : legs_(legs) {}
  int legs() const { return legs_; }

private:
  friend class boost::serialization::access;

  template <typename Archive>
  void serialize(Archive &ar, const unsigned int version) { ar & legs_; }

  int legs_;
};

void save()
{
  text_oarchive oa(ss);
  animal a();
  animal &r = a;
  oa << r;
}

void load()
{
  text_iarchive ia(ss);
  animal a;
  animal &r = a;
  ia >> r;
  std::cout << r.legs() << std::endl;
}

int main()
{
  save();
  load();
}

Serialization of Class Hierarchy Objects

Derived classes must access the function boost::serialization::base_object() inside the member function serialize() to serialize objects based on class hierarchies. This function guarantees that inherited member variables of base classes are correctly serialized.

1. serializing derived classes correctly

#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <iostream>
#include <sstream>

using namespace boost::archive;
std::stringstream ss;

class animal
{
public:
  animal() = default;
  animal(int legs) : legs_(legs) {}
  int legs() const { return legs_; }

private:
  friend class boost::serialization::access;

  template <typename Archive>
  void serialize(Archive &ar, const unsigned int version) { ar & legs_; }

  int legs_;
};

class bird : public animal
{
public:
  bird() = default;
  bird(int legs, bool can_fly) :
    animal(legs), can_fly_{can_fly} {}
  bool can_fly() const { return can_fly_; }

private:
  friend class boost::serialization::access;

  template <typename Archive>
  void serialize(Archive &ar, const unsigned int version)
  {
    ar & boost::serialization::base_object<animal>(*this);
    ar & can_fly_;
  }

  bool can_fly_;
};

void save()
{
  text_oarchive oa(ss);
  bird penguin(, false);
  oa << penguin;
}

void load()
{
  text_iarchive ia(ss);
  bird penguin;
  ia >> penguin;
  std::cout << penguin.legs() << '\n';
  std::cout << std::boolalpha << penguin.can_fly() << '\n';
}

int main()
{
  save();
  load();
  return ;
}

Inherited member variables are serialized by accessing the base class inside the member function serialize() of the derived class and calling boost::serialization::base_object(). You must use this function rather than, for example, static_cast because only boost::serialization::base_object() ensures correct serialization

2. registering derived classes statically with BOOST_CLASS_EXPORT

#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <boost/serialization/export.hpp>
#include <iostream>
#include <sstream>

using namespace boost::archive;

std::stringstream ss;

class animal
{
public:
  animal() = default;
  animal(int legs) : legs_(legs) {}
  virtual int legs() const { return legs_; }
  virtual ~animal() = default;

private:
  friend class boost::serialization::access;

  template <typename Archive>
  void serialize(Archive &ar, const unsigned int version) { ar & legs_; }

  int legs_;
};

class bird : public animal
{
public:
  bird() = default;
  bird(int legs, bool can_fly) :
    animal{legs}, can_fly_(can_fly) {}
  bool can_fly() const { return can_fly_; }

private:
  friend class boost::serialization::access;

  template <typename Archive>
  void serialize(Archive &ar, const unsigned int version)
  {
    ar & boost::serialization::base_object<animal>(*this);
    ar & can_fly_;
  }

  bool can_fly_;
};

BOOST_CLASS_EXPORT(bird)

void save()
{
  text_oarchive oa(ss);
  animal *a = new bird(, false);
  oa << a;
  delete a;
}

void load()
{
  text_iarchive ia(ss);
  animal *a;
  ia >> a;
  std::cout << a->legs() << '\n';
  delete a;
}

int main()
{
  save();
  load();
  return ;
}

To have Boost.Serialization recognize that an object of type bird must be serialized, even though the pointer is of type animal*, the class bird needs to be declared. This is done using the macro BOOST_CLASS_EXPORT, which is defined in boost/serialization/export.hpp. Because the type bird does not appear in the pointer definition, Boost.Serialization cannot serialize an object of type bird correctly without the macro.

The macro BOOST_CLASS_EXPORT must be used if objects of derived classes are to be serialized using a pointer to their corresponding base class. A disadvantage of BOOST_CLASS_EXPORT is that, because of static registration, classes can be registered that may not be used for serialization at all.

3. register_type()

#include <boost/archive/text_oarchive.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <boost/serialization/export.hpp>
#include <iostream>
#include <sstream>

std::stringstream ss;

class animal
{
public:
  animal() = default;
  animal(int legs) : legs_(legs) {}
  virtual int legs() const { return legs_; }
  virtual ~animal() = default;

private:
  friend class boost::serialization::access;

  template <typename Archive>
  void serialize(Archive &ar, const unsigned int version) { ar & legs_; }

  int legs_;
};

class bird : public animal
{
public:
  bird() = default;
  bird(int legs, bool can_fly) :
    animal{legs}, can_fly_(can_fly) {}
  bool can_fly() const { return can_fly_; }

private:
  friend class boost::serialization::access;

  template <typename Archive>
  void serialize(Archive &ar, const unsigned int version)
  {
    ar & boost::serialization::base_object<animal>(*this);
    ar & can_fly_;
  }

  bool can_fly_;
};

void save()
{
  boost::archive::text_oarchive oa(ss);
  oa.register_type<bird>();
  animal *a = new bird(, false);
  oa << a;
  delete a;
}

void load()
{
  boost::archive::text_iarchive ia(ss);
  ia.register_type<bird>();
  animal *a;
  ia >> a;
  std::cout << a->legs() << std::endl;
  delete a;
}

int main()
{
  save();
  load();
  return ;
}

The type to be registered is passed as a template parameter. Note that register_type() must be called both in save() and load().

The advantage of register_type() is that only classes used for serialization must be registered. For example, when developing a library, one does not know which classes a developer may use for serialization later. While the macro BOOST_CLASS_EXPORT makes this easy, it may register types that are not going to be used for serialization.