Serializing user-defined types#

In order to facilitate the sending and receiving of complex datatypes HPX provides a serialization abstraction.

Just like boost, hpx allows users to serialize user-defined types by either providing the serializer as a member function or defining the serialization as a free function.

Unlike Boost HPX doesn’t acknowledge second unsigned int parameter, it is solely there to preserve API compatibility with Boost Serialization

This is tutorial was heavily inspired by Boost’s serialization concepts.

Setup#

The source code for this example can be found here: custom_serialization.cpp.

To compile this program, go to your HPX build directory (see Building HPX for information on configuring and building HPX) and enter:

$ make examples.quickstart.custom_serialization

To run the program type:

$ ./bin/custom_serialization

This should print:

Rectangle(Point(x=0,y=0),Point(x=0,y=5))
gravity.g = 9.81%

Serialization Requirements#

In order to serialize objects in HPX, at least one of the following criteria must be met:

In the case of default constructible objects:

The object is an empty type.
Has a serialization function as shown in this tutorial.
All members are accessible publicly and they can be used in structured binding contexts.

Otherwise:

They need to have special serialization support.

Member function serialization#

struct point_member_serialization
{
    int x{0};
    int y{0};

    // Required when defining the serialization function as private
    // In this case it isn't
    // Provides serialization access to HPX
    friend class hpx::serialization::access;

    // Second argument exists solely for compatibility with boost serialize
    // it is NOT processed by HPX in any way.
    template <typename Archive>
    void serialize(Archive& ar, const unsigned int)
    {
        // clang-format off
        ar & x & y;
        // clang-format on
    }
};

// Allow bitwise serialization
HPX_IS_BITWISE_SERIALIZABLE(point_member_serialization)

Notice that point_member_serialization is defined as bitwise serializable (see Bitwise serialization for bitwise copyable data for more details). HPX is also able to recursively serialize composite classes and structs given that its members are serializable.

struct rectangle_member_serialization
{
    point_member_serialization top_left;
    point_member_serialization lower_right;

    template <typename Archive>
    void serialize(Archive& ar, const unsigned int)
    {
        // clang-format off
        ar & top_left & lower_right;
        // clang-format on
    }
};

Free function serialization#

In order to decouple your models from HPX, HPX also allows for the definition of free function serializers.

struct rectangle_free
{
    point_member_serialization top_left;
    point_member_serialization lower_right;
};

template <typename Archive>
void serialize(Archive& ar, rectangle_free& pt, const unsigned int)
{
    // clang-format off
    ar & pt.lower_right & pt.top_left;
    // clang-format on
}

Even if you can’t modify a class to befriend it, you can still be able to serialize your class provided that your class is default constructable and you are able to reconstruct it yourself.

class point_class
{
public:
    point_class(int x, int y)
      : x(x)
      , y(y)
    {
    }

    point_class() = default;

    [[nodiscard]] int get_x() const noexcept
    {
        return x;
    }

    [[nodiscard]] int get_y() const noexcept
    {
        return y;
    }

private:
    int x;
    int y;
};

template <typename Archive>
void load(Archive& ar, point_class& pt, const unsigned int)
{
    int x, y;
    ar >> x >> y;
    pt = point_class(x, y);
}

template <typename Archive>
void save(Archive& ar, point_class const& pt, const unsigned int)
{
    ar << pt.get_x() << pt.get_y();
}

// This tells HPX that you have spilt your serialize function into
// load and save
HPX_SERIALIZATION_SPLIT_FREE(point_class)

Serializing non default constructable classes#

Some classes don’t provide any default constructor.

class planet_weight_calculator
{
public:
    explicit planet_weight_calculator(double g)
      : g(g)
    {
    }

    template <class Archive>
    friend void save_construct_data(
        Archive&, planet_weight_calculator const*, unsigned int);

    [[nodiscard]] double get_g() const
    {
        return g;
    }

private:
    // Provides serialization access to HPX
    friend class hpx::serialization::access;
    template <class Archive>
    void serialize(Archive&, const unsigned int)
    {
        // Serialization will be done in the save_construct_data
        // Still needs to be defined
    }

    double g;
};

In this case you have to define a save_construct_data and load_construct_data in which you do the serialization yourself.

template <class Archive>
inline void save_construct_data(Archive& ar,
    planet_weight_calculator const* weight_calc, const unsigned int)
{
    ar << weight_calc->g;    // Do all of your serialization here
}

template <class Archive>
inline void load_construct_data(
    Archive& ar, planet_weight_calculator* weight_calc, const unsigned int)
{
    double g;
    ar >> g;

    // ::new(ptr) construct new object at given address
    hpx::construct_at(weight_calc, g);
}

Bitwise serialization for bitwise copyable data#

When sending non arithmetic types not defined by std::is_arithmetic, HPX has to (de)serialize each object separately. However, if the class you are trying to send classes consists only of bitwise copyable datatypes, you may mark your class as such. Then HPX will serialize your object bitwise instead of element wise. This has enormous benefits, especially when sending a vector/array of your class. To define your class as such you need to call HPX_IS_BITWISE_SERIALIZABLE(T) with your desired custom class.

struct point_member_serialization
{
    int x{0};
    int y{0};

    // Required when defining the serialization function as private
    // In this case it isn't
    // Provides serialization access to HPX
    friend class hpx::serialization::access;

    // Second argument exists solely for compatibility with boost serialize
    // it is NOT processed by HPX in any way.
    template <typename Archive>
    void serialize(Archive& ar, const unsigned int)
    {
        // clang-format off
        ar & x & y;
        // clang-format on
    }
};

// Allow bitwise serialization
HPX_IS_BITWISE_SERIALIZABLE(point_member_serialization)

HPX master documentation

Serializing user-defined types

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