boost.serialization and lazy initialization

Question

i need to serialize directory tree. i have no trouble with this type:

std::map<
   std::string, // string(path name)
   std::vector<std::string> // string array(file names in the path)
> tree;

but for the serialization the directory tree with the content i need other type:

std::map<
   std::string, // string(path name)
   std::vector< // files array
      std::pair<
         std::string, // file name
         std::vector< // array of file pieces
            std::pair< // <<<<<<<<<<<<<<<<<<<<<< for this i need lazy initialization
               std::string, // piece buf
               boost::uint32_t // crc32 summ on piece
            >
         >
      >
   >
> tree;

how can i initialize the object of type "std::pair" in the moment of its serialization? i.e. read file piece/calculate crc32 summ.

up

Answer 1

I would replace the std::string in the vector by a custom class, let me say MyFileNames

class MyFileNames : std::string
{
// add forward constructors as needed

};

std::map<
   std::string, // string(path name)
   std::vector<MyFileNames> // string array(file names in the path)
> tree;

And define the save serialization function for MyFileNames by converting the std::string to a

std::pair<
     std::string, // file name
     std::vector< // array of file pieces
        std::pair< // <<<<<<<<<<<<<<<<<<<<<< for this i need lazy initialization
           std::string, // piece buf
           boost::uint32_t // crc32 summ on piece
        >
     >
>

and the serialize this type. This let you evaluate the lazy part only the data is serialized. For the load you could ignore the lazy data, as I suppose that this data can be calculated.

Answer 2

I dont quite understand the question, but #including "boost/serialization/utility.hpp" gives you the implementation for serialising std::pair.

If you want to load the area of your code later on, then I think the best way would be to create a custom pair class:

class custom_pair : std::pair< std::string, // piece buf
               boost::uint32_t > // crc32 summ on piece
{

};

//...
         std::vector< // array of file pieces
            custom_pair // <<<<<<<<<<<<<<<<<<<<<< for this i need lazy initialization
         >
//...

template< class Archive >
void serialize( Archive & ar, custom_pair & p, const unsigned int version ) {
    ar & boost::serialization::make_nvp( "std::pair", std::pair<...>( p ) );
}

template<class Archive>
inline void load_construct_data( Archive & ar, custom_pair * p, const unsigned int file_version ) {
    std::string first;
    boost::uint32_t second;
    ar & boost::serialization::make_nvp( "first", first_ );
    ar & boost::serialization::make_nvp( "second", second_ );
    ::new( t )custom_pair;
    //...
}

template<class Archive>
inline void save_construct_data( Archive & ar, const custom_pair * p, const unsigned int file_version ) {
    ar & boost::serialization::make_nvp( "first", t->first );
    ar & boost::serialization::make_nvp( "second", t->second );
}