Thank you Everybody: François, you idea is cool, but too hard for me:) I traced the serialization library and finally come up with a dirty sulotion, that is after setting file pointer to object's position, direct call class's serialize member function to deserialize the object. I don't know if it's right to do so, but it works for me by now. I post my solution here: struct word_block { std::string word; template<typename Archive> void serialize(Archive & ar, const unsigned int file_version ) { ar & word; } }; int main(int argc, char* argv[]) { const char* filename = "words.idx"; word_block wb1, wb2, wb3, wb2_r; std::ofstream ofs(filename, std::ios_base::binary); boost::archive::binary_oarchive oar(ofs); oar << wb1; size_t wb2_pos = ofs.tellp(); wb2.word = "haha"; oar << wb2; ofs.close(); std::ifstream ifs(filename, std::ios_base::binary); boost::archive::binary_iarchive iar(ifs); ifs.seekg(wb2_pos); wb2_r.serialize(iar, 0); //direct call word_block's serialize member function. return 0; } -----原始邮件----- From: François Mauger Sent: Monday, October 24, 2011 1:13 AM Newsgroups: gmane.comp.lib.boost.user To: boost-users@lists.boost.org Subject: Re: [serialization] how can load the nthelement usingbinary_archive? On 23/10/2011 18:24, Robert Ramey wrote:

gongyiling wrote:

...

Hello, Everyone:

boost::serialization is a cool library. I use it in my project. I encounter a problem now. I serialized a array of objects to file as following:

std::ofstream ofs(filename, std::ios_base::binary); boost::binary_oarchive oar(ofs); oar<< obj[0]<< obj[1]<< ...<< obj[n-1];

Now I want to load the obj[i] (i>=0&& i

struct word_block { std::string word; template<typename Archive> void serialize(Archive& ar, const unsigned int file_version ) { ar& word; } };

int main(int argc, char* argv[]) { const char* filename = "words.idx"; word_block wb1, wb2, wb2_r;

std::ofstream ofs(filename, std::ios_base::binary); boost::archive::binary_oarchive oar(ofs); oar<< wb1; size_t wb2_pos = ofs.tellp(); //record the position of wb2. oar<< wb2; ofs.close();

std::ifstream ifs(filename, std::ios_base::binary); boost::archive::binary_iarchive iar(ifs); ifs.seekg(wb2_pos); //set file pointer to wb2. iar>> wb2_r; //failed with exception input_stream_error. return 0; }

Any idea is appreciated!

I can't see a way to do this without making non-trivial changes in the serialization library

Hi The Boost/serialization library outputs 'sequential access' files. However this is not a blocking point. To achieve the feature you want, it is possible to use a third party I/O library like HDF5 (or ROOT from CERN) that provides random access to some arbitrary data structures (typically buffer of chars) from data files. A possible solution could be (in fact *is*): 1 - associate a Boost output archive to some memory buffer (typically a STL vector<char>) through some special 'ostream' (found in Boost/iostreams) 2 - serialize one unique object from your array in the archive in such a way there is *one* archive *per* object and *not* one archive for the whole array of objects 3 - store the corresponding buffer (size+content) using some native output operations from the external I/O lib and some mapping key (index, label...) that will be used for further random access to the buffer. Deserialization can then be performed : 1 - use some random access input operation from the external lib to rebuild *the* single memory buffer associated to *the* object you want to retrieve (per index, per label... depending on the internal "mapping" of the I/O lib) 2 - associate a Boost input archive to this memory buffer that contains the bytes of the Boost archive for only one object (again Boost/Iostreams is at work). 3 - Finally, use standard Boost/Serialization method to get your object from the archive. No need to say it takes a little time to implement make it work (Boost/iostreams is of great help here) and you need to enter the guts of another I/O library. But it works for me with some home-made library that couples Boost with such high-level third-party I/O library (ROOT I/O lib) and it is quite efficient. You thus take the best of the Boost/Serialization library (easy to write serialization stuff for all your classes in a unified approach) and from a robust I/O lib (buffered I/O ops, automated storage from/into splitted files, random access...). Not sure these comments/ideas may help you today... but if you feel brave... ;-) cheers frc -- François Mauger