Re: [Boost-users] [serialization] how can load the nth element usingbinary_archive?

Subject: Re: [Boost-users] [serialization] how can load the nth element usingbinary_archive?
From: Robert Ramey (ramey_at_[hidden])
Date: 2011-10-23 12:24:35

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<n) from file.  I tried
> recording obj[i]'s
> position in file, then set file stream's pointer to that position and
> trying to load it, but
> failed with exception input_stream_error, can I by any means doing
> that?
> here is my minimun code segments:
>
> 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

Robert Ramey