Question

Hi,
I would like to store binary a std::vector<std::vector<int> > object MATRIX in a file.

out.write((char*)&MATRIX, sizeof(MATRIX));

The problem is, that only the column dimension is fixed. The row dimension changes.
If I read the object out of the binary file, it’s not enough to know only the size, isn’t it? So, initializing e.g. a second matrix with

std::vector<std::vector<int> > MATRIX2;
for ( int i=0;i<column_dim;i++ ) MATRIX2.push_back ( vector<int> ( 0 ) );
ifstream in(cstr, ios::in | ios::binary);

and reading the object data with

ifstream in(cstr, ios::in | ios::binary);    
in.read((char*)& MATRIX2, fSize);

makes no sense, because the compiler has no clue about the structure of the saved data.
My question:
Is there any better way of solving this problem than saving the matrix structure (all info about the row dimensions) in a second file, reading it and creating a MATRIX2 with the appropriate structure which is then filled by using

ifstream in(cstr, ios::in | ios::binary);    
in.read((char*)&nn_H_test, fSize);

?

Answer 1

I will give you a short example on how to do this with boost.

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

std::vector<std::vector<int > > g_matrix;
int main(int argc, char* argv[])
{
    // fill the vector
    std::ofstream ofs("c:\\dump");
    boost::archive::text_oarchive to(ofs);
    to << g_matrix;

    g_matrix.clear();
    std::ifstream ifs("c:\\dump");
    boost::archive::text_iarchive infs(ifs);
    infs >> g_matrix;
    // check your vector. It should be the same
}

If you need it to be more human readable you can also try the xml out from boost.
As always don’t reinvent the wheel.