Writing a vector of vector vector structure to a binary file

Question

std::vector<std::vector<std::vector<sphere_data> >> Sphere_image ; 

I want to write the data in sphere image to binary file.

Could any one tell me how can this be done.

I tried this Code:

ofstream fout("C:/Project/data.dat", ios::out | ios::binary);
fout.write((char*)&Sphere_image[o], Sphere_image.size() *     
sizeof(sphere_data));
fout.close();

Answer 1

When you have nested std::vectors, you don't have contiguity in memory for the whole data structure.

So, you have to iterate through all the nested vectors "dimensions", and assume contiguity of sphere_data instances only in the innermost vector.

So, your line:

fout.write((char*)&Sphere_image[o], Sphere_image.size() *     
sizeof(sphere_data));

must be expanded something like that:

for (const auto& vi : Sphere_image) { // For each outer vector
  for (const auto& vj : vi) {         // For each sub-vector
    // Now you do have contiguity for vj
    fout.write(
        reinterpret_cast<const char*>(vj.data()), 
        vj.size() * sizeof(sphere_data));
  }
}

Note that this assumes that sphere_data is a POD, so e.g. if you have pointer data members inside sphere_data, that won't work.

In this case, you may provide a sphere_data::save(std::ofstream& out) const method, which you can call in the innermost loop. Instances of sphere_data will know how to serialize themselves to binary files. E.g.:

for (const auto& vi : Sphere_image) { // For each outer vector
  for (const auto& vj : vi) {         // For each sub-vector
    for (const auto& sd : vj) {       // For each sphere data
      sd.save(fout);
    }  
  }
}

You may also provide a symmetrical sphere_data::load() method as well.