Convert vector<vector<int>> to QVector<int> or QVector2D<int> or something

I use QDataStream to write the variable to .dat file, so the variable I'm writing should be in the form of QVector or QVector2D

vector<vector<int>> vec;
// save
QFile file("save.dat");
file.open(QIODevice::WriteOnly);
QDataStream out(&file);
out << QVector2D<int>::fromStdVector(vec);

// load
QFile file("save.dat");
file.open(QIODevice::ReadOnly);
QDataStream in(&file);
QVector2D<int> qqi;
in >> qqi; vec = qqi.toStdVector();

Error: QVector2D is not a template: out << ...

You don't need to use Qt types to write into the datastream. Just serialize the data in vec. For that to work you also need to store the sizes of the structures.

int
main( int argc, char** argv ) {
  std::vector<std::vector<int>> vec;
  vec.resize( 2 );
  vec[0] = { 1, 2 };
  vec[1] = { 3, 4, 5 };
  // save
  {
    QFile file( "save.dat" );
    file.open( QIODevice::WriteOnly );
    QDataStream out( &file );
    out << vec.size();
    for( auto& inner : vec ) {
      out << inner.size();
      out.writeBytes(
          reinterpret_cast<const char*>( inner.data() ),
          inner.size() * sizeof( int ) );
    }
  }
  vec.clear();
  {
    // load
    QFile file( "save.dat" );
    file.open( QIODevice::ReadOnly );
    QDataStream in( &file );
    size_t num;
    in >> num;
    vec.resize( num );
    for( auto& inner : vec ) {
      in >> num;
      inner.resize( num );
      auto buffer = reinterpret_cast<char*>( inner.data() );
      uint bytes = num * sizeof( int );
      in.readBytes( buffer, bytes );
    }
  }
}