访问地图矢量的地图元素（C ++）

Question

I've been working on a simple database system managment and I've come up with:

std::map< std::string, std::vector < std::map < std::string,
          boost::variant <std::string, size_t, double bool> > > tables;

I have a map (tables) of vectors (table) of maps (records) and I've allready wrote a function to read a file to it but I'm not really sure how to access the single attributes.

I can print the whole thing with:

for(auto table: tables)
    for(auto record : table.second)
        for(auto attribute : record) {
            std::cout << j.second;

I tried doing something like:

std::cout << tables["credentials"][2]["username"];

This however does not work; it only prints a blank line.

Answer 1

It's most likely that you are using wrong keys of the maps to access the database.

Update your code to print the contents of your database so you can see the keys also in the maps.

for(auto table: tables)
{
   std::cout << "Key: " << table.first << std::endl;
   for(auto record : table.second)
   {
      for(auto attribute : record)
      {
         std::cout << "Key: " << attribute.first
                   << ", Value: " << attribute.second << std::endl;
      }
   }
}