具有不同类型的多个向量

Question

I'm trying to create a small proof-of-concept database system that uses Tables to store data. A "Table" is a collection of columns. Each column can have a different type. Each table can have any number of columns.

Ideally, I'd like something like:

class Table {
  map<string, vector<T>> cols; //string is name of col, vector holds data
}

However, the type of the vector has to be known at compile time, so I can't have multiple types (vector int, vector double, etc) in the same map.

Do I need:

class Table {
  map<string, vector<int>>    int_cols;
  map<string, vector<double>> double_cols;
  //etc...
}

For each type that I'd like to be able to store?
I feel like there has to be a better way to do this.

Answer 1

Actually C++ is not the best choice to manage dynamic types. Consider this member of vector:

reference operator[](size_type index);

If the type is not statically resolved, how will the return value be interpreted under binary level? For a language lack of meta class info support, there is no elegant universal solution for such a problem. However, if the types of the values you intend to store are enumerable, I may suggest you to try boost::any or boost::variant:

map<std::string, boost::any> cols;

It really looks bad when you have to fetch a value:

if (cols[key].type() == typeid(std::vector<int>)) {
    process(cols[key].any_cast<std::vector<int>>());
} else if (cols[key].type() == typeid(std::vector<double>)) {
    process(cols[key].any_cast<std::vector<double>>());
}
...
} else {
    throw std::runtime_error("Oops! Seems that I missed a type :-(");
}