C++ dynamic vector of string array

Question

I want to create a dynamic vector and each vector element is an array of strings.

The data structure I have in mind is something like this:

VECTOR: 
[0] = [str1, str2, str3]
[1] = [str1, str2, str3]
[2] = [str1, str2, str3]

I correctly insert the values in the res variable BUT my code doesn't work properly: the printing loop is executed 4 times but every time it prints only the LAST element. I believe the problem could be: 1) I don't push the strings array properly in the vector; 2) I don't manage correctly the iteration over the vector and over all the string when I want to print it.

This is my code:

std::vector<std::string*> DatabaseConnector::ExecuteQuery(std::string query, std::vector <std::string> columns)
{
    std::vector<std::string*> results;
    std::string res[columns.size() + 1]; // last value = '\0' to signal end of array

    db_params.res = db_params.stmt->executeQuery(query);
    while (db_params.res->next()) // Access column data by alias or column name
    {
        int i = 0;
        for(std::string s : columns)
            res[i++] = db_params.res->getString(s);

        res[i] = "\0";

        results.push_back(res);
    }

    for(auto el :results)
    {
        int i=0;
        while(el[i].compare("") != 0)
             std::cout << el[i++] << " ";

        std::cout << std::endl;
    }

    return results;
};

Answer 1

std::string res[columns.size() + 1]; is a variable length array and you are pushing a pointer to the first element into the vector. You should use std::vector<std::string> res; and std::vector<std::vector<std::string>> results;

std::vector<std::vector<std::string>> DatabaseConnector::ExecuteQuery(std::string query, const std::vector <std::string> &columns)
{
    std::vector<std::vector<std::string>> results;

    db_params.res = db_params.stmt->executeQuery(query);
    while (db_params.res->next()) // Access column data by alias or column name
    {
        std::vector<std::string> res;
        for(std::string s : columns)
            res.push_back(db_params.res->getString(s));

        results.push_back(res);
    }

    for(const auto &el :results)
    {
        for (const auto &res : el)
             std::cout << res << " ";

        std::cout << std::endl;
    }

    return results;
}