Union of two std::vector Read Access Violation

Question

I have a union of two std::vectors and when I call push_back i get a read access violation. I'm pretty new to unions so I bet I'm doing something wrong with them. Any ideas why this is happening?

struct Row
{
    char order_id[30];
    char garbage[994];
    Row() {}
    ~Row() {}
};

struct Row_List
{
    union
    {
        std::vector<Row> divided_row;
        std::vector<char*> combined_row;
    };
    Row_List() {}
    ~Row_List() {}
};

int main(void)
{
    Row_List results;

    char current_row[1024];

    //initialize row to null
    for (int i = 0; i < 1024; ++i)
    {
        current_row[i] = '\0';
    }

//error happens here
    results.combined_row.push_back(current_row);

    return 0;
}

EDIT: my final overall goal is to simplify the SQLGetData (in SQL.h) method by getting it to write the results into a vector of Row. currently, you just have to give it a pointer and it will write the results into that memory location. I wanted a function to return the result into a struct that I specify but each SQL query might have different fields in the result. I thought unions would let me write the results into a char array but then read it as a struct i specify.

I might run an SQL query that returns 10 rows and each row contains an integer and a char array. I might also run a query that returns 100 rows and each row contains a boolean, an integer, and two char arrays.

In the first example I want the results in a vector of structs which contain an integer and a char array. In the second example I want a results in a vector of structs that contain a boolean, an integer, and two char arrays.

The main issue being I want a generic way to write into any struct. I'm fine with passing a list of data type sizes though.

example function to do this:

bool SQL_Query::write_results(std::vector<std::array<char, 1024> >& results_vector)
{
    /*
    user makes a union of (a vector of long chars, and a vector of several small chars, ints etc.)
    they pass the vector of long chars into this function
    */

    //if not connected or if the query isnt successful or if the row structure hasnt been set then do nothing
    if (!m_connected || !m_query_successful || !m_row_structure_set)
    {
        return false;
    }

    //for each row
    while (SQLFetch(m_SQL_statement_handle) == SQL_SUCCESS)
    {
        //declare a single row
        std::array<char, 1024> current_row;

        //initialize row to null
        for (int i = 0; i < 1024; ++i)
        {
            current_row[i] = '\0';
        }

        char* current_location = &current_row[0];

        //for each column
        for (unsigned int current_col = 1; current_col <= m_memory_size_list.size(); ++current_col)
        {
            //get next results max memory size
            int current_buffer_size = m_memory_size_list.at(current_col - 1);

            //write that into current_row at current_location
            SQLGetData(m_SQL_statement_handle, current_col, SQL_C_DEFAULT, current_location, current_buffer_size, NULL);

            //move to next write location in current_row by adding the current max memory size to 
            current_location += current_buffer_size;
        }

        //add single row to results vector
        results_vector.push_back(current_row);
    }

    //return true if everything went right
    return true;
}

Answer 1

Before you use a non-trivial type in a union, you must create it by calling placement new on it:

new(&results.combined_row) std::vector<char*>();

Then you can use it:

results.combined_row.push_back(current_row);

After usage, you need to call the destructor:

results.combined_row.~vector();

Note, that you are not allowed to access other union members, while combined_row is created.