C++ Structure with unknown data types - c++

Question

My program reads user's input and create easy "table". User on the start specifies data types of columns and the number of rows.
User's input:
create table
add attribute string Name
add attribute int Age
rows 3
I need prepare a structure from user's input now. I have something like this:
CTable
{
unsigned attributesCnt;
string * attributesNames;
void ** attributes;
};
So, from the user's input, the progam does these steps:
CTable myTable;
myTable.attributesCnt = 2; // string "Name", int "Age"
myTable.attributesNames = new string[2];
myTable.attributesNames[0] = "Name";
myTable.attributesNames[1] = "Age";
attributes = new void[2]; // 2 attributes
attributes[0] = (void*) new string[3]; // there will be 3 rows
attributes[1] = (void*) new int[3];
I need remember that "attributes[0]" is string and "attributes[1]" is int too.
Is this "right" way?
I would like use only standard libraries.

Answer 1

What you are looking for is a tagged union also called a variant. It allows you to store multiple data types at the same location just like a regular union but includes an additional but separate data member that indicates it's type. The C++ Standard Library does not include variants but they are easy enough to implement.
Once you have a variant you can apply it to your example like below.
myTable.attributesNames[0] = "Name";
myTable.attributesNames[1] = "Age";
// I recommend using std::vector here instead of using new/delete yourself
attributes = new Variant*[2]; // 2 attributes
attributes[0] = new Variant("player name");
attributes[1] = new Variant(player_age);
The following example shows how the variant might be implemented.
struct Variant
{
enum Type
{
INT,
STRINGPTR
};
Type type_;
union
{
int int_;
const char* stringptr_;
} data_;
explicit Variant(int data) : type_(INT)
{
data_.int_ = data;
}
explicit Variant(const char *data) : type_(STRINGPTR)
{
data_.stringptr_ = data;
}
Type getType() const { return type_; }
int getIntValue() const
{
if(type_ != INT)
throw std::runtime_error("Variant is not an int");
return data_.int_;
}
const char *getStringPtr() const
{
if(type_ != STRINGPTR)
throw std::runtime_error("Variane is not a string");
return data_.stringptr_;
}
};
int main()
{
Variant intval(1);
Variant stringval("hello");
std::cout << intval.getIntValue() << std::endl;
std::cout << stringval.getStringPtr() << std::endl;
}