C++ How to store collection of templates objects regardless of tempate

Question

I have a problem with implementing database table library. I have a class Column storing different types.

template <class T>
class Column : iColumn<T>
{ 
... 
}

Table is composed of columns, so I need a collection of them (map with string name as a key and column as value). How shall I implement one collection of all table's columns regardless the template type?

Thanks for any hep.

Answer 1

I think that boost::any might help you there...

Answer 2

Some alternatives:

1. Boost Fusion http://www.boost.org/doc/libs/1_42_0/libs/fusion/doc/html/index.html

2. Create a container of Boost any

3. Make the template class inherit from the same class and store the pointers

4. As this looks like a database thingy, simply use strings instead of different types.

Answer 3

You should have a common interface.

class Column<T>: public IColumn {
  ...
};

std::map<std::string,IColumn*> columns;

Answer 4

You can consider using boost::any or boost::variant for canned up solutions. Or you might consider providing a base column type that is the root of the columns hierarchy. It seems as if all columns do have some things in common (probably a name at the very least). All templated column types would derive from it.

I myself, you go the good-old-way and implement a hierarchy with a visitor pattern (double dispatch) for operations that must be performed in the derived elements and are not present in the base of the hierarchy.

Answer 5

Your types are, from the C++ type system perspective, completely unrelated. To force a relationship, some kind of type erasure is needed; this may either be a commonly implemented interface or a mechanism like Boost.Any as proposed by others.

However, note that putting different things together isn’t a good design to start with – so you should probably put some though into a meaningful common interface that lets you do all the necessary things without excessive downcasting later on.