Quince's cast<...>() function builds and returns an SQL expression that represents a server-side type conversion operation. It's a wrapper for the SQL keyword CAST.

cast<T>(arg) is allowed when T is a mapped type, and the target database's mapping (allowing for any per-database customizations) for T represents it as some single-column SQL type t. Then cast<T>(arg) returns an exprn_mapper<T> that, when executed, evaluates the subexpression that arg represents, and applies the SQL conversion CAST(... AS t) to the result.

Whether you say cast<boost::optional<foo>>(arg) or cast<foo>(arg) makes no difference to the SQL that is actually executed. The only difference is in the type of exprn_mapper you get back. If there is any possiblility that arg could evaluate to an SQL NULL, then it is your responsibility to cast to some boost::optional<...>.