In Table.h:
class Mixed {
public:
explicit Mixed(ColumnType v) {assert(v = COLUMN_TYPE_TABLE); m_type = COLUMN_TYPE_TABLE;}

is missing a "=" after "v =" to assert correctly.

Better yet would be to prevent this run-time error at compile time with something like:
class ColumnTypeTable {};
class Mixed {
public:
Mixed(ColumnTypeTable) { ... }
}
Use:
Mixed m(ColumnTypeTable());