class DbPrivate ....;

class Db {
...
private:
     DbPrivate *d;
};


class IaDbPrivate ...;

class IaDb : public Db {
...
private:
    IaDbPrivate *d;
}


dans IaDb:

void IaDb::sample()
{
    d->m_Data.insert();  <- ne sait pas quel *d prendre...
}