Qt is open-source so...

Qt Code: 
  1. void QSqlDatabase::removeDatabase(const QString& connectionName)
  2. {
  3.     QSqlDatabasePrivate::removeDatabase(connectionName);
  4. }
  5.  
  6. void QSqlDatabasePrivate::removeDatabase(const QString &name)
  7. {
  8.     QConnectionDict *dict = dbDict();
  9.     Q_ASSERT(dict);
  10.     QWriteLocker locker(&dict->lock);
  11.  
  12.     if (!dict->contains(name))
  13.         return;
  14.  
  15.     invalidateDb(dict->take(name), name);
  16. }
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... and ...

Qt Code: 
  1. void QSqlDatabasePrivate::invalidateDb(const QSqlDatabase &db, const QString &name)
  2. {
  3.     if (db.d->ref != 1) {
  4.         qWarning("QSqlDatabasePrivate::removeDatabase: connection '%s' is still in use, "
  5.                  "all queries will cease to work.", name.toLocal8Bit().constData());
  6.         db.d->disable();
  7.         db.d->connName.clear();
  8.     }
  9. }
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