CDaoRecordset::IsBOF

Статья
10/16/2008

Call this member function before you scroll from record to record to learn whether you have gone before the first record of the recordset.

BOOL IsBOF( ) const;

Возвращаемое значение

Nonzero if the recordset contains no records or if you have scrolled backward before the first record; otherwise 0.

Заметки

You can also call IsBOF along with IsEOF to determine whether the recordset contains any records or is empty. Immediately after you call Open, if the recordset contains no records, IsBOF returns nonzero. When you open a recordset that has at least one record, the first record is the current record and IsBOF returns 0.

If the first record is the current record and you call MovePrev, IsBOF will subsequently return nonzero. If IsBOF returns nonzero and you call MovePrev, an exception is thrown. If IsBOF returns nonzero, the current record is undefined, and any action that requires a current record will result in an exception.

Effect of specific methods on IsBOF and IsEOF settings:

Calling Open internally makes the first record in the recordset the current record by calling MoveFirst. Therefore, calling Open on an empty set of records causes IsBOF and IsEOF to return nonzero. (See the following table for the behavior of a failed MoveFirst or MoveLast call.)
All Move operations that successfully locate a record cause both IsBOF and IsEOF to return 0.
An AddNew call followed by an Update call that successfully inserts a new record will cause IsBOF to return 0, but only if IsEOF is already nonzero. The state of IsEOF will always remain unchanged. As defined by the Microsoft Jet database engine, the current record pointer of an empty recordset is at the end of a file, so any new record is inserted after the current record.
Any Delete call, even if it removes the only remaining record from a recordset, will not change the value of IsBOF or IsEOF.

This table shows which Move operations are allowed with different combinations of IsBOF/IsEOF.

	MoveFirst, MoveLast	MovePrev, Move < 0	Move 0	MoveNext, Move > 0
IsBOF=nonzero, IsEOF=0	Allowed	Exception	Exception	Allowed
IsBOF=0, IsEOF=nonzero	Allowed	Allowed	Exception	Exception
Both nonzero	Exception	Exception	Exception	Exception
Both 0	Allowed	Allowed	Allowed	Allowed

Allowing a Move operation does not mean that the operation will successfully locate a record. It merely indicates that an attempt to perform the specified Move operation is allowed and will not generate an exception. The value of the IsBOF and IsEOF member functions may change as a result of the attempted move.

The effect of Move operations that do not locate a record on the value of IsBOF and IsEOF settings is shown in the following table.

	IsBOF	IsEOF
MoveFirst, MoveLast	Nonzero	Nonzero
Move 0	No change	No change
MovePrev, Move < 0	Nonzero	No change
MoveNext, Move > 0	No change	Nonzero