TableLocker.h
Classes
- TableLocker -- Class to hold a (user) lock on a table. (full description)
Interface
- Public Members
- explicit TableLocker (Table& table, Table::LockType = Table::Write, uInt nattempts = 0)
- ~TableLocker()
- Bool hasLock (FileLocker::LockType = FileLocker::Write) const
- Private Members
- TableLocker (const TableLocker&)
- TableLocker& operator= (const TableLocker&)
Review Status
- Reviewed By:
- UNKNOWN
- Date Reviewed:
- before2004/08/25
- Programs:
- Tests:
Prerequisite
Synopsis
Class TableLocker can be used to acquire a (user) lock on a table.
The lock can be a read or write lock.
The destructor only releases the lock if the lock was acquired by the
constructor.
TableLocker simply uses the lock and unlock
function of class Table.
The advantage of TableLocker over these functions is that the
destructor of TableLocker is called automatically by the system,
so unlocking the table does not need to be done explicitly and
cannot be forgotten. Especially in case of exception handling this
can be quite an adavantage.
This class is meant to be used with the UserLocking option.
It can, however, also be used with the other locking options.
In case of PermanentLocking(Wait) it won't do anything at all.
In case of AutoLocking it will acquire and release the lock when
needed. However, it is possible that the system releases an
auto lock before the TableLocker destructor is called.
Example
// Open a table to be updated.
Table myTable ("theTable", TableLock::UserLocking, Table::Update);
// Start of some critical section requiring a lock.
{
TableLocker lock1 (myTable);
... write the data
}
// The TableLocker destructor invoked by } unlocked the table.
Motivation
TableLocker makes it easier to unlock a table.
Member Description
explicit TableLocker (Table& table, Table::LockType = Table::Write, uInt nattempts = 0)
The constructor acquires a read or write lock on a table
which is released by the destructor.
If the table was already locked, the destructor will
not unlock the table.
The number of attempts (default = forever) can be specified when
acquiring the lock does not succeed immediately. When nattempts>1,
the system waits 1 second between each attempt, so nattempts
is more or less equal to a wait period in seconds.
An exception is thrown when the lock cannot be acquired.
If locked, the destructor releases the lock and flushes the data.
Has this process the read or write lock, thus can the table
be read or written safely?
TableLocker (const TableLocker&)
TableLocker& operator= (const TableLocker&)
The copy constructor and assignment are not possible.
Note that only one lock can be held on a table, so copying a
TableLocker object imposes great difficulties which objects should
release the lock.
It can be solved by turning TableLocker into a handle class
with a reference counted body class.
However, that will only be done when the need arises.