TableLocker.h

Classes

TableLocker -- Class to hold a (user) lock on a table. (full description)

class TableLocker

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&)

Description

Review Status

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.

~TableLocker()

If locked, the destructor releases the lock and flushes the data.

Bool hasLock (FileLocker::LockType = FileLocker::Write) const

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.