bslmt_lockguard.h

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/// @file bslmt_lockguard.h
///
/// The content of this file has been pre-processed for Doxygen.
///
 
 
// bslmt_lockguard.h                                                  -*-C++-*-
#ifndef INCLUDED_BSLMT_LOCKGUARD
#define INCLUDED_BSLMT_LOCKGUARD
 
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
 
/// @defgroup bslmt_lockguard bslmt_lockguard
/// @brief  Provide generic scoped guards for synchronization objects.
/// @addtogroup bsl
/// @{
/// @addtogroup bslmt
/// @{
/// @addtogroup bslmt_lockguard
/// @{
///
/// <h1> Outline </h1>
/// * <a href="#bslmt_lockguard-purpose"> Purpose</a>
/// * <a href="#bslmt_lockguard-classes"> Classes </a>
/// * <a href="#bslmt_lockguard-description"> Description </a>
///   * <a href="#bslmt_lockguard-behavior-of-the-release-method"> Behavior of the release Method </a>
///   * <a href="#bslmt_lockguard-usage"> Usage </a>
///     * <a href="#bslmt_lockguard-example-1-basic-usage"> Example 1: Basic Usage </a>
///
/// # Purpose {#bslmt_lockguard-purpose}
/// Provide generic scoped guards for synchronization objects.
///
/// # Classes {#bslmt_lockguard-classes}
///
/// -  bslmt::LockGuard: automatic mutex locking-unlocking
/// -  bslmt::LockGuardUnlock: automatic mutex unlocking-locking
/// -  bslmt::LockGuardTryLock: automatic non-blocking mutex locking-unlocking
/// -  bslmt::UnLockGuard: DEPRECATED
/// -  bslmt::TryLockGuard: DEPRECATED
///
/// @see  bslmt_readlockguard, bslmt_writelockguard
///
/// # Description {#bslmt_lockguard-description}
/// This component provides generic guards, `bslmt::LockGuard`,
/// `bslmt::LockGuardUnlock`, and `bslmt::LockGuardTryLock`, to automatically
/// lock and unlock an external synchronization object.  The synchronization
/// object can be any type (e.g., `bslmt::Mutex` or `bslmt::RecursiveMutex`)
/// that provides the following methods:
/// @code
/// void lock();
/// void unlock();
/// @endcode
/// Both `bslmt::LockGuard` and `bslmt::LockGuardUnlock` implement the
/// "construction is acquisition, destruction is release" idiom.  During
/// construction, `bslmt::LockGuard` automatically calls `lock` on the
/// user-supplied object, and `unlock` when it is destroyed (unless released).
/// `bslmt::LockGuardUnlock` does the opposite -- it invokes the `unlock` method
/// when constructed and the `lock` method when it is destroyed.
///
/// A third type of guard, `bslmt::LockGuardTryLock`, attempts to acquire a
/// lock, and if acquisition succeeds, releases it upon destruction.  Since the
/// acquisition is done at construction time, it is not possible to return a
/// value to indicate success.  Rather, the `bslmt::LockGuardTryLock` contains a
/// pointer to the synchronization object if `tryLock` succeeds, and is null
/// otherwise.  The synchronization object can be any type (e.g., `bslmt::Mutex`
/// or `bslmt::RecursiveMutex`) that provides the following methods:
/// @code
/// int  tryLock();
/// void unlock();
/// @endcode
/// Note that none of these guard types assumes ownership of the external
/// synchronization object.  Also note that objects of all of the guard types
/// may be constructed with a null `lock` whereby the constructed guard objects
/// manage no lock.  The destructor of each of the guard types has no effect if
/// no lock is under management.
///
/// ## Behavior of the release Method {#bslmt_lockguard-behavior-of-the-release-method}
///
///
/// Like all BDE guard classes, each of the three `bslmt::LockGuard*` classes
/// provides a `release` method that terminates the guard's management of any
/// lock object that the guard holds.  The `release` method has *no* *effect*
/// on the state of the lock object.
///
/// In particular, `bslmt::ReadLockGuard::release` does not unlock the lock
/// object under management.  If a user wants to release the lock object *and*
/// unlock the lock object (because the lock is no longer required before the
/// guard goes out of scope), the following idiom can be used:
/// @code
/// // 'guard' is an existing guard of type 'bslmt::LockGuard<my_Lock>',
/// // created in a scope that we do not control.
///
/// {
///     // ... Do work that requires the lock.
///
///     // We know that the lock is no longer needed.
///
///     my_Lock *lock = guard.release();
///
///     // 'lock' is no longer managed, but is *still* *locked*.
///
///     lock->unlock();
///
///     // ... Do work that does not require the lock.
/// }
/// @endcode
///
/// ## Usage {#bslmt_lockguard-usage}
///
///
/// This section illustrates intended use of this component.
///
/// ### Example 1: Basic Usage {#bslmt_lockguard-example-1-basic-usage}
///
///
/// Use this component to ensure that in the event of an exception or exit from
/// any point in a given scope, the synchronization object will be properly
/// unlocked.  The following function, `errorProneFunc`, is overly complex, not
/// exception safe, and contains a bug.
/// @code
/// static void errorProneFunc(my_Object *obj, my_Mutex *mutex)
/// {
///     mutex->lock();
///     if (someCondition) {
///         obj->someMethod();
///         mutex->unlock();
///         return;                                                   // RETURN
///     } else if (someOtherCondition) {
///         obj->someOtherMethod();
///         // MISTAKE! forgot to unlock mutex
///         return;                                                   // RETURN
///     }
///     obj->defaultMethod();
///     mutex->unlock();
///     return;
/// }
/// @endcode
/// The function can be rewritten with a cleaner and safer implementation using
/// a guard object.  The `safeFunc` function is simpler than `errorProneFunc`,
/// is exception safe, and avoids the multiple calls to unlock that can be a
/// source of errors.
/// @code
/// static void safeFunc(my_Object *obj, my_Mutex *mutex)
/// {
///     bslmt::LockGuard<my_Mutex> guard(mutex);
///     if (someCondition) {
///         obj->someMethod();
///         return;                                                   // RETURN
///     } else if (someOtherCondition) {
///         obj->someOtherMethod();
///         // OK, mutex is automatically unlocked
///         return;                                                   // RETURN
///     }
///     obj->defaultMethod();
///     return;
/// }
/// @endcode
/// When blocking while acquiring the lock is not desirable, one may instead use
/// a `bslmt::LockGuardTryLock` in the typical following fashion:
/// @code
/// /// Perform task and return positive value if locking succeeds.  Return
/// /// 0 if locking fails.
/// static int safeButNonBlockingFunc(my_Object *obj, my_Mutex *mutex)
/// {
///     const int RETRIES = 1; // use higher values for higher success rate
///     bslmt::LockGuardTryLock<my_Mutex> guard(mutex, RETRIES);
///     if (guard.ptr()) { // mutex is locked
///         if (someCondition) {
///             obj->someMethod();
///             return 2;                                             // RETURN
///         } else if (someOtherCondition) {
///             obj->someOtherMethod();
///             return 3;                                             // RETURN
///         }
///         obj->defaultMethod();
///         return 1;                                                 // RETURN
///     }
///     return 0;
/// }
/// @endcode
/// Instantiations of `bslmt::LockGuardUnlock` can be interleaved with
/// instantiations of `bslmt::LockGuard` to create both critical sections and
/// regions where the lock is released.
/// @code
/// void f(my_Mutex *mutex)
/// {
///     bslmt::LockGuard<my_Mutex> guard(mutex);
///
///     // critical section here
///
///     {
///         bslmt::LockGuardUnlock<my_Mutex> guard(mutex);
///
///         // mutex is unlocked here
///
///     } // mutex is locked again here
///
///     // critical section here
///
/// } // mutex is unlocked here
/// @endcode
/// Care must be taken so as not to interleave guard objects in such a way as to
/// cause an illegal sequence of calls on a lock (two sequential lock calls or
/// two sequential unlock calls on a non-recursive mutex).
/// @}
/** @} */
/** @} */
 
/** @addtogroup bsl
 * @{
 */
/** @addtogroup bslmt
 * @{
 */
/** @addtogroup bslmt_lockguard
 * @{
 */
 
#include <bslscm_version.h>
 
 
namespace bslmt {
 
                             // ===============
                             // class LockGuard
                             // ===============
 
/// This class template implements a guard for acquisition and release of
/// synchronization resources (i.e., locks).
///
/// See @ref bslmt_lockguard 
template <class T>
class LockGuard {
 
    // DATA
    T *d_lock_p;  // lock guarded by this object (held, not owned)
 
  private:
    // NOT IMPLEMENTED
    LockGuard(const LockGuard<T>&);
    LockGuard<T>& operator=(const LockGuard<T>&);
 
  public:
    // CREATORS
 
    /// Create a scoped guard that conditionally manages the specified
    /// `lock` (if non-null) and invokes `lock->lock()`.  Supplying a null
    /// `lock` has no effect.  The behavior is undefined unless `lock` (if
    /// non-null) is not already locked by this thread.  Note that `lock`
    /// must remain valid throughout the lifetime of this guard, or until
    /// `release` is called.
    explicit LockGuard(T *lock);
 
    /// Create a scoped guard that conditionally manages the specified
    /// `lock` (if non-null) and invokes `lock->lock()` if the specified
    /// `alreadyLockedFlag` is `false`.  Supplying a null `lock` has no
    /// effect.  The behavior is undefined unless the state of `lock` (if
    /// non-null) is consistent with `alreadyLockedFlag`.  Note that
    /// `alreadyLockedFlag` is used to indicate whether `lock` is in an
    /// already-locked state when passed, so if `alreadyLockedFlag` is
    /// `true` the `lock` method will *not* be called on the supplied
    /// `lock`.  Also note that `lock` must remain valid throughout the
    /// lifetime of this guard, or until `release` is called.
    LockGuard(T *lock, bool alreadyLockedFlag);
 
    /// Destroy this scoped guard and invoke the `unlock` method on the
    /// lock object under management by this guard, if any.  If no lock is
    /// currently being managed, this method has no effect.
    ~LockGuard();
 
    // MANIPULATORS
 
    /// Return the address of the modifiable lock object under management by
    /// this guard, and release the lock from further management by this
    /// guard.  If no lock is currently being managed, return 0 with no
    /// other effect.  Note that this operation does *not* unlock the lock
    /// object (if any) that was under management.
    T *release();
 
    // ACCESSORS
 
    /// Return the address of the modifiable lock object under management by
    /// this guard, or 0 if no lock is currently being managed.
    T *ptr() const;
};
 
                          // =====================
                          // class LockGuardUnlock
                          // =====================
 
/// This class template implements a guard for release and reacquisition
/// of synchronization resources (i.e., locks).
///
/// See @ref bslmt_lockguard 
template <class T>
class LockGuardUnlock {
 
    // DATA
    T *d_lock_p;  // lock guarded by this object (held, not owned)
 
  private:
    // NOT IMPLEMENTED
    LockGuardUnlock(const LockGuardUnlock<T>&);
    LockGuardUnlock<T>& operator=(const LockGuardUnlock<T>&);
 
  public:
    // CREATORS
 
    /// Create a scoped guard that conditionally manages the specified
    /// `lock` (if non-null) and invokes `lock->unlock()`.  Supplying a null
    /// `lock` has no effect.  The behavior is undefined unless `lock` (if
    /// non-null) is locked by this thread.  Note that `lock` must remain
    /// valid throughout the lifetime of this guard, or until `release` is
    /// called.
    explicit LockGuardUnlock(T *lock);
 
    /// Create a scoped guard that conditionally manages the specified
    /// `lock` (if non-null) and invokes `lock->unlock()` if the specified
    /// `alreadyUnlockedFlag` is `false`.  Supplying a null `lock` has no
    /// effect.  The behavior is undefined unless the state of `lock` (if
    /// non-null) is consistent with `alreadyUnlockedFlag`.  Note that
    /// `alreadyUnlockedFlag` is used to indicate whether `lock` is in an
    /// already-unlocked state when passed, so if `alreadyUnlockedFlag` is
    /// `true` the `unlock` method will *not* be called on the supplied
    /// `lock`.  Also note that `lock` must remain valid throughout the
    /// lifetime of this guard, or until `release` is called.
    LockGuardUnlock(T *lock, bool alreadyUnlockedFlag);
 
    /// Destroy this scoped guard and invoke the `lock` method on the lock
    /// object under management by this guard, if any.  If no lock is
    /// currently being managed, this method has no effect.
    ~LockGuardUnlock();
 
    // MANIPULATORS
 
    /// Return the address of the modifiable lock object under management by
    /// this guard, and release the lock from further management by this
    /// guard.  If no lock is currently being managed, return 0 with no
    /// other effect.  Note that this operation does *not* lock the lock
    /// object (if any) that was under management.
    T *release();
 
    // ACCESSORS
 
    /// Return the address of the modifiable lock object under management by
    /// this guard, or 0 if no lock is currently being managed.
    T *ptr() const;
};
 
                            // =================
                            // class UnLockGuard
                            // =================
 
/// This class is DEPRECATED.  Use `LockGuardUnlock` instead.
///
/// See @ref bslmt_lockguard 
template <class T>
class UnLockGuard : public LockGuardUnlock<T> {
 
  private:
    // NOT IMPLEMENTED
    UnLockGuard(const UnLockGuard<T>&);
    UnLockGuard<T>& operator=(const UnLockGuard<T>&);
 
  public:
    // CREATORS
 
    /// DEPRECATED: Use `LockGuardUnlock` instead.
    explicit UnLockGuard(T *lock);
 
    /// DEPRECATED: Use `LockGuardUnlock` instead.
    UnLockGuard(T *lock, bool alreadyUnlockedFlag);
};
 
                          // ======================
                          // class LockGuardTryLock
                          // ======================
 
/// This class template implements a guard for tentative acquisition and
/// release of synchronization resources (i.e., locks).
///
/// See @ref bslmt_lockguard 
template <class T>
class LockGuardTryLock {
 
    // DATA
    T *d_lock_p;  // lock guarded by this object (held, not owned)
 
  private:
    // NOT IMPLEMENTED
    LockGuardTryLock(const LockGuardTryLock<T>&);
    LockGuardTryLock<T>& operator=(const LockGuardTryLock<T>&);
 
  public:
    // CREATORS
 
    /// Create a scoped guard that conditionally manages the specified
    /// `lock` (if non-null) and invokes `lock->tryLock()` until the lock is
    /// acquired, or until the optionally specified `attempts` have been
    /// made to acquire the lock.  If `attempts` is not specified only one
    /// attempt is made to acquire the lock.  Supplying a null `lock` has no
    /// effect.  The behavior is undefined unless `lock` (if non-null) is
    /// not already locked by this thread and `0 < attempts`.  Note that
    /// `lock` must remain valid throughout the lifetime of this guard, or
    /// until `release` is called.
    explicit LockGuardTryLock(T *lock, int attempts = 1);
 
    /// Destroy this scoped guard and invoke the `unlock` method on the
    /// lock object under management by this guard, if any.  If no lock is
    /// currently being managed, this method has no effect.
    ~LockGuardTryLock();
 
    // MANIPULATORS
 
    /// Return the address of the modifiable lock object under management by
    /// this guard, and release the lock from further management by this
    /// guard.  If no lock is currently being managed, return 0 with no
    /// other effect.  Note that this operation does *not* unlock the lock
    /// object (if any) that was under management.
    T *release();
 
    // ACCESSORS
 
    /// Return the address of the modifiable lock object under management by
    /// this guard, or 0 if no lock is currently being managed.
    T *ptr() const;
};
 
                            // ==================
                            // class TryLockGuard
                            // ==================
 
/// @deprecated   Use @ref LockGuardTryLock instead.
///
/// See @ref bslmt_lockguard 
template <class T>
class TryLockGuard : public LockGuardTryLock<T> {
 
  private:
    // NOT IMPLEMENTED
    TryLockGuard(const TryLockGuard<T>&);
    TryLockGuard<T>& operator=(const TryLockGuard<T>&);
 
  public:
    // CREATORS
 
    /// @deprecated  Use @ref LockGuardTryLock instead.
    explicit TryLockGuard(T *lock, int attempts = 1);
};
 
// ============================================================================
//                             INLINE DEFINITIONS
// ============================================================================
 
                             // ---------------
                             // class LockGuard
                             // ---------------
 
// CREATORS
template <class T>
inline
LockGuard<T>::LockGuard(T *lock)
: d_lock_p(lock)
{
    if (d_lock_p) {
        d_lock_p->lock();
    }
}
 
template <class T>
inline
LockGuard<T>::LockGuard(T *lock, bool alreadyLockedFlag)
: d_lock_p(lock)
{
    if (d_lock_p && !alreadyLockedFlag) {
        d_lock_p->lock();
    }
}
 
template <class T>
inline
LockGuard<T>::~LockGuard()
{
    if (d_lock_p) {
        d_lock_p->unlock();
    }
}
 
// MANIPULATORS
template <class T>
inline
T *LockGuard<T>::release()
{
    T *lock  = d_lock_p;
    d_lock_p = 0;
    return lock;
}
 
// ACCESSORS
template <class T>
inline
T *LockGuard<T>::ptr() const
{
    return d_lock_p;
}
 
                            // -----------------
                            // class UnLockGuard
                            // -----------------
 
// CREATORS
template <class T>
inline
UnLockGuard<T>::UnLockGuard(T *lock)
: LockGuardUnlock<T>(lock)
{
}
 
template <class T>
inline
UnLockGuard<T>::UnLockGuard(T *lock, bool alreadyUnlockedFlag)
: LockGuardUnlock<T>(lock, alreadyUnlockedFlag)
{
}
 
                            // ------------------
                            // class TryLockGuard
                            // ------------------
 
template <class T>
inline
TryLockGuard<T>::TryLockGuard(T *lock, int attempts)
: LockGuardTryLock<T>(lock, attempts)
{
}
 
                          // ---------------------
                          // class LockGuardUnlock
                          // ---------------------
 
// CREATORS
template <class T>
inline
LockGuardUnlock<T>::LockGuardUnlock(T *lock)
: d_lock_p(lock)
{
    if (d_lock_p) {
        d_lock_p->unlock();
    }
}
 
template <class T>
inline
LockGuardUnlock<T>::LockGuardUnlock(T *lock, bool alreadyUnlockedFlag)
: d_lock_p(lock)
{
    if (d_lock_p && !alreadyUnlockedFlag) {
        d_lock_p->unlock();
    }
}
 
template <class T>
inline
LockGuardUnlock<T>::~LockGuardUnlock()
{
    if (d_lock_p) {
        d_lock_p->lock();
    }
}
 
// MANIPULATORS
template <class T>
inline
T *LockGuardUnlock<T>::release()
{
    T *lock  = d_lock_p;
    d_lock_p = 0;
    return lock;
}
 
// ACCESSORS
template <class T>
inline
T *LockGuardUnlock<T>::ptr() const
{
    return d_lock_p;
}
 
                          // ----------------------
                          // class LockGuardTryLock
                          // ----------------------
 
// CREATORS
template <class T>
LockGuardTryLock<T>::LockGuardTryLock(T *lock, int attempts)
: d_lock_p(0)
{
    if (lock) {
        while (attempts--) {
            if (!lock->tryLock()) {
                d_lock_p = lock;
                break;
            }
        }
    }
}
 
template <class T>
inline
LockGuardTryLock<T>::~LockGuardTryLock()
{
    if (d_lock_p) {
        d_lock_p->unlock();
    }
}
 
// MANIPULATORS
template <class T>
inline
T *LockGuardTryLock<T>::release()
{
    T *lock  = d_lock_p;
    d_lock_p = 0;
    return lock;
}
 
// ACCESSORS
template <class T>
inline
T *LockGuardTryLock<T>::ptr() const
{
    return d_lock_p;
}
 
}  // close package namespace
 
 
#endif
 
// ----------------------------------------------------------------------------
// Copyright 2015 Bloomberg Finance L.P.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// ----------------------------- END-OF-FILE ----------------------------------
 
/** @} */
/** @} */
/** @} */