CachedPointer.icc

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/*
  Copyright (C) 2002-2019 CERN for the benefit of the ATLAS collaboration
*/
// $Id$
/**
 * @file CxxUtils/CachedPointer.icc
 * @author scott snyder <snyder@bnl.gov>
 * @date Nov, 2017, from earlier code in AthLinks.
 * @brief Cached pointer with atomic update.
 */
 
 
namespace CxxUtils {
 
 
/**
 * @brief Default constructor.  Sets the element to null.
 */
template <class T>
inline
CachedPointer<T>::CachedPointer()
  : m_a (nullptr)
{
}
 
 
/**
 * @brief Constructor from an element.
 */
template <class T>
inline
CachedPointer<T>::CachedPointer (pointer_t elt)
  : m_a (elt)
{
}
 
 
/**
 * @brief Copy constructor.
 */
template <class T>
inline
CachedPointer<T>::CachedPointer (const CachedPointer& other) noexcept
  : m_a (other.get())
{
}
 
/**
 * @brief Move constructor.
 */
template <class T>
inline
CachedPointer<T>::CachedPointer ( CachedPointer&& other) noexcept
  : m_a (other.get())
{
   //Does not own pointer so don't need to null
}
 
 
 
/**
 * @brief Assignment.
 */
template <class T>
inline
CachedPointer<T>&
CachedPointer<T>::operator= (const CachedPointer& other)
{
  if (this != &other) {
    store (other.get());
  }
  return *this;
}
 
 
/**
 * @brief Set the element, assuming it is currently null.
 * @param elt The new value for the element.
 *
 * The current value of the element must either be null, in which case,
 * it is set to ELT, or else it must already be equal to ELT, in which
 * case no write occurs.  Anything else is considered an error.
 *
 * The reason for doing things this way is that we (have to) allow
 * a pointer to the underlying element to escape to user code, and we
 * don't want a write to conflict with a non-atomic read.  (Such a read
 * can only happen after the element has been set for the first time.)
 */
template <class T>
inline
void CachedPointer<T>::set (pointer_t elt) const
{
  // Set the element to ELT if it is currently null.
  pointer_t null = nullptr;
  m_a.compare_exchange_strong (null, elt, std::memory_order_relaxed);
  // If the element was originally null, then NULL will still be null.
  // If the element was originally ELT, then no write will have been performed,
  // and NULL will be ELT.  If the element was originally something else,
  // then NULL will be set to that value.
  assert (null == nullptr || null == elt);
}
 
 
/**
 * @brief Store a new value to the element.
 */
template <class T>
inline
void CachedPointer<T>::store (pointer_t elt)
{
  m_a.store (elt, std::memory_order_relaxed);
}
 
 
/**
 * @brief Return the current value of the element.
 */
template <class T>
inline
typename CachedPointer<T>::pointer_t
CachedPointer<T>::get() const
{
  return m_a.load (std::memory_order_relaxed);
}
 
 
/**
 * @brief Return a pointer to the cached element.
 */
template <class T>
inline
const typename CachedPointer<T>::pointer_t*
CachedPointer<T>::ptr() const
{
  return &m_e;
}
 
 
} // namespace CxxUtils