hphp/util/default-ptr.h

   1 /*
   2    +----------------------------------------------------------------------+
   3    | HipHop for PHP                                                       |
   4    +----------------------------------------------------------------------+
   5    | Copyright (c) 2010-present Facebook, Inc. (http://www.facebook.com)  |
   6    +----------------------------------------------------------------------+
   7    | This source file is subject to version 3.01 of the PHP license,      |
   8    | that is bundled with this package in the file LICENSE, and is        |
   9    | available through the world-wide-web at the following url:           |
  10    | http://www.php.net/license/3_01.txt                                  |
  11    | If you did not receive a copy of the PHP license and are unable to   |
  12    | obtain it through the world-wide-web, please send a note to          |
  13    | license@php.net so we can mail you a copy immediately.               |
  14    +----------------------------------------------------------------------+
  15 */
  16
  17 #ifndef incl_HPHP_DEFAULT_PTR_H_
  18 #define incl_HPHP_DEFAULT_PTR_H_
  19
  20 #include <cstddef>
  21 #include <utility>
  22
  23 namespace HPHP {
  24 ///////////////////////////////////////////////////////////////////////////////
  25
  26 /*
  27  * Pointer which can be safely const-dereferenced when null to yield a
  28  * default-constructed value.
  29  */
  30 template <class T>
  31 struct default_ptr {
  32   /*
  33    * Constructors.
  34    */
  35   default_ptr() : m_p{fallback()} {}
  36
  37   /* implicit */ default_ptr(std::nullptr_t) {}
  38
  39   /* implicit */ default_ptr(T* p) : m_p{p ? p : fallback()} {}
  40
  41   /*
  42    * Thread-safe allocation.
  43    */
  44   T* ensureAllocated() {
  45     if (auto p = raw()) return p;
  46     auto ptr = new T();
  47     T* expected = fallback();
  48     if (!m_p.compare_exchange_strong(
  49           expected, ptr, std::memory_order_relaxed)) {
  50       // Already set by someone else, use theirs.
  51       delete ptr;
  52       return expected;
  53     } else {
  54       return ptr;
  55     }
  56   }
  57
  58   /*
  59    * Assignments.
  60    */
  61   default_ptr& operator=(std::nullptr_t /*p*/) {
  62     m_p.store(fallback(), std::memory_order_relaxed);
  63     return *this;
  64   }
  65   default_ptr& operator=(T* p) {
  66     m_p.store(p ? p : fallback(), std::memory_order_relaxed);
  67     return *this;
  68   }
  69
  70   /*
  71    * Observers.
  72    */
  73   const T* get() const {
  74     return m_p.load(std::memory_order_relaxed);
  75   }
  76   const T& operator*() const {
  77     return *get();
  78   }
  79   const T* operator->() const {
  80     return get();
  81   }
  82
  83   T* raw() const {
  84     auto p = m_p.load(std::memory_order_relaxed);
  85     return p == &s_default ? nullptr : p;
  86   }
  87   explicit operator bool() const {
  88     return raw();
  89   }
  90
  91   /*
  92    * Modifiers.
  93    */
  94   void reset(T* p = nullptr) {
  95     operator=(p);
  96   }
  97
  98 private:
  99   /*
 100    * Internals.
 101    */
 102   static T* fallback() {
 103     return const_cast<T*>(&s_default);
 104   }
 105
 106 private:
 107   std::atomic<T*> m_p{const_cast<T*>(&s_default)};
 108   static const T s_default;
 109 };
 110
 111 template <class T>
 112 const T default_ptr<T>::s_default;
 113
 114 ///////////////////////////////////////////////////////////////////////////////
 115 }
 116
 117 #endif // incl_HPHP_DEFAULT_PTR_H_