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() = default;
  36
  37   /* implicit */ default_ptr(std::nullptr_t) {}
  38
  39   /* implicit */ default_ptr(T* p) : m_p{p ? p : fallback()} {}
  40
  41   /*
  42    * Assignments.
  43    */
  44   default_ptr& operator=(std::nullptr_t p) {
  45     m_p = fallback();
  46     return *this;
  47   }
  48   default_ptr& operator=(T* p) {
  49     m_p = p ? p : fallback();
  50     return *this;
  51   }
  52
  53   /*
  54    * Observers.
  55    */
  56   const T* get() const {
  57     return m_p;
  58   }
  59   const T& operator*() const {
  60     return *get();
  61   }
  62   const T* operator->() const {
  63     return get();
  64   }
  65
  66   T* raw() const {
  67     return m_p == &s_default ? nullptr : m_p;
  68   }
  69   explicit operator bool() const {
  70     return raw();
  71   }
  72
  73   /*
  74    * Modifiers.
  75    */
  76   void reset(T* p = nullptr) {
  77     operator=(p);
  78   }
  79
  80 private:
  81   /*
  82    * Internals.
  83    */
  84   static T* fallback() {
  85     return const_cast<T*>(&s_default);
  86   }
  87
  88 private:
  89   T* m_p{const_cast<T*>(&s_default)};
  90   static const T s_default;
  91 };
  92
  93 template <class T>
  94 const T default_ptr<T>::s_default;
  95
  96 ///////////////////////////////////////////////////////////////////////////////
  97 }
  98
  99 #endif // incl_HPHP_DEFAULT_PTR_H_