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| 1 // Copyright 2014 The Chromium Authors. All rights reserved. |
| 2 // Use of this source code is governed by a BSD-style license that can be |
| 3 // found in the LICENSE file. |
| 4 |
| 5 #ifndef BASE_SCOPED_GENERIC_H_ |
| 6 #define BASE_SCOPED_GENERIC_H_ |
| 7 |
| 8 #include <stdlib.h> |
| 9 |
| 10 #include "base/compiler_specific.h" |
| 11 #include "base/move.h" |
| 12 |
| 13 namespace base { |
| 14 |
| 15 // This class acts like ScopedPtr with a custom deleter (although is slightly |
| 16 // less fancy in some of the more escoteric respects) except that it keeps a |
| 17 // copy of the object rather than a pointer, and we require that the contained |
| 18 // object has some kind of "invalid" value. |
| 19 // |
| 20 // Defining a scoper based on this class allows you to get a scoper for |
| 21 // non-pointer types without having to write custom code for set, reset, and |
| 22 // move, etc. and get almost identical semantics that people are used to from |
| 23 // scoped_ptr. |
| 24 // |
| 25 // It is intended that you will typedef this class with an appropriate deleter |
| 26 // to implement clean up tasks for objects that act like pointers from a |
| 27 // resource management standpoint but aren't, such as file descriptors and |
| 28 // various types of operating system handles. Using scoped_ptr for these |
| 29 // things requires that you keep a pointer to the handle valid for the lifetime |
| 30 // of the scoper (which is easy to mess up). |
| 31 // |
| 32 // For an object to be able to be put into a ScopedGeneric, it must support |
| 33 // standard copyable semantics and have a specific "invalid" value. The traits |
| 34 // must define a free function and also the invalid value to assign for |
| 35 // default-constructed and released objects. |
| 36 // |
| 37 // struct FooScopedTraits { |
| 38 // // It's assumed that this is a fast inline function with little-to-no |
| 39 // // penalty for duplicate calls. This must be a static function even |
| 40 // // for stateful traits. |
| 41 // static int InvalidValue() { |
| 42 // return 0; |
| 43 // } |
| 44 // |
| 45 // // This free function will not be called if f == InvalidValue()! |
| 46 // static void Free(int f) { |
| 47 // ::FreeFoo(f); |
| 48 // } |
| 49 // }; |
| 50 // |
| 51 // typedef ScopedGeneric<int, FooScopedTraits> ScopedFoo; |
| 52 template<typename T, typename Traits> |
| 53 class ScopedGeneric { |
| 54 MOVE_ONLY_TYPE_FOR_CPP_03(ScopedGeneric, RValue) |
| 55 |
| 56 private: |
| 57 // This must be first since it's used inline below. |
| 58 // |
| 59 // Use the empty base class optimization to allow us to have a D |
| 60 // member, while avoiding any space overhead for it when D is an |
| 61 // empty class. See e.g. http://www.cantrip.org/emptyopt.html for a good |
| 62 // discussion of this technique. |
| 63 struct Data : public Traits { |
| 64 explicit Data(const T& in) : generic(in) {} |
| 65 Data(const T& in, const Traits& other) : Traits(other), generic(in) {} |
| 66 T generic; |
| 67 }; |
| 68 |
| 69 public: |
| 70 typedef T element_type; |
| 71 typedef Traits traits_type; |
| 72 |
| 73 ScopedGeneric() : data_(traits_type::InvalidValue()) {} |
| 74 |
| 75 // Constructor. Takes responsibility for freeing the resource associated with |
| 76 // the object T. |
| 77 explicit ScopedGeneric(const element_type& value) : data_(value) {} |
| 78 |
| 79 // Constructor. Allows initialization of a stateful traits object. |
| 80 ScopedGeneric(const element_type& value, const traits_type& traits) |
| 81 : data_(value, traits) { |
| 82 } |
| 83 |
| 84 // Move constructor for C++03 move emulation. |
| 85 ScopedGeneric(RValue rvalue) |
| 86 : data_(rvalue.object->release(), rvalue.object->get_traits()) { |
| 87 } |
| 88 |
| 89 ~ScopedGeneric() { |
| 90 FreeIfNecessary(); |
| 91 } |
| 92 |
| 93 // Frees the currently owned object, if any. Then takes ownership of a new |
| 94 // object, if given. Self-resets are not allowd as on scoped_ptr. See |
| 95 // http://crbug.com/162971 |
| 96 void reset(const element_type& value = traits_type::InvalidValue()) { |
| 97 if (data_.generic != traits_type::InvalidValue() && data_.generic == value) |
| 98 abort(); |
| 99 FreeIfNecessary(); |
| 100 data_.generic = value; |
| 101 } |
| 102 |
| 103 void swap(ScopedGeneric& other) { |
| 104 // Standard swap idiom: 'using std::swap' ensures that std::swap is |
| 105 // present in the overload set, but we call swap unqualified so that |
| 106 // any more-specific overloads can be used, if available. |
| 107 using std::swap; |
| 108 swap(static_cast<Traits&>(data_), static_cast<Traits&>(other.data_)); |
| 109 swap(data_.generic, other.data_.generic); |
| 110 } |
| 111 |
| 112 // Release the object. The return value is the current object held by this |
| 113 // object. After this operation, this object will hold a null value, and |
| 114 // will not own the object any more. |
| 115 element_type release() WARN_UNUSED_RESULT { |
| 116 element_type old_generic = data_.generic; |
| 117 data_.generic = traits_type::InvalidValue(); |
| 118 return old_generic; |
| 119 } |
| 120 |
| 121 const element_type& get() const { return data_.generic; } |
| 122 |
| 123 // Returns true if this object doesn't hold the special null value for the |
| 124 // associated data type. |
| 125 bool is_valid() const { return data_.generic != traits_type::InvalidValue(); } |
| 126 |
| 127 bool operator==(const element_type& value) const { |
| 128 return data_.generic == value; |
| 129 } |
| 130 bool operator!=(const element_type& value) const { |
| 131 return data_.generic != value; |
| 132 } |
| 133 |
| 134 Traits& get_traits() { return data_; } |
| 135 const Traits& get_traits() const { return data_; } |
| 136 |
| 137 private: |
| 138 void FreeIfNecessary() { |
| 139 if (data_.generic != traits_type::InvalidValue()) { |
| 140 data_.Free(data_.generic); |
| 141 data_.generic = traits_type::InvalidValue(); |
| 142 } |
| 143 } |
| 144 |
| 145 // Forbid comparison. If U != T, it totally doesn't make sense, and if U == |
| 146 // T, it still doesn't make sense because you should never have the same |
| 147 // object owned by two different ScopedGenerics. |
| 148 template <typename T2, typename Traits2> bool operator==( |
| 149 const ScopedGeneric<T2, Traits2>& p2) const; |
| 150 template <typename T2, typename Traits2> bool operator!=( |
| 151 const ScopedGeneric<T2, Traits2>& p2) const; |
| 152 |
| 153 Data data_; |
| 154 }; |
| 155 |
| 156 template<class T, class Traits> |
| 157 void swap(const ScopedGeneric<T, Traits>& a, |
| 158 const ScopedGeneric<T, Traits>& b) { |
| 159 a.swap(b); |
| 160 } |
| 161 |
| 162 template<class T, class Traits> |
| 163 bool operator==(const T& value, const ScopedGeneric<T, Traits>& scoped) { |
| 164 return value == scoped.get(); |
| 165 } |
| 166 |
| 167 template<class T, class Traits> |
| 168 bool operator!=(const T& value, const ScopedGeneric<T, Traits>& scoped) { |
| 169 return value != scoped.get(); |
| 170 } |
| 171 |
| 172 } // namespace base |
| 173 |
| 174 #endif // BASE_SCOPED_GENERIC_H_ |
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