1 // Copyright 2011 the V8 project 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 V8_MAYBE_HANDLES_H_
6 #define V8_MAYBE_HANDLES_H_
7 
8 #include <type_traits>
9 
10 #include "src/handles.h"
11 
12 namespace v8 {
13 namespace internal {
14 
15 // ----------------------------------------------------------------------------
16 // A Handle can be converted into a MaybeHandle. Converting a MaybeHandle
17 // into a Handle requires checking that it does not point to nullptr.  This
18 // ensures nullptr checks before use.
19 //
20 // Also note that Handles do not provide default equality comparison or hashing
21 // operators on purpose. Such operators would be misleading, because intended
22 // semantics is ambiguous between Handle location and object identity.
23 template <typename T>
24 class MaybeHandle final {
25  public:
MaybeHandle()26   V8_INLINE MaybeHandle() {}
27 
28   // Constructor for handling automatic up casting from Handle.
29   // Ex. Handle<JSArray> can be passed when MaybeHandle<Object> is expected.
30   template <typename S, typename = typename std::enable_if<
31                             std::is_convertible<S*, T*>::value>::type>
MaybeHandle(Handle<S> handle)32   V8_INLINE MaybeHandle(Handle<S> handle)
33       : location_(reinterpret_cast<T**>(handle.location_)) {}
34 
35   // Constructor for handling automatic up casting.
36   // Ex. MaybeHandle<JSArray> can be passed when Handle<Object> is expected.
37   template <typename S, typename = typename std::enable_if<
38                             std::is_convertible<S*, T*>::value>::type>
MaybeHandle(MaybeHandle<S> maybe_handle)39   V8_INLINE MaybeHandle(MaybeHandle<S> maybe_handle)
40       : location_(reinterpret_cast<T**>(maybe_handle.location_)) {}
41 
42   V8_INLINE MaybeHandle(T* object, Isolate* isolate);
43 
Assert()44   V8_INLINE void Assert() const { DCHECK_NOT_NULL(location_); }
Check()45   V8_INLINE void Check() const { CHECK_NOT_NULL(location_); }
46 
ToHandleChecked()47   V8_INLINE Handle<T> ToHandleChecked() const {
48     Check();
49     return Handle<T>(location_);
50   }
51 
52   // Convert to a Handle with a type that can be upcasted to.
53   template <typename S>
ToHandle(Handle<S> * out)54   V8_INLINE bool ToHandle(Handle<S>* out) const {
55     if (location_ == nullptr) {
56       *out = Handle<T>::null();
57       return false;
58     } else {
59       *out = Handle<T>(location_);
60       return true;
61     }
62   }
63 
64   // Returns the raw address where this handle is stored. This should only be
65   // used for hashing handles; do not ever try to dereference it.
address()66   V8_INLINE Address address() const { return bit_cast<Address>(location_); }
67 
is_null()68   bool is_null() const { return location_ == nullptr; }
69 
70  protected:
71   T** location_ = nullptr;
72 
73   // MaybeHandles of different classes are allowed to access each
74   // other's location_.
75   template <typename>
76   friend class MaybeHandle;
77 };
78 
79 // A handle which contains a potentially weak pointer. Keeps it alive (strongly)
80 // while the MaybeObjectHandle is alive.
81 class MaybeObjectHandle {
82  public:
83   inline MaybeObjectHandle();
84   inline MaybeObjectHandle(MaybeObject* object, Isolate* isolate);
85   inline MaybeObjectHandle(Object* object, Isolate* isolate);
86   inline explicit MaybeObjectHandle(Handle<Object> object);
87 
88   static inline MaybeObjectHandle Weak(Object* object, Isolate* isolate);
89   static inline MaybeObjectHandle Weak(Handle<Object> object);
90 
91   inline MaybeObject* operator*() const;
92   inline MaybeObject* operator->() const;
93   inline Handle<Object> object() const;
94 
is_identical_to(const MaybeObjectHandle & other)95   bool is_identical_to(const MaybeObjectHandle& other) const {
96     Handle<Object> this_handle;
97     Handle<Object> other_handle;
98     return reference_type_ == other.reference_type_ &&
99            handle_.ToHandle(&this_handle) ==
100                other.handle_.ToHandle(&other_handle) &&
101            this_handle.is_identical_to(other_handle);
102   }
103 
is_null()104   bool is_null() const { return handle_.is_null(); }
105 
106  private:
107   inline MaybeObjectHandle(Object* object,
108                            HeapObjectReferenceType reference_type,
109                            Isolate* isolate);
110   inline MaybeObjectHandle(Handle<Object> object,
111                            HeapObjectReferenceType reference_type);
112 
113   HeapObjectReferenceType reference_type_;
114   MaybeHandle<Object> handle_;
115 };
116 
117 }  // namespace internal
118 }  // namespace v8
119 
120 #endif  // V8_MAYBE_HANDLES_H_
121