third_party/WebKit/Source/wtf/Optional.h

   1 // Copyright 2015 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 Optional_h
   6 #define Optional_h
   7
   8 #include "wtf/Alignment.h"
   9 #include "wtf/Assertions.h"
  10 #include "wtf/Noncopyable.h"
  11 #include "wtf/StdLibExtras.h"
  12 #include "wtf/Utility.h"
  13
  14 namespace WTF {
  15
  16 // This is a lightweight template similar to std::experimental::optional.
  17 // It currently does not support assignment, swapping, comparison, etc.
  18 //
  19 // Use this instead of OwnPtr for cases where you only want to conditionally
  20 // construct a "scope" object.
  21 //
  22 // Example:
  23 //   Optional<DrawingRecorder> recorder;
  24 //   if (shouldDraw)
  25 //       recorder.emplace(constructor, args, here);
  26 //   // recorder destroyed at end of scope
  27 //
  28 // Note in particular that unlike a pointer, though, dereferencing a const
  29 // optional yields a const reference.
  30
  31 template <typename T>
  32 class Optional {
  33     WTF_MAKE_NONCOPYABLE(Optional);
  34 public:
  35     Optional() : m_ptr(nullptr) { }
  36     ~Optional()
  37     {
  38         if (m_ptr)
  39             m_ptr->~T();
  40     }
  41
  42     typedef T* Optional::*UnspecifiedBoolType;
  43     operator UnspecifiedBoolType() const { return m_ptr ? &Optional::m_ptr : nullptr; }
  44
  45     T& operator*() { ASSERT_WITH_SECURITY_IMPLICATION(m_ptr); return *m_ptr; }
  46     const T& operator*() const { ASSERT_WITH_SECURITY_IMPLICATION(m_ptr); return *m_ptr; }
  47     T* operator->() { ASSERT_WITH_SECURITY_IMPLICATION(m_ptr); return m_ptr; }
  48     const T* operator->() const { ASSERT_WITH_SECURITY_IMPLICATION(m_ptr); return m_ptr; }
  49
  50     template <typename... Args>
  51     void emplace(Args&&... args)
  52     {
  53         RELEASE_ASSERT(!m_ptr);
  54         m_ptr = reinterpret_cast_ptr<T*>(&m_storage.buffer);
  55         new (m_ptr) T(forward<Args>(args)...);
  56     }
  57
  58 private:
  59     T* m_ptr;
  60     AlignedBuffer<sizeof(T), WTF_ALIGN_OF(T)> m_storage;
  61 };
  62
  63 } // namespace WTF
  64
  65 using WTF::Optional;
  66
  67 #endif // Optional_h