Add m-esr52 at 52.6.0

1 files changed, 551 insertions, 0 deletions

diff --git a/mfbt/Maybe.h b/mfbt/Maybe.h
new file mode 100644
index 000000000..2a601ac49
--- /dev/null
+++ b/mfbt/Maybe.h
@@ -0,0 +1,551 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* A class for optional values and in-place lazy construction. */
+
+#ifndef mozilla_Maybe_h
+#define mozilla_Maybe_h
+
+#include "mozilla/Alignment.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/Move.h"
+#include "mozilla/TypeTraits.h"
+
+#include <new>  // for placement new
+#include <type_traits>
+
+namespace mozilla {
+
+struct Nothing { };
+
+/*
+ * Maybe is a container class which contains either zero or one elements. It
+ * serves two roles. It can represent values which are *semantically* optional,
+ * augmenting a type with an explicit 'Nothing' value. In this role, it provides
+ * methods that make it easy to work with values that may be missing, along with
+ * equality and comparison operators so that Maybe values can be stored in
+ * containers. Maybe values can be constructed conveniently in expressions using
+ * type inference, as follows:
+ *
+ *   void doSomething(Maybe<Foo> aFoo) {
+ *     if (aFoo)                  // Make sure that aFoo contains a value...
+ *       aFoo->takeAction();      // and then use |aFoo->| to access it.
+ *   }                            // |*aFoo| also works!
+ *
+ *   doSomething(Nothing());      // Passes a Maybe<Foo> containing no value.
+ *   doSomething(Some(Foo(100))); // Passes a Maybe<Foo> containing |Foo(100)|.
+ *
+ * You'll note that it's important to check whether a Maybe contains a value
+ * before using it, using conversion to bool, |isSome()|, or |isNothing()|. You
+ * can avoid these checks, and sometimes write more readable code, using
+ * |valueOr()|, |ptrOr()|, and |refOr()|, which allow you to retrieve the value
+ * in the Maybe and provide a default for the 'Nothing' case.  You can also use
+ * |apply()| to call a function only if the Maybe holds a value, and |map()| to
+ * transform the value in the Maybe, returning another Maybe with a possibly
+ * different type.
+ *
+ * Maybe's other role is to support lazily constructing objects without using
+ * dynamic storage. A Maybe directly contains storage for a value, but it's
+ * empty by default. |emplace()|, as mentioned above, can be used to construct a
+ * value in Maybe's storage.  The value a Maybe contains can be destroyed by
+ * calling |reset()|; this will happen automatically if a Maybe is destroyed
+ * while holding a value.
+ *
+ * It's a common idiom in C++ to use a pointer as a 'Maybe' type, with a null
+ * value meaning 'Nothing' and any other value meaning 'Some'. You can convert
+ * from such a pointer to a Maybe value using 'ToMaybe()'.
+ *
+ * Maybe is inspired by similar types in the standard library of many other
+ * languages (e.g. Haskell's Maybe and Rust's Option). In the C++ world it's
+ * very similar to std::optional, which was proposed for C++14 and originated in
+ * Boost. The most important differences between Maybe and std::optional are:
+ *
+ *   - std::optional<T> may be compared with T. We deliberately forbid that.
+ *   - std::optional allows in-place construction without a separate call to
+ *     |emplace()| by using a dummy |in_place_t| value to tag the appropriate
+ *     constructor.
+ *   - std::optional has |valueOr()|, equivalent to Maybe's |valueOr()|, but
+ *     lacks corresponding methods for |refOr()| and |ptrOr()|.
+ *   - std::optional lacks |map()| and |apply()|, making it less suitable for
+ *     functional-style code.
+ *   - std::optional lacks many convenience functions that Maybe has. Most
+ *     unfortunately, it lacks equivalents of the type-inferred constructor
+ *     functions |Some()| and |Nothing()|.
+ *
+ * N.B. GCC has missed optimizations with Maybe in the past and may generate
+ * extra branches/loads/stores. Use with caution on hot paths; it's not known
+ * whether or not this is still a problem.
+ */
+template<class T>
+class Maybe
+{
+  bool mIsSome;
+  AlignedStorage2<T> mStorage;
+
+public:
+  typedef T ValueType;
+
+  Maybe() : mIsSome(false) { }
+  ~Maybe() { reset(); }
+
+  MOZ_IMPLICIT Maybe(Nothing) : mIsSome(false) { }
+
+  Maybe(const Maybe& aOther)
+    : mIsSome(false)
+  {
+    if (aOther.mIsSome) {
+      emplace(*aOther);
+    }
+  }
+
+  /**
+   * Maybe<T*> can be copy-constructed from a Maybe<U*> if U* and T* are
+   * compatible, or from Maybe<decltype(nullptr)>.
+   */
+  template<typename U,
+           typename =
+             typename std::enable_if<std::is_pointer<T>::value &&
+                                     (std::is_same<U, decltype(nullptr)>::value ||
+                                      (std::is_pointer<U>::value &&
+                                       std::is_base_of<typename std::remove_pointer<T>::type,
+                                                       typename std::remove_pointer<U>::type>::value))>::type>
+  MOZ_IMPLICIT
+  Maybe(const Maybe<U>& aOther)
+    : mIsSome(false)
+  {
+    if (aOther.isSome()) {
+      emplace(*aOther);
+    }
+  }
+
+  Maybe(Maybe&& aOther)
+    : mIsSome(false)
+  {
+    if (aOther.mIsSome) {
+      emplace(Move(*aOther));
+      aOther.reset();
+    }
+  }
+
+  /**
+   * Maybe<T*> can be move-constructed from a Maybe<U*> if U* and T* are
+   * compatible, or from Maybe<decltype(nullptr)>.
+   */
+  template<typename U,
+           typename =
+             typename std::enable_if<std::is_pointer<T>::value &&
+                                     (std::is_same<U, decltype(nullptr)>::value ||
+                                      (std::is_pointer<U>::value &&
+                                       std::is_base_of<typename std::remove_pointer<T>::type,
+                                                       typename std::remove_pointer<U>::type>::value))>::type>
+  MOZ_IMPLICIT
+  Maybe(Maybe<U>&& aOther)
+    : mIsSome(false)
+  {
+    if (aOther.isSome()) {
+      emplace(Move(*aOther));
+      aOther.reset();
+    }
+  }
+
+  Maybe& operator=(const Maybe& aOther)
+  {
+    if (&aOther != this) {
+      if (aOther.mIsSome) {
+        if (mIsSome) {
+          // XXX(seth): The correct code for this branch, below, can't be used
+          // due to a bug in Visual Studio 2010. See bug 1052940.
+          /*
+          ref() = aOther.ref();
+          */
+          reset();
+          emplace(*aOther);
+        } else {
+          emplace(*aOther);
+        }
+      } else {
+        reset();
+      }
+    }
+    return *this;
+  }
+
+  Maybe& operator=(Maybe&& aOther)
+  {
+    MOZ_ASSERT(this != &aOther, "Self-moves are prohibited");
+
+    if (aOther.mIsSome) {
+      if (mIsSome) {
+        ref() = Move(aOther.ref());
+      } else {
+        emplace(Move(*aOther));
+      }
+      aOther.reset();
+    } else {
+      reset();
+    }
+
+    return *this;
+  }
+
+  /* Methods that check whether this Maybe contains a value */
+  explicit operator bool() const { return isSome(); }
+  bool isSome() const { return mIsSome; }
+  bool isNothing() const { return !mIsSome; }
+
+  /* Returns the contents of this Maybe<T> by value. Unsafe unless |isSome()|. */
+  T value() const
+  {
+    MOZ_ASSERT(mIsSome);
+    return ref();
+  }
+
+  /*
+   * Returns the contents of this Maybe<T> by value. If |isNothing()|, returns
+   * the default value provided.
+   */
+  template<typename V>
+  T valueOr(V&& aDefault) const
+  {
+    if (isSome()) {
+      return ref();
+    }
+    return Forward<V>(aDefault);
+  }
+
+  /*
+   * Returns the contents of this Maybe<T> by value. If |isNothing()|, returns
+   * the value returned from the function or functor provided.
+   */
+  template<typename F>
+  T valueOrFrom(F&& aFunc) const
+  {
+    if (isSome()) {
+      return ref();
+    }
+    return aFunc();
+  }
+
+  /* Returns the contents of this Maybe<T> by pointer. Unsafe unless |isSome()|. */
+  T* ptr()
+  {
+    MOZ_ASSERT(mIsSome);
+    return &ref();
+  }
+
+  const T* ptr() const
+  {
+    MOZ_ASSERT(mIsSome);
+    return &ref();
+  }
+
+  /*
+   * Returns the contents of this Maybe<T> by pointer. If |isNothing()|,
+   * returns the default value provided.
+   */
+  T* ptrOr(T* aDefault)
+  {
+    if (isSome()) {
+      return ptr();
+    }
+    return aDefault;
+  }
+
+  const T* ptrOr(const T* aDefault) const
+  {
+    if (isSome()) {
+      return ptr();
+    }
+    return aDefault;
+  }
+
+  /*
+   * Returns the contents of this Maybe<T> by pointer. If |isNothing()|,
+   * returns the value returned from the function or functor provided.
+   */
+  template<typename F>
+  T* ptrOrFrom(F&& aFunc)
+  {
+    if (isSome()) {
+      return ptr();
+    }
+    return aFunc();
+  }
+
+  template<typename F>
+  const T* ptrOrFrom(F&& aFunc) const
+  {
+    if (isSome()) {
+      return ptr();
+    }
+    return aFunc();
+  }
+
+  T* operator->()
+  {
+    MOZ_ASSERT(mIsSome);
+    return ptr();
+  }
+
+  const T* operator->() const
+  {
+    MOZ_ASSERT(mIsSome);
+    return ptr();
+  }
+
+  /* Returns the contents of this Maybe<T> by ref. Unsafe unless |isSome()|. */
+  T& ref()
+  {
+    MOZ_ASSERT(mIsSome);
+    return *mStorage.addr();
+  }
+
+  const T& ref() const
+  {
+    MOZ_ASSERT(mIsSome);
+    return *mStorage.addr();
+  }
+
+  /*
+   * Returns the contents of this Maybe<T> by ref. If |isNothing()|, returns
+   * the default value provided.
+   */
+  T& refOr(T& aDefault)
+  {
+    if (isSome()) {
+      return ref();
+    }
+    return aDefault;
+  }
+
+  const T& refOr(const T& aDefault) const
+  {
+    if (isSome()) {
+      return ref();
+    }
+    return aDefault;
+  }
+
+  /*
+   * Returns the contents of this Maybe<T> by ref. If |isNothing()|, returns the
+   * value returned from the function or functor provided.
+   */
+  template<typename F>
+  T& refOrFrom(F&& aFunc)
+  {
+    if (isSome()) {
+      return ref();
+    }
+    return aFunc();
+  }
+
+  template<typename F>
+  const T& refOrFrom(F&& aFunc) const
+  {
+    if (isSome()) {
+      return ref();
+    }
+    return aFunc();
+  }
+
+  T& operator*()
+  {
+    MOZ_ASSERT(mIsSome);
+    return ref();
+  }
+
+  const T& operator*() const
+  {
+    MOZ_ASSERT(mIsSome);
+    return ref();
+  }
+
+  /* If |isSome()|, runs the provided function or functor on the contents of
+   * this Maybe. */
+  template<typename Func>
+  Maybe& apply(Func aFunc)
+  {
+    if (isSome()) {
+      aFunc(ref());
+    }
+    return *this;
+  }
+
+  template<typename Func>
+  const Maybe& apply(Func aFunc) const
+  {
+    if (isSome()) {
+      aFunc(ref());
+    }
+    return *this;
+  }
+
+  /*
+   * If |isSome()|, runs the provided function and returns the result wrapped
+   * in a Maybe. If |isNothing()|, returns an empty Maybe value.
+   */
+  template<typename Func>
+  auto map(Func aFunc) -> Maybe<decltype(aFunc(DeclVal<Maybe<T>>().ref()))>
+  {
+    using ReturnType = decltype(aFunc(ref()));
+    if (isSome()) {
+      Maybe<ReturnType> val;
+      val.emplace(aFunc(ref()));
+      return val;
+    }
+    return Maybe<ReturnType>();
+  }
+
+  template<typename Func>
+  auto map(Func aFunc) const -> Maybe<decltype(aFunc(DeclVal<Maybe<T>>().ref()))>
+  {
+    using ReturnType = decltype(aFunc(ref()));
+    if (isSome()) {
+      Maybe<ReturnType> val;
+      val.emplace(aFunc(ref()));
+      return val;
+    }
+    return Maybe<ReturnType>();
+  }
+
+  /* If |isSome()|, empties this Maybe and destroys its contents. */
+  void reset()
+  {
+    if (isSome()) {
+      ref().T::~T();
+      mIsSome = false;
+    }
+  }
+
+  /*
+   * Constructs a T value in-place in this empty Maybe<T>'s storage. The
+   * arguments to |emplace()| are the parameters to T's constructor.
+   */
+  template<typename... Args>
+  void emplace(Args&&... aArgs)
+  {
+    MOZ_ASSERT(!mIsSome);
+    ::new (mStorage.addr()) T(Forward<Args>(aArgs)...);
+    mIsSome = true;
+  }
+};
+
+/*
+ * Some() creates a Maybe<T> value containing the provided T value. If T has a
+ * move constructor, it's used to make this as efficient as possible.
+ *
+ * Some() selects the type of Maybe it returns by removing any const, volatile,
+ * or reference qualifiers from the type of the value you pass to it. This gives
+ * it more intuitive behavior when used in expressions, but it also means that
+ * if you need to construct a Maybe value that holds a const, volatile, or
+ * reference value, you need to use emplace() instead.
+ */
+template<typename T>
+Maybe<typename RemoveCV<typename RemoveReference<T>::Type>::Type>
+Some(T&& aValue)
+{
+  typedef typename RemoveCV<typename RemoveReference<T>::Type>::Type U;
+  Maybe<U> value;
+  value.emplace(Forward<T>(aValue));
+  return value;
+}
+
+template<typename T>
+Maybe<typename RemoveCV<typename RemoveReference<T>::Type>::Type>
+ToMaybe(T* aPtr)
+{
+  if (aPtr) {
+    return Some(*aPtr);
+  }
+  return Nothing();
+}
+
+/*
+ * Two Maybe<T> values are equal if
+ * - both are Nothing, or
+ * - both are Some, and the values they contain are equal.
+ */
+template<typename T> bool
+operator==(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
+{
+  if (aLHS.isNothing() != aRHS.isNothing()) {
+    return false;
+  }
+  return aLHS.isNothing() || *aLHS == *aRHS;
+}
+
+template<typename T> bool
+operator!=(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
+{
+  return !(aLHS == aRHS);
+}
+
+/*
+ * We support comparison to Nothing to allow reasonable expressions like:
+ *   if (maybeValue == Nothing()) { ... }
+ */
+template<typename T> bool
+operator==(const Maybe<T>& aLHS, const Nothing& aRHS)
+{
+  return aLHS.isNothing();
+}
+
+template<typename T> bool
+operator!=(const Maybe<T>& aLHS, const Nothing& aRHS)
+{
+  return !(aLHS == aRHS);
+}
+
+template<typename T> bool
+operator==(const Nothing& aLHS, const Maybe<T>& aRHS)
+{
+  return aRHS.isNothing();
+}
+
+template<typename T> bool
+operator!=(const Nothing& aLHS, const Maybe<T>& aRHS)
+{
+  return !(aLHS == aRHS);
+}
+
+/*
+ * Maybe<T> values are ordered in the same way T values are ordered, except that
+ * Nothing comes before anything else.
+ */
+template<typename T> bool
+operator<(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
+{
+  if (aLHS.isNothing()) {
+    return aRHS.isSome();
+  }
+  if (aRHS.isNothing()) {
+    return false;
+  }
+  return *aLHS < *aRHS;
+}
+
+template<typename T> bool
+operator>(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
+{
+  return !(aLHS < aRHS || aLHS == aRHS);
+}
+
+template<typename T> bool
+operator<=(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
+{
+  return aLHS < aRHS || aLHS == aRHS;
+}
+
+template<typename T> bool
+operator>=(const Maybe<T>& aLHS, const Maybe<T>& aRHS)
+{
+  return !(aLHS < aRHS);
+}
+
+} // namespace mozilla
+
+#endif /* mozilla_Maybe_h */