mfbt/Maybe.h
author Mike Hommey <mh+mozilla@glandium.org>
Wed, 05 Apr 2017 13:48:52 +0900
changeset 402137 7ad4be04d16e0d86508c15c104f9686f2493061a
parent 393442 e0f2e64261cc3e1a535b1500f13cefe7e32be2dd
child 403490 054f46576d6da24893099b40c0d78881da96d19d
permissions -rw-r--r--
Bug 1344038 - Move the gio protocol handler under netwerk/protocol. r=chmanchester,karlt Historically, we had support for some GNOME VFS protocols through the gnomevfs library, and this was under extension. This may not have been built by default when it was introduced, but GNOME upstream moved those things into Gtk itself, and we then got support for the new Gio-based protocol, similar to what we had through the gnomevfs library. Time passes, and we switched off the gnomevfs library entirely, and enabled the Gio-based protocol handlers by default. We then removed everything related to the gnomevfs library. Fast forward to now, and disabling Gio support in Firefox just doesn't make sense, and leaving the gio protocol handler as an extension doesn't make sense either. As it is a protocol handler, its natural place is under netwerk/protocol, which is where we're moving it here. The netwerk/protocol subdirectories being handled automatically, we don't need to add the moved directory in any DIRS variable.

/* -*- 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/OperatorNewExtensions.h"
#include "mozilla/TypeTraits.h"

#include <new>  // for placement new
#include <ostream>
#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 MOZ_NON_PARAM Maybe
{
  alignas(T) unsigned char mStorage[sizeof(T)];
  char mIsSome; // not bool -- guarantees minimal space consumption

  // GCC fails due to -Werror=strict-aliasing if |mStorage| is directly cast to
  // T*.  Indirecting through these functions addresses the problem.
  void* data() { return mStorage; }
  const void* data() const { return mStorage; }

public:
  using ValueType = T;

  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 is convertible to T.
   */
  template<typename U,
           typename =
             typename std::enable_if<std::is_convertible<U, T>::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 is convertible to T.
   */
  template<typename U,
           typename =
             typename std::enable_if<std::is_convertible<U, T>::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) {
          ref() = aOther.ref();
        } else {
          emplace(*aOther);
        }
      } else {
        reset();
      }
    }
    return *this;
  }

  template<typename U,
           typename =
             typename std::enable_if<std::is_convertible<U, T>::value>::type>
  Maybe& operator=(const Maybe<U>& aOther)
  {
    if (aOther.isSome()) {
      if (mIsSome) {
        ref() = aOther.ref();
      } 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;
  }

  template<typename U,
           typename =
             typename std::enable_if<std::is_convertible<U, T>::value>::type>
  Maybe& operator=(Maybe<U>&& aOther)
  {
    if (aOther.isSome()) {
      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 *static_cast<T*>(data());
  }

  const T& ref() const
  {
    MOZ_ASSERT(mIsSome);
    return *static_cast<const T*>(data());
  }

  /*
   * 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 (KnownNotNull, data()) T(Forward<Args>(aArgs)...);
    mIsSome = true;
  }

  friend std::ostream&
  operator<<(std::ostream& aStream, const Maybe<T>& aMaybe)
  {
    if (aMaybe) {
      aStream << aMaybe.ref();
    } else {
      aStream << "<Nothing>";
    }
    return aStream;
  }
};

/*
 * 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,
         typename U = typename std::remove_cv<
           typename std::remove_reference<T>::type>::type>
Maybe<U>
Some(T&& aValue)
{
  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 */