mfbt/NotNull.h
author Mike Hommey <mh+mozilla@glandium.org>
Wed, 05 Apr 2017 13:48:52 +0900
changeset 402137 7ad4be04d16e0d86508c15c104f9686f2493061a
parent 340471 19f6a028f736c65e4848d901d33d6bad68294ee6
child 416501 3411f1c818f69059bfb93b4fcb105f41647846b6
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/. */

#ifndef mozilla_NotNull_h
#define mozilla_NotNull_h

// It's often unclear if a particular pointer, be it raw (T*) or smart
// (RefPtr<T>, nsCOMPtr<T>, etc.) can be null. This leads to missing null
// checks (which can cause crashes) and unnecessary null checks (which clutter
// the code).
//
// C++ has a built-in alternative that avoids these problems: references. This
// module defines another alternative, NotNull, which can be used in cases
// where references are not suitable.
//
// In the comments below we use the word "handle" to cover all varieties of
// pointers and references.
//
// References
// ----------
// References are always non-null. (You can do |T& r = *p;| where |p| is null,
// but that's undefined behaviour. C++ doesn't provide any built-in, ironclad
// guarantee of non-nullness.)
//
// A reference works well when you need a temporary handle to an existing
// single object, e.g. for passing a handle to a function, or as a local handle
// within another object. (In Rust parlance, this is a "borrow".)
//
// A reference is less appropriate in the following cases.
//
// - As a primary handle to an object. E.g. code such as this is possible but
//   strange: |T& t = *new T(); ...; delete &t;|
//
// - As a handle to an array. It's common for |T*| to refer to either a single
//   |T| or an array of |T|, but |T&| cannot refer to an array of |T| because
//   you can't index off a reference (at least, not without first converting it
//   to a pointer).
//
// - When the handle identity is meaningful, e.g. if you have a hashtable of
//   handles, because you have to use |&| on the reference to convert it to a
//   pointer.
//
// - Some people don't like using non-const references as function parameters,
//   because it is not clear at the call site that the argument might be
//   modified.
//
// - When you need "smart" behaviour. E.g. we lack reference equivalents to
//   RefPtr and nsCOMPtr.
//
// - When interfacing with code that uses pointers a lot, sometimes using a
//   reference just feels like an odd fit.
//
// Furthermore, a reference is impossible in the following cases.
//
// - When the handle is rebound to another object. References don't allow this.
//
// - When the handle has type |void|. |void&| is not allowed.
//
// NotNull is an alternative that can be used in any of the above cases except
// for the last one, where the handle type is |void|. See below.

#include "mozilla/Assertions.h"

namespace mozilla {

// NotNull can be used to wrap a "base" pointer (raw or smart) to indicate it
// is not null. Some examples:
//
// - NotNull<char*>
// - NotNull<RefPtr<Event>>
// - NotNull<nsCOMPtr<Event>>
//
// NotNull has the following notable properties.
//
// - It has zero space overhead.
//
// - It must be initialized explicitly. There is no default initialization.
//
// - It auto-converts to the base pointer type.
//
// - It does not auto-convert from a base pointer. Implicit conversion from a
//   less-constrained type (e.g. T*) to a more-constrained type (e.g.
//   NotNull<T*>) is dangerous. Creation and assignment from a base pointer can
//   only be done with WrapNotNull(), which makes them impossible to overlook,
//   both when writing and reading code.
//
// - When initialized (or assigned) it is checked, and if it is null we abort.
//   This guarantees that it cannot be null.
//
// - |operator bool()| is deleted. This means you cannot check a NotNull in a
//   boolean context, which eliminates the possibility of unnecessary null
//   checks.
//
// NotNull currently doesn't work with UniquePtr. See
// https://github.com/Microsoft/GSL/issues/89 for some discussion.
//
template <typename T>
class NotNull
{
  template <typename U> friend NotNull<U> WrapNotNull(U aBasePtr);

  T mBasePtr;

  // This constructor is only used by WrapNotNull().
  template <typename U>
  explicit NotNull(U aBasePtr) : mBasePtr(aBasePtr) {}

public:
  // Disallow default construction.
  NotNull() = delete;

  // Construct/assign from another NotNull with a compatible base pointer type.
  template <typename U>
  MOZ_IMPLICIT NotNull(const NotNull<U>& aOther) : mBasePtr(aOther.get()) {}

  // Default copy/move construction and assignment.
  NotNull(const NotNull<T>&) = default;
  NotNull<T>& operator=(const NotNull<T>&) = default;
  NotNull(NotNull<T>&&) = default;
  NotNull<T>& operator=(NotNull<T>&&) = default;

  // Disallow null checks, which are unnecessary for this type.
  explicit operator bool() const = delete;

  // Explicit conversion to a base pointer. Use only to resolve ambiguity or to
  // get a castable pointer.
  const T& get() const { return mBasePtr; }

  // Implicit conversion to a base pointer. Preferable to get().
  operator const T&() const { return get(); }

  // Dereference operators.
  const T& operator->() const { return get(); }
  decltype(*mBasePtr) operator*() const { return *mBasePtr; }
};

template <typename T>
NotNull<T>
WrapNotNull(const T aBasePtr)
{
  NotNull<T> notNull(aBasePtr);
  MOZ_RELEASE_ASSERT(aBasePtr);
  return notNull;
}

// Compare two NotNulls.
template <typename T, typename U>
inline bool
operator==(const NotNull<T>& aLhs, const NotNull<U>& aRhs)
{
  return aLhs.get() == aRhs.get();
}
template <typename T, typename U>
inline bool
operator!=(const NotNull<T>& aLhs, const NotNull<U>& aRhs)
{
  return aLhs.get() != aRhs.get();
}

// Compare a NotNull to a base pointer.
template <typename T, typename U>
inline bool
operator==(const NotNull<T>& aLhs, const U& aRhs)
{
  return aLhs.get() == aRhs;
}
template <typename T, typename U>
inline bool
operator!=(const NotNull<T>& aLhs, const U& aRhs)
{
  return aLhs.get() != aRhs;
}

// Compare a base pointer to a NotNull.
template <typename T, typename U>
inline bool
operator==(const T& aLhs, const NotNull<U>& aRhs)
{
  return aLhs == aRhs.get();
}
template <typename T, typename U>
inline bool
operator!=(const T& aLhs, const NotNull<U>& aRhs)
{
  return aLhs != aRhs.get();
}

// Disallow comparing a NotNull to a nullptr.
template <typename T>
bool
operator==(const NotNull<T>&, decltype(nullptr)) = delete;
template <typename T>
bool
operator!=(const NotNull<T>&, decltype(nullptr)) = delete;

// Disallow comparing a nullptr to a NotNull.
template <typename T>
bool
operator==(decltype(nullptr), const NotNull<T>&) = delete;
template <typename T>
bool
operator!=(decltype(nullptr), const NotNull<T>&) = delete;

} // namespace mozilla

#endif /* mozilla_NotNull_h */