The Tor Browser: mfbt/Move.h@97036ab72558 (annotated)

mfbt/Move.h@97036ab72558 (annotated)

mfbt/Move.h

Tue, 06 Jan 2015 21:39:09 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Tue, 06 Jan 2015 21:39:09 +0100
branch: TOR_BUG_9701
changeset 8: 97036ab72558
permissions: -rw-r--r--

Conditionally force memory storage according to privacy.thirdparty.isolate;
This solves Tor bug #9701, complying with disk avoidance documented in
https://www.torproject.org/projects/torbrowser/design/#disk-avoidance.

 /* -*- 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/. */
 /* C++11-style, but C++98-usable, "move references" implementation. */
 #ifndef mozilla_Move_h
 #define mozilla_Move_h
 #include "mozilla/TypeTraits.h"
 namespace mozilla {
 /*
  * "Move" References
  *
  * Some types can be copied much more efficiently if we know the original's
  * value need not be preserved --- that is, if we are doing a "move", not a
  * "copy". For example, if we have:
  *
  *   Vector<T> u;
  *   Vector<T> v(u);
  *
  * the constructor for v must apply a copy constructor to each element of u ---
  * taking time linear in the length of u. However, if we know we will not need u
  * any more once v has been initialized, then we could initialize v very
  * efficiently simply by stealing u's dynamically allocated buffer and giving it
  * to v --- a constant-time operation, regardless of the size of u.
  *
  * Moves often appear in container implementations. For example, when we append
  * to a vector, we may need to resize its buffer. This entails moving each of
  * its extant elements from the old, smaller buffer to the new, larger buffer.
  * But once the elements have been migrated, we're just going to throw away the
  * old buffer; we don't care if they still have their values. So if the vector's
  * element type can implement "move" more efficiently than "copy", the vector
  * resizing should by all means use a "move" operation. Hash tables should also
  * use moves when resizing their internal array as entries are added and
  * removed.
  *
  * The details of the optimization, and whether it's worth applying, vary
  * from one type to the next: copying an 'int' is as cheap as moving it, so
  * there's no benefit in distinguishing 'int' moves from copies. And while
  * some constructor calls for complex types are moves, many really have to
  * be copies, and can't be optimized this way. So we need:
  *
  * 1) a way for a type (like Vector) to announce that it can be moved more
  *    efficiently than it can be copied, and provide an implementation of that
  *    move operation; and
  *
  * 2) a way for a particular invocation of a copy constructor to say that it's
  *    really a move, not a copy, and that the value of the original isn't
  *    important afterwards (although it must still be safe to destroy).
  *
  * If a constructor has a single argument of type 'T&&' (an 'rvalue reference
  * to T'), that indicates that it is a 'move constructor'. That's 1). It should
  * move, not copy, its argument into the object being constructed. It may leave
  * the original in any safely-destructible state.
  *
  * If a constructor's argument is an rvalue, as in 'C(f(x))' or 'C(x + y)', as
  * opposed to an lvalue, as in 'C(x)', then overload resolution will prefer the
  * move constructor, if there is one. The 'mozilla::Move' function, defined in
  * this file, is an identity function you can use in a constructor invocation to
  * make any argument into an rvalue, like this: C(Move(x)). That's 2). (You
  * could use any function that works, but 'Move' indicates your intention
  * clearly.)
  *
  * Where we might define a copy constructor for a class C like this:
  *
  *   C(const C& rhs) { ... copy rhs to this ... }
  *
  * we would declare a move constructor like this:
  *
  *   C(C&& rhs) { .. move rhs to this ... }
  *
  * And where we might perform a copy like this:
  *
  *   C c2(c1);
  *
  * we would perform a move like this:
  *
  *   C c2(Move(c1));
  *
  * Note that 'T&&' implicitly converts to 'T&'. So you can pass a 'T&&' to an
  * ordinary copy constructor for a type that doesn't support a special move
  * constructor, and you'll just get a copy. This means that templates can use
  * Move whenever they know they won't use the original value any more, even if
  * they're not sure whether the type at hand has a specialized move constructor.
  * If it doesn't, the 'T&&' will just convert to a 'T&', and the ordinary copy
  * constructor will apply.
  *
  * A class with a move constructor can also provide a move assignment operator.
  * A generic definition would run this's destructor, and then apply the move
  * constructor to *this's memory. A typical definition:
  *
  *   C& operator=(C&& rhs) {
  *     MOZ_ASSERT(&rhs != this, "self-moves are prohibited");
  *     this->~C();
  *     new(this) C(Move(rhs));
  *     return *this;
  *   }
  *
  * With that in place, one can write move assignments like this:
  *
  *   c2 = Move(c1);
  *
  * This destroys c2, moves c1's value to c2, and leaves c1 in an undefined but
  * destructible state.
  *
  * As we say, a move must leave the original in a "destructible" state. The
  * original's destructor will still be called, so if a move doesn't
  * actually steal all its resources, that's fine. We require only that the
  * move destination must take on the original's value; and that destructing
  * the original must not break the move destination.
  *
  * (Opinions differ on whether move assignment operators should deal with move
  * assignment of an object onto itself. It seems wise to either handle that
  * case, or assert that it does not occur.)
  *
  * Forwarding:
  *
  * Sometimes we want copy construction or assignment if we're passed an ordinary
  * value, but move construction if passed an rvalue reference. For example, if
  * our constructor takes two arguments and either could usefully be a move, it
  * seems silly to write out all four combinations:
  *
  *   C::C(X&  x, Y&  y) : x(x),       y(y)       { }
  *   C::C(X&  x, Y&& y) : x(x),       y(Move(y)) { }
  *   C::C(X&& x, Y&  y) : x(Move(x)), y(y)       { }
  *   C::C(X&& x, Y&& y) : x(Move(x)), y(Move(y)) { }
  *
  * To avoid this, C++11 has tweaks to make it possible to write what you mean.
  * The four constructor overloads above can be written as one constructor
  * template like so[0]:
  *
  *   template <typename XArg, typename YArg>
  *   C::C(XArg&& x, YArg&& y) : x(Forward<XArg>(x)), y(Forward<YArg>(y)) { }
  *
  * ("'Don't Repeat Yourself'? What's that?")
  *
  * This takes advantage of two new rules in C++11:
  *
  * - First, when a function template takes an argument that is an rvalue
  *   reference to a template argument (like 'XArg&& x' and 'YArg&& y' above),
  *   then when the argument is applied to an lvalue, the template argument
  *   resolves to 'T &'; and when it is applied to an rvalue, the template
  *   argument resolves to 'T &&'. Thus, in a call to C::C like:
  *
  *      X foo(int);
  *      Y yy;
  *
  *      C(foo(5), yy)
  *
  *   XArg would resolve to 'X&&', and YArg would resolve to 'Y&'.
  *
  * - Second, Whereas C++ used to forbid references to references, C++11 defines
  *   'collapsing rules': 'T& &', 'T&& &', and 'T& &&' (that is, any combination
  *   involving an lvalue reference) now collapse to simply 'T&'; and 'T&& &&'
  *   collapses to 'T&&'.
  *
  *   Thus, in the call above, 'XArg&&' is 'X&& &&', collapsing to 'X&&'; and
  *   'YArg&&' is 'Y& &&', which collapses to 'Y &'. Because the arguments are
  *   declared as rvalue references to template arguments, the rvalue-ness
  *   "shines through" where present.
  *
  * Then, the 'Forward<T>' function --- you must invoke 'Forward' with its type
  * argument --- returns an lvalue reference or an rvalue reference to its
  * argument, depending on what T is. In our unified constructor definition, that
  * means that we'll invoke either the copy or move constructors for x and y,
  * depending on what we gave C's constructor. In our call, we'll move 'foo()'
  * into 'x', but copy 'yy' into 'y'.
  *
  * This header file defines Move and Forward in the mozilla namespace. It's up
  * to individual containers to annotate moves as such, by calling Move; and it's
  * up to individual types to define move constructors and assignment operators
  * when valuable.
  *
  * (C++11 says that the <utility> header file should define 'std::move' and
  * 'std::forward', which are just like our 'Move' and 'Forward'; but those
  * definitions aren't available in that header on all our platforms, so we
  * define them ourselves here.)
  *
  * 0. This pattern is known as "perfect forwarding".  Interestingly, it is not
  *    actually perfect, and it can't forward all possible argument expressions!
  *    There are two issues: one that's a C++11 issue, and one that's a legacy
  *    compiler issue.
  *
  *    The C++11 issue is that you can't form a reference to a bit-field.  As a
  *    workaround, assign the bit-field to a local variable and use that:
  *
  *      // C is as above
  *      struct S { int x : 1; } s;
  *      C(s.x, 0); // BAD: s.x is a reference to a bit-field, can't form those
  *      int tmp = s.x;
  *      C(tmp, 0); // OK: tmp not a bit-field
  *
  *    The legacy issue is that when we don't have true nullptr and must emulate
  *    it (gcc 4.4/4.5), forwarding |nullptr| results in an |int| or |long|
  *    forwarded reference.  But such a reference, even if its value is a null
  *    pointer constant expression, is not itself a null pointer constant
  *    expression.  This causes -Werror=conversion-null errors and pointer-to-
  *    integer comparison errors.  Until we always have true nullptr, users of
  *    forwarding methods must not pass |nullptr| to them.
  */
 /**
  * Identical to std::Move(); this is necessary until our stlport supports
  * std::move().
  */
 template<typename T>
 inline typename RemoveReference<T>::Type&&
 Move(T&& a)
 {
   return static_cast<typename RemoveReference<T>::Type&&>(a);
 }
 /**
  * These two overloads are identical to std::forward(); they are necessary until
  * our stlport supports std::forward().
  */
 template<typename T>
 inline T&&
 Forward(typename RemoveReference<T>::Type& a)
 {
   return static_cast<T&&>(a);
 }
 template<typename T>
 inline T&&
 Forward(typename RemoveReference<T>::Type&& t)
 {
   static_assert(!IsLvalueReference<T>::value,
                 "misuse of Forward detected!  try the other overload");
   return static_cast<T&&>(t);
 }
 /** Swap |t| and |u| using move-construction if possible. */
 template<typename T>
 inline void
 Swap(T& t, T& u)
 {
   T tmp(Move(t));
   t = Move(u);
   u = Move(tmp);
 }
 } // namespace mozilla
 #endif /* mozilla_Move_h */

The Tor Browser / annotate

mfbt/Move.h@97036ab72558 (annotated)

mfbt/Move.h