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239 lines
9.5 KiB
C++
239 lines
9.5 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/* C++11-style, but C++98-usable, "move references" implementation. */
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#ifndef mozilla_Move_h
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#define mozilla_Move_h
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#include "mozilla/TypeTraits.h"
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namespace mozilla {
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/*
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* "Move" References
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*
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* Some types can be copied much more efficiently if we know the original's
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* value need not be preserved --- that is, if we are doing a "move", not a
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* "copy". For example, if we have:
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*
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* Vector<T> u;
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* Vector<T> v(u);
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*
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* the constructor for v must apply a copy constructor to each element of u ---
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* taking time linear in the length of u. However, if we know we will not need u
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* any more once v has been initialized, then we could initialize v very
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* efficiently simply by stealing u's dynamically allocated buffer and giving it
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* to v --- a constant-time operation, regardless of the size of u.
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*
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* Moves often appear in container implementations. For example, when we append
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* to a vector, we may need to resize its buffer. This entails moving each of
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* its extant elements from the old, smaller buffer to the new, larger buffer.
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* But once the elements have been migrated, we're just going to throw away the
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* old buffer; we don't care if they still have their values. So if the vector's
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* element type can implement "move" more efficiently than "copy", the vector
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* resizing should by all means use a "move" operation. Hash tables should also
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* use moves when resizing their internal array as entries are added and
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* removed.
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*
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* The details of the optimization, and whether it's worth applying, vary
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* from one type to the next: copying an 'int' is as cheap as moving it, so
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* there's no benefit in distinguishing 'int' moves from copies. And while
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* some constructor calls for complex types are moves, many really have to
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* be copies, and can't be optimized this way. So we need:
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*
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* 1) a way for a type (like Vector) to announce that it can be moved more
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* efficiently than it can be copied, and provide an implementation of that
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* move operation; and
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*
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* 2) a way for a particular invocation of a copy constructor to say that it's
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* really a move, not a copy, and that the value of the original isn't
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* important afterwards (although it must still be safe to destroy).
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*
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* If a constructor has a single argument of type 'T&&' (an 'rvalue reference
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* to T'), that indicates that it is a 'move constructor'. That's 1). It should
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* move, not copy, its argument into the object being constructed. It may leave
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* the original in any safely-destructible state.
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*
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* If a constructor's argument is an rvalue, as in 'C(f(x))' or 'C(x + y)', as
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* opposed to an lvalue, as in 'C(x)', then overload resolution will prefer the
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* move constructor, if there is one. The 'mozilla::Move' function, defined in
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* this file, is an identity function you can use in a constructor invocation to
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* make any argument into an rvalue, like this: C(Move(x)). That's 2). (You
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* could use any function that works, but 'Move' indicates your intention
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* clearly.)
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*
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* Where we might define a copy constructor for a class C like this:
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*
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* C(const C& rhs) { ... copy rhs to this ... }
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*
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* we would declare a move constructor like this:
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*
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* C(C&& rhs) { .. move rhs to this ... }
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*
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* And where we might perform a copy like this:
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*
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* C c2(c1);
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*
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* we would perform a move like this:
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*
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* C c2(Move(c1));
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*
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* Note that 'T&&' implicitly converts to 'T&'. So you can pass a 'T&&' to an
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* ordinary copy constructor for a type that doesn't support a special move
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* constructor, and you'll just get a copy. This means that templates can use
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* Move whenever they know they won't use the original value any more, even if
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* they're not sure whether the type at hand has a specialized move constructor.
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* If it doesn't, the 'T&&' will just convert to a 'T&', and the ordinary copy
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* constructor will apply.
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*
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* A class with a move constructor can also provide a move assignment operator.
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* A generic definition would run this's destructor, and then apply the move
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* constructor to *this's memory. A typical definition:
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*
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* C& operator=(C&& rhs) {
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* MOZ_ASSERT(&rhs != this, "self-moves are prohibited");
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* this->~C();
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* new(this) C(Move(rhs));
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* return *this;
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* }
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*
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* With that in place, one can write move assignments like this:
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*
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* c2 = Move(c1);
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*
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* This destroys c2, moves c1's value to c2, and leaves c1 in an undefined but
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* destructible state.
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*
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* As we say, a move must leave the original in a "destructible" state. The
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* original's destructor will still be called, so if a move doesn't
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* actually steal all its resources, that's fine. We require only that the
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* move destination must take on the original's value; and that destructing
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* the original must not break the move destination.
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*
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* (Opinions differ on whether move assignment operators should deal with move
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* assignment of an object onto itself. It seems wise to either handle that
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* case, or assert that it does not occur.)
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*
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* Forwarding:
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*
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* Sometimes we want copy construction or assignment if we're passed an ordinary
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* value, but move construction if passed an rvalue reference. For example, if
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* our constructor takes two arguments and either could usefully be a move, it
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* seems silly to write out all four combinations:
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*
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* C::C(X& x, Y& y) : x(x), y(y) { }
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* C::C(X& x, Y&& y) : x(x), y(Move(y)) { }
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* C::C(X&& x, Y& y) : x(Move(x)), y(y) { }
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* C::C(X&& x, Y&& y) : x(Move(x)), y(Move(y)) { }
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*
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* To avoid this, C++11 has tweaks to make it possible to write what you mean.
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* The four constructor overloads above can be written as one constructor
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* template like so[0]:
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*
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* template <typename XArg, typename YArg>
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* C::C(XArg&& x, YArg&& y) : x(Forward<XArg>(x)), y(Forward<YArg>(y)) { }
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*
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* ("'Don't Repeat Yourself'? What's that?")
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*
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* This takes advantage of two new rules in C++11:
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*
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* - First, when a function template takes an argument that is an rvalue
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* reference to a template argument (like 'XArg&& x' and 'YArg&& y' above),
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* then when the argument is applied to an lvalue, the template argument
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* resolves to 'T&'; and when it is applied to an rvalue, the template
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* argument resolves to 'T'. Thus, in a call to C::C like:
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*
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* X foo(int);
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* Y yy;
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*
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* C(foo(5), yy)
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*
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* XArg would resolve to 'X', and YArg would resolve to 'Y&'.
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*
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* - Second, Whereas C++ used to forbid references to references, C++11 defines
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* 'collapsing rules': 'T& &', 'T&& &', and 'T& &&' (that is, any combination
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* involving an lvalue reference) now collapse to simply 'T&'; and 'T&& &&'
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* collapses to 'T&&'.
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*
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* Thus, in the call above, 'XArg&&' is 'X&&'; and 'YArg&&' is 'Y& &&', which
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* collapses to 'Y&'. Because the arguments are declared as rvalue references
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* to template arguments, the lvalue-ness "shines through" where present.
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*
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* Then, the 'Forward<T>' function --- you must invoke 'Forward' with its type
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* argument --- returns an lvalue reference or an rvalue reference to its
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* argument, depending on what T is. In our unified constructor definition, that
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* means that we'll invoke either the copy or move constructors for x and y,
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* depending on what we gave C's constructor. In our call, we'll move 'foo()'
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* into 'x', but copy 'yy' into 'y'.
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*
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* This header file defines Move and Forward in the mozilla namespace. It's up
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* to individual containers to annotate moves as such, by calling Move; and it's
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* up to individual types to define move constructors and assignment operators
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* when valuable.
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*
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* (C++11 says that the <utility> header file should define 'std::move' and
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* 'std::forward', which are just like our 'Move' and 'Forward'; but those
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* definitions aren't available in that header on all our platforms, so we
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* define them ourselves here.)
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*
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* 0. This pattern is known as "perfect forwarding". Interestingly, it is not
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* actually perfect, and it can't forward all possible argument expressions!
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* There is a C++11 issue: you can't form a reference to a bit-field. As a
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* workaround, assign the bit-field to a local variable and use that:
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*
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* // C is as above
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* struct S { int x : 1; } s;
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* C(s.x, 0); // BAD: s.x is a reference to a bit-field, can't form those
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* int tmp = s.x;
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* C(tmp, 0); // OK: tmp not a bit-field
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*/
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/**
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* Identical to std::Move(); this is necessary until our stlport supports
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* std::move().
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*/
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template<typename T>
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inline typename RemoveReference<T>::Type&&
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Move(T&& aX)
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{
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return static_cast<typename RemoveReference<T>::Type&&>(aX);
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}
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/**
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* These two overloads are identical to std::forward(); they are necessary until
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* our stlport supports std::forward().
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*/
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template<typename T>
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inline T&&
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Forward(typename RemoveReference<T>::Type& aX)
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{
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return static_cast<T&&>(aX);
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}
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template<typename T>
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inline T&&
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Forward(typename RemoveReference<T>::Type&& aX)
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{
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static_assert(!IsLvalueReference<T>::value,
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"misuse of Forward detected! try the other overload");
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return static_cast<T&&>(aX);
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}
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/** Swap |aX| and |aY| using move-construction if possible. */
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template<typename T>
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inline void
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Swap(T& aX, T& aY)
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{
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T tmp(Move(aX));
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aX = Move(aY);
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aY = Move(tmp);
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}
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} // namespace mozilla
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#endif /* mozilla_Move_h */
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