mirror of https://github.com/axmolengine/axmol.git
167 lines
6.1 KiB
C++
167 lines
6.1 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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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 also need to
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* be resized.
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*
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* The details of the optimization, and whether it's worth applying, vary from
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* one type to the next. And while some constructor calls are moves, many really
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* are copies, and can't be optimized this way. So we need:
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*
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* 1) a way for a particular invocation of a copy constructor to say that it's
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* really a move, and that the value of the original isn't important
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* afterwards (although it must still be safe to destroy); and
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*
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* 2) 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.
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*
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* The Move(T&) function takes a reference to a T, and returns a MoveRef<T>
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* referring to the same value; that's 1). A MoveRef<T> is simply a reference
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* to a T, annotated to say that a copy constructor applied to it may move that
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* T, instead of copying it. Finally, a constructor that accepts an MoveRef<T>
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* should perform a more efficient move, instead of a copy, providing 2).
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*
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* So, 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(MoveRef<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 MoveRef<T> implicitly converts to T&, so you can pass a MoveRef<T>
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* to an ordinary copy constructor for a type that doesn't support a special
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* move constructor, and you'll just get a copy. This means that templates can
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* use Move whenever they know they won't use the original value any more, even
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* if they're not sure whether the type at hand has a specialized move
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* constructor. If it doesn't, the MoveRef<T> will just convert to a T&, and
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* the ordinary copy 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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* which runs this's destructor, and then applies the move constructor to
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* *this's memory. A typical definition:
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*
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* C& operator=(MoveRef<C> rhs) {
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* this->~C();
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* new(this) C(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 c1, 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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* This header file defines MoveRef and Move 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.
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*
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* One hint: if you're writing a move constructor where the type has members
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* that should be moved themselves, it's much nicer to write this:
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*
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* C(MoveRef<C> c) : x(Move(c->x)), y(Move(c->y)) { }
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*
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* than the equivalent:
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*
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* C(MoveRef<C> c) { new(&x) X(Move(c->x)); new(&y) Y(Move(c->y)); }
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*
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* especially since GNU C++ fails to notice that this does indeed initialize x
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* and y, which may matter if they're const.
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*/
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template<typename T>
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class MoveRef
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{
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T* pointer;
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public:
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explicit MoveRef(T& t) : pointer(&t) { }
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T& operator*() const { return *pointer; }
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T* operator->() const { return pointer; }
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operator T& () const { return *pointer; }
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};
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template<typename T>
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inline MoveRef<T>
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Move(T& t)
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{
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return MoveRef<T>(t);
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}
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template<typename T>
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inline MoveRef<T>
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Move(const T& t)
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{
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// With some versions of gcc, for a class C, there's an (incorrect) ambiguity
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// between the C(const C&) constructor and the default C(C&&) C++11 move
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// constructor, when the constructor is called with a const C& argument.
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//
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// This ambiguity manifests with the Move implementation above when Move is
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// passed const U& for some class U. Calling Move(const U&) returns a
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// MoveRef<const U&>, which is then commonly passed to the U constructor,
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// triggering an implicit conversion to const U&. gcc doesn't know whether to
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// call U(const U&) or U(U&&), so it wrongly reports a compile error.
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//
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// http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50442 has since been fixed, so
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// this is no longer an issue for up-to-date compilers. But there's no harm
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// in keeping it around for older compilers, so we might as well. See also
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// bug 686280.
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return MoveRef<T>(const_cast<T&>(t));
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}
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/** Swap |t| and |u| using move-construction if possible. */
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template<typename T>
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inline void
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Swap(T& t, T& u)
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{
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T tmp(Move(t));
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t = Move(u);
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u = Move(tmp);
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}
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} // namespace mozilla
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#endif /* mozilla_Move_h */
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