208 lines
8.0 KiB
C++
208 lines
8.0 KiB
C++
// Copyright (c) 2012 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#ifndef BASE_MOVE_H_
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#define BASE_MOVE_H_
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// Macro with the boilerplate that makes a type move-only in C++03.
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//
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// USAGE
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//
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// This macro should be used instead of DISALLOW_COPY_AND_ASSIGN to create
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// a "move-only" type. Unlike DISALLOW_COPY_AND_ASSIGN, this macro should be
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// the first line in a class declaration.
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//
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// A class using this macro must call .Pass() (or somehow be an r-value already)
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// before it can be:
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//
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// * Passed as a function argument
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// * Used as the right-hand side of an assignment
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// * Returned from a function
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//
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// Each class will still need to define their own "move constructor" and "move
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// operator=" to make this useful. Here's an example of the macro, the move
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// constructor, and the move operator= from the scoped_ptr class:
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//
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// template <typename T>
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// class scoped_ptr {
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// MOVE_ONLY_TYPE_FOR_CPP_03(scoped_ptr, RValue)
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// public:
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// scoped_ptr(RValue& other) : ptr_(other.release()) { }
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// scoped_ptr& operator=(RValue& other) {
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// swap(other);
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// return *this;
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// }
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// };
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//
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// Note that the constructor must NOT be marked explicit.
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//
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// For consistency, the second parameter to the macro should always be RValue
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// unless you have a strong reason to do otherwise. It is only exposed as a
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// macro parameter so that the move constructor and move operator= don't look
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// like they're using a phantom type.
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//
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//
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// HOW THIS WORKS
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//
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// For a thorough explanation of this technique, see:
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//
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// http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Move_Constructor
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//
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// The summary is that we take advantage of 2 properties:
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//
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// 1) non-const references will not bind to r-values.
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// 2) C++ can apply one user-defined conversion when initializing a
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// variable.
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//
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// The first lets us disable the copy constructor and assignment operator
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// by declaring private version of them with a non-const reference parameter.
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//
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// For l-values, direct initialization still fails like in
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// DISALLOW_COPY_AND_ASSIGN because the copy constructor and assignment
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// operators are private.
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//
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// For r-values, the situation is different. The copy constructor and
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// assignment operator are not viable due to (1), so we are trying to call
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// a non-existent constructor and non-existing operator= rather than a private
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// one. Since we have not committed an error quite yet, we can provide an
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// alternate conversion sequence and a constructor. We add
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//
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// * a private struct named "RValue"
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// * a user-defined conversion "operator RValue()"
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// * a "move constructor" and "move operator=" that take the RValue& as
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// their sole parameter.
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//
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// Only r-values will trigger this sequence and execute our "move constructor"
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// or "move operator=." L-values will match the private copy constructor and
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// operator= first giving a "private in this context" error. This combination
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// gives us a move-only type.
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//
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// For signaling a destructive transfer of data from an l-value, we provide a
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// method named Pass() which creates an r-value for the current instance
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// triggering the move constructor or move operator=.
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//
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// Other ways to get r-values is to use the result of an expression like a
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// function call.
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//
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// Here's an example with comments explaining what gets triggered where:
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//
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// class Foo {
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// MOVE_ONLY_TYPE_FOR_CPP_03(Foo, RValue);
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//
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// public:
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// ... API ...
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// Foo(RValue other); // Move constructor.
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// Foo& operator=(RValue rhs); // Move operator=
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// };
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//
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// Foo MakeFoo(); // Function that returns a Foo.
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//
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// Foo f;
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// Foo f_copy(f); // ERROR: Foo(Foo&) is private in this context.
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// Foo f_assign;
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// f_assign = f; // ERROR: operator=(Foo&) is private in this context.
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//
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//
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// Foo f(MakeFoo()); // R-value so alternate conversion executed.
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// Foo f_copy(f.Pass()); // R-value so alternate conversion executed.
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// f = f_copy.Pass(); // R-value so alternate conversion executed.
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//
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//
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// IMPLEMENTATION SUBTLETIES WITH RValue
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//
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// The RValue struct is just a container for a pointer back to the original
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// object. It should only ever be created as a temporary, and no external
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// class should ever declare it or use it in a parameter.
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//
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// It is tempting to want to use the RValue type in function parameters, but
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// excluding the limited usage here for the move constructor and move
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// operator=, doing so would mean that the function could take both r-values
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// and l-values equially which is unexpected. See COMPARED To Boost.Move for
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// more details.
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//
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// An alternate, and incorrect, implementation of the RValue class used by
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// Boost.Move makes RValue a fieldless child of the move-only type. RValue&
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// is then used in place of RValue in the various operators. The RValue& is
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// "created" by doing *reinterpret_cast<RValue*>(this). This has the appeal
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// of never creating a temporary RValue struct even with optimizations
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// disabled. Also, by virtue of inheritance you can treat the RValue
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// reference as if it were the move-only type itself. Unfortunately,
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// using the result of this reinterpret_cast<> is actually undefined behavior
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// due to C++98 5.2.10.7. In certain compilers (e.g., NaCl) the optimizer
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// will generate non-working code.
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//
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// In optimized builds, both implementations generate the same assembly so we
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// choose the one that adheres to the standard.
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//
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//
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// COMPARED TO C++11
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//
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// In C++11, you would implement this functionality using an r-value reference
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// and our .Pass() method would be replaced with a call to std::move().
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//
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// This emulation also has a deficiency where it uses up the single
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// user-defined conversion allowed by C++ during initialization. This can
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// cause problems in some API edge cases. For instance, in scoped_ptr, it is
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// impossible to make a function "void Foo(scoped_ptr<Parent> p)" accept a
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// value of type scoped_ptr<Child> even if you add a constructor to
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// scoped_ptr<> that would make it look like it should work. C++11 does not
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// have this deficiency.
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//
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//
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// COMPARED TO Boost.Move
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//
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// Our implementation similar to Boost.Move, but we keep the RValue struct
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// private to the move-only type, and we don't use the reinterpret_cast<> hack.
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//
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// In Boost.Move, RValue is the boost::rv<> template. This type can be used
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// when writing APIs like:
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//
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// void MyFunc(boost::rv<Foo>& f)
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//
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// that can take advantage of rv<> to avoid extra copies of a type. However you
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// would still be able to call this version of MyFunc with an l-value:
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//
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// Foo f;
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// MyFunc(f); // Uh oh, we probably just destroyed |f| w/o calling Pass().
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//
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// unless someone is very careful to also declare a parallel override like:
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//
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// void MyFunc(const Foo& f)
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//
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// that would catch the l-values first. This was declared unsafe in C++11 and
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// a C++11 compiler will explicitly fail MyFunc(f). Unfortunately, we cannot
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// ensure this in C++03.
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//
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// Since we have no need for writing such APIs yet, our implementation keeps
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// RValue private and uses a .Pass() method to do the conversion instead of
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// trying to write a version of "std::move()." Writing an API like std::move()
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// would require the RValue struct to be public.
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//
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//
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// CAVEATS
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//
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// If you include a move-only type as a field inside a class that does not
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// explicitly declare a copy constructor, the containing class's implicit
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// copy constructor will change from Containing(const Containing&) to
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// Containing(Containing&). This can cause some unexpected errors.
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//
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// http://llvm.org/bugs/show_bug.cgi?id=11528
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//
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// The workaround is to explicitly declare your copy constructor.
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//
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#define MOVE_ONLY_TYPE_FOR_CPP_03(type, rvalue_type) \
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private: \
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struct rvalue_type { \
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explicit rvalue_type(type* object) : object(object) {} \
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type* object; \
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}; \
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type(type&); \
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void operator=(type&); \
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public: \
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operator rvalue_type() { return rvalue_type(this); } \
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type Pass() { return type(rvalue_type(this)); } \
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private:
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#endif // BASE_MOVE_H_
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