Remove webrtc/base/move.h, and make types move-only manually
In days of yore, move.h contained complicated macros for approximating
move-only behavior in C++03. But since we live in the future now, and
can rely on C++11 features---including real move semantics!---it makes
more sense to just write the handful of required lines by hand in each
move-only class.
(We only live in the near future, though, not in some sci-fi
intergalactic civilization singularity type future, so we have to
define Pass() methods for these classes since we're not allowed to use
std::move().)
R=tommi@webrtc.org
Review URL: https://webrtc-codereview.appspot.com/45099004
Cr-Commit-Position: refs/heads/master@{#9060}
diff --git a/webrtc/base/move.h b/webrtc/base/move.h
deleted file mode 100644
index 198badf..0000000
--- a/webrtc/base/move.h
+++ /dev/null
@@ -1,237 +0,0 @@
-/*
- * Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
- *
- * Use of this source code is governed by a BSD-style license
- * that can be found in the LICENSE file in the root of the source
- * tree. An additional intellectual property rights grant can be found
- * in the file PATENTS. All contributing project authors may
- * be found in the AUTHORS file in the root of the source tree.
- */
-
-// Borrowed from Chromium's src/base/move.h.
-
-#ifndef WEBRTC_BASE_MOVE_H_
-#define WEBRTC_BASE_MOVE_H_
-
-#include "webrtc/typedefs.h"
-
-// Macro with the boilerplate that makes a type move-only in C++03.
-//
-// USAGE
-//
-// This macro should be used instead of DISALLOW_COPY_AND_ASSIGN to create
-// a "move-only" type. Unlike DISALLOW_COPY_AND_ASSIGN, this macro should be
-// the first line in a class declaration.
-//
-// A class using this macro must call .Pass() (or somehow be an r-value already)
-// before it can be:
-//
-// * Passed as a function argument
-// * Used as the right-hand side of an assignment
-// * Returned from a function
-//
-// Each class will still need to define their own "move constructor" and "move
-// operator=" to make this useful. Here's an example of the macro, the move
-// constructor, and the move operator= from the scoped_ptr class:
-//
-// template <typename T>
-// class scoped_ptr {
-// RTC_MOVE_ONLY_TYPE_FOR_CPP_03(scoped_ptr, RValue)
-// public:
-// scoped_ptr(RValue& other) : ptr_(other.release()) { }
-// scoped_ptr& operator=(RValue& other) {
-// swap(other);
-// return *this;
-// }
-// };
-//
-// Note that the constructor must NOT be marked explicit.
-//
-// For consistency, the second parameter to the macro should always be RValue
-// unless you have a strong reason to do otherwise. It is only exposed as a
-// macro parameter so that the move constructor and move operator= don't look
-// like they're using a phantom type.
-//
-//
-// HOW THIS WORKS
-//
-// For a thorough explanation of this technique, see:
-//
-// http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Move_Constructor
-//
-// The summary is that we take advantage of 2 properties:
-//
-// 1) non-const references will not bind to r-values.
-// 2) C++ can apply one user-defined conversion when initializing a
-// variable.
-//
-// The first lets us disable the copy constructor and assignment operator
-// by declaring private version of them with a non-const reference parameter.
-//
-// For l-values, direct initialization still fails like in
-// DISALLOW_COPY_AND_ASSIGN because the copy constructor and assignment
-// operators are private.
-//
-// For r-values, the situation is different. The copy constructor and
-// assignment operator are not viable due to (1), so we are trying to call
-// a non-existent constructor and non-existing operator= rather than a private
-// one. Since we have not committed an error quite yet, we can provide an
-// alternate conversion sequence and a constructor. We add
-//
-// * a private struct named "RValue"
-// * a user-defined conversion "operator RValue()"
-// * a "move constructor" and "move operator=" that take the RValue& as
-// their sole parameter.
-//
-// Only r-values will trigger this sequence and execute our "move constructor"
-// or "move operator=." L-values will match the private copy constructor and
-// operator= first giving a "private in this context" error. This combination
-// gives us a move-only type.
-//
-// For signaling a destructive transfer of data from an l-value, we provide a
-// method named Pass() which creates an r-value for the current instance
-// triggering the move constructor or move operator=.
-//
-// Other ways to get r-values is to use the result of an expression like a
-// function call.
-//
-// Here's an example with comments explaining what gets triggered where:
-//
-// class Foo {
-// RTC_MOVE_ONLY_TYPE_FOR_CPP_03(Foo, RValue);
-//
-// public:
-// ... API ...
-// Foo(RValue other); // Move constructor.
-// Foo& operator=(RValue rhs); // Move operator=
-// };
-//
-// Foo MakeFoo(); // Function that returns a Foo.
-//
-// Foo f;
-// Foo f_copy(f); // ERROR: Foo(Foo&) is private in this context.
-// Foo f_assign;
-// f_assign = f; // ERROR: operator=(Foo&) is private in this context.
-//
-//
-// Foo f(MakeFoo()); // R-value so alternate conversion executed.
-// Foo f_copy(f.Pass()); // R-value so alternate conversion executed.
-// f = f_copy.Pass(); // R-value so alternate conversion executed.
-//
-//
-// IMPLEMENTATION SUBTLETIES WITH RValue
-//
-// The RValue struct is just a container for a pointer back to the original
-// object. It should only ever be created as a temporary, and no external
-// class should ever declare it or use it in a parameter.
-//
-// It is tempting to want to use the RValue type in function parameters, but
-// excluding the limited usage here for the move constructor and move
-// operator=, doing so would mean that the function could take both r-values
-// and l-values equially which is unexpected. See COMPARED To Boost.Move for
-// more details.
-//
-// An alternate, and incorrect, implementation of the RValue class used by
-// Boost.Move makes RValue a fieldless child of the move-only type. RValue&
-// is then used in place of RValue in the various operators. The RValue& is
-// "created" by doing *reinterpret_cast<RValue*>(this). This has the appeal
-// of never creating a temporary RValue struct even with optimizations
-// disabled. Also, by virtue of inheritance you can treat the RValue
-// reference as if it were the move-only type itself. Unfortunately,
-// using the result of this reinterpret_cast<> is actually undefined behavior
-// due to C++98 5.2.10.7. In certain compilers (e.g., NaCl) the optimizer
-// will generate non-working code.
-//
-// In optimized builds, both implementations generate the same assembly so we
-// choose the one that adheres to the standard.
-//
-//
-// WHY HAVE typedef void MoveOnlyTypeForCPP03
-//
-// Callback<>/Bind() needs to understand movable-but-not-copyable semantics
-// to call .Pass() appropriately when it is expected to transfer the value.
-// The cryptic typedef MoveOnlyTypeForCPP03 is added to make this check
-// easy and automatic in helper templates for Callback<>/Bind().
-// See IsMoveOnlyType template and its usage in base/callback_internal.h
-// for more details.
-//
-//
-// COMPARED TO C++11
-//
-// In C++11, you would implement this functionality using an r-value reference
-// and our .Pass() method would be replaced with a call to std::move().
-//
-// This emulation also has a deficiency where it uses up the single
-// user-defined conversion allowed by C++ during initialization. This can
-// cause problems in some API edge cases. For instance, in scoped_ptr, it is
-// impossible to make a function "void Foo(scoped_ptr<Parent> p)" accept a
-// value of type scoped_ptr<Child> even if you add a constructor to
-// scoped_ptr<> that would make it look like it should work. C++11 does not
-// have this deficiency.
-//
-//
-// COMPARED TO Boost.Move
-//
-// Our implementation similar to Boost.Move, but we keep the RValue struct
-// private to the move-only type, and we don't use the reinterpret_cast<> hack.
-//
-// In Boost.Move, RValue is the boost::rv<> template. This type can be used
-// when writing APIs like:
-//
-// void MyFunc(boost::rv<Foo>& f)
-//
-// that can take advantage of rv<> to avoid extra copies of a type. However you
-// would still be able to call this version of MyFunc with an l-value:
-//
-// Foo f;
-// MyFunc(f); // Uh oh, we probably just destroyed |f| w/o calling Pass().
-//
-// unless someone is very careful to also declare a parallel override like:
-//
-// void MyFunc(const Foo& f)
-//
-// that would catch the l-values first. This was declared unsafe in C++11 and
-// a C++11 compiler will explicitly fail MyFunc(f). Unfortunately, we cannot
-// ensure this in C++03.
-//
-// Since we have no need for writing such APIs yet, our implementation keeps
-// RValue private and uses a .Pass() method to do the conversion instead of
-// trying to write a version of "std::move()." Writing an API like std::move()
-// would require the RValue struct to be public.
-//
-//
-// CAVEATS
-//
-// If you include a move-only type as a field inside a class that does not
-// explicitly declare a copy constructor, the containing class's implicit
-// copy constructor will change from Containing(const Containing&) to
-// Containing(Containing&). This can cause some unexpected errors.
-//
-// http://llvm.org/bugs/show_bug.cgi?id=11528
-//
-// The workaround is to explicitly declare your copy constructor.
-//
-#define RTC_MOVE_ONLY_TYPE_FOR_CPP_03(type, rvalue_type) \
- private: \
- struct rvalue_type { \
- explicit rvalue_type(type* object) : object(object) {} \
- type* object; \
- }; \
- type(type&); \
- void operator=(type&); \
- public: \
- operator rvalue_type() { return rvalue_type(this); } \
- type Pass() WARN_UNUSED_RESULT { return type(rvalue_type(this)); } \
- typedef void MoveOnlyTypeForCPP03; \
- private:
-
-#define RTC_MOVE_ONLY_TYPE_WITH_MOVE_CONSTRUCTOR_FOR_CPP_03(type) \
- private: \
- type(type&); \
- void operator=(type&); \
- public: \
- type&& Pass() WARN_UNUSED_RESULT { return static_cast<type&&>(*this); } \
- typedef void MoveOnlyTypeForCPP03; \
- private:
-
-#endif // WEBRTC_BASE_MOVE_H_
diff --git a/webrtc/base/scoped_ptr.h b/webrtc/base/scoped_ptr.h
index 02fa15d..676343e 100644
--- a/webrtc/base/scoped_ptr.h
+++ b/webrtc/base/scoped_ptr.h
@@ -105,7 +105,6 @@
#include <algorithm> // For std::swap().
#include "webrtc/base/constructormagic.h"
-#include "webrtc/base/move.h"
#include "webrtc/base/template_util.h"
#include "webrtc/typedefs.h"
@@ -322,7 +321,6 @@
// types.
template <class T, class D = rtc::DefaultDeleter<T> >
class scoped_ptr {
- RTC_MOVE_ONLY_TYPE_WITH_MOVE_CONSTRUCTOR_FOR_CPP_03(scoped_ptr)
// TODO(ajm): If we ever import RefCountedBase, this check needs to be
// enabled.
@@ -386,6 +384,13 @@
return *this;
}
+ // Deleted copy constructor and copy assignment, to make the type move-only.
+ scoped_ptr(const scoped_ptr& other) = delete;
+ scoped_ptr& operator=(const scoped_ptr& other) = delete;
+
+ // Get an rvalue reference. (sp.Pass() does the same thing as std::move(sp).)
+ scoped_ptr&& Pass() { return static_cast<scoped_ptr&&>(*this); }
+
// Reset. Deletes the currently owned object, if any.
// Then takes ownership of a new object, if given.
void reset(element_type* p = nullptr) { impl_.reset(p); }
@@ -470,8 +475,6 @@
template <class T, class D>
class scoped_ptr<T[], D> {
- RTC_MOVE_ONLY_TYPE_WITH_MOVE_CONSTRUCTOR_FOR_CPP_03(scoped_ptr)
-
public:
// The element and deleter types.
typedef T element_type;
@@ -514,6 +517,13 @@
return *this;
}
+ // Deleted copy constructor and copy assignment, to make the type move-only.
+ scoped_ptr(const scoped_ptr& other) = delete;
+ scoped_ptr& operator=(const scoped_ptr& other) = delete;
+
+ // Get an rvalue reference. (sp.Pass() does the same thing as std::move(sp).)
+ scoped_ptr&& Pass() { return static_cast<scoped_ptr&&>(*this); }
+
// Reset. Deletes the currently owned array, if any.
// Then takes ownership of a new object, if given.
void reset(element_type* array = nullptr) { impl_.reset(array); }
diff --git a/webrtc/system_wrappers/interface/scoped_vector.h b/webrtc/system_wrappers/interface/scoped_vector.h
index a3409b6..1e12645 100644
--- a/webrtc/system_wrappers/interface/scoped_vector.h
+++ b/webrtc/system_wrappers/interface/scoped_vector.h
@@ -16,7 +16,6 @@
#include <vector>
#include "webrtc/base/checks.h"
-#include "webrtc/base/move.h"
#include "webrtc/system_wrappers/interface/stl_util.h"
namespace webrtc {
@@ -25,8 +24,6 @@
// destructor.
template <class T>
class ScopedVector {
- RTC_MOVE_ONLY_TYPE_FOR_CPP_03(ScopedVector, RValue)
-
public:
typedef typename std::vector<T*>::allocator_type allocator_type;
typedef typename std::vector<T*>::size_type size_type;
@@ -44,13 +41,25 @@
ScopedVector() {}
~ScopedVector() { clear(); }
- ScopedVector(RValue other) { swap(*other.object); }
- ScopedVector& operator=(RValue rhs) {
- swap(*rhs.object);
+ // Move construction and assignment.
+ ScopedVector(ScopedVector&& other) {
+ *this = static_cast<ScopedVector&&>(other);
+ }
+ ScopedVector& operator=(ScopedVector&& other) {
+ std::swap(v_, other.v_); // The arguments are std::vectors, so std::swap
+ // is the one that we want.
+ other.clear();
return *this;
}
+ // Deleted copy constructor and copy assignment, to make the type move-only.
+ ScopedVector(const ScopedVector& other) = delete;
+ ScopedVector& operator=(const ScopedVector& other) = delete;
+
+ // Get an rvalue reference. (sv.Pass() does the same thing as std::move(sv).)
+ ScopedVector&& Pass() { return static_cast<ScopedVector&&>(*this); }
+
reference operator[](size_t index) { return v_[index]; }
const_reference operator[](size_t index) const { return v_[index]; }
