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Unified Diff: base/move.h

Issue 1407443002: Remove old C++03 move emulation code. (Closed) Base URL: https://chromium.googlesource.com/chromium/src.git@master
Patch Set: std::move and reflow Created 5 years, 1 month ago
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Index: base/move.h
diff --git a/base/move.h b/base/move.h
index ce8a8e13a7924fd92128d2dd1a2a4087c1f5a46a..fe0517eb363105727b4ea6c9d1775359ce5abf23 100644
--- a/base/move.h
+++ b/base/move.h
@@ -9,7 +9,7 @@
#include "base/compiler_specific.h"
-// Macro with the boilerplate that makes a type move-only in C++03.
+// Macro with the boilerplate that makes a type move-only in C++11.
//
// USAGE
//
@@ -24,121 +24,21 @@
// * 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:
+// 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 a hypothetical scoped_ptr class:
//
// template <typename T>
// class scoped_ptr {
-// MOVE_ONLY_TYPE_FOR_CPP_03(scoped_ptr, RValue)
+// MOVE_ONLY_TYPE_WITH_MOVE_CONSTRUCTOR_FOR_CPP_03(type);
// public:
-// scoped_ptr(RValue& other) : ptr_(other.release()) { }
-// scoped_ptr& operator=(RValue& other) {
-// swap(other);
+// scoped_ptr(scoped_ptr&& other) : ptr_(other.release()) { }
+// scoped_ptr& operator=(scoped_ptr&& other) {
+// reset(other.release());
// 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 {
-// 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
//
@@ -148,84 +48,19 @@
// 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 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 MOVE_ONLY_TYPE_WITH_MOVE_CONSTRUCTOR_FOR_CPP_03(type) \
- private: \
- type(const type&); \
- void operator=(const type&); \
- public: \
+#define MOVE_ONLY_TYPE_FOR_CPP_03(type) \
+ MOVE_ONLY_TYPE_WITH_MOVE_CONSTRUCTOR_FOR_CPP_03(type)
+
+#define MOVE_ONLY_TYPE_WITH_MOVE_CONSTRUCTOR_FOR_CPP_03(type) \
+ private: \
+ type(const type&) = delete; \
+ void operator=(const type&) = delete; \
+ \
+ public: \
type&& Pass() WARN_UNUSED_RESULT { return std::move(*this); } \
- typedef void MoveOnlyTypeForCPP03; \
+ typedef void MoveOnlyTypeForCPP03; \
+ \
private:
#define TYPE_WITH_MOVE_CONSTRUCTOR_FOR_CPP_03(type) \
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