Add Pass(), which implements move semantics, to scoped_ptr, scoped_array, and scoped_ptr_malloc.

base/memory/scoped_ptr.h

Index: base/memory/scoped_ptr.h
diff --git a/base/memory/scoped_ptr.h b/base/memory/scoped_ptr.h
index 5ac6846f3964e2ea929cef92d065454f23a64a4a..a356a34f31bef1ee52f81c55195c5f1dfe33073b 100644
--- a/base/memory/scoped_ptr.h
+++ b/base/memory/scoped_ptr.h
@@ -32,6 +32,41 @@
 //     foo.get()->Method();  // Foo::Method on the 0th element.
 //     foo[10].Method();     // Foo::Method on the 10th element.
 //   }
+//
+// These scopers also implement part of the functionality of C++11 unique_ptr
+// in that they are "moveable but not copyable."  You can use the scopers in
+// the parameter and return types of functions to signify ownership transfer
+// in to and out of a function.  When calling a function that has a scoper
+// as the argument type, it must be called with the result of an analogous
+// scoper's Pass() function or another funciton that generates a temporary;

[willchan no longer on Chromium, 2011/12/07 16:24:34]

function

[awong, 2011/12/08 21:04:10]

Done.

+// passing by copy will NOT work.  Here is an example using scoped_ptr:
+//
+//   void TakesOwnership(scoped_ptr<Foo> arg) {
+//     // Do something with arg
+//   }
+//   scoped_ptr<Foo> CreateFoo() {
+//     // No need for calling Pass() because we are constructing a temporary
+//     // for the return value.
+//     return scoped_ptr<Foo>(new Foo("new"));
+//   }
+//   scoped_ptr<Foo> PassThru(scoped_ptr<Foo> arg) {
+//     return arg.Pass();
+//   }
+//
+//   {
+//     scoped_ptr<Foo> ptr(new Foo("yay"));  // ptr manages Foo("yay)"
+//     TakesOwnership(ptr.Pass());           // ptr no longer owns Foo("yay").
+//     scoped_ptr<Foo> ptr2 = CreateFoo();   // ptr2 owns the return Foo.
+//     scoped_ptr<Foo> ptr3 =                // ptr3 now owns what was in ptr2.
+//         PassThru(ptr2.Pass());            // ptr2 is correspondly NULL.
+//   }
+//
+// Notice that if you do not call Pass() when returning from PassThru(), or
+// when invoking TakesOwnership(), the code will not compile because scopers
+// are not copyable; they only implement move semantics which require calling
+// the Pass() function to signify a destructive transfer of state. CreateFoo()
+// is different though because we are constructing a temporary on the return
+// line and thus can avoid needing to call Pass().
 
 #ifndef BASE_MEMORY_SCOPED_PTR_H_
 #define BASE_MEMORY_SCOPED_PTR_H_
@@ -47,6 +82,28 @@
 
 #include "base/compiler_specific.h"
 
+// Macro with the boilerplate C++03 move emulation for a class.
+//
+// In C++11, this is done via rvalue references.  Here, we use C++03 move
+// emulation to fake an rvalue reference.  For a more thorough explanation
+// of the technique, see:
+//
+//   http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Move_Constructor
+//
+#define CPP_03_MOVE_EMULATION(scoper, field) \
+ public: \

[willchan no longer on Chromium, 2011/12/07 19:25:24]

this line is unnecessary, right?

[awong, 2011/12/08 21:04:10]

Fixed to make it necessary. :)

+  struct RValue { \
+    explicit RValue(scoper& obj) : obj_(obj) {} \
+    scoper& obj_; \
+  }; \
+  operator RValue() { return RValue(*this); } \
+  scoper(RValue proxy) : field(proxy.obj_.release()) { } \
+  scoper& operator=(RValue proxy) { \
+    swap(proxy.obj_); \
+    return *this; \
+  } \
+  scoper Pass() { return scoper(RValue(*this)); }
+
 // A scoped_ptr<T> is like a T*, except that the destructor of scoped_ptr<T>
 // automatically deletes the pointer it holds (if any).
 // That is, scoped_ptr<T> owns the T object that it points to.
@@ -122,6 +179,8 @@ class scoped_ptr {
     return retVal;
   }
 
+  CPP_03_MOVE_EMULATION(scoped_ptr, ptr_);
+
  private:
   C* ptr_;
 
@@ -131,9 +190,10 @@ class scoped_ptr {
   template <class C2> bool operator==(scoped_ptr<C2> const& p2) const;
   template <class C2> bool operator!=(scoped_ptr<C2> const& p2) const;
 
-  // Disallow evil constructors
-  scoped_ptr(const scoped_ptr&);
-  void operator=(const scoped_ptr&);
+  // Disallow evil constructors.  Note that MUST NOT take a const& because we
+  // are implmenting move semantics.  See the CPP_03_MOVE_EMULATION macro.

[willchan no longer on Chromium, 2011/12/07 19:25:24]

implementing

[awong, 2011/12/08 21:04:10]

Done.

+  scoped_ptr(scoped_ptr&);
+  void operator=(scoped_ptr&);
 };
 
 // Free functions
@@ -229,6 +289,8 @@ class scoped_array {
     return retVal;
   }
 
+  CPP_03_MOVE_EMULATION(scoped_array, array_);
+
  private:
   C* array_;
 
@@ -236,9 +298,10 @@ class scoped_array {
   template <class C2> bool operator==(scoped_array<C2> const& p2) const;
   template <class C2> bool operator!=(scoped_array<C2> const& p2) const;
 
-  // Disallow evil constructors
-  scoped_array(const scoped_array&);
-  void operator=(const scoped_array&);
+  // Disallow evil constructors.  Note that MUST NOT take a const& because we
+  // are implmenting move semantics.  See the CPP_03_MOVE_EMULATION macro.
+  scoped_array(scoped_array&);
+  void operator=(scoped_array&);
 };
 
 // Free functions
@@ -347,6 +410,8 @@ class scoped_ptr_malloc {
     return tmp;
   }
 
+  CPP_03_MOVE_EMULATION(scoped_ptr_malloc, ptr_);
+
  private:
   C* ptr_;
 
@@ -356,11 +421,14 @@ class scoped_ptr_malloc {
   template <class C2, class GP>
   bool operator!=(scoped_ptr_malloc<C2, GP> const& p) const;
 
-  // Disallow evil constructors
-  scoped_ptr_malloc(const scoped_ptr_malloc&);
-  void operator=(const scoped_ptr_malloc&);
+  // Disallow evil constructors.  Note that MUST NOT take a const& because we
+  // are implmenting move semantics.  See the CPP_03_MOVE_EMULATION macro.

[willchan no longer on Chromium, 2011/12/07 19:25:24]

implementing

[awong, 2011/12/08 21:04:10]

Done.

+  scoped_ptr_malloc(scoped_ptr_malloc&);
+  void operator=(scoped_ptr_malloc&);
 };
 
+#undef CPP_03_MOVE_EMULATION
+
 template<class C, class FP> inline
 void swap(scoped_ptr_malloc<C, FP>& a, scoped_ptr_malloc<C, FP>& b) {
   a.swap(b);