Add std::unique_ptr conversions to scoped_ptr.

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 c4d75a4b0583d1c75404c1b4b16ec1d369a89bb4..a7ff92235235383051010a94a51622a49fba6fc7 100644
--- a/base/memory/scoped_ptr.h
+++ b/base/memory/scoped_ptr.h
@@ -86,6 +86,8 @@
 
 #include <algorithm>  // For std::swap().
 #include <iosfwd>
+#include <memory>
+#include <utility>
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
@@ -99,61 +101,6 @@ class RefCountedBase;
 class RefCountedThreadSafeBase;
 }  // namespace subtle
 
-// Function object which deletes its parameter, which must be a pointer.
-// If C is an array type, invokes 'delete[]' on the parameter; otherwise,
-// invokes 'delete'. The default deleter for scoped_ptr<T>.
-template <class T>
-struct DefaultDeleter {

[dcheng, 2015/11/11 16:18:34]

I tried using a type alias, but apparently the standard doesn't permit that
("elaborated type refers to a type alias template"). 

-  DefaultDeleter() {}
-  template <typename U> DefaultDeleter(const DefaultDeleter<U>& other) {
-    // IMPLEMENTATION NOTE: C++11 20.7.1.1.2p2 only provides this constructor
-    // if U* is implicitly convertible to T* and U is not an array type.
-    //
-    // Correct implementation should use SFINAE to disable this
-    // constructor. However, since there are no other 1-argument constructors,
-    // using a COMPILE_ASSERT() based on is_convertible<> and requiring
-    // complete types is simpler and will cause compile failures for equivalent
-    // misuses.
-    //
-    // Note, the is_convertible<U*, T*> check also ensures that U is not an
-    // array. T is guaranteed to be a non-array, so any U* where U is an array
-    // cannot convert to T*.
-    enum { T_must_be_complete = sizeof(T) };
-    enum { U_must_be_complete = sizeof(U) };
-    COMPILE_ASSERT((base::is_convertible<U*, T*>::value),
-                   U_ptr_must_implicitly_convert_to_T_ptr);
-  }
-  inline void operator()(T* ptr) const {
-    enum { type_must_be_complete = sizeof(T) };
-    delete ptr;
-  }
-};
-
-// Specialization of DefaultDeleter for array types.
-template <class T>
-struct DefaultDeleter<T[]> {
-  inline void operator()(T* ptr) const {
-    enum { type_must_be_complete = sizeof(T) };
-    delete[] ptr;
-  }
-
- private:
-  // Disable this operator for any U != T because it is undefined to execute
-  // an array delete when the static type of the array mismatches the dynamic
-  // type.
-  //
-  // References:
-  //   C++98 [expr.delete]p3
-  //   http://cplusplus.github.com/LWG/lwg-defects.html#938
-  template <typename U> void operator()(U* array) const;
-};
-
-template <class T, int n>
-struct DefaultDeleter<T[n]> {
-  // Never allow someone to declare something like scoped_ptr<int[10]>.
-  COMPILE_ASSERT(sizeof(T) == -1, do_not_use_array_with_size_as_type);
-};
-
 // Function object which invokes 'free' on its parameter, which must be
 // a pointer. Can be used to store malloc-allocated pointers in scoped_ptr:
 //
@@ -208,6 +155,12 @@ class scoped_ptr_impl {
   }
 
   template <typename U, typename V>
+  void TakeState(std::unique_ptr<U, V>* other) {
+    reset(other->release());
+    get_deleter() = other->get_deleter();
+  }
+
+  template <typename U, typename V>
   void TakeState(scoped_ptr_impl<U, V>* other) {
     // See comment in templated constructor above regarding lack of support
     // for move-only deleters.
@@ -303,7 +256,7 @@ class scoped_ptr_impl {
 // unique_ptr<> features. Known deficiencies include not supporting move-only
 // deleteres, function pointers as deleters, and deleters with reference
 // types.
-template <class T, class D = base::DefaultDeleter<T> >
+template <class T, class D = std::default_delete<T>>
 class scoped_ptr {
   MOVE_ONLY_TYPE_WITH_MOVE_CONSTRUCTOR_FOR_CPP_03(scoped_ptr)
 
@@ -422,6 +375,22 @@ class scoped_ptr {
     return impl_.release();
   }
 
+  // Conversion shims to help with the std::unique_ptr transition.
+  template <typename U, typename V>
+  scoped_ptr(std::unique_ptr<U, V>&& p)
+      : impl_(p.release(), p.get_deleter()) {}
+
+  template <typename U, typename V>
+  scoped_ptr& operator=(std::unique_ptr<U, V>&& p) {
+    impl_.TakeState(&p);
+    return *this;
+  }
+
+  template <typename U, typename V>
+  operator std::unique_ptr<U, V>() && {

[danakj, 2015/11/14 01:29:31]

Vlad pointed out that this doesn't work yet in MSVC 2013:
https://msdn.microsoft.com/en-us/library/hh567368.aspx#corelanguagetable

It's a nice thought though. :(

Other options include a ToUnique(scoped_ptr*) function? Or maybe
ToUnique(scoped_ptr&&) but ya. Not ideal..

Or scoped_ptr::PassToUnique().

Maybe that would be enough, if we make base::Callback hold a unique_ptr
internally (base::Passed converts unique or scoped ptr into a unique_ptr
inside). Then when the Callback::Run() happens, it could implicitly convert to a
scoped_ptr if the function wanted that?

In code transitioning to unique_ptr, they'd have to call some explicit thing at
the place they wanted to move to unique_ptr, but not when they went back again.
It's again not awesome, but it's something.

Other ideas?

[dcheng, 2015/11/14 01:37:10]

Argh. That's obnoxious. Having asymmetrical ways to convert back and forth would
feel really clunky, especially in //base.

Maybe the best approach is to just make the interface match as closely as
possible and then typedef it.

+    return std::unique_ptr<U, V>(release(), get_deleter());
+  }
+
  private:
   // Needed to reach into |impl_| in the constructor.
   template <typename U, typename V> friend class scoped_ptr;
@@ -527,6 +496,19 @@ class scoped_ptr<T[], D> {
     return impl_.release();
   }
 
+  // Conversion shims to help with the std::unique_ptr transition.
+  scoped_ptr(std::unique_ptr<T[], D>&& p)

[dcheng, 2015/11/11 16:18:34]

To keep things simple, the T[] specializations just forbid conversions of any
sort.

[danakj, 2015/11/14 01:29:31]

Can you put that in the comment?

+      : impl_(p.release(), p.get_deleter()) {}
+
+  scoped_ptr& operator=(std::unique_ptr<T[], D>&& p) {
+    impl_.TakeState(&p);
+    return *this;
+  }
+
+  operator std::unique_ptr<T[], D>() && {

[danakj, 2015/11/14 01:29:31]

explain the && here too

[danakj, 2015/11/14 01:30:06]

oops, nevermind this one.

+    return std::unique_ptr<T[], D>(release(), get_deleter());
+  }
+
  private:
   // Force element_type to be a complete type.
   enum { type_must_be_complete = sizeof(element_type) };