Adds ScopedHandleBase::From

mojo/public/cpp/system/core.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MOJO_PUBLIC_CPP_SYSTEM_CORE_H_
 #define MOJO_PUBLIC_CPP_SYSTEM_CORE_H_
 
 #include <assert.h>
 #include <stddef.h>
 
@@ skipping 31 matching lines @@
 
   // Move-only constructor and operator=.
   ScopedHandleBase(RValue other) : handle_(other.object->release()) {}
   ScopedHandleBase& operator=(RValue other) {
     handle_ = other.object->release();
     return *this;
   }
 
   const HandleType& get() const { return handle_; }
 
+  template <typename PassedHandleType>
+  static ScopedHandleBase<HandleType> From(
+      ScopedHandleBase<PassedHandleType> other) {
+    MOJO_COMPILE_ASSERT(
+        sizeof(static_cast<PassedHandleType*>(static_cast<HandleType*>(0))),
+        HandleType_is_not_a_subtype_of_PassedHandleType);
+    return ScopedHandleBase<HandleType>(
+        static_cast<HandleType>(other.release().value()));

[darin (slow to review), 2014/04/04 22:40:49]

Are you sure about the .value() call here? Doesn't that strip the
PassedHandleType down to a MojoHandle, which any HandleType can be initialized
from? It feels like this might allow you to initialize a
ScopedHandleBase<MessagePipeHandle> from a
ScopedHandleBase<DataPipeConsumerHandle>.

I would have expected this to work out if you leave off the .value() call.

[darin (slow to review), 2014/04/04 22:56:14]

Nevermind. I somehow missed the compile assert :-P

+  }
+
   void swap(ScopedHandleBase& other) {
     handle_.swap(other.handle_);
   }
 
   HandleType release() MOJO_WARN_UNUSED_RESULT {
     HandleType rv;
     rv.swap(handle_);
     return rv;
   }
 
@@ skipping 396 matching lines @@
       CreateSharedBuffer(&options, num_bytes, &handle);
   assert(result == MOJO_RESULT_OK);
 }
 
 inline SharedBuffer::~SharedBuffer() {
 }
 
 }  // namespace mojo
 
 #endif  // MOJO_PUBLIC_CPP_SYSTEM_CORE_H_