scoped_refptr: support opaque pointed-to classes via extern template

base/memory/ref_counted.h

Index: base/memory/ref_counted.h
diff --git a/base/memory/ref_counted.h b/base/memory/ref_counted.h
index be4919c1a51908699c260936f4f6b3c62a98d9ab..9ca7451861328cdf48feebefd7041982f4bc0531 100644
--- a/base/memory/ref_counted.h
+++ b/base/memory/ref_counted.h
@@ -276,23 +276,23 @@ class scoped_refptr {
 
   scoped_refptr(T* p) : ptr_(p) {
     if (ptr_)
-      ptr_->AddRef();
+      AddRef(ptr_);
   }
 
   scoped_refptr(const scoped_refptr<T>& r) : ptr_(r.ptr_) {
     if (ptr_)
-      ptr_->AddRef();
+      AddRef(ptr_);
   }
 
   template <typename U>
   scoped_refptr(const scoped_refptr<U>& r) : ptr_(r.get()) {
     if (ptr_)
-      ptr_->AddRef();
+      AddRef(ptr_);
   }
 
   ~scoped_refptr() {
     if (ptr_)
-      ptr_->Release();
+      Release(ptr_);
   }
 
   T* get() const { return ptr_; }
@@ -316,11 +316,11 @@ class scoped_refptr {
   scoped_refptr<T>& operator=(T* p) {
     // AddRef first so that self assignment should work
     if (p)
-      p->AddRef();
+      AddRef(p);
     T* old_ptr = ptr_;
     ptr_ = p;
     if (old_ptr)
-      old_ptr->Release();
+      Release(old_ptr);
     return *this;
   }
 
@@ -362,8 +362,26 @@ class scoped_refptr {
 
  protected:
   T* ptr_;
+
+ private:
+  // Non-inline helpers to allow:
+  //     class Opaque;
+  //     extern template class scoped_refptr<Opaque>;
+  // Otherwise the compiler will complain that Opaque is an incomplete type.

[danakj, 2014/10/09 00:06:57]

Can you add a test that demonstrates this working?

[mdempsky, 2014/10/09 00:21:33]

Sure.  Do you know if there are any existing idioms for testing
cross-compilation-unit behavior I can follow?

My inclination is to write something like:

  ref_counted_unittest_helper.h:
    class OpaqueHelper;
    scoped_refptr<OpaqueHelper> MakeOpaque();
    void UseOpaque(scoped_refptr<OpaqueHelper>);
    extern template class scoped_refptr<OpaqueHelper>;

  ref_counted_unittest_helper.cc:
    class OpaqueHelper : public base::RefCounted<OpaqueHelper> {
      ...
    };
    scoped_refptr<OpaqueHelper> MakeOpaque() {
      return new OpaqueHelper();
    }
    void UseOpaque(scoped_refptr<OpaqueHelper> p) {
      ...
    }
    template class scoped_refptr<Opaque>;

  ref_counted_unittest.cc:
    TEST(RefCountedUnitTest, ScopedRefPtrToOpaque) {
      scoped_refptr<OpaqueHelper> p = MakeOpaque();
      UseOpaque(p);

      scoped_refptr<OpaqueHelper> q;
      q = p;
      UseOpaque(p);
      UseOpaque(q);
    }

and add the new _helper files to only the test target.  Does that sound
reasonable?

[danakj, 2014/10/09 00:31:47]

That sounds like how I'd do it, putting the helper files in base/test/. But is
extern template class still needed to make this compile after this change?

[mdempsky, 2014/10/09 00:52:48]

SGTM
Yes. The "extern template class" declaration informs the compiler it can assume
another compilation unit will instantiate that template class; otherwise, it has
to conservatively instantiate all needed templates itself, and would complain
that ptr->AddRef() is dereferencing a pointer to an incomplete type.

+  static void AddRef(T* ptr);
+  static void Release(T* ptr);
 };
 
+template <typename T>
+void scoped_refptr<T>::AddRef(T* ptr) {
+  ptr->AddRef();

[danakj, 2014/10/09 00:06:57]

What does the code look like with/out this change in an optimized build? Is
there any difference? (I expect not but please check.)

+}
+
+template <typename T>
+void scoped_refptr<T>::Release(T* ptr) {
+  ptr->Release();
+}
+
 // Handy utility for creating a scoped_refptr<T> out of a T* explicitly without
 // having to retype all the template arguments
 template <typename T>