Enable boolean testing of scoped_refptr<T>.

base/memory/ref_counted.h

 // Copyright (c) 2012 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 BASE_MEMORY_REF_COUNTED_H_
 #define BASE_MEMORY_REF_COUNTED_H_
 
 #include <cassert>
 #include <iosfwd>
 
@@ skipping 315 matching lines @@
   void swap(T** pp) {
     T* p = ptr_;
     ptr_ = *pp;
     *pp = p;
   }
 
   void swap(scoped_refptr<T>& r) {
     swap(&r.ptr_);
   }
 
+ private:
+  // Allow scoped_refptr<T> to be used in boolean expressions, but not
+  // implicitly convertible to a real bool (which is dangerous).
+  //
+  // Note that this trick is only safe when the == and != operators
+  // are declared explicitly, as otherwise "refptr1 == refptr2"
+  // will compile but do the wrong thing (i.e., convert to Testable
+  // and then do the comparison).
+  typedef T* scoped_refptr::*Testable;
+
+ public:
+  operator Testable() const { return ptr_ ? &scoped_refptr::ptr_ : nullptr; }
+
   template <typename U>
   bool operator==(const scoped_refptr<U>& rhs) const {
     return ptr_ == rhs.get();
   }
 
   template <typename U>
   bool operator!=(const scoped_refptr<U>& rhs) const {
     return !operator==(rhs);
   }
 
@@ skipping 52 matching lines @@
 bool operator!=(const T* lhs, const scoped_refptr<U>& rhs) {
   return !operator==(lhs, rhs);
 }
 
 template <typename T>
 std::ostream& operator<<(std::ostream& out, const scoped_refptr<T>& p) {
   return out << p.get();
 }
 
 #endif  // BASE_MEMORY_REF_COUNTED_H_