Enable boolean testing of scoped_refptr<T>.

BUG=110610

Review URL: https://codereview.chromium.org/758803002

Cr-Commit-Position: refs/heads/master@{#305763}

base/memory/ref_counted.h

@@ -333,6 +333,19 @@ class scoped_refptr {
     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();

base/memory/ref_counted_unittest.cc

@@ -10,9 +10,19 @@
 namespace {
 
 class SelfAssign : public base::RefCounted<SelfAssign> {
+ protected:
+  virtual ~SelfAssign() {}
+
+ private:
   friend class base::RefCounted<SelfAssign>;
+};
+
+class Derived : public SelfAssign {
+ protected:
+  ~Derived() override {}
 
-  ~SelfAssign() {}
+ private:
+  friend class base::RefCounted<Derived>;
 };
 
 class CheckDerivedMemberAccess : public scoped_refptr<SelfAssign> {
@@ -72,3 +82,34 @@ TEST(RefCountedUnitTest, ScopedRefPtrToOpaque) {
   base::TestOpaqueRefCounted(p);
   base::TestOpaqueRefCounted(q);
 }
+
+TEST(RefCountedUnitTest, BooleanTesting) {
+  scoped_refptr<SelfAssign> p;
+  EXPECT_FALSE(p);
+  p = new SelfAssign;
+  EXPECT_TRUE(p);
+}
+
+TEST(RefCountedUnitTest, Equality) {
+  scoped_refptr<SelfAssign> p1(new SelfAssign);
+  scoped_refptr<SelfAssign> p2(new SelfAssign);
+
+  EXPECT_EQ(p1, p1);
+  EXPECT_EQ(p2, p2);
+
+  EXPECT_NE(p1, p2);
+  EXPECT_NE(p2, p1);
+}
+
+TEST(RefCountedUnitTest, ConvertibleEquality) {
+  scoped_refptr<Derived> p1(new Derived);
+  scoped_refptr<SelfAssign> p2;
+
+  EXPECT_NE(p1, p2);
+  EXPECT_NE(p2, p1);
+
+  p2 = p1;
+
+  EXPECT_EQ(p1, p2);
+  EXPECT_EQ(p2, p1);
+}