jni: Allow nullptr to be converted to JavaRef.

base/android/scoped_java_ref.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_ANDROID_SCOPED_JAVA_REF_H_
 #define BASE_ANDROID_SCOPED_JAVA_REF_H_
 
 #include <jni.h>
 #include <stddef.h>
 
@@ skipping 24 matching lines @@
 
 // Forward declare the generic java reference template class.
 template<typename T> class JavaRef;
 
 // Template specialization of JavaRef, which acts as the base class for all
 // other JavaRef<> template types. This allows you to e.g. pass
 // ScopedJavaLocalRef<jstring> into a function taking const JavaRef<jobject>&
 template<>
 class BASE_EXPORT JavaRef<jobject> {
  public:
+  // ALlow nullptr to be converted to JavaRef. This avoids having to declare an

[Yaron, 2015/12/02 19:42:10]

Allow

+  // empty ScopedJavaLocalRef just to pass null to a function with a JavaRef
+  // parameter, and makes C++ "nullptr" and Java "null" equivalent.
+  JavaRef(std::nullptr_t) : JavaRef() {}
+
+  // Public to allow destruction of temporary JavaRef objects created by the
+  // nullptr conversion. Don't add anything else here; it's inlined.
+  ~JavaRef() {}
+
   jobject obj() const { return obj_; }
 
   bool is_null() const { return obj_ == NULL; }
 
  protected:
   // Initializes a NULL reference. Don't add anything else here; it's inlined.
   JavaRef() : obj_(NULL) {}
 
   // Takes ownership of the |obj| reference passed; requires it to be a local
   // reference type.
 #if DCHECK_IS_ON()
   // Implementation contains a DCHECK; implement out-of-line when DCHECK_IS_ON.
   JavaRef(JNIEnv* env, jobject obj);
 #else
   // Don't add anything else here; it's inlined.
   JavaRef(JNIEnv* env, jobject obj) : obj_(obj) {}
 #endif
 
-  // Don't add anything else here; it's inlined.
-  ~JavaRef() {}
-
   // The following are implementation detail convenience methods, for
   // use by the sub-classes.
   JNIEnv* SetNewLocalRef(JNIEnv* env, jobject obj);
   void SetNewGlobalRef(JNIEnv* env, jobject obj);
   void ResetLocalRef(JNIEnv* env);
   void ResetGlobalRef();
   jobject ReleaseInternal();
 
  private:
   jobject obj_;
 
   DISALLOW_COPY_AND_ASSIGN(JavaRef);
 };
 
 // Generic base class for ScopedJavaLocalRef and ScopedJavaGlobalRef. Useful
 // for allowing functions to accept a reference without having to mandate
 // whether it is a local or global type.
 template<typename T>
 class JavaRef : public JavaRef<jobject> {
  public:
+  JavaRef(std::nullptr_t) : JavaRef<jobject>(nullptr) {}
+  ~JavaRef() {}
+
   T obj() const { return static_cast<T>(JavaRef<jobject>::obj()); }
 
  protected:
   JavaRef() {}
-  ~JavaRef() {}
 
   JavaRef(JNIEnv* env, T obj) : JavaRef<jobject>(env, obj) {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(JavaRef);
 };
 
 // Holds a local reference to a JNI method parameter.
 // Method parameters should not be deleted, and so this class exists purely to
 // wrap them as a JavaRef<T> in the JNI binding generator. Do not create
 // instances manually.
 template<typename T>
 class JavaParamRef : public JavaRef<T> {
  public:
   // Assumes that |obj| is a parameter passed to a JNI method from Java.
   // Does not assume ownership as parameters should not be deleted.
   JavaParamRef(JNIEnv* env, T obj) : JavaRef<T>(env, obj) {}
 
   // Allow nullptr to be converted to JavaParamRef. Some unit tests call JNI
   // methods directly from C++ and pass null for objects which are not actually
   // used by the implementation (e.g. the caller object); allow this to keep
   // working.
-  JavaParamRef(std::nullptr_t) : JavaRef<T>() {}
+  JavaParamRef(std::nullptr_t) : JavaRef<T>(nullptr) {}
 
   ~JavaParamRef() {}
 
   // TODO(torne): remove this cast once we're using JavaRef consistently.
   // http://crbug.com/506850
   operator T() const { return JavaRef<T>::obj(); }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(JavaParamRef);
 };
@@ skipping 136 matching lines @@
   // is *not* a transfer of ownership and Release() should not be used.
   T Release() {
     return static_cast<T>(this->ReleaseInternal());
   }
 };
 
 }  // namespace android
 }  // namespace base
 
 #endif  // BASE_ANDROID_SCOPED_JAVA_REF_H_