Catch intptr_t to int conversion on 64-bit platforms.

base/android/jni_int_wrapper.h

+// Copyright (c) 2014 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_JNI_INT_WRAPPER_H_
+#define BASE_ANDROID_JNI_INT_WRAPPER_H_
+
+#include <stdint.h>
+
+// Wrapper used to receive int when calling Java from native.
+// The wrapper disallows automatic conversion of long to int.
+// This is to avoid a common anti-pattern where a Java int is used
+// to receive a native pointer. Please use a Java long to receive
+// native pointers, so that the code works on both 32-bit and 64-bit
+// platforms. Note the wrapper allows other lossy conversions into
+// jint that could be consider anti-patterns, such as from size_t.

[bulach, 2014/05/02 16:55:56]

I think we could do with a 
#ifdef NDEBUG 
// Don't add any indirection on release mode.
typedef jint JniIntWrapper;
#else
// On debug mode, use the extra indirection to capture
// the type-narrowing above.
#endi

+
+class jni_int_wrapper {

[bulach, 2014/05/02 16:55:56]

nit: should be JniIntWrapper

+ public:
+  jni_int_wrapper() : i_(0) {}
+  jni_int_wrapper(int i) : i_(i) {}
+  jni_int_wrapper(const jni_int_wrapper& ji) : i_(ji.i_) {}
+  template <class T> jni_int_wrapper(const T& t) : i_(t) {}
+  jint asJint() const { return i_; }

[bulach, 2014/05/02 16:55:56]

nit: and here, as_jint()

+ private:
+  // If you get an "is private" error at the line below it is because you used
+  // an implicit conversion to convert a long to an int when calling Java.
+  // We disallow this, as a common anti-pattern allows converting a native
+  // pointer (intptr_t) to a Java int. Please use a Java long to represent
+  // a native pointer. If you want a lossy conversion, please use an
+  // explicit conversion in your C++ code. Note an error is only seen when
+  // compiling on a 64-bit platform, as intptr_t is indistinguishable from
+  // int on 32-bit platforms.
+  jni_int_wrapper(long);
+  jint i_;
+};
+
+#endif  // BASE_ANDROID_JNI_INT_WRAPPER_H_