Reading comments from .proto files using a Protocol Buffers descriptor object

I am currently revisiting a project using Google Protocol Buffers.

In the project I want to make use of the features Descriptors and Reflection of Protocol Buffers.

The official documentation states that the comments of .proto files can be read:

1. With the function DebugStringWithOptions(), called on a message or descriptor.
2. With the function GetSourceLocation(), called on a descriptor.

I am unable to retrieve comments, so I think I am doing something completely wrong or that feature isn't fully implemented in Protocol Buffers, yet.

Here are some code snippets:

google::protobuf::DebugStringOptions options;
options.include_comments = true;
std::cout << "google::protobuf::Descriptor::DebugStringWithOptions(): "
          << message.descriptor()->DebugStringWithOptions(options) << std::endl
          << std::endl;

const google::protobuf::FieldDescriptor* field_descriptor{
    message.descriptor()->field(1)};

// TODO(wolters): Why doesn't this work?
google::protobuf::SourceLocation* source_location{
    new google::protobuf::SourceLocation};
field_descriptor->GetSourceLocation(source_location);

// if (field_descriptor->GetSourceLocation(source_location)) {
std::cout << "start_line: " << source_location->leading_comments
          << std::endl;
std::cout << "end_line: " << source_location->leading_comments << std::endl;
std::cout << "start_column: " << source_location->start_column << std::endl;
std::cout << "end_column: " << source_location->end_column << std::endl;
std::cout << "leading_comments: " << source_location->leading_comments
          << std::endl;
std::cout << "trailing_comments: " << source_location->trailing_comments
          << std::endl;
// }

I've tried using the following two syntaxes for comments in the .proto file, but none of them seems to work:

MessageHeader header = 1;  // The header of this `Message`.

/**
 * The header of this `Message`.
 */
MessageHeader header = 1;

I am using GCC 4.7.1 (with C++11 support enabled) and the latest Protocol Buffers version 3.0.0-alpha-4.1.

Can someone guide me into the correct direction and/or provide me an working example?