roscpp: topic.h is not compatible with c++11

[taketwo]

This line gives the following error when compiled with c++11 option:

/opt/ros/indigo/include/ros/topic.h|64 col 12| error: no viable conversion from 'MConstPtr' (aka 'shared_ptr<const stdr_msgs::RobotIndexedVectorMsg_<std::allocator<void> > >') to 'bool'
         return message_;

The reason is that in C++11, shared_ptr has an explicit operator bool which means that a shared_ptr can't be implicitly converted to a bool. A trivial fix could be applied to make the code compatible with both the new and the old stardards:

return static_cast<bool>(message_);

Alternatively, one could write:

return message_.get() != 0;

Please let me know which version is preferable and I will submit a pull request.

[dirk-thomas]

The first proposed fix lgtm. Please provide a pull request against the indigo-devel branch. Thanks.