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What's the difference between PointCloudConstPtr and PointCloud::ConstPtr

In subscriber callbacks our software stack uses a mixture of two function signatures. In the case of subscribing to a sensor_msgs::PointCloud, we use the following two signatures:

  1. void callback(const sensor_msgs::PointCloud::ConstPtr& msg)
  2. void callback(const sensor_msgs::PointCloudConstPtr& msg)

These are both defined in the autogenerated header PointCloud.h

template <class ContainerAllocator>
struct PointCloud_
  // ---- Snip ----
  typedef boost::shared_ptr< ::sensor_msgs::PointCloud_<ContainerAllocator> const> ConstPtr;
typedef ::sensor_msgs::PointCloud_<std::allocator<void> > PointCloud;

typedef boost::shared_ptr< ::sensor_msgs::PointCloud > PointCloudPtr;
typedef boost::shared_ptr< ::sensor_msgs::PointCloud const> PointCloudConstPtr;

Expanding the typedefs myself, I get the following

  1. sensor_msgs::PointCloud::ConstPtr

    boost::shared_ptr< ::sensor_msgs::PointCloud_<ContainerAllocator> const>

  2. sensor_msgs::PointCloudConstPtr
    boost::shared_ptr< ::sensor_msgs::PointCloud_<std::allocator<void> > const>

It's not clear to me what ContainerAllocator gets set to in case 1, and I'm also not sure what the purpose of ContainerAllocater is. Why would I choose one incantation over the other?