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

asked 2018-06-07 11:21:23 -0600

vpradeep gravatar image

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?

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answered 2018-06-07 20:57:59 -0600

ahendrix gravatar image

These are the same.

PointCloud_::ConstPtr is templated on the ContainerAllocator, but the PointCloud typedef specifies a default container allocator of std::allocator<void>, so they're the same.

I'm not sure why there are two aliases for the same symbol.

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Makes sense. You're right that PointCloud::ConstPtr uses std::allocator<void>, and hence they're the same. I guess I just tripped myself up when attempting to expand the typedefs by hand. Thanks for the clarification.

vpradeep gravatar imagevpradeep ( 2018-06-07 22:06:19 -0600 )edit

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Asked: 2018-06-07 11:21:23 -0600

Seen: 408 times

Last updated: Jun 07 '18