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Get Const::PTR from message

asked 2014-11-04 09:10:15 -0500

Ruud gravatar image

Hi all,

For a specific function that is not a callback, I require a const sensor_msgs::ImageConstPtr& image_pointer to work with that is derived from a regular sensor_msgs::Image source_image that I already have.

How would one get such a constant image pointer from a regular message?

Thanks! Ruud

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answered 2014-11-04 09:34:06 -0500

Wolf gravatar image

If its no problem for your that the data is copied you can do something like:

sensor_msgs::ImageConstPtr your_const_pointer( new sensor_msgs::Image( source_image ) );

Note that, this calls the copy constructor of sensor_msgs::Image thereby copying the underlying data. (But I guess its Ok for you because you want ConstPtr)

If you need a shared pointer that can modify the data in your original object this is not possible in this way because you cannot (safely) create a shared_pointer for a object you do not have the ownership for ( Unless you already have a shared_pointer (, which you don't,) and the object you intend to point to does not implement boost enable_shared_from_this<y> (which message types do not) ) ....

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Thanks! That worked.

Ruud gravatar image Ruud  ( 2014-11-04 09:48:30 -0500 )edit

answered 2014-11-04 09:35:42 -0500

paulbovbel gravatar image

updated 2014-11-04 09:38:58 -0500

For reference, ImagePtr and ImageConstPtr are defined as:

typedef boost::shared_ptr< ::sensor_msgs::Image> ImagePtr;
typedef boost::shared_ptr< ::sensor_msgs::Image const> ImageConstPtr;

The quickest legal solution would be to create an ImageConstPtr with a null deleter, so that when the shared_ptr is destructed, it doesn't try to deallocate the memory. You would also have to be careful that the shared_ptr doesn't outlive your source_image object.

ImageConstPtr image_pointer(*source_image, boost::null_deleter());

However, this is not by any means the 'right way'. The best thing to do would be to initialize your source_image object as an ImagePtr in the first place, and simply pass it to the function expecting an ImageConstPtr.

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Or use Wolf's suggestion to copy the data :)

paulbovbel gravatar image paulbovbel  ( 2014-11-04 09:40:51 -0500 )edit

This is a great trick when you are willing to trade a bit of safety/readability for performance.

xkortex gravatar image xkortex  ( 2021-01-31 23:17:00 -0500 )edit

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Asked: 2014-11-04 09:10:15 -0500

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Last updated: Nov 04 '14