Streaming non yarp object through yarp ports

[GiulioRomualdi]

I would like to stream a Vector (Matrix) through a yarp::os::BufferedPort, as far as I understood I could:

1. Store the data in a yarp::sig::Vector (yarp::sig::Matrix);
2. Define a custom vector / matrix that inherits from yarp::os::Portable class.

However iDynTree, Eigen, openCV, etc... define their vectors (Matrix). So, every time I would like to send the vector (matrix) through a yarp::os::BufferedPort I should instantiate a yarp::sig::Vector (yarp::sig::Matrix) and copy the value and this is time-consuming.

It would be nice to introduce a portable based on Eigen::Map, indeed every vector (matrix) can be easily wrapped using an Eigen::Map<Eigen::MatrixXd>(.) method. For instance, in yarp and iDynTree there are already functions that convert a Vector (Matrix) in Eigen:Map<> object.

What do you think? Is it worth spending time on this?
@traversaro @Nicogene @prashanthr05 @kouroshD

Related issue: #76

[Nicogene]

Hi @GiulioRomualdi ,

You are right, in order to stream a custom type through the yarp network, that type should inherits from yarp::os::Portable classe, and you have to implement the methods write() and read(), where respectively you encode and decode respectively your custom data structure.

You are right, converting from a datastructure of another library could be time consuming, unless you simply copy the raw pointer of external data.

This has been possible in the YARP_cv library(see #1932) because exists the setExternal method in yarp::sig::Image.

On the other hand yarp::sig::Vector has not a method like this, and like std::vector, is a data structure that manage the memory, it doesn't use memory manage by external objects.

Actually #1582 was intended to add setExternal in Vector, but after a long discussion we decided that it would be better to implement a separate class called VectorMap that uses external memory, in order to make the memory management clear and explicity.
The idea was to do something similar to std::string_view.

Unfortunately we never found the time to add this class that would be very useful indeed.

[traversaro]

Related PR on iDynTree: robotology/idyntree#734 . Having a similar MatrixView class also in YARP that inherits from yarp::os::Portable may be indeed quite convenient.

[drdanz]

Related: #1603

I think all types in general require a big refactor. In order to split the "storage" part of the types from their serialization