What are they ?

Opaque types are a way to enable oroGen to handle types that it cannot handle completely automatically. The general idea is that you provide oroGen with a “marshalling structure” that (1) it can understand and (2) can hold all the data that the “real type” holds. Then, you have to implement two convertion functions: one that converts from the marshalling type, and one to the marshalling type.

So, it involves doing one copy. What is the gain ?

Unlike handwritten typekits, another solution to this problem, opaque types provide you with the advantage that other types that use opaque types (i.e. structures with fields that are from opaque types, std::vector, arrays) will be automatically handled by oroGen. I.e. you write the convertion function for the types that oroGen can’t handle and let it do the rest of the work.

Moreover, oroGen will be able to generate typekits for all the transports it can handle.

Finally, the convertion to and from the marshalling type is only done in transports. Thanks to RTT’s use of plain C++ structures to implement its dataflow, there is no need for convertions in intra-process dataflow.

How to use them

You first have to create a wrapper type (a.k.a. “intermediate type”) for the opaque. In the case of Eigen::Vector3d, a suitable wrapper would be

namespace wrappers
{
 struct Vector3d
 {
     double x, y, z;
 };
}

This wrapper then needs to be loaded with #import_types_from. Finally, one can use #opaque_type to declare the opaque.

import_types_from "wrappers/my_opaque_wrapper.h"
opaque_type '/Eigen/Vector3d", "/wrappers/Vector3d"

where wrappers::Vector3d is defined in the my_opaque_wrapper.h header. Moreover, if getting the definition of the opaque type requires new include directories that are not yet added to the typekit through the using_library mechanism, you will have to detect them in the Ruby code and add them with the :include option

import_types_from "wrappers/my_opaque_wrapper.h"
opaque_type '/Eigen/Vector3d", "/wrappers/Vector3d", :includes => eigen_prefix

Once you have re-generated the project, a typekit/ directory is created with Opaques.cpp and Opaques.hpp in it. These files hold the convertion functions that should be used by oroGen to convert the opaque to the wrapper and the wrapper to the opaque.

As usual, if you add new opaques to an orogen project that already has some, you will need to copy the new toIntermediate/fromIntermediate convertion functions from templates/typekit/Opaques.cpp

The m-types: automatically generated intermediate types

As explained, once you have defined an opaque type, oroGen will take care of other types that use this opaque. For instance

struct Position
{
 base::Time time;
 Eigen::Vector3d position;
};

can be used in your task interfaces without any modifications. This works for structures, std::vector and static-size arrays.