Rock

the Robot Construction Kit

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Type definitions

One of the first thing that a component designer has to think about is defining the data structures that will be used in the component’s interfaces:

  • in the communication between components (ports)
  • in the configuration of the component (properties)
  • in the control of the component (operations)

From your component definitions, oroGen will generate the necessary RTT plugins to support:

  • XML marshalling as .xml or .cpf
  • CORBA marshalling to use with the CORBA transport
  • Typelib marshalling to use with the data logger
  • the classes needed for the Ruby bindings

In principle, if you are using oroGen, all the tools that are offered by the Rock toolchain should work seamlessly. If they don’t, it’s because you found a bug !

The type-handling part of oroGen is available as a standalone tool for people that want to develop RTT components directly. This tools is called typeGen and presented at the end of this page.

Defining types

In oroGen, the types are described in C++. However, not all C++ types can be used in the data flow. To be usable in the data flow, a type must:

  • be default constructible and copyable (i.e. have a constructor that have no arguments and can be copied).
  • have no private fields
  • have no parent class (this might be removed in a future version of oroGen)

Moreover, oroGen has a special support for the std::string and std::vector standard classes, so you can use them freely.

Example: defining a Time class

namespace base {
  struct Time
  {
    uint64_t microseconds;
    static Time fromMilliseconds(uint64_t ms);
    Time operator +(Time const& other);
  };
}

Note that, for now, all types that are defined in the orogen project must be defined inline: orogen does not support separating the implementation from the definition. Types that are defined in separate libraries are fine, though.

Known Limitations

  • structures and classes with private members are not handled (a warning is issued)
  • structures and classes that have a parent class are not handled (a warning is issued)
  • char, short, 64 bit integers and float are forbidden as argument types for an operation. Use int instead of char or short, and double instead of float. Unfortunately there is no equivalent for 64 bit integers. This is due to limitations in the RTT default typekit.
  • structs that contain 64 bit integers won’t be marshallable as XML. This is because the CPF file format does not have 64bit integer support either.
  • there is no typelib marshalling - and hence no logging - for base types (int, float, …). They work fine if used as fields in a structure though.

What to do if your type cannot be understood by oroGen ?

For types that can’t be directly managed by oroGen, a special mechanism allows to give a way for oroGen to manipulate the data. This is unfortunately only available to the users of oroGen – not to the users of typeGen – for now. See the opaque types section for more information.

Defining types in an oroGen project

In an oroGen project, one adds one or many following statement to load a C/C++ file that defines the project’s types:

import_types_from "myproject.h"

Be aware that myproject.h must be self-consistent, i.e. that it must include all the headers that are necessary to define the types that it wants to define.

Moreover, only the types that are directly defined in myproject.h will be exported in the typekit. It means that the include order actually matters (that’s a bug, not a feature).

In other words, if other_header.h is included in myproject.h and you want the types of other_header.h to be exported in the typekit, then you need to do

import_types_from "other_header.h"
import_types_from "myproject.h"

and not

import_types_from "myproject.h"
import_types_from "other_header.h"

Finally, one can directly use types defined in a library, provided that this library gives a pkg-config file for dependency discovery.

Let’s consider a ‘drivers/hokuyo’ package that would define a hokuyo::Statistics structure. Assuming that this package (1) installs a hokuyo.pc file, and (2) installs the relevant header as “hokuyo.hpp”, it is possible to do

using_library "hokuyo"
import_types_from "hokuyo.hpp"

The issue of loading types from other oroGen projects is handled in here

The special case of templates

Templates are not directly understood by oroGen. Howver, explicit instanciations of them can be used.

Unfortunately, typedef’ing the type that you need is not enough for gccxml – the tool that parses C++ for oroGen. You have to use the instanciated template directly in a structure. To work around this, you can define a structure whose name contains gccxml_workaround to get the template instanciated, and then define the typedefs that you will actually use in your typekits and oroGen task interfaces.

For instance, with

template <typename Scalar, int DIM>
struct Vector {
  Scalar values[DIM];
};

struct __orogen_workaround {
  Vector<3> vector3;
  Vector<4> vector4;
};

one can use Vector<3> in its orogen interface, and in other structures.

Standalone typekits

If you don’t want to use oroGen. The orogen package offers the typegen tool to generate the typekits for people that want to write their components themselves.

To generate a typekit using typegen, do the following:

  • write your types as described in this page
  • save all of them under the same subdirectory, and put only these types there
  • run

    typegen typekit_name inputs

    where inputs is the input directory.

typeGen will generate a typekit/ directory which includes both the typekit code and the cmake code that is needed to compile it.

All the caveats described in the oroGen section apply:

  • import order might matter. If you are in a situation where it does matter, you will have to list the header files one by one on the command line. Note that it is fine to do

    typegen typekit_name header_file input_directory

    to ensure that header_file will be loaded before all the other files in the directory.

  • it is possible to load headers from libraries that define pkg-config files and from other typegen/orogen project. Use the -i option instead of using using_library and import_types_from.