Part V: Developing and Testing the ROS Message Endpoint

[Author's note, July 28, 2010: Introduced Boost::shared_ptr to manage reference to message endpoint so as to enable postponed construction, as will be required in Part VI.]

While this series (Part I, II, III, IV) has been specifically written to address writing well-designed packages for ROS, we’ve actually seen very little of ROS itself thus far. In fact, outside of the use of roscreate to generate the package basics and sensor_msgs::LaserScan for communicating laser scan data from the reader up to the application services layer, there’s been no indication that this application was actually intended to work with ROS now or ever. Ironically, this is exactly what we’d expect to see in a well designed ROS package.

Each layer that we’ve developed – as initially outlined in Part I – is logically separated from each other’s context of responsibility. To illustrate, the upper layers do not directly depend on “service” layers, such as message endpoints. Instead, the lower layers depend on abstract service interfaces declared in the upper layers. This dependency inversion was enabled in Part IV with the creation of ILaserScanEndpoint, a separated interface. If all of this dependency inversion and separated interface mumbo-jumbo has your head spinning at all, take some time to delve deeper into this subject in Dependency Injection 101.

While the actual message endpoint interface was created, only a test double was developed for testing the application service layer’s functionality. Accordingly, in this post, the concrete message endpoint “service,” which implements its separated interface, will be developed and tested. That’s right…we’ll finally actually talk to ROS! You can skip to the chase and download the source for this post.

Before digging into the code, it’s important to take a moment to better understand the purpose and usefulness of the message endpoint. The message endpoint encapsulates communications to the messaging middleware similarly to how a data repository encapsulates communications to a database. By encapsulating such communications, the rest of the application (ROS package, in our case) may remain blissfully oblivious to details such as how to publish messages to a topic or translate between messages and domain layer objects.

This separation of concerns helps to keep the application cleanly decoupled from the messaging middleware. Another benefit of this approach is enabling the development and testing of nearly the entirety of the application/package before “wiring” it up to the messaging middleware itself. This typically results in more reusable and readable code. If you haven’t already, I would encourage you to read the article Message-Based Systems for Maintainable, Asynchronous Development for a more complete discussion on message endpoints.

Onward!

Target Class Diagram

The following diagram shows what the package will look like after completing the steps in this post…it’s beginning to look oddly familiar to the package diagram discussed in Part I of this series, isn’t it? If you’ve been following along, most of the elements have already been completed; only the concrete LaserScanEndpoint and LaserScanEndpointTests will need to be introduced along with a slight modification to the TestRunner.

1. Setup the Package Skeleton, Domain Layer and Application Services Layer

If not done already, follow the steps in Part II, Part III, and Part IV to create the package and develop/test the domain and application service layers. (Or just download the code from Part IV as a starting point to save some time.)

2. Create the message endpoint header class.

Create src/message_endpoints/LaserScanEndpoint.hpp containing the following code:

// LaserScanEndpoint.hpp
 
#ifndef GUARD_LaserScanEndpoint
#define GUARD_LaserScanEndpoint
 
#include <ros/ros.h>
#include "sensor_msgs/LaserScan.h"
#include "ILaserScanEndpoint.hpp"
 
namespace ladar_reporter_message_endpoints
{
  class LaserScanEndpoint : public ladar_reporter_core::ILaserScanEndpoint
  { 
    public:
      LaserScanEndpoint();
 
      void publish(const sensor_msgs::LaserScan& laserScan) const;
 
    private:
      // Create handle to node
      ros::NodeHandle _ladarReporterNode;
 
      ros::Publisher _laserReportPublisher;
  };
}
 
#endif /* GUARD_LaserScanEndpoint */

The message endpoint header simply implements ILaserScanEndpoint and sets up handlers for holding the ROS NodeHandle and Publisher. The more interesting bits are found in the implementation details…

3. Create the message endpoint implementation class.

Create src/message_endpoints/LaserScanEndpoint.cpp containing the following code:

// LaserScanEndpoint.cpp
 
#include <ros/ros.h>
#include "sensor_msgs/LaserScan.h"
#include "LaserScanEndpoint.hpp"
 
namespace ladar_reporter_message_endpoints
{
  LaserScanEndpoint::LaserScanEndpoint() 
    // Setup topic for publishing laser scans to
    : _laserReportPublisher(
      _ladarReporterNode.advertise<sensor_msgs::LaserScan>("laser_report", 100)) { }
 
  void LaserScanEndpoint::publish(const sensor_msgs::LaserScan& laserScan) const {
    _laserReportPublisher.publish(laserScan);
    ros::spinOnce();
    ROS_INFO("Published laser scan to laser_report topic with angle_min of: %f", laserScan.angle_min);
  };
}

As you can see, there’s really not much to the actual publication process…which is what we were hoping for. The message endpoint should simply be a light way means to send and receive messages to/from the messaging middleware. This message endpoint does so as follows:

• The constructor initializes the publisher by advertising on a topic with the name of “laser_report.”
• The publish() function simply takes the received laserSan and moves it along to be published via ROS. Although not necessary for this specific package code, the call to spinOnce() will be important when the package has callbacks based on messages received. (See http://www.ros.org/wiki/ROS/Tutorials/WritingPublisherSubscriber(c%2B%2B) for more details.)

4. Configure CMake to Include the Header and Implementation

With the header and implementation classes completed, we need to make a couple of minor modifications to CMake for their inclusion in the build.

1. Open /ladar_reporter/CMakeLists.txt. Within the include_directories statement, add an include for the following directory to include the concrete message endpoint header:

   "src/message_endpoints"

2. Open /ladar_reporter/src/CMakeLists.txt. Add an inclusion for the message_endpoints directory at the end of the file:

   add_subdirectory(message_endpoints)

3. Now, in order to create the message endpoints class library itself, a new CMake file is needed under /ladar_reporter/src/message_endpoints. Accordingly, create a new CMakeLists.txt under /ladar_reporter/src/message_endpoints, containing the following:

   # Create the library
   add_library(
     ladar_reporter_message_endpoints
     LaserScanEndpoint.cpp
   )

5. Build the message endpoints Class Library

In a terminal window, cd to /ladar_reporter and run make. The class library should build and link successfully.

Like with everything else thus far…it’s now time to test our new functionality.

6. Unit Test the LaserScanEndpoint Functionality

While testing up to this point has been pretty straight-forward, we now need to incorporate ROS package initialization within the test itself.

1. Our package is going to act as a single ROS node; accordingly, we need to modify /ladar_reporter/test/TestRunner.cpp to initialize ROS within the package and to register itself as a node which we’ll call “ladar_reporter.” While I’m hard-coding the node name within the test itself, you may want to consider putting the name into a config file as it’ll likely be referenced in multiple places; having it centralized within a config file gets rid of the magic string, making it easier to change while reducing the likelihood of typing it wrong in some hard-to-track-down spot.

   Open /ladar_reporter/test/TestRunner.cpp and modify the code to reflect the following:

   #include <gtest/gtest.h> #include <ros/ros.h>   // Run all the tests that were declared with TEST() int main(int argc, char **argv){ testing::InitGoogleTest(&argc, argv);   // Initialize ROS and set node name ros::init(argc, argv, "ladar_reporter");   return RUN_ALL_TESTS(); }

2. Create a new testing class, /ladar_reporter/test/message_endpoints/LaserScanEndpointTests.cpp:

   // LaserScanEndpointTests.cpp   #include <gtest/gtest.h> #include "sensor_msgs/LaserScan.h" #include "LaserScanEndpoint.hpp" #include "LaserScanReportingService.hpp"   using namespace ladar_reporter_application_services; using namespace ladar_reporter_message_endpoints;   namespace ladar_reporter_test_message_endpoints { // Define unit test to verify ability to publish laser scans // to ROS using the concrete message endpoint. TEST(LaserScanEndpointTests, canPublishLaserScanWithEndpoint) { // Establish Context LaserScanEndpoint laserScanEndpoint; sensor_msgs::LaserScan laserScan;   // Give ROS time to fully initialize and for the laserScanEndpoint to advertise sleep(1);   // Act laserScan.angle_min = 1; laserScanEndpoint.publish(laserScan);   laserScan.angle_min = 2; laserScanEndpoint.publish(laserScan);   // Assert // Nothing to assert other than using terminal windows to // watch publication activity. Alternatively, for better testing, // you could create a subscriber and subscribe to the reports // You could then track how many reports were received and // assert checks, accordingly. }   // Define unit test to verify ability to leverage the reporting // service using the concrete message endpoint. This is more of a // package integration test than a unit test, making sure that all // of the pieces are playing together nicely within the package. TEST(LaserScanEndpointTests, canStartAndStopLaserScanReportingServiceWithEndpoint) { // Establish Context boost::shared_ptr<LaserScanEndpoint> laserScanEndpoint = boost::shared_ptr<LaserScanEndpoint>(new LaserScanEndpoint()); LaserScanReportingService laserScanReportingService(laserScanEndpoint);   // Give ROS time to fully initialize and for the laserScanEndpoint to advertise sleep(1);   // Act laserScanReportingService.beginReporting(); sleep(4); laserScanReportingService.stopReporting();   // Assert // See assertion note above from // LaserScanEndpointTests.canPublishLaserScanWithEndpoint } }

   The comments within the test class above should clarify what is occurring. But in summary, the canPublishLaserScanWithEndpoint test bypasses all of the layers and tests the publishing of messages directly via the message endpoint. The canStartAndStopLaserScanReportingServiceWithEndpoint test takes this much further and injects the LaserScanEndpoint message endpoint into the LaserScanReportingService application service and starts/stops the laser scan reprorting, accordingly. This latter test should be seen more as an integration test rather than a unit test as it tests the results of all of the layers working together.

3. Open /ladar_reporter/test/CMakeLists.txt. Within the rosbuild_add_executable statement, add the following to include the message endpoint tests in the build:

   ./message_endpoints/LaserScanEndpointTests.cpp

4. While we’re at it, we’ll also need to link the new message_endpoints class library to the unit testing executable; accordingly, also within /ladar_reporter/test/CMakeLists.txt, modify target_link_libraries to reflect the following:

   # Link the libraries
   target_link_libraries(
     ladar_reporter_tests
     ladar_reporter_application_services
     ladar_reporter_message_endpoints
     # Important that core comes after application_services due to direction of dependencies
     ladar_reporter_core
   )

5. Verify that everything builds OK by running make within a terminal from the root folder, /ladar_reporter.
6. Now for the fun part…time to run our tests with ROS running. The steps will follow closely to those described in the ROS tutorial, Examining Pulisher/Subscriber:
   1. Open a new terminal and run roscore to begin ROS
   2. In the terminal that you’ve been building in…

      make
      cd bin
      ./ladar_reporter_tests

   With a little luck, you should see a few messaging being published to ROS while running the unit tests. And just to prove it…

   1. With roscore still running, open a third terminal window and run rostopic echo /laser_report. You’ll likely be warned that the topic is not publishing yet…let’s change that.
   2. Back in your original terminal, the one you ran the unit tests in, rerun ./ladar_reporter_tests. You should now being seeing laser scans being echoed to the third terminal window for the LaserScanEndpointTests canPublishLaserScanWithEndpoint and canStartAndStopLaserScanReportingServiceWithEndpoint. Seriously now, how cool is that?

The first five parts of this series conclude the primary elements of developing well-designed packages for Robot Operating System (ROS) using proven design patterns and proper separation of concerns. Obviously, this is not a trivially simple approach to developing ROS packages; indeed, it would be overkill for very simple packages. But as packages grow in size, scope, and complexity, techniques described in this series should help to establish a maintainable, extensible package which doesn’t get too unruly as it evolves. In Part VI, the final part in this series, we’ll look at adding a simple UI layer, using wxWidgets, to interact with the package functionality.

Enjoy!
Billy McCafferty

Download the source for this post.

Part IV: Developing and Testing the Application Services

[Author's note, July 28, 2010: Introduced Boost::shared_ptr to manage reference to message endpoint so as to enable postponed construction, as will be required in Part VI.]

Ah, we’ve made it to the application services layer.  After defining the architecture, setting up the package, and implementing the core layer which contains the domain logic of the application, we’re ready to take on developing the application services layer.  In short, the service layer provides the external API to the underlying domain layer.  Ideally, one should be able to develop the entirety of the “non-UI” portions of an application and have it exposed via the application services layer.  In other words, one should be able to swap out the front end – say from a web front end to a Flash front end – without having the application services layer effected. If you’d like to go ahead and download the source for this article, click here.

The application services layer is analogous to a task or coordination manager; i.e., it doesn’t know how to carry out the low level details of a particular action but it does know who is responsible for carrying out particular tasks.  Accordingly, the application services layer of the package (or any application for that matter) is mostly made up of a number of publicly accessible methods which, in turn, pass responsibilities on to external service dependencies (e.g., a message endpoint) and the domain layer for execution.

With that said, the services layer should still eschew direct dependencies on external services, such as communications with a messaging framework (e.g., ROS).  Accordingly, in this post, we’ll materialize the application services layer of our package and give it two responsibilities:

• Coordinate with the domain layer to begin laser report reading and handle the raised reports, accordingly;
• Forward laser reports on to the appropriate message endpoint(s) to have the information disseminated over the message based system.

But (there’s always a “but” isn’t there), while the application services layer should be responsible for passing messages received from the domain layer on to the messaging middleware, it should not have a direct dependency on that messaging middleware itself.  This decoupling facilitates testing of the application services layer with test doubles, keeps a clean separation of concerns between task coordination and messaging, and provides greater flexibility with being able to modify/upgrade the messaging layer without affecting the application services layer.

A moment needs to be taken to clarify the differences among application services, domain services, and “external resource” services.

• Application services are our current concern.  The word “services” in their convoluted title is misleading and should not be inferred as being web services, RESTful services, or the like.  A better name for describing the application services responsibilities is “task coordination layer,” “workflow management layer,” or “not-real-services application services layer.”  We’ll examine a bit in this post exactly how the application services layer can be properly leveraged.
• Domain services are really utility classes used by the core layer (the concern of our previous post) for processing data or encapsulating a specific, reusable task or behavior.  Domain services almost never have state (i.e., properties) and sometimes expose behavior as static methods.  A good example of a domain service would be a calculator class (e.g., Kalman filter calculator) or a string utility class.
• External resource services are what we typically think of when we hear the word “service.”  An external resource service is any class which depends upon…well…an external resource.  Examples of an external resource include databases, resource files (e.g., maps), messaging middleware, third party web services, and even the laser reading class that we simulated in Part II of this series.  Ideally, external resource services should be abstracted, allowing consumers of the services to interact with them via interfaces or abstract classes.  In this way, layers remain decoupled from the implementation details of the services, reflecting the benefits of proper OOP design.  Proper abstraction of service layers usually involves separated interface and dependency injection.  We’ll see in this post how these techniques are leveraged to decouple the application services layer from the message endpoints (which are indeed external resource services).

Before we delve in, let’s briefly review what we plan to accomplish:

• An application service class – LaserScanReportingService – will be developed to provide a public API for starting and stopping the laser reader.  In addition to exposing this behavior, the class will also listen for laser scan reads and pass the laser scans onto a message endpoint for publication to ROS.
• A message endpoint interface will be introduced to provide an abstraction to the implementation details of the concrete message endpoint itself.  The application service class will communicate with the message endpoint via this abstraction to allow for unit testing the application services layer in isolation of the messaging middleware.
• Unit tests will be written to verify the functionality of the application service class.  Furthermore, a message endpoint test double, a stub in this case, will be added to simulate communications with the messaging middleware.  Again, this will allow us to verify the task coordination behavior of the application services layer without concerning ourselves with messaging middleware integration details.

Enough with the chatter, let’s see some code!

Target Class Diagram

The following diagram shows what the package will look like after completing the steps in this post. While the individual elements will be discussed in more detail; the class diagram should serve as a good bird’s eye view of the current objectives. The elements with green check marks were completed in previous posts.

1. Setup the Package Skeleton and Domain Layer

If not done already, follow the steps in Part II and Part III to create the package and develop/test the domain layer.

2. Create an interface for the message endpoint service which the application service layer will leverage to communicate with ROS.

Create src/core/message_endpoint_interfaces/ILaserScanEndpoint.hpp containing the following code:

// ILaserScanEndpoint.hpp
 
#ifndef GUARD_ILaserScanEndpoint
#define GUARD_ILaserScanEndpoint
 
#include "sensor_msgs/LaserScan.h"
 
namespace ladar_reporter_core
{
  class ILaserScanEndpoint
  {
    public:
      // Virtual destructor to pass pointer ownership without exposing base class [Meyers, 2005, Item 7]
      virtual ~ILaserScanEndpoint() {}
      virtual void publish(const sensor_msgs::LaserScan& laserScan) const = 0;
  };
}
 
#endif /* GUARD_ILaserScanEndpoint */

There shouldn’t be anything too surprising in this interface.  It simply exposes a method to publish a laser scan to the underlying messaging middleware.  Why not put the interface in the application services layer, which intends to use it?  A couple of good reasons come to mind: 1) since it’s a pure interface, having elements within core aware of it (or even directly dependent upon it) does not introduce any further coupling to the underlying external resource, the messaging middleware, and 2) in very simply packages, an application services layer might be overkill, so keeping the “external resource service interface” (there’s a mouthful) in the core layer facilitates either approach without having to move anything around if the selected approach changes during development.

3. Create the application service header class.

Create src/application_services/LaserScanReportingService.hpp containing the following code:

// LaserScanReportingService.hpp
 
#ifndef GUARD_LaserScanReportingService
#define GUARD_LaserScanReportingService
 
#include <boost/shared_ptr.hpp>
#include "ILaserScanEndpoint.hpp"
 
namespace ladar_reporter_application_services
{
  class LaserScanReportingService
  {
    public:
      explicit LaserScanReportingService(boost::shared_ptr<ladar_reporter_core::ILaserScanEndpoint> laserScanEndpoint);
 
      void beginReporting() const;
      void stopReporting() const;
 
    private:
      // Forward declare the implementation class
      class LaserScanReportingServiceImpl;
      boost::shared_ptr<LaserScanReportingServiceImpl> _pImpl;
  };
}
 
#endif /* GUARD_LaserScanReportingService */

A few notes:

• Line 7:  Includes the previously defined message endpoint interface.
• Line 14:  Provides a constructor for the application service class, accepting an implementation of ILaserScanEndpoint, accordingly.
• Lines 16-17:  Exposes methods for exposing behavior found within the core layer.
• Lines 21-22:  Declaration for a pImpl (private implementation) that will allow the application service class to be updated frequently while minimizing the rebuilding of other class libraries which depend upon it.  Using the pImpl pattern will likely be overkill in smaller packages, but can be an appreciable timesaver as packages grow in size. It has been included in the sample code here as a example reference for when it might be more appropriately required.

4.  Create the application service implementation class.

Create src/application_services/LaserScanReportingService.cpp containing the following code:

// LaserScanReportingService.cpp
 
#include "sensor_msgs/LaserScan.h"
#include "ILaserScanListener.hpp"
#include "LaserScanReader.hpp"
#include "LaserScanReportingService.hpp"
 
using namespace ladar_reporter_core;
 
namespace ladar_reporter_application_services
{
  // Private implementation of LaserScanReportingService
  class LaserScanReportingService::LaserScanReportingServiceImpl : public ladar_reporter_core::ILaserScanListener
  {
    public:
      explicit LaserScanReportingServiceImpl(boost::shared_ptr<ILaserScanEndpoint> laserScanEndpoint);
 
      void onLaserScanAvailableEvent(const sensor_msgs::LaserScan& laserScan) const;
      LaserScanReader laserScanReader;
 
    private:
      boost::shared_ptr<ILaserScanEndpoint> _laserScanEndpoint;
  };
 
  LaserScanReportingService::LaserScanReportingService(boost::shared_ptr<ILaserScanEndpoint> laserScanEndpoint)
    : _pImpl(new LaserScanReportingServiceImpl(laserScanEndpoint)) {
 
    // Wire up the reader to the handler of laser scan reports
    _pImpl->laserScanReader.attach(*_pImpl);
 }
 
  LaserScanReportingService::LaserScanReportingServiceImpl::LaserScanReportingServiceImpl(boost::shared_ptr<ILaserScanEndpoint> laserScanEndpoint)
    : _laserScanEndpoint(laserScanEndpoint) { }
 
  void LaserScanReportingService::beginReporting() const {
    _pImpl->laserScanReader.beginReading();
  }
 
  void LaserScanReportingService::stopReporting() const {
    _pImpl->laserScanReader.stopReading();
  }
 
  void LaserScanReportingService::LaserScanReportingServiceImpl::onLaserScanAvailableEvent(const sensor_msgs::LaserScan& laserScan) const {
    // Send laserScan to the message end point
    _laserScanEndpoint->publish(laserScan);
  };
}

A few notes:

• LaserScanReportingService::LaserScanReportingServiceImpl defines the pImpl implementation.  As shown, this pImpl implementation accepts a ILaserScanEndpoint via its constructor and also adds members for communicating with LaserScanReader and with the provided message end point.  As discussed earlier in this post, it’s preferable to abstract dependencies on external resource services using interfaces or abstract classes, as was done with ILaserScanEndpoint.  Quite arguably, LaserScanReader is also an external resource service.  But since the core of our package is in providing a means to read laser scans from a particular laser range finder, I made a purity concession, if you will, allowing the application services layer to have a direct dependency on the reader itself.  Alternatively, an interface could be introduced for the LaserScanReader; but since the very heart of this package is in reading and reporting laser scans, I felt this concession to be warranted…or at least justifiably defendable.  With that said, concessions such as this should be carefully thought out and discussed with the project team to determine the pros and cons of either approach and to ensure that everyone has a clear understanding of the motivations of the decision.  (If the team – or yourself – does not understand or can explain why an architectural decision was made, it’s a good chance that the architectural decision will not be adhered to or may become a source of code rot.)

  The other item to note is that the pImpl class implements ILaserScanListener; accordingly, this class will act as the observer for receiving and handling laser scan reports raised from LaserScanReader.

• The constructor of LaserScanReportingService accepts a reference to ILaserScanEndpoint, passes that reference on to the constructor of LaserScanReportingServiceImpl and attaches the observer to LaserScanReader to listen for raised laser scans.
• The constructor of LaserScanReportingServiceImpl accepts a reference to ILaserScanEndpoint and initializes the laserScanEndpoint member to that reference, accordingly.
• beginReporting and stopReporting provide pass through methods to the underlying behavior exposed by laserScanReader.
• Finally, onLaserScanAvailableEvent provides an event handler to accept a laser scan from LaserScanReader and pass it on to laserScanEndpoint’s publish function.

As mentioned previously, the above implementation could be done without a private implementation design pattern, but this serves to illustrate how such a pattern may be leveraged when warranted.

5. Configure CMake to Include the Header and Implementation

With the header and implementation classes completed, we need to make a couple of minor modifications to CMake for their inclusion in the build.

1. Open /ladar_reporter/CMakeLists.txt. Within the include_directories statement, add an include for the following directory to include the message endpoint and application service headers:

   "src/core/message_endpoint_interfaces"
   "src/application_services"

2. Open /ladar_reporter/src/CMakeLists.txt. Add an inclusion for the application_services directory at the end of the file:

   add_subdirectory(application_services)

3. Now, in order to create the application services class library itself, a new CMake file is needed under /ladar_reporter/src/application_services. Accordingly, create a new CMakeLists.txt under /ladar_reporter/src/application_services, containing the following:

   # Create the library
   add_library(
     ladar_reporter_application_services
     LaserScanReportingService.cpp
   )

6. Build the application services Class Library

In a terminal window, cd to /ladar_reporter and run make. The class library should build and link successfully.

Like before, we’re not done yet…it’s now time to test our new functionality.

7. Unit Test the LaserScanReportingService Functionality

When we go to test the functionality of the laser scan reporting application service, it will likely be quickly noticed that there’s a missing dependency which will be needed to test the functionality of this class: a concrete implementation of ILaserScanEndpoint.hpp. The job of the message endpoint class will be to take laser scans and publish them to the appropriate topic on ROS. But we’re just not there yet…what we’d really like to do is to be able to test the functionality of the application service layer – LaserScanReportingService.cpp to be specific – without necessitating the presence of the message endpoint and ROS itself. While we’ll have to cross that bridge eventually (in Part V to be exact), we’re not currently interested in doing integration tests. Instead, we’re only interested in testing the behavior of the application service regardless of its integration with ROS.

Accordingly, a “stub” object will be employed to stand in for an actual message endpoint. In this case, the stub object is nothing more than a concrete implementation of ILaserScanEndpoint.hpp; but instead of publishing the laser scan to ROS, it’ll do something testable, such as keep a tally, or possibly a temporary queue, of laser scans “published” which can then be verified with testing asserts. If you’re new to unit testing, you’ll want to read about test doubles, Martin Fowler’s Mocks Aren’t Stubs, and XUnit Test Patterns for a more comprehensive treatment of the subject.

Onward with testing…

1. Create /ladar_reporter/test/message_endpoints/test_doubles/LaserScanEndpointStub.hpp containing the following code to declare and define the message endpoint stub:

   // LaserScanEndpointStub.hpp   #ifndef GUARD_LaserScanEndpointStub #define GUARD_LaserScanEndpointStub   #include "sensor_msgs/LaserScan.h" #include "ILaserScanEndpoint.hpp"   namespace ladar_reporter_test_message_endpoints_test_doubles { class LaserScanEndpointStub : public ladar_reporter_core::ILaserScanEndpoint { public: LaserScanEndpointStub() : countOfLaserScansPublished(0) { }   void publish(const sensor_msgs::LaserScan& laserScan) const { countOfLaserScansPublished++;   // Output the laser scan seq number to the terminal; this isn't the // unit test, but merely a helpful means to show what's going on. std::cout << "Laser scan sent to LaserScanReceiver with angle_min of: " << laserScan.angle_min << std::endl; };   // Extend ILaserScanEndpoint for unit testing needs. // May be modified by const functions but maintains logical constness [Meyers, 2005, Item 3]. mutable int countOfLaserScansPublished; }; }   #endif /* GUARD_LaserScanEndpointStub */

   If you’re paying attention (of course you are!), you’ll notice that this code looks very similar to the code used within the LaserScanReaderTests.cpp class’ fake event handler from Part III. In fact, it’s the exact same code but now it’s encapsulated within a message endpoint stub. Admittedly, this is not a very good testing endpoint (since it won’t make it easier to verify that the laser scans have been “published” via testing asserts), but it does provide an easy means to visually note, while the tests are running, how the message endpoint stub is behaving. More importantly, seeing the laser scans output to the screen will show the application service layer is interacting with the message endpoint appropriately…which is exactly what we’re trying to verify.

   While the LaserScanEndpointStub class could have been coded inline within the unit test class (discussed next), encapsulating it within an external class allows the stub to be reused by other tests as well. This may not be necessary in all cases – in which coding it inline would be just fine – but I typically find myself reusing stubs and mocks to avoid duplicated code in my testing layer; besides, keeping it within an external test double keeps the code arguably cleaner.

2. Create /ladar_reporter/test/application_services/LaserScanReportingServiceTests.cpp containing the following testing code:

   // LaserScanReportingServiceTests.cpp   #include <gtest/gtest.h> #include "LaserScanEndpointStub.hpp" #include "LaserScanReportingService.hpp"   using namespace ladar_reporter_application_services; using namespace ladar_reporter_test_message_endpoints_test_doubles;   namespace ladar_reporter_test_application_services { // Define the unit test to verify ability to leverage the reporting service using the messege endpoint stub TEST(LaserScanReportingServiceTests, canStartAndStopLaserScanReportingService) { // Establish Context boost::shared_ptr<LaserScanEndpointStub> laserScanEndpointStub = boost::shared_ptr<LaserScanEndpointStub>(new LaserScanEndpointStub()); LaserScanReportingService laserScanReportingService(laserScanEndpointStub);   // Act laserScanReportingService.beginReporting(); sleep(2); laserScanReportingService.stopReporting();   // Assert   // Since we just ran the reader for 2 seconds, we should expect a few readings. // Arguably, this test is a bit light but makes sure laser scans are being pushed // to the message endpoint for publication. EXPECT_TRUE(laserScanEndpointStub->countOfLaserScansPublished > 0 && laserScanEndpointStub->countOfLaserScansPublished <= 10); } }

3. Open /ladar_reporter/test/CMakeLists.txt. Modify the contents to reflect the following:

   include_directories(
     "message_endpoints/test_doubles"
   )
   
   rosbuild_add_executable(
     ladar_reporter_tests
     TestRunner.cpp
     ./core/LaserScanReaderTests.cpp
     ./application_services/LaserScanReportingServiceTests.cpp
   )
   
   rosbuild_add_gtest_build_flags(ladar_reporter_tests)
   
   # Link the libraries
   target_link_libraries(
     ladar_reporter_tests
     ladar_reporter_application_services
     # Important that core comes after application_services due to direction of dependencies
     ladar_reporter_core
   )

4. With everything in place, head back to the terminal and run:

   make
   cd bin
   ./ladar_reporter_tests

While running the tests, you should see a few messages published to the message endpoint stub. This demonstrates that all of the interactions among our core and application service layers are occurring exactly as expected. Now for the fun part…

In Part V, we’ll finally take a look at using all of these pieces together in order to publish messages to a ROS topic via a message endpoint.

Enjoy!
Billy McCafferty

Download the source for this article.

Part III:  Developing and Testing the Domain Layer

[Author's note, July 28, 2010: Fixed minor bug in LaserScanReader.cpp wherein it couldn't be restarted after being stopped; had to reset _stopRequested to false.]

In Part II of this series, we created the humble beginnings of the package and added folders to accommodate all of the layers of our end product, a well-designed ROS package that reports (fake) laser scan reports.  In this post, the domain layer of the package will be fleshed out along with unit tests to verify the model and functionality, accordingly.  The entire focus will be on implementing just one of the requirements initially described in Part I:  The package will read laser reports coming from a laser range-finder. If you’d like to download the resulting source for this article, click here.

That certainly sounds easy enough.  Disregarding the previous discussions concerning architecture, the gut reaction might be to start adding code to main(), simply taking the results from the range-finder, turning them directly into a ROS message, and publishing the messages to the appropriate ROS topic.  This myopic “get ‘er done” approach quickly gets out of hand as main() turns into a tangled mess of code managing a variety of responsibilities.  Object oriented principles aside, having all of these separate concerns mashed into main turns the little package into a maintenance nightmare with little ability to reuse code.  As mentioned, the first concern that we’ll want to tackle is the ability to read laser range-finder reports.  We’ll tackle this requirement by encapsulating the range-finder integration code within a class called LaserScanReader.cpp.  By doing so, all of the communications to the range-finder are properly encapsulated within one or more classes, making the integration code easier to reuse and maintain.  To keep our focus on the overall architecture, and to avoid the need to have a physical range-finder handy, we’ll simulate range-finder communications within LaserScanReader.cpp.  Certainly an added benefit of this approach, if we were doing this for a real-world package, is that one group could work on the “rest” of the package while another group works on the actually range-finder communications; so when ready, LaserScanReader.cpp could be switched out with the “real” range-finder integration code.

Prerequisites:

• ROS Box Turtle or later
• Familiarity with C++

Before proceeding, recall that, as described in Part I, the domain layer of the package should not have knowledge concerning how to communicate with the messaging middleware directly (e.g., ROS).  This implies that the domain layer should have no direct dependencies on the messaging middleware.  This allows the domain layer to be more easily reused with another messaging middleware solution.  Additionally, keeping this clean separation of concerns facilitates the testing of the domain layer independently from its interactions with the messaging middleware.  Accordingly, the simple domain layer developed in this post will adhere to this guidance along with full testing for verification of capabilities as well.

Our LaserScanReader class will expose two methods, beginReading() and stopReading(), along with an observer hook to provide a call-back to be invoked whenever a new reading is available.  For now, we won’t worry about what exactly will be called back in the completed package, as that’ll be a concern of the application services layer; but we’ll need to prepare for it by including an interface for the laser scan observer.

Target Class Diagram

The following diagram shows what the package will look like after completing the steps in this post. While the individual elements will be discussed in more detail; the class diagram should serve as a good bird’s eye view of the current objectives.

1. Setup the Package Skeleton

If not done already, follow the steps in Part II to create the beginnings of the package.

2. Create the ILaserScanListener Observer Header

Whenever a laser scan is read, it’ll need to be given to whomever is interested in it. It should not be the responsibility of the laser scan reader to predict who will want the laser scans. Accordingly, an observer interface should be introduced which the laser scan reader will communicate through to raise laser scan events to an arbitrary number of listeners.

Add a new interface header file to /ladar_reporter/src/core called ILaserScanListener.hpp containing the following code:

// ILaserScanListener.hpp
 
#ifndef GUARD_ILaserScanListener
#define GUARD_ILaserScanListener
 
#include "sensor_msgs/LaserScan.h"
 
namespace ladar_reporter_core
{
  class ILaserScanListener
  {
    public:
      // Virtual destructor to pass pointer ownership without exposing base class [Meyers, 2005, Item 7]
      virtual ~ILaserScanListener() {}
      virtual void onLaserScanAvailableEvent(const sensor_msgs::LaserScan& laserScan) const = 0;
  };
}
 
#endif /* GUARD_ILaserScanListener */

As you can see, this C++ interface (or as close as you can get to an interface in C++) simply exposes a single function to handle laser scan events.

3.  Create the LaserScanReader Header

Add a new class header file to /ladar_reporter/src/core called LaserScanReader.hpp containing the following code, which we’ll discuss in detail below.

// LaserScanReader.hpp
 
#ifndef GUARD_LaserScanReader
#define GUARD_LaserScanReader
 
#include <pthread.h>
#include <vector>
#include "sensor_msgs/LaserScan.h"
#include "ILaserScanListener.hpp"
 
namespace ladar_reporter_core
{
  class LaserScanReader
  {
    public:
      LaserScanReader();
 
      void beginReading();
      void stopReading();
 
      // Provides a call-back mechanism for objects interested in receiving scans
      void attach(ILaserScanListener& laserScanListener);
 
    private:
      void readLaserScans();
      void notifyLaserScanListeners(const sensor_msgs::LaserScan& laserScan);
 
      std::vector<ILaserScanListener*> _laserScanListeners;
 
      // Basic threading support as suggested by Jeremy Friesner at
      // http://stackoverflow.com/questions/1151582/pthread-function-from-a-class
      volatile bool _stopRequested;
      volatile bool _running;
      pthread_t _thread;
      static void * readLaserScansFunction(void * This) {
        ((LaserScanReader *)This)->readLaserScans();
        return 0;
      }
  };
}
 
#endif /* GUARD_LaserScanReader */

Let’s now review the more interesting parts of the header class:

• Lines 3-4:  Standard header guard so that the class is not declared multiple times.
• Line 8:  Include the laser scan message from ROS.  As discussed previously, the domain layer should not have any direct dependencies on the messaging infrastructure.  The question here is whether or not the domain layer’s knowledge of the laser scan message class violates this principle.  When I initially developed this example code, I wrote a domain-specific equivalent of the laser scan message, placed within /core, so as to avoid any direct dependencies on anything ROS related.  Continuing in this vein, it would then be required that the message endpoint contain a message translator to  convert the domain-specific laser scan class into the corresponding ROS laser scan message, for subsequent publication to ROS.  The motivation for taking this “theoretically pure” approach was to reduce coupling between the domain layer and the messaging infrastructure; one of the advantages being to enable the ability to swap out the messaging infrastructure with another messaging option while having no impact on the domain layer of the package.
  With that said, ROS makes available a very well organized model which is useful to the domain beyond the concerns of messaging itself.  For example, our laser scan reader class is only concerned with  reading information from the laser scanning device (e.g., Sick LMS111 or Hokuyo URG-04LX-UG01) and raising the laser scan event data.  Accordingly, regardless of the chosen messaging infrasturcture, the laser reader ultimately needs to encapsulate the laser scan information within a class for intra-package data passing.  The reader could use a custom (domain-specific) class to encapsulate this information, but we’d end up replicating many of the ROS message classes along with an equivalent number of message translators, without much decoupling benefit.  To illustrate, in the unlikely event that the ROS messaging infrastructure needed to be swapped out with another messaging solution, the domain layer could continue using the strongly typed ROS message classes in the domain, using message translator within the message endpoints, to translate between the ROS message classes (and other domain-specific classes we may be using) with the messages required by the messaging infrastructure.  If ROS messages were not strongly typed, being a collection of name/value pairs instead, for example, then I would avoid referencing ROS message classes from within the domain layer.
  So in our example package here, we will slightly ease the rule of strict domain-layer/ROS decoupling and allow the use of ROS message classes from within the domain layer while still avoiding the introduction of any ROS communication details to the domain, such as how to publish to a topic.  This theoretical vs. practical compromise is a discussion that should be had with any project team to carefully decide how strictly decoupling should be enforced on a given package and specifically where the rule may be eased, if and when appropriate.  In this case, the benefits of using the strongly typed ROS message classes were weighed against the introduction of domain-layer coupling to those classes, accordingly.  Keep in mind that any introduction of coupling with the messaging infrastructure should be carefully reviewed, and well justified, before doing so.
• Lines 18-19:  Declare the methods beginReading() and stopReading() which will provide the ability to start and stop the reader, respectively.
• Line 22:  Provide a means for laser scan listeners (observers) to register to receive laser scan reports when available.
• Line 25:  The readLaserScans() function actually does the work of reading laser reports from the laser scanner and raising the laser data events.  We’ll see in the implementation that it’ll be invoked within the beginReading() function.
• Line 26: notifyLaserScanListeners() has the responsibility of informing all laser scan listeners that a new laser scan is available.
• Line 28: _laserScanListeners will hold a reference to every laser scan listerner.
• Lines 32-38:  These members will facilitate the ability to read and raise laser scan events asynchronously. (As a side note, I spent an abhorrent amount of time trying to get Boost::Thread and Boost::signals2 in place for the threading and callback mechanisms within the ROS context. While I got it working most of the time, the intermittent segfault finally made me throw in the towel in favor of pthreads and observer pattern; more on this below.)

4.  Create the LaserScanReader Class Implementation

Add a new class file to /ladar_reporter/src/core called LaserScanReader.cpp containing the following code, which we’ll discuss in detail below.

// LaserScanReader.cpp
 
#include "LaserScanReader.hpp"
 
namespace ladar_reporter_core
{
  LaserScanReader::LaserScanReader()
    : _stopRequested(false), _running(false) {
    _laserScanListeners.reserve(1);
  }
 
  void LaserScanReader::attach(ILaserScanListener& laserScanListener) {
    _laserScanListeners.push_back(&laserScanListener);
  }
 
  void LaserScanReader::beginReading() {
    if (! _running) {
      _running = true;
      _stopRequested = false;
      // Spawn async thread for reading laser scans
      pthread_create(&_thread, 0, readLaserScansFunction, this);
    }
  }
 
  void LaserScanReader::stopReading() {
    if (_running) {
      _running = false;
      _stopRequested = true;
      // Wait to return until _thread has completed
      pthread_join(_thread, 0);
    }
  }
 
  void LaserScanReader::readLaserScans() {
    int i = 0;
 
    while (! _stopRequested) {
      sensor_msgs::LaserScan laserScan;
      // Just set the angle_min to include some data
      laserScan.angle_min = ++i;
      notifyLaserScanListeners(laserScan);
      sleep(1);
    }
  }
 
  void LaserScanReader::notifyLaserScanListeners(const sensor_msgs::LaserScan& laserScan) {
    for (int i= 0; i < _laserScanListeners.size(); i++) {
        _laserScanListeners[i]->onLaserScanAvailableEvent(laserScan);
    }
  }
}

Let’s now review the more interesting parts of the class implementation:

• LaserScanReader constructor:  The constructor for the LaserScanReader class initializes the _stopRequested member to false.  Since the reading of laser scans will be performed asynchronously, this private member acts as a poor man’s state machine, facilitating a means to stop the reading of laser scans if set to true. In addition to this, the constructor reserves a wee bit of expected space to avoid too much space being allocated upon the first add to the vector.
• beginReading():  This function will be called to begin the process of reading laser scans from the laser range finder.  The first step of this process sets the _stopRequested member to ensure that the readings won’t stop until requested to do so.  Next, a new thread is begun to execute the readLaserScans() function asynchronously.  I spent a very long amount of time researching different approaches for the simple thread management needed in this class; the most simple and stable solution I found was on stackoverflow.com. (Please let me know if you have a simpler, cleaner – and still stable – solution.)
• stopReading():  This function does not explicitly stop the laser scan reading process.  Instead, it simply sets the _stopRequested member to false to inform the asynchronous reading process to terminate. The pthread_join() function is called to wait until the thread represented by the thread object has completed.
• readLaserScans():  In a real package, this function would communicate with the physical range finding device, collect data, and raise laser scan information via the onLaserScanAvailableEvent.  In our example code here, we’ll simply create a number of fake laser scan reports as long as the _stopRequested member does not equal true.

  Note again that by using the observer pattern to raise laser scan events to listeners, the implementation code need not be aware of who a priori is listening for laser scans, only that a means exists to raise laser scan events, accordingly.  Arguably, readLaserScans() could publish laser scans directly to ROS topics. But this approach comes with the drawback of tightly coupling the laser scan reading functionality to the ROS messaging system, it violates the Single Responsibility Principle, and it makes it more difficult to do “any other stuff” (whatever that may be) with the laser scans before they’re handed off to ROS for publishing without further cluttering readLaserScans().  As the package evolves, we’ll see that an application services layer will be put in place to coordinate the task of handling the laser scan events and passing the information onto a message endpoint for publication to ROS.

• notifyLaserScanListeners(): Raises the laser scan to any observers. Be sure to note that this does not publish laser scans to ROS, but only raises them to registered observers within the package.

5.  Add a ROS sensor_msgs Dependency to manifest.xml

Since the code above refers to sensor_msgs::LaserScan, a dependency needs to be added for the package to use this class, accordingly:

• Under /ladar_reporter, open manifest.xml and add the following line just before the closing </package> tag:

  <depend package="sensor_msgs" />

6.  Configure CMake to Include both the Header and Implementation

With the header and implementation classes completed, we need to make a couple of minor modifications to CMake for their inclusion in the build.

First, open /ladar_reporter/CMakeLists.txt and make the following modifications:

1. Under the first line, which sets the cmake_minimum_required, add a new line to set the project name:

   project(ladar_reporter)

2. At the end of the CMake file:

   # Include subdirectories for headers
   include_directories(
     "src/core"
   )
   
   add_subdirectory(src)

In doing so, the LaserScanReader header has been included for consumption by other classes and application layers.  This inclusion has been done in the root CMakeLists.txt as this will make the headers available to the unit tests as well.  For more complex packages, this approach of including the headers in the root CMakeLists.txt, to make them globally accessible, may become a bit messy; it may be more appropriate to put the header inclusions in only the CMakeLists.txt which actually require the respective headers if the package is larger.

At this point, a new CMake file is needed under /ladar_report/src:

• Create a new CMakeLists.txt under /ladar_reporter/src, containing the following:

  # Include subdirectories with their own CMakeLists.txt
  add_subdirectory(core)

You’ll quickly notice that this CMake file has merely passed the buck of defining class libraries further down the chain.  Accordingly, a CMakeLists.txt will be setup for each of the package layers including application_services, core, message_endpoints, and ui.  All of these layers will be compiled into separate class libraries and, finally, an executable.  Arguably, all of these layers could be combined into a single executable with a single CMakeLists.txt file.  But keeping them in separate class libraries keeps a clean separation of concerns in their respective responsibilities and makes each aspect of our package more easily testable in isolation from the other layers.

Next, in order to create the core class library, a new CMake file is needed under /ladar_reporter/src/core:

• Create a new CMakeLists.txt under /ladar_reporter/src/core, containing the following:

  # Create the core library
  add_library(
    ladar_reporter_core
    LaserScanReader.cpp
  )

We’re now ready to compile the class library for the “core” layer of the package…

7.  Build the core Class Library

In a terminal window, cd to /ladar_reporter and run make.  The class library should build and link successfully.

Woohoo!  Done, right?  Well, not yet…time to test our new functionality.

8.  Unit Test the LadarScanReader Functionality

So far, you’ve had to simply assume that a successful build means everything is working as expected.  Obviously, when developing a ROS package, we’ll want a bit more reassurance than a successful build to be confident that the developed capabilities are working as expected.  Accordingly, unit tests should be developed to test the functionality; in the case at hand, a unit test will be developed to initialize, begin and stop the laser reading cycle to ensure that it is raising laser scan events as designed.

The standard ROS testing tool is gtest; this is a great choice as gtest is very easy to setup and provides informative output during unit test execution.  To setup and run unit tests for the package, only two elements are needed:  a “test runner” to act as the unit tests’ main and to execute all of the package unit tests, and the unit tests themselves which may be spread out among a variety of folders and classes.  When unit testing, I typically write one unit testing class (a test fixture) per class being tested.  Furthermore, I include one unit test for each public function or behavior of the class being tested.  As for a couple of other unit testing best practices, be sure to keep the unit tests independent from each other – they should be able to be run in isolation without being dependent on the running of other unit tests.  Additionally, each unit test is organized as three stages of the test:  “establish context” wherein the testing context is setup, “act” wherein the desired behavior is invoked, and “assert” wherein the results of the behavior are verified. Finally, no testing code should be added to the “production” source code itself; all tests should be maintained in a separate executable to keep a clean separation of concerns between application code and tests.  We’ll see an example of this as we proceed.

1. To begin testing the laser scan reader, first define the test runner by creating /ladar_reporter/test/TestRunner.cpp containing the following:

   #include <gtest/gtest.h>   // Run all the tests that were declared with TEST() int main(int argc, char **argv){ testing::InitGoogleTest(&argc, argv); return RUN_ALL_TESTS(); }

2. Now that the unit tests’ main is in place, create /ladar_reporter/test/core/LaserScanReaderTests.cpp containing the following testing code:

   #include <gtest/gtest.h> #include "LaserScanReader.hpp" #include "ILaserScanListener.hpp" #include "sensor_msgs/LaserScan.h"   using namespace ladar_reporter_core;   namespace ladar_reporter_test_core { // Will be used by unit test to handle laser scans struct LaserScanReceiver : public ladar_reporter_core::ILaserScanListener { LaserScanReceiver() : countOfLaserScansReceived(0) { }   mutable int countOfLaserScansReceived;   void onLaserScanAvailableEvent(const sensor_msgs::LaserScan& laserScan) const { countOfLaserScansReceived++;   // Output the laser scan seq number to the terminal; this isn't the // unit test, but merely a helpful means to show what's going on. std::cout << "Laser scan sent to LaserScanReceiver with angle_min of: " << laserScan.angle_min << std::endl; } };   // Define the unit test to verify ability to start and stop the reader TEST(LaserScanReaderTests, canStartAndStopLaserScanReader) { // Establish Context LaserScanReader laserScanReader; LaserScanReceiver laserScanReceiver;   // Wire up the reader to the handler of laser scan reports laserScanReader.attach(laserScanReceiver);   // Act laserScanReader.beginReading(); // Let it perform readings for a couple of seconds sleep(2); laserScanReader.stopReading();   // Assert   // Since we just ran the reader for 2 seconds, we should expect a few readings. // Arguably, this test is a bit light but makes sure laser scans are being reported. EXPECT_TRUE(laserScanReceiver.countOfLaserScansReceived > 0 && laserScanReceiver.countOfLaserScansReceived <= 10); } }

3. Then to include the new test in the build, open /ladar_reporter/CMakeLists.txt and add the following to the end of the file:

   add_subdirectory(test)

   As seen before, this has just passed the buck onto subsequent CMake file(s) to include the appropriate files in the build – which is preferred. Ideally, each CMakeLists.txt should only know about its immediate “surroundings” and not have the concerns of subfolders added to it.

4. Next, create a new CMakeLists.txt file under /ladar_reporter/test containing the following:

   rosbuild_add_executable(
     ladar_reporter_tests
     TestRunner.cpp
     ./core/LaserScanReaderTests.cpp
   )
   
   rosbuild_add_gtest_build_flags(ladar_reporter_tests)
   
   # Link the libraries
   target_link_libraries(
     ladar_reporter_tests
     ladar_reporter_core
   )

   This CMake file instructs the make process to create a stand-alone gtest executable called ladar_reporter_tests, made up of the two files indicated. (As a side note, ROS has a macro called rosbuild_add_gtest which will include and run the tests automatically when compiled with the command make test. I have reliably encountered segfaults when using Boost asynchronous threading and building with rosbuild_add_gtest. Interestingly, I do not run into such problems if I build and run the testing executable separately from the make process. I have thoroughly researched this topic (i.e., thrown laptops through windows), and have found no solution. Moral of the story: use rosbuild_add_executable to create your test executable.)

5. With everything in place, head back to the terminal and run:

   make
   cd bin
   ./ladar_reporter_tests

When the test runs, you should see a few laser scan angle_min values get printed to the terminal along with the final report that the test successfully passed.

Now that we’ve done all of the above to complete the core domain layer of the package, let’s review what have was accomplished:

• Header and implementation classes were developed for communicating with the (fake) laser scan range finder;
• An observer interface was established to allow (yet unseen) observers listen for new laser scans;
• A unit test was developed, outside of the package source, to test the reader functionality;
• CMake was configured to compile the artifacts and to run tests with gtest; and
• Most importantly, we’ve been able to develop and test our core functionality without needing to involve the ROS messaging system (outside of the use of its LaserScan message class) and without having to prematurely develop any UI layers to test the completed code.

In Part IV, we’ll be setting our sights on developing and testing the application services layer of the application with a test double standing in for the ROS messaging system.

Enjoy!
Billy McCafferty

Download the source for this article.

The pImpl Idiom is a useful technique for hiding implementation details of C++ classes away from being exposed in the header file.  A couple of the primary benefits include decoupling of the implementation from the interface of the object (as declared in the header) and reduced compilation time if only the pImpl class is modified.  As a rule of thumb, when using pImpl Idiom, there should be clean separation of any implementation details from the containing class.  Spreading implementation details across both the containing class and the pImpl class muddies the separation of concerns and can quickly lead to maintenance issues, with unclear direction as to what should go into the parent vs. the pImpl class.  Accordingly, public methods declared in the parent should simply act as pass-through methods to pImpl.  I’ve seen arguments that any public method which does not require access to the internal state of pImpl may instead be defined on the parent and not require a pass-through; this sounds reasonable while still providing a gauge for cleanly deciding where implementation definitions should reside (i.e., if it’s a public method which doesn’t require access to pImpl state, define it on the parent).  To that end, there may be scenarios wherein the pImpl class needs to invoke a method or property on the parent.  Accordingly, this post shows how to pass a reference to the parent object to the pImpl object while avoiding a copy of the parent in the process.

The first step is in defining the header class which will expose the public methods and properties along with a forward declaration of the pImpl class:

// LaserScanReader.hpp
 
#ifndef GUARD_LaserScanReader
#define GUARD_LaserScanReader
 
#include <boost/shared_ptr.hpp>
 
namespace ladar_reporter_core
{
  class LaserScanReader
  {
    public:
      ...
 
    private:
      // Forward declare the implementation class
      class LaserScanReaderImpl;
      boost::shared_ptr<LaserScanReaderImpl> _pImpl;
  };
}
 
#endif /* GUARD_LaserScanReader */

Note that the _pImpl member is stored as a boost::shared_ptr so that _pImpl will be destructed automatically when it loses all references to it.

Now that the parent has been declared, we can turn our attention towards the implementation class.

// LaserScanReader.cpp
 
#include "LaserScanReader.hpp"
 
namespace ladar_reporter_core
{
  // Private implementation
  class LaserScanReader::LaserScanReaderImpl
  {
    public:
      explicit LaserScanReaderImpl(const LaserScanReader& laserScanReader);
 
    private:
      // Provides access to methods/properties of parent; e.g., _laserScanReader.someProperty
      const LaserScanReader& _laserScanReader;
  };
 
  LaserScanReader::LaserScanReader()
    // Passes a reference of LaserScanReader to LaserScanReaderImpl (not a copy)
    : _pImpl(new LaserScanReaderImpl(*this)) { }
 
  LaserScanReader::LaserScanReaderImpl::LaserScanReaderImpl(const LaserScanReader& laserScanReader)
    // Initialize the reference to the LaserScanReader (parent object of _pImpl)
    : _laserScanReader(laserScanReader) { }
}

Let’s now look at a few of the interesting points in more detail.

• The constructor declaration of LaserScanReaderImpl takes a const reference to LaserScanReader, avoiding copying the parent object when passed to it.
• The private member _laserScanReader holds the reference to the parent object, providing access to it’s public methods and properties, accordingly.
• The constructor definition of LaserScanReader passes a pointer to itself to the constructor of LaserScanReaderImpl.  While it appears to pass a value pointer, thus creating a copy of itself as the parameter, the fact that the constructor is expecting a reference to LaserScanReader avoids copying the object, accordingly.
• The constructor definition of LaserScanReaderImpl accepts a reference to the parent object, initializing the member _laserScanReader with that reference, accordingly.

With that, the pImpl class now has access to the parent object’s properties and methods via its _laserScanReader member.

Billy McCafferty