Slideshow from C++ Meetup Bologna 2014, about the central role of Dependency Injection in OO software. The slide deck contains detailed explanation about dependency injection in general and C++ frameworks in particular.

1. www.italiancpp.org
Going native with less
coupling
Dependency Injection in C++

2. Italian C++ Community
Style 1: [Fake] OO Design
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CONTROLLER /
MANAGER
Dumb
object
Dumb
object
Dumb
object
Dumb
object
Dumb
object
Dumb
object
Dumb
object
Dumb
object

3. Italian C++ Community
Style 2: [True] OO design
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Smart
Object
Smart
Object
Smart
Object
Smart
Object
Smart
Object
Smart
Object
Smart
Object
Smart
Object
Smart
Object
Smart
Object
Smart
Object
Smart
Object
Smart
Object
Smart
Object

4. Italian C++ Community
OO Architecture Benefits
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Up Scalability (extension)
Down Scalability
(contraction)
Reusability
Safety (testability)

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The Wiring
Issue
In a True Modular Architecture, wiring is critical

6. Italian C++ Community
The Wiring Issue
When you've got a Good Architecture, you deal with
requirements changes by adding/removing/substituting objects
So, we need a simple way to:
Change the type of the objects
Create new objects
Modify the wiring of the objects
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7. Italian C++ Community i=7
Life without a
CONTROLLER
Example taken from:
Carlo Pescio's blog – March 2012

8. Italian C++ Community
First Design
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MinePlant
SumpPump
+Drain()
PumpEngine
+On()
+Off()
DigitalOutput
+Write()
GasSensor
<<interface>>
+IsCritical()
GasAlarm
+Watch()
1..*
Alarm
+On()
+Off()
SafeEngine

9. Italian C++ Community
Requirements Change
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MinePlant
SumpPump
+Drain()
PumpEngine
+On()
+Off()
DigitalOutput
+Write()
GasSensor
<<interface>>
+IsCritical()
GasAlarm
+Watch()
1..*
Alarm
+On()
+Off()
SafeEngine

10. Italian C++ Community
Extension
The design is robust: I only need to add a class
But…
Who creates SafeEngine instead of PumpEngine?
How does SafeEngine get the pointer to the
GasSensor (the same used by GasAlarm)?
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11. Italian C++ Community
Solution #1: Local Creation
Each class creates its own
dependencies
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12. Italian C++ Community i=12
MinePlant
SumpPump
+Drain()
PumpEngine
+On()
+Off()
DigitalOutput
+Write()
GasSensor
<<interface>>
+IsCritical()
GasAlarm
+Watch()
1..*
Alarm
+On()
+Off()
SafeEngine <<create>>

13. Italian C++ Community
Solution #1: Consequences
The SumpPump constructor creates a SafeEngine
instead of a PumpEngine.
… but SafeEngine needs a pointer to the GasSensor
instance already used by GasAlarm.
So, we must pass it as a parameter to SumpPump
constructor.
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14. Italian C++ Community
Solution #1: Properties
If I need to change the concrete
type, I have to modify the client.
It's difficult to reuse the same client
class (even in the same application).
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15. Italian C++ Community
Solution #1: Summary
SafeEngine class added
SumpPump constructor modified
MinePlant modified
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16. Italian C++ Community
Solution #2: Factory
(not the GoF factory)
Create objects without exposing
the instantiation logic to the client
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17. Italian C++ Community i=17
<<create>>

18. Italian C++ Community
Solution #2: Consequences
The SumpPump constructor takes a Factory as
parameter.
PumpEngineFactory instantiates a PumpEngine.
SafeEngineFactory instantiates a SafeEngine.
SafeEngine still needs a pointer to the GasSensor
instance already used by GasAlarm, so we must pass
it to the SafeEngineFactory constructor.
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19. Italian C++ Community
Solution #2: Summary
SafeEngine class added
SafeEngineFactory added
MinePlant modified
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20. Italian C++ Community
Solution #3: Service Locator
It’s a registry containing the
instances to use
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21. Italian C++ Community
Solution #3: Service Locator
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class ServiceLocator
{
public:
static ServiceLocator& Instance();
shared_ptr< PumpEngine > Engine();
void Engine( const shared_ptr< PumpEngine >& engine );
private:
...
};

22. Italian C++ Community
Solution #3: Service Locator
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// MinePlant:
auto e =
make_shared< SafeEngine >( engineOutput, gasSensor );
ServiceLocator::Instance().Engine( e );
// SumpPump:
SumpPump::SumpPump() :
engine( ServiceLocator::Instance().Engine() )
{
}

23. Italian C++ Community
Solution #3: Properties
Clients aware of the locator
Dependencies not explicit / evident
Dependencies not checked by compiler
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24. Italian C++ Community
Solution #3: Summary
SafeEngine class added
MinePlant modified
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25. Italian C++ Community
Solution #4: Dependency Injection
Dependency Injection is when
you have something setting the
dependencies for you.
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26. Italian C++ Community
Solution #4: Dependency Injection
Classes don't create their own dependencies
They're passed from outside
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27. Italian C++ Community
Dependency Injection
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auto gasSensor = ...
auto alarmOutput = make_shared<DigitalOutput>("/dev/ttyS0");
auto alarm = make_shared<Alarm>(alarmOutput);
auto gasAlarm = make_shared<GasAlarm>(gasSensor,alarm);
auto engineOutput = make_shared<DigitalOutput>("/dev/ttyS1");
auto engine = make_shared<PumpEngine>(engineOutput);
auto pump = make_shared<SumpPump>(engine);

28. Italian C++ Community
Dependency Injection
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auto gasSensor = ...
auto alarmOutput = make_shared<DigitalOutput>("/dev/ttyS0");
auto alarm = make_shared<Alarm>(alarmOutput);
auto gasAlarm = make_shared<GasAlarm>(gasSensor,alarm);
auto engineOutput = make_shared<DigitalOutput>("/dev/ttyS1");
// auto engine = make_shared<PumpEngine>(engineOutput);
auto engine = make_shared<SafeEngine>(engineOutput,gasSensor);
auto pump = make_shared<SumpPump>(engine);

29. Italian C++ Community
Solution #4: Properties
Complete separation between:
application logic (classes)
wiring (main/builder)
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30. Italian C++ Community
Solution #4: Summary
SafeEngine class added
MinePlant modified (one liner)
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31. Italian C++ Community i=31
Can we do
BETTER
SafeEngine must be added anyway.
… can we remove the one liner in MinePlant?
?

32. Italian C++ Community i=32
Configuration
DrivenWIRING
moving creation and wiring outside the code,
in a configuration file

33. Italian C++ Community
Why?
To easily get extensibility/contraction
(without having to touch zillion files and
recompile everything)
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34. Italian C++ Community
From Identifiers to Strings
Improving Previous Solution:
Objects creation from string
Objects identified by name
Objects connected by name
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35. Italian C++ Community
Run-time Reflection Missing…
Create("Foo") vs new Foo
Enumerate the dependencies
“Inject” the right object address in a
class dependency
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36. Italian C++ Community
Solution #5: Dependency Injection +
Dependency Injection is when you
have something setting the
dependencies for you.
…this something is usually a
framework.
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37. Italian C++ Community
Existing libraries (C++)
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QtIOCContainer
Sauce
DICPP
Hypodermic2
Pococapsule
Main issues:
Compile time injection only
Code generators needed

38. Italian C++ Community
Enter Wallaroo Library
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wallaroo.googlecode.com
wallarooC++ Dependency Injection

39. Italian C++ Community
Creating objects
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Catalog catalog;
...
catalog.Create("alarmOutput","DigitalOutput","/dev/ttyS0");
catalog.Create("alarm","Alarm");
catalog.Create("gasAlarm","GasAlarm");
catalog.Create("engineOutput","DigitalOutput","/dev/ttyS1");
catalog.Create("pump","SumpPump");
catalog.Create("engine","SafeEngine");

40. Italian C++ Community
Creating objects (from cfg)
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<parts>
<part>
<name>pump</name>
<class>SumpPump</class>
</part>
<part>
<name>engine</name>
<class>SafeEngine</class>
</part>
</parts>
...
Catalog catalog;
XmlConfiguration
file("wiring.xml");
file.Fill( catalog );
...

41. Italian C++ Community
Object lookup by name
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shared_ptr< SumpPump > pump = catalog[ "pump" ];

42. Italian C++ Community
Connect Things by name (DSL)
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Catalog catalog;
// fill catalog
...
use(catalog["alarmOutput"]).as("out").of(catalog["alarm" ]);
use(catalog["safeEngine"]).as("engine").of(catalog["pump"]);

43. Italian C++ Community
Connect Things by name (DSL)
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Catalog catalog;
// fill catalog
...
wallaroo_within( catalog )
{
use( "alarmOutput" ).as( "out" ).of( "alarm" );
use( "safeEngine" ).as( "engine" ).of( "pump" );
}

44. Italian C++ Community
Connect Things by name (from cfg)
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<wiring>
<wire>
<source>alarm</source>
<dest>alarmOutput</dest>
<collaborator>out</collaborator>
</wire>
<wire>
<source>pump</source>
<dest>safeEngine</dest>
<collaborator>engine</collaborator>
</wire>
</wiring>
Catalog catalog;
...
XmlConfiguration
file( "wiring.xml" );
file.Fill( catalog );
catalog.CheckWiring();
...

45. Italian C++ Community
Class Declaration
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#include "wallaroo/registered.h"
using namespace wallaroo;
class SumpPump : public Part
{
public:
SumpPump( int id );
private:
Collaborator< Engine > engine;
};

46. Italian C++ Community
Class Registration
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WALLAROO_REGISTER( SumpPump, int )
SumpPump::SumpPump( int id ) :
engine( "engine", RegistrationToken() )
{
...
}
// other methods definition here
...

47. Italian C++ Community
Shared Libraries – AKA plugins (code)
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Plugin::Load( "safeengine" + Plugin::Suffix() );
// Plugin::Suffix() expands to .dll or .so according
to the OS

48. Italian C++ Community
Shared Libraries – AKA plugins (cfg)
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<plugins>
<shared>safeengine</shared>
</plugins>
Catalog catalog;
XmlConfiguration file( "wiring.xml" );
// load the shared libraries specified in the configuration file:
file.LoadPlugins();
file.Fill( catalog );
// throws a WiringError exception if any dependency is missing:
catalog.CheckWiring();

49. Italian C++ Community
Collections
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class Car : public wallaroo::Part
{
...
private:
Collaborator< Engine > engine;
Collaborator< AirConditioning, optional > airConditioning;
Collaborator< Airbag, collection > airbags;
Collaborator< Speaker, collection, std::list > speakers;
Collaborator< Seat, bounded_collection< 2, 6 > > seats;
};

50. Italian C++ Community
Checks
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if ( !catalog.IsWiringOk() )
{
// error handling
}
catalog.CheckWiring() // throws exception

51. Italian C++ Community
Initialization
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class Part
{
...
public:
virtual void Init() {}
...
};
catalog.Init() // calls Part::Init for each part in catalog

52. Italian C++ Community
Wallaroo Internals
WALLAROO_REGISTER declares a static object.
Its constructor creates a factory and puts it in a table, with the class
name as key.
Catalog::Create uses the factory to put a new instance in the catalog.
wallaroo::Part has a table of <name, Collaborator>
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53. Italian C++ Community
Wallaroo Internals
shared_ptr< Foo > foo = catalog[ “foo” ];
catalog[ “foo” ] returns a class that defines operator
shared_ptr< T >()
Collaborator uses weak_ptr for the dependency
Collaborator defines operator shared_ptr() and operator->()
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54. Italian C++ Community
Wallaroo Internals
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wallaroo_within( catalog )
{
use("alarmOutput").as("out").of("alarm");
use("safeEngine").as("engine").of("pump");
}
for ( Context c(catalog);c.FirstTime();c.Terminate() )
{
use("alarmOutput").as("out").of("alarm");
use("safeEngine").as("engine").of("pump");
}

55. Italian C++ Community
Wallaroo Strengths
Lightweight (header file only)
Portable
Type Safe
DSL syntax for object creation and wiring
Configuration driven wiring (xml and json)
Shared library support (plugin)
C++11/14 or boost interface
No code generators
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56. Italian C++ Community
Design is a balance of forces
Intrusive VS Non Intrusive
Non Intrusive Solutions can manage existing classes but
require code generators for configuration driven wiring
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57. Italian C++ Community
Design is a balance of forces
Configuration-driven wiring
VS static type checking
By moving the wiring in a configuration file, we give up the
static type checking.
But it’s ok, since you build your system at startup.
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58. Italian C++ Community
Action Points
Real OOD (no controllers / managers)
Manual Dependency Injection
Wallaroo (configuration-driven wiring)
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59. Italian C++ Community
References & Credits
Me: @DPallastrelli
Me: it.linkedin.com/in/pallad
Rate me: https://joind.in/12277
Wallaroo: wallaroo.googlecode.com
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MinePlant example from Carlo Pescio's blog (http://www.carlopescio.com/2012/03/life-
without-controller-case-1.html)
Wiring Picture: By Gael Mace (Own work (Personal photograph))
[CC-BY-3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons