Service implementation returning a provider. More...

#include <ServiceProviderWrappers.h>

Inheritance diagram for gar::ServiceProviderImplementationWrapper< PROVIDER, INTERFACE >:

Public Types
using	concrete_provider_type = PROVIDER
	type of service provider implementation More...

using	service_interface_type = INTERFACE
	art service interface class More...

using	provider_type = typename service_interface_type::provider_type
	type of service provider interface More...

using	Parameters = ::art::ServiceTable< typename concrete_provider_type::Config >
	Type of configuration parameter (for art description) More...

Public Member Functions
	ServiceProviderImplementationWrapper (Parameters const &config,::art::ActivityRegistry &)
	Constructor (using a configuration table) More...

Private Member Functions
virtual provider_type const *	do_provider () const override
	Returns a constant pointer to the service provider. More...

Private Attributes
std::unique_ptr< concrete_provider_type >	prov
	service provider More...

Detailed Description

template<typename PROVIDER, typename INTERFACE>
class gar::ServiceProviderImplementationWrapper< PROVIDER, INTERFACE >

Service implementation returning a provider.

Template Parameters

INTERFACE	type of art service being implemented
PROVIDER	type of service provider to be returned

See also: ServiceProviderInterfaceWrapper

This class is suitable for a simple art service implementing an interface. This is the case of a service that can have multiple implementations, for example an experiment-specific access to a database. In this case, the art service is described by an interface, and many services can inherit and implement it.

In the factorization model recommended in GArSoft, this hierarchy of art service implementations derived by a common interface is mirrored in a matching hierarchy of service provider implementation derived by a common service provider interface. In this model, services are basically trivial classes that return a pointer to the provider through a prescribed interface. The only relevant exception is that services may also register framework callbacks.

Given the nature of the service returning a provider, its structure is repetitive. This class implements most of that repetitive frame. The resulting service is not reacting to any framework events.

The configuration of the service is passed directly to the provider.

Given that we want to have a service that implements the service MyServiceInterface and returns a provider implementation "A" called MyProviderA, the simplest way to create it with this template is:

#include "CoreUtils/ServiceProviderWrappers.h"
#include "path/to/MyProviderA.h"
#include "path/to/MyServiceInterface.h"

#include "art/Framework/Services/Registry/ServiceDeclarationMacros.h"
#include "art/Framework/Services/Registry/ServiceDefinitionMacros.h"

namespace myprov {

  using MyServiceA = gar::ServiceProviderImplementationWrapper
    <MyProviderA, MyServiceInterface>;

}

DECLARE_ART_SERVICE_INTERFACE_IMPL
  (myprov::MyServiceA, myprov::MyService, LEGACY)

The class MyServiceInterface is expected to have been defined via ServiceProviderInterfaceWrapper. An implementation file is necessary too, that will look like:

#include "path/to/MyServiceA.h"

DEFINE_ART_SERVICE_INTERFACE_IMPL
  (myprov::MyServiceA, myprov::MyService)

If callback registration is needed, a class can derive from this template and still gain a bit of boilerplate facility. That is:

a provider_type definition (required by gar::providerFrom())
a provider() method to access the provider (required by gar::providerFrom())
a Parameters definition (used by art to print accepted configuration)
a constructor supporting FHiCL configuration validation
transparent life management of the provider instance
a concrete_provider_type definition (that is PROVIDER)
a service_interface_type definition (that is INTERFACE)

Requirements on the service provider (PROVIDER):

a data type Config being the configuration object. This is the object wrapped by fhicl::Table when performing FHiCL validation.
a constructor with as argument a constant reference to a Config object. The provider should be configured by that constructor.

Requirements on the service interface (INTERFACE):

a data type provider_type representing the abstract interface of the service provider
a virtual method do_provider() where the retrieval of the provider is expected to happen; this method is overridden

Definition at line 241 of file ServiceProviderWrappers.h.