ServiceHandle.h

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#ifndef art_Framework_Services_Registry_ServiceHandle_h
#define art_Framework_Services_Registry_ServiceHandle_h
// vim: set sw=2 expandtab :

// =======================================================================
// ServiceHandle
//
// Smart pointer used to give easy access to Services.
//
// Note invocation only requires one template argument, but the
// constructor will require zero or one arguments depending on the scope
// of the service (LEGACY, SHARED).
//
// Technical notes:
//
//    Since ServiceHelper<T> instantiations correspond to
//    specializations created by macro calls, only a template argument
//    'T' that is non-const qualified will match any specialization.
//    However, const-only access to a service can be provided via
//    ServiceHandle<MyService const> as long as the const-ness of the
//    template argument is stripped (via std::remove_const_t<T>)
//    before serving as an argument to ServiceHelper.
// =======================================================================

#include "art/Framework/Services/Registry/ServiceRegistry.h"
#include "art/Framework/Services/Registry/ServiceScope.h"
#include "art/Framework/Services/Registry/detail/ServiceHandleAllowed.h"
#include "art/Framework/Services/Registry/detail/ServiceHelper.h"

#include <type_traits>

namespace art {

  template <typename T,
            ServiceScope SCOPE =
              detail::ServiceHelper<std::remove_const_t<T>>::scope_val>
  class ServiceHandle {
  public:
    static_assert(
      detail::handle_allowed_v<T>,
      "\n\nart-error: You cannot create a ServiceHandle for this type.\n"
      "           Please contact artists@fnal.gov for guidance.\n");

    ServiceHandle()
    try : instance{&ServiceRegistry::instance().get<std::remove_const_t<T>>()} {
    }
    catch (Exception const& x) {
      throw Exception(errors::ServiceNotFound)
        << "Unable to create ServiceHandle.\n"
        << "Perhaps the FHiCL configuration does not specify the necessary "
           "service?\n"
        << "The class of the service is noted below...\n"
        << x;
    }

    T*
    operator->() const
    {
      return instance;
    }

    T&
    operator*() const
    {
      return *instance;
    }

    T*
    get() const
    {
      return instance;
    }

  private:
    T* instance;
  };

} // namespace art

#endif /* art_Framework_Services_Registry_ServiceHandle_h */

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