backend/interface.hh

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#ifndef DUNE_PDELAB_BACKEND_INTERFACE_HH
#define DUNE_PDELAB_BACKEND_INTERFACE_HH
 
#include <type_traits>
#include <utility>
 
namespace Dune {
  namespace PDELab {
    namespace Backend {
 
#ifndef DOXYGEN
 
      namespace impl {
 
        // This class needs to be specialized by each backend and return the correct
        // vector wrapper type via the nested type named "type"
        template<typename Backend, typename GridFunctionSpace, typename FieldType>
        struct BackendVectorSelectorHelper
        {};
 
        template<typename GridFunctionSpace, typename FieldType>
        struct BackendVectorSelector
        {
          typedef typename GridFunctionSpace::Traits::Backend Backend;
          typedef typename BackendVectorSelectorHelper<Backend, GridFunctionSpace, FieldType>::Type Type;
        };
 
        template<typename Backend, typename VU, typename VV, typename E>
        struct BackendMatrixSelector
        {
          typedef typename Backend::template MatrixHelper<VV,VU,E>::type Type;
        };
 
        // marker mixin type used by the generic implementation below to decide whether a type
        // is a PDELab wrapper around a native object - for internal use only!
        struct WrapperBase
        {};
 
        // mixin class for PDELab wrappers around vectors and matrices.
        // All backend wrappers should inherit from this class and set the
        // template parameter to the type of the native container of the backend that they are
        // wrapping.
        // Moreover, they have to provide methods
        //
        // NativeContainer& native()
        // const NativeContainer& native() const
        //
        // that provide access to the wrapped data structure. These two methods should be private;
        // in that case, the Wrapper<NativeContainer> mixin must be a friend of the wrapper implementation
        // so that it can access the native() methods.
        template<typename NativeContainer>
        struct Wrapper
          : public WrapperBase
        {
 
          using native_type = NativeContainer;
 
          template<typename U>
          static auto access_native(U&& u) -> decltype(u.native())
          {
            // make sure the wrapper actually returns the right type of object
            static_assert(
              std::is_same<
                typename std::decay<
                  decltype(u.native())
                  >::type,
                NativeContainer
                >::value,
              "u.native() must return a cv-qualified xvalue of type T"
              );
 
            return u.native();
          }
 
        };
 
      } // namespace impl
 
    } // namespace Backend
 
 
    namespace Backend{
 
#endif // DOXYGEN
 
 
      template<typename GridFunctionSpace, typename FieldType>
      using Vector = typename impl::BackendVectorSelector<GridFunctionSpace, FieldType>::Type;
 
      template<typename Backend, typename VU, typename VV, typename E>
      using Matrix = typename impl::BackendMatrixSelector<Backend, VU, VV, E>::Type;
 
 
      namespace {
 
        // helper TMP to deduce the native type of a wrapper. It works by checking whether
        // T inherits from WrapperBase. In that case, it returns the nested typedef native_type,
        // otherwise T is returned unchanged.
        template<typename T>
        struct native_type
        {
 
          // We need to defer the (possible) extraction of the nested type until we are sure
          // that the nested type actually exists. That'w what these lazy_... structs are for:
          // a std::conditional picks the correct version, and the actual evaluation only happens
          // after the evaluation of the std::conditional.
 
          struct lazy_identity
          {
            template<typename U>
            struct evaluate
            {
              using type = U;
            };
 
          };
 
          struct lazy_native_type
          {
 
            template<typename U>
            struct evaluate
            {
              using type = typename U::native_type;
            };
 
          };
 
          using type = typename std::conditional<
            std::is_base_of<impl::WrapperBase,T>::value,
            lazy_native_type,
            lazy_identity
            >::type::template evaluate<T>::type;
        };
 
      }
 
      template<typename T>
      using Native = typename native_type<T>::type;
 
#ifdef DOXYEN
 
      template<typename T>
      Native<T> native(T&& t);
 
#else // DOXYGEN
 
      // version for mutable reference to wrapper
      template<typename T>
      typename std::enable_if<
        std::is_base_of<impl::WrapperBase,T>::value,
        Native<T>&
        >::type
      native(T& t)
      {
        return impl::Wrapper<Native<T>>::access_native(t);
      }
 
      // version for const reference to wrapper
      template<typename T>
      typename std::enable_if<
        std::is_base_of<impl::WrapperBase,T>::value,
        const Native<T>&
        >::type
      native(const T& t)
      {
        return impl::Wrapper<Native<T>>::access_native(t);
      }
 
      // version for non-wrapper class. Important: Don't drop the std::decay<>!
      template<typename T>
      typename std::enable_if<
        !std::is_base_of<impl::WrapperBase,typename std::decay<T>::type>::value,
        decltype(std::forward<T>(std::declval<T&&>()))
        >::type
      native(T&& t)
      {
        return std::forward<T>(t);
      }
 
#endif // DOXYGEN
 
    } // namespace Backend
  } // namespace PDELab
} // namespace Dune
 
#endif // DUNE_PDELAB_BACKEND_INTERFACE_HH