traits.h

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#ifndef canvas_Persistency_Common_traits_h
#define canvas_Persistency_Common_traits_h

/*----------------------------------------------------------------------
  Definition of traits templates used in the event data model.
  ----------------------------------------------------------------------*/

#include "canvas/Utilities/Exception.h"
#include "cetlib/container_algorithms.h"
#include "cetlib/map_vector.h"
#include "cetlib/metaprogramming.h"
#include "cetlib_except/demangle.h"

#include <deque>
#include <limits>
#include <list>
#include <set>
#include <string>
#include <type_traits>
#include <typeinfo>
#include <utility>
#include <vector>

namespace art {

  // The trait struct template key_traits<K> is used to carry
  // information relevant to the type K when used as a 'key' in
  // PtrVector and its related classes and templates.
  //
  // The general case works only for integral types K; for more
  // 'esoteric' types, one must introduce an explicit specialization.
  // That specialization must initialize the static data member
  // 'value'.

  template <typename K>
  struct key_traits {
    using key_type = K;
    static constexpr key_type value{
      std::numeric_limits<typename key_traits<K>::key_type>::max()};
  };

  // Partial specialization for std::pair
  template <typename U, typename V>
  struct key_traits<std::pair<U, V>> {
    using key_type = std::pair<U, V>;
    static const key_type value;
  };

  // If we ever need to support instantiations of std::basic_string
  // other than std::string, this is the place to do it.
  // For value, we make a 1-character long string that contains an
  // unprintable character; we are hoping nobody ever uses such a
  // string as a legal key.
  template <>
  struct key_traits<std::string> {
    using key_type = std::string;
    static const key_type value;
  };

  template <typename T, typename = void>
  struct has_value_type : std::false_type {};

  template <typename T>
  struct has_value_type<T, std::void_t<typename T::value_type>>
    : std::true_type {
    using element_type = typename T::value_type;
  };

  template <typename T, typename = void>
  struct has_mapped_type : std::false_type {};

  template <typename T>
  struct has_mapped_type<T, std::void_t<typename T::mapped_type>>
    : std::true_type {
    using element_type = typename T::mapped_type;
  };

  // A type supports a view if it has a nested 'value_type' or 'mapped_type'
  // type name.
  template <typename T, typename = void>
  struct SupportsView : std::false_type {
    static std::type_info const*
    type_id()
    {
      return nullptr;
    }
  };

  template <typename T>
  struct SupportsView<
    T,
    std::enable_if_t<(has_value_type<T>::value && !has_mapped_type<T>::value)>>
    : std::true_type {
    using element_type = typename has_value_type<T>::element_type;
    static std::type_info const*
    type_id()
    {
      return &typeid(element_type);
    }
  };

  template <typename T>
  struct SupportsView<T, std::enable_if_t<has_mapped_type<T>::value>>
    : std::true_type {
    using element_type = typename has_mapped_type<T>::element_type;
    static std::type_info const*
    type_id()
    {
      return &typeid(element_type);
    }
  };

  // The trait struct template has_getView<T> is used to
  // indicate whether or not the type T has a member function
  //
  //      std::vector<void const*> T::getView() const
  //
  // We assume the 'general case' for T is to not support getView.
  // Classes which do support getView must specialize this trait.

  template <typename T>
  void const*
  address_of(T const& t) noexcept
  {
    return &t;
  }

  template <typename T>
  void const*
  address_of_second(typename cet::map_vector<T>::value_type const& t) noexcept
  {
    return &t.second;
  }

  template <typename T, typename = void>
  struct has_getView {};

  template <typename T>
  struct has_getView<
    T,
    cet::enable_if_function_exists_t<void (T::*)(std::vector<void const*>&),
                                     &T::getView>> {};

  template <typename T>
  struct CannotGetView {
    [[noreturn]] static std::vector<void const*>
    get(T const&)
    {
      throw Exception(errors::ProductDoesNotSupportViews)
        << "Product type " << cet::demangle_symbol(typeid(T).name())
        << " has no getView() capability.\n";
    }
  };

  template <typename T, typename = void>
  struct MaybeGetView : CannotGetView<T> {};

  template <typename T>
  struct MaybeGetView<T, std::enable_if_t<has_getView<T>::value>> {
    static auto
    get(T const& product)
    {
      return product.getView();
    }
  };

  template <typename T, typename A>
  struct MaybeGetView<std::vector<T, A>> {
    static auto
    get(std::vector<T> const& product)
    {
      std::vector<void const*> view;
      cet::transform_all(product, back_inserter(view), address_of<T>);
      return view;
    }
  };

  template <typename A>
  struct MaybeGetView<std::vector<bool, A>>
    : CannotGetView<std::vector<bool, A>> {};

  template <typename T, typename A>
  struct MaybeGetView<std::list<T, A>> {
    static auto
    get(std::list<T> const& product)
    {
      std::vector<void const*> view;
      cet::transform_all(product, back_inserter(view), address_of<T>);
      return view;
    }
  };

  template <typename T, typename A>
  struct MaybeGetView<std::deque<T, A>> {
    static auto
    get(std::deque<T> const& product)
    {
      std::vector<void const*> view;
      cet::transform_all(product, back_inserter(view), address_of<T>);
      return view;
    }
  };

  template <typename T, typename A>
  struct MaybeGetView<std::set<T, A>> {
    static auto
    get(std::set<T> const& product)
    {
      std::vector<void const*> view;
      cet::transform_all(product, back_inserter(view), address_of<T>);
      return view;
    }
  };

  template <typename T>
  struct MaybeGetView<cet::map_vector<T>> {
    static auto
    get(cet::map_vector<T> const& product)
    {
      std::vector<void const*> view;
      cet::transform_all(product, back_inserter(view), address_of_second<T>);
      return view;
    }
  };

  //
  // The trait struct template has_setPtr<T> is used to
  // indicate whether or not the type T has a member function
  //
  //      void T::setPtr(std::type_info const&, void const*&) const
  //
  // We assume the 'general case' for T is to not support setPtr.
  // Classes which do support setPtr must specialize this trait.
  //

  template <typename T>
  struct has_setPtr : std::false_type {};
  template <typename T, typename A>
  struct has_setPtr<std::vector<T, A>> : std::true_type {};
  template <typename A>
  struct has_setPtr<std::vector<bool, A>> : std::false_type {};
  template <typename T, typename A>
  struct has_setPtr<std::list<T, A>> : std::true_type {};
  template <typename T, typename A>
  struct has_setPtr<std::deque<T, A>> : std::true_type {};
  template <typename T, typename A>
  struct has_setPtr<std::set<T, A>> : std::true_type {};
  template <typename T>
  struct has_setPtr<cet::map_vector<T>> : std::true_type {};
}

#endif /* canvas_Persistency_Common_traits_h */

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