TypeTraits.h

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// Copyright 2019-2020 CERN and copyright holders of ALICE O2.
// See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
// All rights not expressly granted are reserved.
//
// This software is distributed under the terms of the GNU General Public
// License v3 (GPL Version 3), copied verbatim in the file "COPYING".
//
// In applying this license CERN does not waive the privileges and immunities
// granted to it by virtue of its status as an Intergovernmental Organization
// or submit itself to any jurisdiction.
#ifndef O2_FRAMEWORK_TYPETRAITS_H_
#define O2_FRAMEWORK_TYPETRAITS_H_
 
#include <type_traits>
#include <vector>
#include <memory>
#include "Framework/Traits.h"
 
#include <gsl/gsl>
 
namespace o2::framework
{
// helper struct to mark a type as non-messageable by defining a type alias
// with name 'non-messageable'
struct MarkAsNonMessageable {
};
 
// detect if a type is forced to be non-messageable, this is done by defining
// a type alias with name 'non-messageable' of the type MarkAsNonMessageable
template <typename T, typename _ = void>
struct is_forced_non_messageable : public std::false_type {
};
 
// specialization to detect the type a lias
template <typename T>
struct is_forced_non_messageable<
  T,
  typename std::enable_if<std::is_same<typename T::non_messageable, MarkAsNonMessageable>::value>::type> : public std::true_type {
};
 
// TODO: extend this to exclude structs with pointer data members
// see e.g. https://stackoverflow.com/questions/32880990/how-to-check-if-class-has-pointers-in-c14
template <typename T>
struct is_messageable : std::conditional<std::is_trivially_copyable<T>::value && //
                                           !std::is_polymorphic<T>::value &&     //
                                           !std::is_pointer<T>::value &&         //
                                           !is_forced_non_messageable<T>::value, //
                                         std::true_type,
                                         std::false_type>::type {
};
 
// FIXME: it apears that gsl:span matches the criteria for being messageable, regardless of the
// underlying type, but our goal is to identify structures that can be sent without serialization.
// needs investigation
template <typename T>
struct is_messageable<gsl::span<T>> : std::false_type {
};
 
// Detect a container by checking on the container properties
// this is the default trait implementation inheriting from false_type
template <typename T, typename _ = void>
struct is_container : std::false_type {
};
 
// helper to be substituted if the specified template arguments are
// available
template <typename... Ts>
struct class_member_checker {
};
 
// the specialization for container types inheriting from true_type
// the helper can be substituted if all the specified members are available
// in the type
template <typename T>
struct is_container<
  T,
  std::conditional_t<
    false,
    class_member_checker<
      typename T::value_type,
      typename T::size_type,
      typename T::allocator_type,
      typename T::iterator,
      typename T::const_iterator,
      decltype(std::declval<T>().size()),
      decltype(std::declval<T>().begin()),
      decltype(std::declval<T>().end()),
      decltype(std::declval<T>().cbegin()),
      decltype(std::declval<T>().cend())>,
    void>> : public std::true_type {
};
 
// Detect whether a container class has a type definition `value_type` of messageable type
template <typename T, typename _ = void>
struct has_messageable_value_type : std::false_type {
};
template <typename T>
struct has_messageable_value_type<T, std::conditional_t<false, typename T::value_type, void>> : is_messageable<typename T::value_type> {
};
 
template <typename T, typename _ = void>
struct is_span : std::false_type {
};
template <typename T>
struct is_span<T, std::conditional_t<false, typename T::value_type, void>> : std::is_same<gsl::span<typename T::value_type>, T> {
};
 
// Detect whether a class has a ROOT dictionary
// This member detector idiom is implemented using SFINAE idiom to look for
// a 'Class()' method.
// This is actually only covering classes using the ClasDef/Imp macros. ROOT
// serialization however is also possible for types only having the link
// in the LinkDef file. Such types can only be detected at runtime.
template <typename T, typename _ = void>
struct has_root_dictionary : std::false_type {
};
 
template <typename T>
struct has_root_dictionary<
  T,
  std::conditional_t<
    false,
    class_member_checker<
      decltype(std::declval<T>().Class())>,
    void>> : public std::true_type {
};
 
// Detect whether a container class has a type definition `mapped_type` with a root dictionary
template <typename T, typename _ = void>
struct has_root_dictionary_mapped_type : std::false_type {
};
 
template <typename T>
struct has_root_dictionary_mapped_type<T, std::conditional_t<false, typename T::mapped_type, void>> : has_root_dictionary<typename T::mapped_type> {
};
 
// specialization for containers
// covers cases with T::value_type having ROOT dictionary, meaning that
// std::map is not supported out of the box
//
// Note: this is only a SFINAE idiom if the default type of the second template
// argument in the declaration is the same as the conditionally produced one in
// the specialization (void in this case)
template <typename T>
class has_root_dictionary<T, typename std::enable_if<is_container<T>::value>::type>
  : public has_root_dictionary<std::remove_pointer_t<typename T::value_type>>
{
};
 
} // namespace o2::framework
#endif // FRAMEWORK_TYPETRAITS_H