View.h

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

// ====================================================================
// The class template View<T> provides a means to obtain pointers (of
// type T const*) into an arbitrary collection in an Event.
//
// A View<T> is *valid* if the container data product pointer is set.
// A valid View may still hold an empty vector of pointes; this means
// that either the referenced container was empty, or that the View's
// vector was emptied after the View was created.
//
// While View<T> is not a persistent class, one can fill a
// PtrVector<T> from a View<T>, as long as no new pointers have been
// added to the View<T>.
// ====================================================================

#include "art/Framework/Principal/fwd.h"
#include "canvas/Persistency/Common/PtrVector.h"
#include "cetlib/container_algorithms.h"

#include <string>
#include <utility>

namespace art {

  class DataViewImpl;
  class InputTag;

  template <typename T>
  class View {

    // Give access to private ctor.
    template <typename Element>
    friend bool DataViewImpl::getView(std::string const&,
                                      std::string const&,
                                      std::string const&,
                                      View<Element>&) const;

  public:
    using collection_type = std::vector<T const*>;
    using value_type = typename collection_type::value_type;
    using const_iterator = typename collection_type::const_iterator;
    using iterator = typename collection_type::iterator;
    using size_type = typename collection_type::size_type;

    View() = default;
    View(std::vector<T const*>, ProductID const, EDProduct const*);

    auto
    isValid() const noexcept
    {
      return prod_ != nullptr;
    }

    auto
    id() const
    {
      return id_;
    }

    // Fill a PtrVector<T> with Ptrs to each element of the container data
    // product that can be reached from the internal vector<T const*>.
    void fill(PtrVector<T>& pv) const;

    auto&
    vals() noexcept
    {
      return vals_;
    }

    auto const&
    vals() const noexcept
    {
      return vals_;
    }

    operator auto&() noexcept { return vals_; }
    operator auto const&() const noexcept { return vals_; }

    auto
    begin() noexcept
    {
      return vals_.begin();
    }

    auto
    end() noexcept
    {
      return vals_.end();
    }

    auto
    begin() const noexcept
    {
      return vals_.begin();
    }

    auto
    end() const noexcept
    {
      return vals_.end();
    }

    auto
    cbegin() const noexcept
    {
      return vals_.cbegin();
    }

    auto
    cend() const noexcept
    {
      return vals_.cend();
    }

    auto
    size() const noexcept
    {
      return vals_.size();
    }

  private:
    // Vector of pointers to elements in the container product (this is the view
    // itself).
    std::vector<T const*> vals_{};

    // The container id.
    ProductID id_{};

    // The container itself, we do not own.
    EDProduct const* prod_{nullptr};
  };

  template <typename T>
  View<T>::View(std::vector<T const*> v, ProductID const id, EDProduct const* p)
    : vals_{move(v)}, id_{id}, prod_{p}
  {}

  // Fill a PtrVector<T> with Ptrs to each element of the container
  // data product that can be reached from the internal vector<T const*>.
  template <typename T>
  void
  View<T>::fill(PtrVector<T>& pv) const
  {
    if (not isValid()) {
      return;
    }
    typename std::vector<T const*>::size_type i{};
    for (auto a : prod_->getView()) {
      // We do this as a sloppy guard against the container data
      // product having been replaced between the View<T> ctor and the
      // fill call. The user is supposed to do event.getView<T>()
      // immediately followed by View<T>::fill(), but well. This can
      // be O(n^2) so maybe it is not worth it in the case of large
      // containers.
      if (cet::search_all(vals_, a)) {
        pv.emplace_back(id_, reinterpret_cast<T const*>(a), i);
        ++i;
      }
    }
  }

} // namespace art

#endif /* art_Framework_Principal_View_h */

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