ReturnByValue.h

// This file is part of Eigen, a lightweight C++ template library
// for linear algebra.
//
// Copyright (C) 2009-2010 Gael Guennebaud <gael.guennebaud@inria.fr>
// Copyright (C) 2009-2010 Benoit Jacob <jacob.benoit.1@gmail.com>
//
// This Source Code Form is subject to the terms of the Mozilla
// Public License v. 2.0. If a copy of the MPL was not distributed
// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
// SPDX-License-Identifier: MPL-2.0
 
#ifndef EIGEN_RETURNBYVALUE_H
#define EIGEN_RETURNBYVALUE_H
 
// IWYU pragma: private
#include "./InternalHeaderCheck.h"
 
namespace Eigen {
 
namespace internal {
 
template <typename Derived>
struct traits<ReturnByValue<Derived> > : public traits<typename traits<Derived>::ReturnType> {
  enum {
    // We're disabling the DirectAccess because e.g. the constructor of
    // the Block-with-DirectAccess expression requires to have a coeffRef method.
    // Also, this avoids having to implement stride support.
    Flags = (traits<typename traits<Derived>::ReturnType>::Flags | EvalBeforeNestingBit) & ~DirectAccessBit
  };
};
 
/* The ReturnByValue object doesn't even have a coeff() method.
 * So the only way that nesting it in an expression can work, is by evaluating it into a plain matrix.
 * So internal::nested always gives the plain return matrix type.
 *
 * FIXME: this specialization may be redundant with EvalBeforeNestingBit.
 * Answer: EvalBeforeNestingBit should be deprecated since we have the evaluators
 */
template <typename Derived, int n, typename PlainObject>
struct nested_eval<ReturnByValue<Derived>, n, PlainObject> {
  using type = typename traits<Derived>::ReturnType;
};
 
}  // end namespace internal

template <typename Derived>
class ReturnByValue : public internal::dense_xpr_base<ReturnByValue<Derived> >::type, internal::no_assignment_operator {
 public:
  using ReturnType = typename internal::traits<Derived>::ReturnType;
 
  using Base = typename internal::dense_xpr_base<ReturnByValue>::type;
  EIGEN_DENSE_PUBLIC_INTERFACE(ReturnByValue)
 
  template <typename Dest>
  EIGEN_DEVICE_FUNC inline void evalTo(Dest& dst) const {
    static_cast<const Derived*>(this)->evalTo(dst);
  }
  EIGEN_DEVICE_FUNC constexpr Index rows() const noexcept { return static_cast<const Derived*>(this)->rows(); }
  EIGEN_DEVICE_FUNC constexpr Index cols() const noexcept { return static_cast<const Derived*>(this)->cols(); }
 
#ifndef EIGEN_PARSED_BY_DOXYGEN
#define Unusable \
  YOU_ARE_TRYING_TO_ACCESS_A_SINGLE_COEFFICIENT_IN_A_SPECIAL_EXPRESSION_WHERE_THAT_IS_NOT_ALLOWED_BECAUSE_THAT_WOULD_BE_INEFFICIENT
  class Unusable {
    Unusable(const Unusable&) {}
    Unusable& operator=(const Unusable&) { return *this; }
  };
  const Unusable& coeff(Index) const { return *reinterpret_cast<const Unusable*>(this); }
  const Unusable& coeff(Index, Index) const { return *reinterpret_cast<const Unusable*>(this); }
  Unusable& coeffRef(Index) { return *reinterpret_cast<Unusable*>(this); }
  Unusable& coeffRef(Index, Index) { return *reinterpret_cast<Unusable*>(this); }
#undef Unusable
#endif
};
 
template <typename Derived>
template <typename OtherDerived>
EIGEN_DEVICE_FUNC Derived& DenseBase<Derived>::operator=(const ReturnByValue<OtherDerived>& other) {
  other.evalTo(derived());
  return derived();
}
 
namespace internal {
 
// Expression is evaluated in a temporary; default implementation of Assignment is bypassed so that
// when a ReturnByValue expression is assigned, the evaluator is not constructed.
// TODO: Finalize port to new regime; ReturnByValue should not exist in the expression world
 
template <typename Derived>
struct evaluator<ReturnByValue<Derived> > : public evaluator<typename internal::traits<Derived>::ReturnType> {
  using XprType = ReturnByValue<Derived>;
  using PlainObject = typename internal::traits<Derived>::ReturnType;
  using Base = evaluator<PlainObject>;
 
  EIGEN_DEVICE_FUNC explicit evaluator(const XprType& xpr) : m_result(xpr.rows(), xpr.cols()) {
    internal::construct_at<Base>(this, m_result);
    xpr.evalTo(m_result);
  }
 
 protected:
  PlainObject m_result;
};
 
}  // end namespace internal
 
}  // end namespace Eigen
 
#endif  // EIGEN_RETURNBYVALUE_H