Eigen/src/Core/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/.

#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, we don't want to have to implement the stride stuff.
    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: I don't understand why we need this specialization: isn't this taken care of by the 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> {
  typedef typename traits<Derived>::ReturnType type;
};

}  // end namespace internal

/** \class ReturnByValue
 * \ingroup Core_Module
 *
 */
template <typename Derived>
class ReturnByValue : public internal::dense_xpr_base<ReturnByValue<Derived> >::type, internal::no_assignment_operator {
 public:
  typedef typename internal::traits<Derived>::ReturnType ReturnType;

  typedef typename internal::dense_xpr_base<ReturnByValue>::type Base;
  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 EIGEN_CONSTEXPR inline Index rows() const EIGEN_NOEXCEPT {
    return static_cast<const Derived*>(this)->rows();
  }
  EIGEN_DEVICE_FUNC EIGEN_CONSTEXPR inline Index cols() const EIGEN_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> {
  typedef ReturnByValue<Derived> XprType;
  typedef typename internal::traits<Derived>::ReturnType PlainObject;
  typedef evaluator<PlainObject> Base;

  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