Eigen/src/Core/NoAlias.h

// This file is part of Eigen, a lightweight C++ template library
// for linear algebra.
//
// Copyright (C) 2009 Gael Guennebaud <gael.guennebaud@inria.fr>
//
// 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_NOALIAS_H
#define EIGEN_NOALIAS_H

// IWYU pragma: private
#include "./InternalHeaderCheck.h"

namespace Eigen {

/** \class NoAlias
 * \ingroup Core_Module
 *
 * \brief Pseudo expression providing an operator = assuming no aliasing
 *
 * \tparam ExpressionType the type of the object on which to do the lazy assignment
 *
 * This class represents an expression with special assignment operators
 * assuming no aliasing between the target expression and the source expression.
 * More precisely it alloas to bypass the EvalBeforeAssignBit flag of the source expression.
 * It is the return type of MatrixBase::noalias()
 * and most of the time this is the only way it is used.
 *
 * \sa MatrixBase::noalias()
 */
template <typename ExpressionType, template <typename> class StorageBase>
class NoAlias {
 public:
  typedef typename ExpressionType::Scalar Scalar;

  EIGEN_DEVICE_FUNC explicit NoAlias(ExpressionType& expression) : m_expression(expression) {}

  template <typename OtherDerived>
  EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE ExpressionType& operator=(const StorageBase<OtherDerived>& other) {
    call_assignment_no_alias(m_expression, other.derived(),
                             internal::assign_op<Scalar, typename OtherDerived::Scalar>());
    return m_expression;
  }

  template <typename OtherDerived>
  EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE ExpressionType& operator+=(const StorageBase<OtherDerived>& other) {
    call_assignment_no_alias(m_expression, other.derived(),
                             internal::add_assign_op<Scalar, typename OtherDerived::Scalar>());
    return m_expression;
  }

  template <typename OtherDerived>
  EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE ExpressionType& operator-=(const StorageBase<OtherDerived>& other) {
    call_assignment_no_alias(m_expression, other.derived(),
                             internal::sub_assign_op<Scalar, typename OtherDerived::Scalar>());
    return m_expression;
  }

  EIGEN_DEVICE_FUNC ExpressionType& expression() const { return m_expression; }

 protected:
  ExpressionType& m_expression;
};

/** \returns a pseudo expression of \c *this with an operator= assuming
 * no aliasing between \c *this and the source expression.
 *
 * More precisely, noalias() allows to bypass the EvalBeforeAssignBit flag.
 * Currently, even though several expressions may alias, only product
 * expressions have this flag. Therefore, noalias() is only useful when
 * the source expression contains a matrix product.
 *
 * Here are some examples where noalias is useful:
 * \code
 * D.noalias()  = A * B;
 * D.noalias() += A.transpose() * B;
 * D.noalias() -= 2 * A * B.adjoint();
 * \endcode
 *
 * On the other hand the following example will lead to a \b wrong result:
 * \code
 * A.noalias() = A * B;
 * \endcode
 * because the result matrix A is also an operand of the matrix product. Therefore,
 * there is no alternative than evaluating A * B in a temporary, that is the default
 * behavior when you write:
 * \code
 * A = A * B;
 * \endcode
 *
 * \sa class NoAlias
 */
template <typename Derived>
NoAlias<Derived, MatrixBase> EIGEN_DEVICE_FUNC MatrixBase<Derived>::noalias() {
  return NoAlias<Derived, Eigen::MatrixBase>(derived());
}

}  // end namespace Eigen

#endif  // EIGEN_NOALIAS_H