add ReturnByValue pseudo expression for in-place evaluation with a

return-by-value API style (will soon use it for the transform products)

Eigen/Core

@@ -121,6 +121,7 @@ namespace Eigen {
 
 #include "src/Core/MatrixStorage.h"
 #include "src/Core/NestByValue.h"
+#include "src/Core/ReturnByValue.h"
 #include "src/Core/Flagged.h"
 #include "src/Core/Matrix.h"
 #include "src/Core/Cwise.h"

Eigen/src/Core/Matrix.h

@@ -289,9 +289,13 @@ class Matrix
       */
     EIGEN_STRONG_INLINE Matrix& operator=(const Matrix& other)
     {
-      return _set(other);
+      return  _set(other);
     }
 
+    template<typename OtherDerived,typename OtherEvalType>
+    EIGEN_STRONG_INLINE Matrix& operator=(const ReturnByValue<OtherDerived,OtherEvalType>& func)
+    { return Base::operator=(func); }
+
     EIGEN_INHERIT_ASSIGNMENT_OPERATOR(Matrix, +=)
     EIGEN_INHERIT_ASSIGNMENT_OPERATOR(Matrix, -=)
     EIGEN_INHERIT_SCALAR_ASSIGNMENT_OPERATOR(Matrix, *=)
@@ -412,6 +416,10 @@ class Matrix
       _check_template_params();
       _set_noalias(other);
     }
+    /** Copy constructor with in-place evaluation */
+    template<typename OtherDerived,typename OtherEvalType>
+    EIGEN_STRONG_INLINE Matrix(const ReturnByValue<OtherDerived,OtherEvalType>& other)
+    { other.evalTo(*this); }
     /** Destructor */
     inline ~Matrix() {}
 

Eigen/src/Core/MatrixBase.h

@@ -630,6 +630,9 @@ template<typename Derived> class MatrixBase
     template<typename Derived1, typename Derived2>
     Derived& lazyAssign(const SparseProduct<Derived1,Derived2,DenseTimeSparseProduct>& product);
 
+    template<typename OtherDerived,typename OtherEvalType>
+    Derived& operator=(const ReturnByValue<OtherDerived,OtherEvalType>& func);
+
     #ifdef EIGEN_MATRIXBASE_PLUGIN
     #include EIGEN_MATRIXBASE_PLUGIN
     #endif

Eigen/src/Core/ReturnByValue.h

@@ -0,0 +1,63 @@
+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra. Eigen itself is part of the KDE project.
+//
+// Copyright (C) 2009 Gael Guennebaud <g.gael@free.fr>
+//
+// Eigen is free software; you can redistribute it and/or
+// modify it under the terms of the GNU Lesser General Public
+// License as published by the Free Software Foundation; either
+// version 3 of the License, or (at your option) any later version.
+//
+// Alternatively, you can redistribute it and/or
+// modify it under the terms of the GNU General Public License as
+// published by the Free Software Foundation; either version 2 of
+// the License, or (at your option) any later version.
+//
+// Eigen is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+// FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License or the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public
+// License and a copy of the GNU General Public License along with
+// Eigen. If not, see <http://www.gnu.org/licenses/>.
+
+#ifndef EIGEN_RETURNBYVALUE_H
+#define EIGEN_RETURNBYVALUE_H
+
+/** \class ReturnByValue
+  *
+  */
+template<typename Functor,typename EvalType>
+struct ei_traits<ReturnByValue<Functor,EvalType> > : public ei_traits<EvalType>
+{
+  enum {
+    Flags = ei_traits<EvalType>::Flags | EvalBeforeNestingBit
+  };
+};
+
+template<typename Functor,typename EvalType,int n>
+struct ei_nested<ReturnByValue<Functor,EvalType>, n, EvalType>
+{
+  typedef EvalType type;
+};
+
+template<typename Functor, typename EvalType> class ReturnByValue
+  : public MatrixBase<ReturnByValue<Functor,EvalType> >
+{
+  public:
+    EIGEN_GENERIC_PUBLIC_INTERFACE(ReturnByValue)
+    template<typename Dest>
+    inline void evalTo(Dest& dst) const
+    { static_cast<const Functor*>(this)->evalTo(dst); }
+};
+
+template<typename Derived>
+template<typename OtherDerived,typename OtherEvalType>
+Derived& MatrixBase<Derived>::operator=(const ReturnByValue<OtherDerived,OtherEvalType>& other)
+{
+  other.evalTo(derived());
+  return derived();
+}
+
+#endif // EIGEN_RETURNBYVALUE_H

Eigen/src/Core/util/ForwardDeclarations.h

@@ -56,6 +56,7 @@ template<typename MatrixType, unsigned int Mode> class Extract;
 template<typename ExpressionType> class Cwise;
 template<typename ExpressionType> class WithFormat;
 template<typename MatrixType> struct CommaInitializer;
+template<typename Functor, typename EvalType> class ReturnByValue;
 
 
 template<typename Lhs, typename Rhs> struct ei_product_mode;