After resurrecting nestByValue(), I decided to see whether the numerous copies could be removed by move ctors. Mostly for curiosity as I'm not sure nestByValue() is a really useful feature as it was broken since 3.3.0 and nobody complained! OK, so let's consider this toy example: auto get_xpr_with_temps(const MatrixXd& a) { return a.rowwise().reverse().eval().nestByValue() + (a+a).eval().nestByValue(); } Currently this creates 6 temporaries, and 4 copies. Ouch! Those can be reduced to 2 temporaries only (the ideal) if we do the following changes: 1) Make all functions return non const expressions, including eval(). So far, they return const object to prevent stupid code like: expr.eval().normalized(); 2) Add ctors from rvalue refs to all expressions, e.g.: CwiseBinaryOp(Lhs&& aLhs, Rhs&& aRhs, const BinaryOp& func = BinaryOp()) : m_lhs(std::move(aLhs)), m_rhs(std::move(aRhs)), m_functor(func) {} (plus the 2 hybrid versions for binary expressions). 3) Add overloads members/operators for rvalue ref, e.g., for EIGEN_MAKE_CWISE_BINARY_OP(), we have to add: template<typename OtherDerived> \ EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE EIGEN_CWISE_BINARY_RETURN_TYPE(Derived,OtherDerived,OPNAME) \ (METHOD)(EIGEN_CURRENT_STORAGE_BASE_CLASS<OtherDerived> &&other) && \ { \ return EIGEN_CWISE_BINARY_RETURN_TYPE(Derived,OtherDerived,OPNAME)(std::move(derived()), std::move(other.derived())); \ } \ /* + the 2 hybrid versions */ The regular const operator/member must be qualified by "const &" instead of "const" only. That's all. Whereas points 1) and 2) are easily manageable, point 3) represents a lot of boilerplate code. E.g., think about all the block variants! Moreover, this only addresses heap-allocated temporaries, not fixed size ones.
With the aforementioned changes we could even get rid of explicit nestByValue() by introducing them implicitly when detecting a rvalue ref of type T for which T usually prefers to be nested by reference. The user would still have to turn named local temporaries into a xvalue using move(). To this end we could define two helpers (not tested): template<typename T> using nest_by_value_t = typename conditional<prefer_nesting_by_ref<T>::value,NestByValue<T>,T>::type; template<typename T, typename = enable_if<prefer_nesting_by_ref<T>::value> > auto nest_by_value(T&& x) { return NestByValue<T>(std::move(x)); } template<typename T, typename = enable_if<!prefer_nesting_by_ref<T>::value> > T&& nest_by_value(T&& x) { return std::move(x); } Then, for instance, in operator+: auto DenseBase<Derived>::operator+(DenseStorage<Other> &&other) && { return CwiseBinaryOp<scalar_sum_op<...>, const nest_by_value_t<Derived>, const nest_by_value_t<Other> >( nest_by_value(std::move(derived())), nest_by_value(std::move(other.derived())) ); }
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