Hi all, I am trying to use proto for the evaluation of expressions involved in a Finite Element Method. The idea is that developers can write expressions that resemble the mathematical formulation of the problem, but that get processed efficiently to use matrix operations on fixed-size matrices. The matrix library that is used as backend is Eigen (http://eigen.tuxfamily.org).

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In my first attempt, I used proto::eval with a context that understands some custom terminals and functions, and delegates other stuff (like the math operators) to the proto default context. Unfortunately, this approach works only on expressions involving at most 2 matrices, since in more complex cases Eigen constructs its own nested expression templates and these may refer to their operands by reference. The operands may be temporaries, however, (as generated by a nested call to proto::eval), so the program either crashes or gives the wrong result. In a release compile, everything works, presumably because all the nested proto calls get inlined. In my second attempt, I tried to use an object transform, hoping the temporaries would persist until the evaluation happens. The code looks like this: template<typename Left, typename Right> struct MultExpr { typedef typename Eigen::ProductReturnType<Left, Right>::Type type; }; struct EvaluateExpr : boost::proto::or_ < boost::proto::when < boost::proto::multiplies<boost::proto::_, boost::proto::_>, MultExpr < EvaluateExpr(boost::proto::_left), EvaluateExpr(boost::proto::_right) >(EvaluateExpr(boost::proto::_left), EvaluateExpr(boost::proto::_right)) >, boost::proto::when < boost::proto::_, ContextEvaluator > > {}; An example of the expression to evaluate: transpose(mapped_gradient(nodes)) * transpose(jacobian_adjoint(nodes)) * jacobian_adjoint(nodes) where nodes is a terminal that can be evaluated using a context. In the EvaluateExpr grammar, ContextEvaluator is a primitive transform that simply delegates evaluation to proto::eval, using the data argument as context. The context stores the matrices involved, so any references to matrices returned are safe to use. The Eigen product expression is now constructed using object transform MultExpr, but it turns out to exhibit the same problem with temporaries. Does anyone see a solution to this? At the moment I am thinking of having the EvaluateExpr transform build a fusion::cons list to store all the intermediate expressions, and refer to those by reference in subsequent expressions, but I'm not sure how to pull this off yet, or if there is a simpler way. I have also asked the question on the Eigen forum, together with a small sample that exposes the temporary variable problem without using proto: http://forum.kde.org/viewtopic.php?f=74&t=91119 Kind regards, -- Bart _______________________________________________ proto mailing list proto@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/proto