Convert Op to actual values

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Horus Registered Member Posts 296 Karma 0 OS	Convert Op to actual values Mon Nov 21, 2016 4:35 pm Hello, I have this piece of code Code: Select all `#include <Eigen/Dense> int main(int argc, char argv[]) { Eigen::Vector3d x(1, 2, 3); double pdx = (x.array() + 1).data(); }` It does not compile, because: % clang++ -std=c++11 test.cpp test.cpp:6:33: error: no member named 'data' in 'Eigen::CwiseUnaryOp<Eigen::internal::scalar_add_op<double>, const Eigen::ArrayWrapper<Eigen::Matrix<double, 3, 1, 0, 3, 1> > >' double* pdx = (x.array() + 1).data(); ~~~~~~~~~~~~~~~ ^ 1 error generated. What is the best way to work around that? Can I force evaluation of the CwiseUnaryOp to something which has actual values and can return the underlying data? Thanks!
ggael Moderator Posts 3447 Karma 19 OS	Re: Convert Op to actual values Mon Nov 21, 2016 9:51 pm You could do ptr = (x.array() + 1).eval().data(); but that would be useless as ptr would then immediately reference a dead object. So you need to name the temporary: Vector3d tmp = x.array() + 1; ptr = tmp.data();
Horus Registered Member Posts 296 Karma 0 OS	Re: Convert Op to actual values Tue Nov 22, 2016 8:19 am It also works using a static cast: Code: Select all `double* pdx = static_cast<Eigen::VectorXd>(x.array() + 1).data();`
ggael Moderator Posts 3447 Karma 19 OS	Re: Convert Op to actual values Tue Nov 22, 2016 9:26 am Please don't do that !! because right after this statement pdx reference a freed memory. It might appear to work if no memory operation wrote in this buffer, but in general this won't work, and running your program within a memory debugger (e.g. valgrind) will show you the issue.
Horus Registered Member Posts 296 Karma 0 OS	Re: Convert Op to actual values Tue Nov 22, 2016 10:07 am Ok, I see you're right. It would be nice to have a method that force an expression template to be evaluated, so that you can call methods like data() on it.
ggael Moderator Posts 3447 Karma 19 OS	Re: Convert Op to actual values Tue Nov 22, 2016 10:50 am That's the .eval() method, but you cannot use it in your context.