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eigen3

Return an expression containing ArrayWrapper

I am writing C++/Python hybrid. The library that glue the two parts support Eigen matrix/array but not tensor.

Is it safe to do something like this?

#include <iostream>
#include <Eigen/Eigen>
using namespace Eigen;

template<typename D>
auto f(DenseBase<D>& x, const Index i) {
    // x2 is destroyed when the program leaves 
    // this function.
    ArrayWrapper<D> x2(x.derived());  
    return x2.middleCols(i * 3, 3);
}

int main() {
    ArrayXf a(3, 9);
    a = 0;
    f(x, 1) = 1;
    std::cout << x << "\n";
}

Or, is it better to do this?

template<typename D>
auto f(DenseBase<D>& x, const Index i) {
    return x.derived().array().middleCols(i * 3, 3);
}
Solution

  • Both version are the same, and both are safe. This is because proxy expressions like ArrayWrapper or Block as returned by middleCols() are nested by value, not by reference.