eigen_structured_bindings.cpp

#include <iostream>
#include <Eigen/Dense>
#include <concepts>

namespace Eigen {
    template <int D, typename Derived>
        inline auto& get(Eigen::DenseCoeffsBase<Derived, WriteAccessors>& p) { return p.coeffRef(D); }

    template <int D, typename Derived>
        inline const auto get(const Eigen::DenseCoeffsBase<Derived,ReadOnlyAccessors>& p) { return p.coeff(D); }

    template <typename Derived>
        concept StaticSizePlainObjectBase = std::derived_from<Derived,Eigen::PlainObjectBase<Derived>> &&
        (Derived::RowsAtCompileTime >0 && Derived::ColsAtCompileTime > 0);

    template <typename Derived>
        concept StaticSizeEigenBase = std::derived_from<Derived,Eigen::EigenBase<Derived>> &&
        (Derived::RowsAtCompileTime >0 && Derived::ColsAtCompileTime > 0);
}
namespace std {

    template <typename Derived> requires Eigen::StaticSizeEigenBase<Derived>
        struct tuple_size<Derived> {
            constexpr static int value = Derived::RowsAtCompileTime * Derived::ColsAtCompileTime;
        };
    template <size_t D, typename Derived> requires Eigen::StaticSizeEigenBase<Derived> &&
        (D < Derived::RowsAtCompileTime * Derived::ColsAtCompileTime)
        struct tuple_element<D,Derived> {
            using type = typename Derived::Scalar;
        };
}


int main(int argc, char * argv[]) {
    // We can use a structured binding to populate an eigen vector!
    Eigen::Vector3d p;
    auto& [x,y,z] = p;
    x = 2;
    y = 3;
    z = 4;

    // works for Array, const, int, and size 2!
    const Eigen::Array2i q = p.head<2>().cast<int>().array();
    auto& [u,v] = q;

    // even works on expression templates!
    auto [u2,v2] = 2*q;

    std::cout << p.transpose() << std::endl;
    std::cout << u << "," << v << std::endl;
    std::cout << u2 << "," << v2 << std::endl;
    
    // Can use to read blocks, but not write... yet! 
    Eigen::Matrix3f A;
    A.setIdentity();
    auto [m,n,o] = A.col(2);
    std::cout << m << "," << n << "," << o << std::endl;
    
}