[eigen] [Sparse] element acces in a cwise unary op on an Eigen::Sparse

[<bernard.hugueney@xxxxxxxxxx>]

Hi,
I'd like to have access on elements of a cwise unary op, but the 
compiler
cannot find a coeff member function.
the folowing code :
#include <iostream>
#include <Eigen/Core>
#include <Eigen/Sparse>
#include <complex>

template<typename Derived>
void useCoeff( Eigen::EigenBase<Derived> const a){
    std::cout<<a.coeff(0,0)<<std::endl;
}

int main(int argc, char* argv[])
{
    Eigen::SparseMatrix<double> a(2,2);
    std::cout<<a.coeff(0,0)<<std::endl;
    useCoeff(a.cast<std::complex<double> >());
    return 0;
}

gives the error :
tst-coeff.cxx: In function ‘void useCoeff(Eigen::EigenBase<Derived>) 
[with Derived = 
Eigen::CwiseUnaryOp<Eigen::internal::scalar_cast_op<double, 
std::complex<double> >, Eigen::SparseMatrix<double> >]’:
tst-coeff.cxx:15:45:   instantiated from here
tst-coeff.cxx:8:5: error: ‘const class 
Eigen::CwiseUnaryOp<Eigen::internal::scalar_cast_op<double, 
std::complex<double> >, Eigen::SparseMatrix<double> >’ has no member 
named ‘coeff’

How am I to access elements of my cast expression ?

Any help greatly appreciated :)

Best regards,

Bernard