复数转换

Question

我正在尝试用C ++编写一些代码，而且我在使用复数时遇到了困难。这是代码：

#include <iostream>
#include <iomanip>
#include <complex>
#include <cmath>

using namespace std;
using namespace std::complex_literals;

double Diff (double pos,double kvector,double amp){
std::complex <double> E_complex_field=exp(-(1i)*kvector*pos);
std::complex <double> E_complexplus_field=exp(-(1i)*kvector*(pos+amp));
std::complex <double> Derivada=(E_complexplus_field-E_complex_field)/(double)amp;
return Derivada;
}

int main()
{   
    cout<<fixed <<setprecision(22);
    int N=100;
    double PI,lambda,k_vector,Epso,Eo,alfa_complex,x,paso,Fx,d;
    PI=acos(-1);    lambda=500*pow(10,-9);  k_vector=(1/(double)lambda);    Epso=1;     Eo=1;   paso=pow(10,-8);    d=pow(10,-4);
    complex<double> phase =(1i *k_vector*x);    complex<double> alfa=1.+(1i)*pow(10,-21);
    complex<double> E_field=Eo*exp(phase);      complex<double> Ec_field=conj(E_field);
                                alfa_complex=imag(alfa);
    x=0;
    for(int i=0;i<N;i++){
        Fx= (0.5)*real(Epso*pow(Eo,2)*alfa*E_field*Diff(x,k_vector,paso));
        x=x+d;
    }

}

代码中的错误如下：

Der_num.cpp:13:8: error: cannot convert ‘std::complex<double>’ to ‘double’ in return.

我如何获得Derivada的价值？