NAG Library Manual, Mark 27.2
```/* C05QC_T1W_F C++ Header Example Program.
*
* Copyright 2021 Numerical Algorithms Group.
* Mark 27.2, 2021.
*/

#include <dco.hpp>
#include <iostream>
#include <math.h>
#include <nag.h>
#include <nagx02.h>
#include <nagx04.h>
#include <stdio.h>
#include <string>
using namespace std;

extern "C"
{
static void NAG_CALL fcn(void *&                 ad_handle,
const Integer &         n,
Integer                 iuser[],
Integer &               iflag);
}

int main(void)
{
// Scalars
int           exit_status = 0;
const Integer maxfev = 2000, ml = 1, mode = 2, mu = 1, n = 7, nprint = 0;

cout << "C05QC_T1W_F C++ Header Example Program Results\n\n";

// problem parameters and starting value

ruser[0] = -1.0;
ruser[1] = 3.0;
ruser[2] = -2.0;
ruser[3] = -2.0;
ruser[4] = -1.0;

// Create AD configuration data object
Integer ifail     = 0;

nagad_t1w_w_rtype diag[n], fjac[n * n], epsfcn, factor, fvec[n], qtf[n],
r[n * (n + 1) / 2], xtol;
double  dr[5 * n];
Integer iuser[1], nfev;

xtol   = sqrt(X02AJC);
epsfcn = 0.;
factor = 100.;

for (int i = 0; i < 5; ++i)
{
for (int j = 0; j < n; ++j)
{
x[j]    = -1.0;
diag[j] = 1.;
}

dco::derivative(ruser[i]) = 0.5;

ifail = 0;
mode, diag, factor, nprint, nfev, fjac, r, qtf, -1, iuser,
-1, ruser, ifail);

for (int j = 0; j < n; ++j)
{
dr[i * n + j] = 2. * dco::derivative(x[j]);
}

dco::derivative(ruser[i]) = 0.;
}

cout.setf(ios::scientific, ios::floatfield);
cout.precision(4);
cout << "           Solution:\n";
for (int i = 0; i < n; ++i)
{
cout.width(10);
cout << i + 1;
cout.width(20);
cout << dco::value(x[i]) << endl;
}

cout << "\n Derivatives calculated: First order tangents\n";
cout << " Computational mode    : algorithmic\n";
cout << "\n Derivatives are of solution w.r.t function params\n\n";

NagError fail;
INIT_FAIL(fail);
x04cac(Nag_ColMajor, Nag_GeneralMatrix, Nag_NonUnitDiag, n, 5, dr, n,
"    dx/druser", 0, &fail);

// Remove computational data object

return exit_status;
}

static void NAG_CALL fcn(void *&                 ad_handle,
const Integer &         n,
Integer                 iuser[],
Integer &               iflag)
{
if (iflag != 0)
{
for (int i = 0; i < n; ++i)
{
fvec[i] = (ruser[1] + ruser[2] * x[i]) * x[i] - ruser[4];
}
for (int i = 1; i < n; ++i)
{
fvec[i] = fvec[i] + ruser[0] * x[i - 1];
}
for (int i = 0; i < n - 1; ++i)
{
fvec[i] = fvec[i] + ruser[3] * x[i + 1];
}
}
iflag = 0;
return;
}
```