NAG Library Manual, Mark 27.2
```/* C05QC_P0W_F C++ Header Example Program.
*
* Copyright 2021 Numerical Algorithms Group.
* Mark 27.2, 2021.
*/

#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,
const double   x[],
double         fvec[],
Integer        iuser[],
double         ruser[],
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_P0W_F C++ Header Example Program Results\n\n";

// problem parameters and starting value
double ruser[5], x[7];

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

for (int i = 0; i < n; ++i)
{
x[i] = -1.0;
}

Integer ifail     = 0;
double  diag[n], fjac[n * n], epsfcn, factor, fvec[n], qtf[n],
r[n * (n + 1) / 2], xtol;
Integer iuser[1], nfev;

xtol   = sqrt(X02AJC);
epsfcn = 0.;
factor = 100.;
for (int i = 0; i < n; ++i)
{
diag[i] = 1.;
}

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

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 << x[i] << endl;
}

return exit_status;
}

static void NAG_CALL fcn(void *&        ad_handle,
const Integer &n,
const double   x[],
double         fvec[],
Integer        iuser[],
double         ruser[],
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;
}
```