NAG Library Manual, Mark 27.2
/* D01TC_P0W_F C++ Header Example Program.
*
* Copyright 2021 Numerical Algorithms Group.
* Mark 27.2, 2021.
*/

#include <iostream>
#include <math.h>
#include <nag.h>
#include <stdio.h>
using namespace std;

extern "C"
{
static void NAG_CALL fun(void *&        ad_handle,
const Integer &ndim,
const double   x[],
double &       ret,
Integer        iuser[],
double         ruser[]);
}

int main(void)
{
// Scalars
int     exit_status = 0;
Integer ndim        = 4;

cout << "D01TC_P0W_F C++ Header Example Program Results\n\n";

// Allocate memory
Integer *nptvec = 0;
double * abscis = 0, *weight = 0;
Integer  lwa = 0;

nptvec = new Integer[ndim];
for (int i = 0; i < ndim; i++)
{
nptvec[i] = 4;
lwa       = lwa + nptvec[i];
}
abscis = new double[lwa];
weight = new double[lwa];

Integer ifail     = 0;

// Evaluate primal weights and abscisae in each Dimension
int j = 0;
for (int i = 0; i < ndim; i++)
{

Integer ifail = 0, quadtype = 0;
double  a, b, c, d;
switch (i)
{
case 0:
a        = 1.0;
b        = 2.0;
c        = 0.0;
d        = 0.0;
break;
case 1:
a        = 0.0;
b        = 2.0;
c        = 0.0;
d        = 0.0;
break;
case 2:
a        = 0.0;
b        = 0.5;
c        = 0.0;
d        = 0.0;
break;
case 3:
a        = 1.0;
b        = 2.0;
c        = 0.0;
d        = 2.0;
break;
}
&abscis[j], ifail);
j = j + nptvec[i];
}

// Call the passive routine
ifail = 0;
double  ans;
double  ruser[1];
Integer iuser[1];
iuser, -1, ruser, ifail);

cout.setf(ios::right);
cout.precision(4);
cout << "\n Solution, x = ";
cout.width(12);
cout << ans << endl;

delete[] nptvec;
delete[] abscis;
delete[] weight;
return exit_status;
}

static void NAG_CALL fun(void *&        ad_handle,
const Integer &ndim,
const double   x[],
double &       ret,
Integer        iuser[],
double         ruser[])
{
double p1 = 6.0, p2 = 8.0;
ret = (pow(x[0] * x[1] * x[2], p1) / pow(x[3] + 2.0, p2)) *
exp(-2.0 * x[1] - 0.5 * x[2] * x[2]);
return;
}