How to call ** var in C/C++ 10.5 chapter numerical recipes - c++

Question

I have a problem with calling this function:
void powell(float p[], float **xi, int n,
float ftol, int *iter, float *fret,
float (*func)(float []))
I don't know which argument must be under **xi to run my code.
Whole function below:
void powell(float p[], float** xi, int n, float ftol, int* iter, float* fret, float (*func)(float[]))
{
void linmin(float p[], float xi[], int n, float* fret, float (*func)(float[]));
int i, ibig, j;
float del, fp, fptt, t, *pt, *ptt, *xit;
pt = vector(1, n);
ptt = vector(1, n);
xit = vector(1, n);
*fret = (*func)(p);
for (j = 1; j <= n; j++)
pt[j] = p[j];
for (*iter = 1;; ++(*iter)) {
fp = (*fret);
ibig = 0;
del = 0.0;
for (i = 1; i <= n; i++) {
for (j = 1; j <= n; j++)
xit[j] = xi[j][i];
fptt = (*fret);
linmin(p, xit, n, fret, func);
if (fptt - (*fret) > del) {
del = fptt - (*fret);
ibig = i;
}
}
if (2.0 * (fp - (*fret)) <= ftol * (fabs(fp) + fabs(*fret)) + TINY) {
free_vector(xit, 1, n);
free_vector(ptt, 1, n);
free_vector(pt, 1, n);
return;
}
if (*iter == ITMAX)
nrerror("powell exceeding maximum iterations.");
for (j = 1; j <= n; j++) {
ptt[j] = 2.0 * p[j] - pt[j];
xit[j] = p[j] - pt[j];
pt[j] = p[j];
}
fptt = (*func)(ptt);
if (fptt < fp) {
t = 2.0 * (fp - 2.0 * (*fret) + fptt) * SQR(fp - (*fret) - del) - del * SQR(fp - fptt);
if (t < 0.0) {
linmin(p, xit, n, fret, func);
for (j = 1; j <= n; j++) {
xi[j][ibig] = xi[j][n];
xi[j][n] = xit[j];
}
}
}
}
}
Thanks in advance.

Answer 1

A double pointer means that the function wants the address of a pointer.
void my_function(int **p_pointer)
{
*p_pointer = new int[42];
}
int main(void)
{
int * pointer = nullptr;
my_function(&pointer);
return 0;
}
In C++, the double pointer can be avoided by using reference:
void another_function(int *& pointer)
{
pointer = new int [256];
}
int main(void)
{
int p = nullptr;
another_function(p);
return 0;
}
One of the primary concerns with pointers is that they can point to anywhere, a defined location or not. Testing a pointer for validity is complex because it depends on the range (or ranges) that are valid for the current platform. With references, the reference is valid, by definition, so no validity checks need to be performed.