Question

Is there a way to figure out where in an array a pointer is?

Lets say we have done this:

int nNums[10] = {'11','51','23', ... };   // Some random sequence
int* pInt = &nNums[4];                     // Some index in the sequence.

...

pInt++;      // Assuming we have lost track of the index by this stage.

...

Is there a way to determine what element index in the array pInt is ‘pointing’ to without walking the array again?

Answer 1

Yes:

ptrdiff_t index = pInt - nNums;

When pointers to elements of an array are subtracted, it is the same as subtracting the subscripts.

The type ptrdiff_t is defined in <stddef.h> (in C++ it should be std::ptrdiff_t and <cstddef> should be used).