ptrdiff.cpp

// To compile: clang++ -std=c++11 ptrdiff.cpp -o ptrdiff
// To run: ./ptrdiff

#include <vector>
#include <numeric>
#include <cassert>
#include <iostream>

int main() {

  // Create a vector where v[x] == x
  std::vector<int> v(100);
  std::iota(v.begin(), v.end(), 0);

  // &v[0] - first == 0 --> first = &v[0]
  int *first = &v[0];

  for(int &e : v) {
    // Vectors are stored in contiguous memory, like a C array.
    assert(&e - first == e);
  }

  std::cout << "All good." << std::endl;
  return 0;
}

// NOTE: Pointers are assumed to be within &v[0] to &v[v.size() - 1],
// so if this assumption can be violated, you may need to check it.
// Unaligned pointers will be rounded down.
// For pointers, `(T *)a - (T *)b` is equivalent to `((char *)a - (char *)b) / sizeof(T)`,
// but the division can be computed efficiently as a shift when sizeof(T) is a power of 2.
// So it might be worth ensure that.