How can you set the address of elements in an array to point to specific elements in another array?

Question

Let's say we have an array with 10 elements and we want to extract a 6-element sub-array from it that contains the first three items and the last three items. I don't want the extracted sub-array to be an independent array but I want its items to point to the same locations in memory as the corresponding items in the original array. How can I do that?

Answer 1

You can use an array of pointers to point to the elements in the other one:

int a[10] = { 5, 3, 7, 4, 9, 2, 0, 3, 3, 6 };
int* b[6];
b[0] = &a[0];
b[1] = &a[1];

and so on. You have to be sure that a lives t least as long as b.

A more idiomatic solution might be to use a range type to hold a pointer to the first and one past the last elements of interest. The range would be iterable in the same way as a standard library container. See boost.range for example, or you can roll out a minimal and simple version using pointers as iterators. This would satisfy your use-case. For example:

struct range
{
  typedef const int* const_iterator;
  const_iterator begin() const {return begin_;}
  const_iterator end() const { return end_; }
  range(const int* begin, const int* end) : begin_(begin), end_(end) {}
  std::size_t size() const {return end_ - begin_ };
private:
  int* begin_;
  int* end_;
};

Then

int a[10] = { 5, 3, 7, 4, 9, 2, 0, 3, 3, 6 };
range b(&a[0], &a[3);
for (range::const_iterator i = b.begin(); i != b.end(); ++i)
  std::cout << *i << " ";
std::cout << std::endl;