The only type used in the demos is vector<pair<int,int>>. Vectors are guaranteed to be contiguous in memory. I'm pretty sure pairs are too, but that's easy to test:

    #include "stdio.h"
    #include <utility>
    printf("%zu\n", sizeof(std::pair<int, int>));

    8

So it's the obvious layout you would see in C with an array of struct { int x; int y }.

The cling interpreter is good for this kind of thing.

20.2.2 Pairs &para; 1 (ISO/IEC 14882:2003):

  The library provides a template for heterogeneous pairs of values. The library also provides a matching
  function template to simplify their construction.

      template <class T1, class T2>
      struct pair {
        typedef T1 first_type;
        typedef T2 second_type;
      
        T1 first;
        T2 second;
        pair();
        pair(const T1& x, const T2& y);
        template<class U, class V> pair(const pair<U, V> &p);
      };
      
      pair();

tl;dr: It looks like the 03 standard requires it to be equivalent to the plant C version in terms of layout, although that could involve padding between first and second if e.g. T1 = short and T2 = int, just like in the equivalent C code.