Difference between size_t and std::size_t

Question

What are the differences between size_t and std::size_t in terms of where they are declared, when they should be used and any other differentiating

Answer 1

std::size_t is in fact stddef.h's size_t.

cstddef gives the following:

#include 
namespace std 
{
  using ::ptrdiff_t;
  using ::size_t;
}

...effectively bringing the previous definition into the std namespace.