A comparison of using brackets or "at" to access elements of an STL vector

stl_bounds_at.cpp

#include<iostream>
#include<vector>

/** There are two ways to access the ith element of an STL vector, the usual
 *  v[i] syntax or using v.at(i).
 *  
 *  The former doesn't check array boundaries, so something like v[v.size()+1]
 *  works and gives you whatever happens to be sitting in that memory location.
 *
 *  v.at() has the same purpose, but actually throws an exception
 *  (std::out_of_range) when you're outside array boundaries. This may be
 *  helpful for debugging buffer overflows, a common source of C++ headaches.
 *
 *  Both v[i] and v.at(i) are of constant complexity.
 */

int main()
{
  std::vector<int> a(5);

  // Accessing the (a.size()+1)th element with brackets returns junk
  std::cout << a[6] << std::endl;

  // Accessing the (a.size()+1)th element with at throws an exception
  std::cout << a.at(6) << std::endl;

  return 0;
}