Add the at() method to C++ vectors.

* There is no reason not to have this.

headers/cpp/stl_vector.h

@@ -31,6 +31,8 @@
 #ifndef __SGI_STL_INTERNAL_VECTOR_H
 #define __SGI_STL_INTERNAL_VECTOR_H
 
+#include <stdexcept>
+
 __STL_BEGIN_NAMESPACE 
 
 #if defined(__sgi) && !defined(__GNUC__) && (_MIPS_SIM != _MIPS_SIM_ABI32)
@@ -218,6 +220,18 @@ public:
   reference operator[](size_type __n) { return *(begin() + __n); }
   const_reference operator[](size_type __n) const { return *(begin() + __n); }
 
+  reference at(size_type __n) {
+	  if (__n >= size())
+		  throw std::out_of_range("vector::at() out_of_range");
+	  return *(begin() + __n);
+  }
+
+  const_reference at(size_type __n) const {
+	  if (__n >= size())
+		  throw std::out_of_range("vector::at() out_of_range");
+	  return *(begin() + __n);
+  }
+
   explicit vector(const allocator_type& __a = allocator_type())
     : _Base(__a) {}