vector<string> v;

// Fill up v with some data

vector<string>::iterator p = v.begin();

p += 5;  // Now p refers to the 5th element

p[5];    // Now p refers to the 10th element

p -= 10; // Back to the beginning...

p[5] will return the 10th element (which is not used), but p itself will still refer to the 5th element after doing this. Hence the subsequent statement "p -= 10" is a bug: p will point before v.begin(). I verified this by running a simple test program compiled with g++ 4.2.1 through valgrind. It did report an access violation when dereferencing p after the last statement.