The operator[] method of std::vector does not do any bounds checking on the index. It is safer to use the at() method, which does do bounds checking.

Use the at() method, rather than operator[].

Some uses of operator[] are safe because they are protected by a bounds check. The query recognises the following safe coding patterns:

if (!x.empty()) { ...x[0]... }
if (x.length()) { ...x[0]... }
if (x.size() > 2) { ...x[2]... }
if (x.size() == 2) { ...x[1]... }
if (x.size() != 0) { ...x[0]... }
if (i < x.size()) { ... x[i] ... }
if (!x.empty()) { ... x[x.size() - 1] ... }

#1706 was caused by a lack of bounds-checking on this array access. The bug was fixed calling the at() method instead.