In C++11 and beyond does std::string::operator[] do bounds checking?

Question

I have seen many times that std::string::operator[] does not do any bounds checking. Even What is the difference between string::at and string::operator[]?, as

Answer 1

The wording is slightly confusing, but if you study it in detail you'll find that it's actually very precise.

It says this:

• The precondition is that the argument to [] is either = n or it's < n.
• Assuming that precondition is satisfied:
  • If it's < n then you get the character you asked for.
  • "Otherwise" (i.e. if it's n) then you get charT() (i.e. the null character).

But no rule is defined for when you break the precondition, and the check for = n can be satisfied implicitly (but isn't explicitly mandated to be) by actually storing a charT() at position n.

So implementations don't need to perform any bounds checking… and the common ones won't.