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[]?, asked in 2013, the answers say that operator[] does not do any bounds checking.

My issue with this is if I look at the standard (in this case draft N3797) in [string.access] we have

const_reference operator[](size_type pos) const; reference operator[](size_type pos); 

1. Requires: pos <= size().
2. Returns: *(begin() + pos) if pos < size(). Otherwise, returns a reference to an object of type charT with value charT(), where modifying the object leads to undefined behavior.
3. Throws: Nothing.
4. Complexity: constant time.

This leads me to believe that operator[] has to do some sort of bounds checking to determine if it needs to return a element of the string or a default charT. Is this assumption correct and operator[] is now required to do bounds checking?

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.