c++stringc++11c++14arrayaccess

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


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?


Solution

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

    It says this:

    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.