c++ - Behaviour of attempting to access a character at index -1 for std::string? -

is there guaranteed behaviour attempting access character @ -1 in std::string?

eg if do:

for (int = 0; < str.size(); i++) {   if (str[i-1] == 'a' && str[i+1] == 'c') {     //etc   } } 

then on first iteration str[-1], standard happen? know says in c++11 str[str.size()] return null character, sites cppreference , cplusplus don't other out of bounds accesses.

(if run code above, nothing bad happens, want make sure standard)

is there guaranteed behaviour access character @ -1 in std::string?

formally, depends. in practice, undefined behaviour, there no guarantees.

the parameter of operator[] unsigned, -1 maximum value type can hold (std::numeric_limits<std::string::size_type>::max()). since maximum value std::string::size() can hold, defined only in (extremely improbable) case string has largest size allowed, in case reference null terminator returned.

from c++11 standard, , n3936 draft of c++14 standard,

21.4.5 basic_string element access [string.access]

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 reference object of type chart value chart(), modifying object leads undefined behavior. ....