std::basic_string::operator[]
From cppreference.com
                    
                                        
                    < cpp | string | basic string
                    
                                                            
                    |   reference       operator[]( size_type pos ); 
 | 
||
|   const_reference operator[]( size_type pos ) const; 
 | 
||
Returns a reference to the character at specified location pos. No bounds checking is performed.
If pos == size(),
- The const version returns a reference to the character with value CharT() (the null character). (until C++11)
 -  Both versions return a reference to the character with value CharT() (the null character). For the non-const version of 
operator[], modifications to this character result in undefined behavior. (since C++11) 
Contents | 
[edit] Parameters
| pos | - | position of the character to return | 
[edit] Return value
reference to the requested character
[edit] Complexity
Constant
[edit] See also
|     access specified character with bounds checking  (public member function)  | 
|