The std::string accessors (back, front, at, and operator[]) have const and non-const overloads, as below:
char& x();
const char& x() const;
Why does the second version return a const reference, as opposed to simply returning the char by value (as a copy)?
According to the rules of thumb on how to pass objects around, shouldn't small objects be passed by value when there's no need to modify the original?
Because the caller might want a reference to the char, that reflects any changes made to it through non-const avenues.
std::string str = "hello";
char const& front_ref = static_cast<std::string const&>(str).front();
str[0] = 'x';
std::cout << front_ref; // prints x
Because context.
You're dealing with a container and functions with names that imply specific positional data.
std::string s = "hello world?";
const auto& s1 = s.back();
s.back() = '!';
Returning a reference here provides flexibility, it's also consistent with the non-const variants and other stl containers. After all these functions are actually members of std::basic_string<char>.
Remember that "rule of thumb" is a guideline not a rule.
std::vectorfor instance).