Question

In C++11 basic_string::c_str is defined to be exactly the same as basic_string::data, which is in turn defined to be exactly the same as *(begin() + n) and *(&*begin() + n) (when 0 <= n < size()).

I cannot find anything that requires the string to always have a null character at its end.

Does this mean that c_str() is no longer guaranteed to produce a null-terminated string?

Answer 1

Strings are now required to use null-terminated buffers internally. Look at the definition of operator[] (21.4.5):

Requires: pos <= size().

Returns: *(begin() + pos) if pos <
size(), otherwise a reference to an object of type T with value
charT(); the referenced value shall not be modified.

Looking back at c_str (21.4.7.1/1), we see that it is defined in terms of operator[]:

Returns: A pointer p such that p + i == &operator[](i) for each i in [0,size()].

And both c_str and data are required to be O(1), so the implementation is effectively forced to use null-terminated buffers.

Additionally, as David Rodríguez – dribeas points out in the comments, the return value requirement also means that you can use &operator[](0) as a synonym for c_str(), so the terminating null character must lie in the same buffer (since *(p + size()) must be equal to charT()); this also means that even if the terminator is initialised lazily, it’s not possible to observe the buffer in the intermediate state.