Will std::string always be null-terminated in C++11?

&str[0] is safe to use — so long as you do not assume it points to a null-terminated string.

Since C++11 the requirements include (section [string.accessors]):

• str.data() and str.c_str() point to a null-terminated string.
• &str[i] == str.data() + i , for 0 <= i <= str.size()
  • note that this implies the storage is contiguous.

However, there is no requirement that &str[0] + str.size() points to a null terminator.

A conforming implementation must place the null terminator contiguously in storage when data(), c_str() or operator[](str.size()) are called; but there is no requirement to place it in any other situation, such as calls to operator[] with other arguments.

To save you on reading the long chat discussion below: The objection was been raised that if c_str() were to write a null terminator, it would cause a data race under res.on.data.races#3 ; and I disagreed that it would be a data race .