c++ - Legal to overwrite std::string's null terminator?

Question

In C++11, we know that std::string is guaranteed to be both contiguous and null-terminated (or more pedantically, terminated by charT(), which in the case of char is the null character 0).

There is this C API I need to use that fills in a string by pointer. It writes the whole string + null terminator. In C++03, I was always forced to use a vector<char>, because I couldn't assume that string was contiguous or null-terminated. But in C++11 (assuming a properly conforming basic_string class, which is still iffy in some standard libraries), I can.

Or can I? When I do this:

std::string str(length);

The string will allocate length+1 bytes, with the last filled in by the null-terminator. That's good. But when I pass this off to the C API, it's going to write length+1 characters. It's going to overwrite the null-terminator.

Admittedly, it's going to overwrite the null-terminator with a null character. Odds are good that this will work (indeed, I can't imagine how it couldn't work).

But I don't care about what "works". I want to know, according to the spec, whether it's OK to overwrite the null-terminator with a null character?

question from:https://stackoverflow.com/questions/12740403/legal-to-overwrite-stdstrings-null-terminator

Answer 1

LWG 2475 made this valid by editing the specification of operator[](size()) (inserted text in bold):

Otherwise, returns a reference to an object of type charT with value charT(), where modifying the object to any value other than charT() leads to undefined behavior.