Is it safe to return std::wstring from a DLL?

Question

According to some older StackOverflow questions ( Unable to pass std::wstring across DLL , C++ DLL returning pointer to std::list<std::wstring> ) it's not considered safe for a C++ DLL to return a std::wstring because there's no guarantee the main program has the same definition of std::wstring and therefore it might cause a crash.

However, in http://en.cppreference.com/w/cpp/string/basic_string , it seems std::wstring can be used interchangeably with a WCHAR array now:

(Since C++11) The elements of a basic_string are stored contiguously, that is, for a basic_string s, &*(s.begin() + n) == &*s.begin() + n for any n in [0, s.size()), or, equivalently, a pointer to s[0] can be passed to functions that expect a pointer to the first element of a CharT[] array.

I've tested this by passing &s[0] to a WINAPI function that expected a WCHAR* buffer and it appeared to work (the std::wstring was correctly populated with the results of the WINAPI). So since std::wstring can apparently be treated like a WCHAR array now, I decided to revisit this question: can a std::wstring be safely returned from a DLL? Why or why not?

Answer 1

Nothing has changed with regards passing C++ objects across DLL boundaries. That is still not allowed for the same reason as before. The module on the other side of the boundary may have a different definition of the class.

The fact that &s[0] is a valid modifiable pointer to character array is not really relevant. Because a std::basic_string is a lot more than just an array of characters.

Remember that each implementation of std::basic_string can have different internal storage. Can have a different implementation for operator[]. Can be allocated off a different heap. And so on.

I think it is safe to assume that it will never be valid to pass C++ objects across general DLL boundaries. It is only viable if you guarantee that both sides of the boundary are linked against the same runtime instance.