Why data() appends a NULL character at the end of string?

As we know member function data() defined by string won't append a NULL character at the end of string. But I found it still appends a NULL character at the end of string. Please check the following code,

Code:

int main() 
{
	string s = "Mike";

	char* q = const_cast<char*>(s.data());

	while(*q)
	{
		q++;
	}

	return 0;
}

Debug this piece of code, you will find out there is a NULL character at the end of string. Why?