len = vsnprintf((char*)result.c_str(), tlen, format, list);
const CharT* c_str() const;
Returns a pointer to a null-terminated character array with data equivalent to those stored in the string.
The pointer is such that the range [c_str(); c_str() + size()] is valid and the values in it correspond to the
values stored in the string with an additional null character after the last position.
The pointer obtained from c_str() may be invalidated by:
o Passing a non-const reference to the string to any standard library function, or
o Calling non-const member functions on the string, excluding operator, at(), front(), back(), begin(),
rbegin(), end() and rend().
Writing to the character array accessed through c_str() is undefined behavior.