First of all, it's important to understand that attempting to make use of a dangling pointer is not guaranteed to fail in any obvious way. It can appear to "work" just as often as it can crash spectacularly.
Secondly, yes, that code is invalid and you shouldn't do it. The stringstream's lifetime is not important because std::stringstream::str() returns by value (i.e. a copy of the internal buffer), but then you're still subject to that string going out of scope before you can make use of its C-string pointer.
