stringstream::str copy lifetime

Question

Something a lot of C++ programmers miss (read: me) when first using stringstreams is the fact that the copy returned by stringstream::str() is a temporary, which lasts until the end of the expression it's used in. However, I don't understand:

1. How this is done. Looking in the sstream header for libstdc++, I only see a copy being made and returned. How is the lifetime restricted?
2. Why this is the desired behavior, especially since it is such a common gotcha. If a copy is being made anyway, why can't I take ownership of it?

Note that this is not a duplicate of stringstream, string, and char* conversion confusion. That goes over explaining the behavior and workarounds; I'm looking for mechanics and rationale.

Answer 1

This is not a problem of stringstream. It is the problem of c_str function - it returns pointer to a char* representation of std::string, but it lives only during lifetime of an original string. When you call char *str = ss.str().c_str() the following actually happens:

string tmp = ss.str();
char *str = tmp.c_str();
tmp.~string (); // after that line the pointer `str` is no longer valid

c_str is a dangerous function provided only for compatibility and speed purposes and you should avoid using it.