stringstream::str copy lifetime
c++, memory, stringstream
Solution
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tmp.~string (); // after that line the pointer `str` is no longer valid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.
Problem
Something a lot of C++ programmers miss (read: me) when first using `stringstream`s 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: - 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? - 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.