Re: Exception class with shift operator
eca wrote:
On Jan 8, 3:22 pm, Ulrich Eckhardt <dooms...@knuut.de> wrote:
virtual const char* what() const throw()
{ return my_ss.str().c_str(); }
Have you tried this? IIRC, stringstream::str() returns by value,
which would obviously be fatal!
You are definitely right, this is really dangerous.
It worked in my simple tests because I used to store the returned
const char * content in a std::string:
std::string s = e.what();
std::cout << s; // this should work
No, that's just coincidence. The string returned by the call to str() is
destroyed after the return statement, but before the initialisation of the
string 's' in the calling function. Note that the memory it points to it
typically still mapped into the process and its content doesn't change
either, so it /seems/ that it works!
However now it is clear to me that the following code won't work:
const char *s = e.what();
// s is now invalid!
std::cout << s; // this would crash
Try it! I wouldn't be surprised if it seemed to work, too!
[...] Further, you are throwing the stringstream along with the
exception, though it isn't needed at that time any more. Lastly,
if my guess about stringstream::str() is true, you will have to store
the string in parallel, which sounds like even more waste to me.
A possible answer to both items that seems really interesting to me
has been suggested by Alberto Ganesh Barbati in a later post,
consisting in using a shared_ptr to stringstream as a member instead
of the stringstream itself, and a (mutable) string. This seems to me
a very clever compromise.
I'm not sure. The point is that this requires another dynamic allocation
that should in fact be unnecessary, but it should generate correct code
still. Note that in general it wouldn't be correct, because it actually
causes the stringstream to be shared between possibly multiple objects,
but exceptions are typically not stored or copied, so it probably won't
matter. In order to get copying correct, you could allocate the
stringstream on demand and only copy the resulting string when copying.
Uli
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