Re: C++ exception error messages

From:
"Bo Persson" <bop@gmb.dk>
Newsgroups:
comp.lang.c++.moderated
Date:
Tue, 28 Sep 2010 03:18:19 CST
Message-ID:
<8gc3jkF2t3U1@mid.individual.net>
Gert-Jan de Vos wrote:

On Sep 26, 1:24 am, "Bo Persson" <b...@gmb.dk> wrote:

tf wrote:

An awkward situation with exception handling occurred to me this
morning.
I frequently see (and I'll admit to writing it myself sometimes)
error detection code which does something along these lines:

 if(badness) {
    std::ostringstream err;
    err << "Badness " << errcode << " has occurred while frobbing"
        << " nitzes. blah blah blah";
    throw SomeException(err.str().c_str());
 }

or just using snprintf:

 if(badness) {
    char errc[1024];
    snprintf(errc, 1024, "badness %d occurred ...", errcode);
    throw SomeException(errc);
 }

Yet, of course, by the time we get to a:

 catch(const SomeException& se) { ... }

clause somewhere up the call stack, the above stack has unwound.
In the first case, the std::string from err.str() has had its
destructor run, so our saved pointer is bogus; in the latter,
there's no destructor for char* of course but the 'errc' array is
"gone".


Why don't you just create a new exception BadnessOccurred, dervied
from SomeException, and carrying the errcode? Its what() member can
produce the message, if and when it is called.

if (badness)
   throw BadnessOccurred(errcode);


The problem is the signature of std::exception::what():

virtual const char* what() const throw();

A derived class overriding what() has to:
1) return a pointer to the message string
2) without modifying the exception's state
3) and without throwing anything

I don't know how to dynamically construct a message given these
constraints:

1) suggests using a member string or char array as a formatting
  buffer.
2) suggests a mutable member variable formatting buffer
3) makes it impossible to dynamically allocate this buffer

What am I missing?


That as the class designer you are producing the error message
yourself. That might make it possible to allocate a big enough buffer,
or have the message short enough to fit in a std::string using the
short string optimization. Possibly as a mutable member.

The original code using sprintf also makes the assumptions that 1)
1024 bytes is always enough, and 2) the stack is big enough for that.
How do we know that?

Bo Persson

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