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CS253 Old-fashioned Error Handling

The Old Days

• Before try/recover, throw/catch, and the like became popular, many other methods of error handling were used.
• Many are still used.
• We’ll look at several old techniques, paying attention to whether objects get properly destroyed.
• These examples use my Loud class, which announces constructors & destructors

Out-of-band value

• One popular technique is to return an “out-of-band” value to indicate an error.
• This value must not be one of the normal, non-error, return values.
• For example, a locate() function that returns a pointer to some datum might return nullptr to indicate a failure.

Out-of-band value

Loud a='a';

double invert(double num) {
    Loud b('b');
    if (num == 0)
        return NAN;
    return 1/num;
}

int main() {
    Loud c('c');
    double result = invert(0);
    if (isnan(result))
        return 1;
    cout << result << '\n';
}

Loud::Loud() [c='a']
Loud::Loud() [c='c']
Loud::Loud() [c='b']
Loud::~Loud() [c='b']
Loud::~Loud() [c='c']
Loud::~Loud() [c='a']

• Did all the objects get properly cleaned up?
• NAN is the floating-point Not-A-Number value. isnan() detects it. Maybe return INFINITY?

That works, but …

• That works. However:
  • You need an out-of-band value. This isn’t always possible.
  • The caller of foo() needs to check for errors, and return early.
  • So does his caller.
  • So does his caller.
  • …

exit

Loud a='a';

double invert(double num) {
    Loud b('b');
    if (num == 0)
        exit(1);
    return 1/num;
}

int main() {
    Loud c('c');
    cout << invert(0) << '\n';
}

Loud::Loud() [c='a']
Loud::Loud() [c='c']
Loud::Loud() [c='b']
Loud::~Loud() [c='a']

• That was brutal.
• Dtor was called for global object, but not for local objects.

abort

Loud a='a';

double invert(double num) {
    Loud b('b');
    if (num == 0)
        abort();
    return 1/num;
}

int main() {
    Loud c('c');
    cout << invert(0) << '\n';
}

Loud::Loud() [c='a']
Loud::Loud() [c='c']
Loud::Loud() [c='b']
SIGABRT: Aborted

• Worse: no dtors called at all.
• Hey, it sent a signal! Like SIGINT (keyboard interrupt), but it’s SIGABRT (abnormal termation).

assert

Loud a='a';

double invert(double num) {
    Loud b('b');
    assert(num);
    return 1/num;
}

int main() {
    Loud c('c');
    cout << invert(0) << '\n';
}

Loud::Loud() [c='a']
Loud::Loud() [c='c']
Loud::Loud() [c='b']
a.out: c.cc:5: double invert(double): Assertion `num' failed.
SIGABRT: Aborted

I suppose that we have low expectations for a failed assertion.

Division by Zero

Loud a='a';
static int z = 0;

double invert(double num) {
    Loud b('b');
    if (num == 0)
        z = 1/z;
    return 1/num;
}

int main() {
    Loud c('c');
    cout << invert(0) << '\n';
}

Loud::Loud() [c='a']
Loud::Loud() [c='c']
Loud::Loud() [c='b']
SIGFPE: Floating point exception

• Another new signal: SIGFPE (floating-point exception, even though it was an integer operation).
• Since it was a signal termination, like abort(), no dtors were called.

The Horror That Is Setjmp/Longjmp

Loud a='a';
jmp_buf env;

double invert(double num) {
    Loud b('b');
    if (num == 0)
        longjmp(env, 456);
    return 1/num;
}

int main() {
    Loud c('c');
    if (int val = setjmp(env)) {
        cout << "error=" << val << "!\n";
        return 1;
    }
    cout << invert(0) << '\n';
}

Loud::Loud() [c='a']
Loud::Loud() [c='c']
Loud::Loud() [c='b']
error=456!
Loud::~Loud() [c='c']
Loud::~Loud() [c='a']

• Lord, take me now—I done seen everything!