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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

• For a function that normally returns a value, as opposed to a mere success/failure status indication, a popular technique is to return an “out-of-band” value to indicate an error, also called called a sentinel value.
• This value must not be one of the normal, non-error, return values.
• Examples:
  • A locate() function that returns a pointer to some datum might return nullptr to indicate a failure.
  • An integer square root function could return −1 to indicate that you attempted to take the square root of a negative number.
  • A floating-point square root function could return NAN (Not-A-Number) to indicate that you attempted to take the square root of a negative number.

Out-of-band value

Loud a='a';

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

int main() {
    Loud c('c');
    double result = invert(0);
    if (result == INFINITY)
        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?

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.
  • …

Unacceptable out-of-band values

Remember that nullptr and NULL aren’t magic (and null doesn’t exist). You can only return a null pointer if you’re returning a pointer. It won’t work for returning a number, object, or reference.

double foo() {
    return nullptr;
}

int main() {
    cout << foo();
}

c.cc:2: error: cannot convert ‘std::nullptr_t’ to ‘double’ in return

For a floating-point value, NAN (not a number) is often a good sentinel value.

Unacceptable out-of-band values

For an int, nullptr isn’t possible, and NAN is only for floating-point values:

int bar() {
    return nullptr;
}

int main() {
    cout << bar();
}

c.cc:2: error: cannot convert ‘std::nullptr_t’ to ‘int’ in return

A good sentinel depends on the possible return values. -1 is fine for an integer square root routine, and a billion works for historical years.

Unacceptable out-of-band values

This compiles, because string has a ctor that takes a const char *, and nullptr converts to that. However, it fails at runtime:

string baz() {
    return nullptr;
}

int main() {
    cout << baz();
}

terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_M_construct null not valid
SIGABRT: Aborted

For a string, "" is often a good sentinel value. Otherwise pick something that can’t occur in the data, e.g., "//**NOT POSSIBLE**//".

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.
• abort() sent the SIGABRT (abnormal termation) signal, similar to SIGINT (keyboard interrupt).

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.
• assert() calls abort(), so similar results.

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

• Division by zero is undefined behavior. On this computer, it generates the SIGFPE (floating-point exception) signal.
• 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!