Show Lecture.Old-fashionedErrorHandling as a slide show.

CS253 Old-fashioned Error Handling

#includes

To use NAN and isnan(), you need to:

#include <cmath>

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, or some other impossible operation.

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))  // failure?
        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 NULL; }
double bar() { return nullptr; }
int main() { }

c.cc:1: warning: converting to non-pointer type ‘double’ from NULL
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 zero works for the year a loan is due.

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

• We deserve that–string isn’t a pointer, so why the !@#$ did we return nullptr?
• An empty string is often a good sentinel value. Otherwise pick something that can’t occur in the data, e.g., "%$☹ NO FIRST NAME ☹$%".

Giving Up

• The alternative to having an return value to indicate failure is simply complaining and giving up right on the spot.
• This is bad sportsmanship, and bad style. It implies that the code that detected the error knows how to best handle the error.
• On the other hand, sometimes, it’s the most straightforward thing to do.
• Programming is an art, not a science.

exit

Loud a='a';

double invert(double num) {
    Loud b('b');
    if (num == 0) {
        cerr << "Can’t invert zero!\n";
        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']
Can’t invert zero!
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) {
        cerr << "Can’t invert zero!\n";
        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']
Can’t invert zero!
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!=0);
    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!=0' failed.
SIGABRT: Aborted

• I suppose that we have low expectations for a failed assertion.
• assert() calls abort(), and so has similar results.