CS253 Heckendorn

Heckendorn’s Rule

Heckendorn’s Rule is based on a saying of the Computer Scientist Robert Heckendorn:

Never emit a constant error message

Example

#include <iostream>
#include <fstream>          // for ifstream

using namespace std;

int main() {
    const string pwfile = "/etc/shadow";

    ifstream in(pwfile);
    if (!in) {
        cerr << "Bad file\n";
        return 1;
    }
    return 0;
}

Bad file

Frustration

Imagine the frustration of the user who reads this uninformative message.

Which file is bad?
What was wrong with it?
What was the program trying to do with the file?
What program was running, anyway?

More Information

There’s always more information that you can provide:

program name from argv[0]
filename
operation attempted (open for reading, open for writing, deletion, rename)
failure code via strerror or perror

Example

#include <iostream>         // for cerr
#include <fstream>          // for ifstream
#include <cerrno>           // for errno
#include <cstring>          // for strerror

using namespace std;

int main(int /* argc */, char *argv[]) {
    const string pwfile = "/etc/shadow";
    ifstream in(pwfile);
    if (!in) {
        const auto save_errno = errno;  // cerr might change errno
        cerr << argv[0] << ": can’t open shadow password file "
             << pwfile << " for reading: "
             << strerror(save_errno) << '\n';
        return 1;
    }
    return 0;
}

./a.out: can’t open shadow password file /etc/shadow for reading: Permission denied

CS253: Software Development with C++

Spring 2018

Heckendorn

CS253 Heckendorn

Heckendorn’s Rule

Example

Frustration

More Information

Example