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Heckendorn

CS253 Heckendorn’s Rule

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 char 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