Heckendorn’s Law

Heckendorn’s Law is based on a saying of the great Robert Heckendorn:

That is, don’t do this:

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

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?

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

Here’s how to do it:

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

    using namespace std;

    int main(int /* argc */, char *argv[]) {
        const char pwfile[] = "/etc/shadow";      // Why not a std::string?

        ifstream in(pwfile);
        if (!in) {
            cerr << argv[0] << ": can’t read shadow password file ";
            perror(pwfile);
            return 1;
        }

        return 0;
    }