Re: encapsulating directory operations

On Sun, 2025-06-08 at 16:55 +0000, Scott Lurndal wrote:

 
For example:
 
c_file_card_unit::c_file_card_unit(ulong channel, ulong unit, const char *name,
                                   c_logger *lp, c_card_dlp *dlp, bool punch,
                                   bool hollerith, const char *binpath)
    : c_card_unit(channel, unit, lp, dlp, punch)
{
    int flags = punch?O_RDWR|O_APPEND:O_RDONLY;
    int diag;
    uint8 header[CARD_SIZE_COLUMNS];
 
    f_file        = NULL;
    f_inputhopper = 0ul;
    f_binfd       = -1;
    snprintf(f_binary_path, sizeof(f_binary_path), "%s", binpath);
 
    diag = stat(name, &cu_stat);
    if (diag == -1) {
        if ((errno == ENOENT) && punch) {
            flags |= O_CREAT|O_EXCL;
        }
    }
 
    cu_fd = open(name, flags, 0600);
    if (cu_fd == -1) {
        fprintf(stdout, "%4.4lu/%2.2lu Unable to open '%s': %s\n",
                cu_channel, cu_unit, name, strerror(errno));
        return;
    }
 
    diag = fcntl(cu_fd, F_SETFD, FD_CLOEXEC);
    if (diag == -1) {
        lp->log("%4.4lu/%2.2lu Unable to set FD_CLOEXEC on '%s': %s\n",
                      cu_channel, cu_unit, name, strerror(errno));
        diag = close(cu_fd);
        cu_fd = -1;
        return;
    }
 
...
}

I would like just share my comment of the code (just skim the code).
 
Assume c_file_card_unit(..) is a normal function:
1. It should be obvious throwing is no good than returning error.
2. Error is issued in severl places, post condition is a concern.
3. Error handling itself (code and propagation) has to (or better be) error free.
4. Cancellation point/Async-signal safe issues.

bool
c_file_card_unit::is_ready(void)
{
    return (cu_fd != -1) && ((f_file != NULL) && !feof(f_file));
}
 
So, if is_ready() returns false after the constructor runs,
the creator of the object knows the creation failed.

If the code in C++ constructor, returning error encountered is no good, 'contract' is broken.
A ctor like this should be considered too complicated but fine in non-serious code.