Re: C++ FAQ

From:

Francis Glassborow <francis.glassborow@btinternet.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Sat, 4 Jul 2009 14:55:58 CST

Message-ID:

<mLidncgWq9m2_dLXnZ2dnUVZ8vqdnZ2d@bt.com>

Seungbeom Kim wrote:

Francis Glassborow wrote:

I don't know the details of that challenge, but I find it surprised
that you haven't seen an example, especially in C. How do you do
cleanup in case of an error in the middle of successive allocation
of resources, when there are no destructors available?

    s = socket(...);
    if (s < 0) goto cleanup;

    r = connect(...);
    if (r < 0) goto cleanupS;

    f = open(...);
    if (f < 0) goto cleanupS;

    while (...) {
        if (some error) goto cleanupF;
    }

cleanupF:
    close(f);

cleanupS:
    close(s);

cleanup:
    /* other common stuff */
    return result;

Certainly you could write a goto-less version (with nested ifs or some
code duplication), but I don't see a better a better alternative here.

I don't enjoy using goto in general, but I feel no guilt as I use it
if it clearly conveys my intention and the alternatives are not as
appealing.

Yes and we are in the subjective area. What appeals to one person has
the reverse effect on another

    s = socket(...);
    if (s >= 0){
      r = connect(...);
      if (r >= 0){
        f = open(...);
        if (f >= 0){
          while (...) {
            if (some error) {
              // process error;
              break;
            }
          }
        close(f);
        }
      }
      close(s);
    }
    /* other common stuff */
    return result;
}

And though the above code avoids using goto it does have that horrible
nest of if statements. And as you say, with destructors we can avoid
those. Of course the above code seems to assume that there is some
acceptable return value that can be preset before we start.

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