Fnordology

Edited repost of this stackoverflow reply since several people have asked me about this lately.

There are good things to say about having a single exit-point, just as there are bad things to say about the inevitable "arrow" programming that results for some types of problems.

The ideal function/method should have a straight execution path with a single exit-point, no conditionals, no side effects and exactly one return point. This is a good thing to strive for, as few "wiring bugs" can hide in that kind of code; but this is not always possible or feasible.

If using multiple exit points during input validation or resource allocation, I try to put all the 'error-exits' very visibly at the top of the function in a guard clause manner.

Both the Spartan Programming article of the "SSDSLPedia" and the single function exit point article of the "Portland Pattern Repository's Wiki" have some insightful arguments around this. Also, of course, there is this post to consider.

If you really want a single exit-point (in any non-exception-enabled language) for example in order to release resources in one single place, I find the careful application of goto to be good; see for example this rather contrived example (compressed to save screen real-estate):

int f(int y) {
    int value = -1;
    void *data = NULL;

    if (y < 0)
        goto clean;

    if ((data = malloc(123)) == NULL)
        goto clean;

    /* More code */

    value = 1;
clean:
   free(data);
   return value;
}

Personally I, in general, dislike arrow programming more than I dislike multiple exit-points, although both are useful when applied correctly. The best, of course, is to structure your program to require neither. Breaking down your function into multiple chunks usually help :)

Although when doing so, I find I end up with multiple exit points anyway as in this example, where some larger function has been broken down into several smaller functions:

int g(int y) {
  value = 0;

  if ((value = g0(y, value)) == -1)
    return -1;

  if ((value = g1(y, value)) == -1)
    return -1;

  return g2(y, value);
}

Depending on the project or coding guidelines, most of the boiler-plate code could be replaced by macros. As a side note, breaking it down this way makes the functions g0, g1 ,g2 very easy to test individually.

Obviously, in an OO and exception-enabled language, I wouldn't use if-statements like that (or at all, if I could get away with it with little enough effort), and the code would be much more plain. And non-arrowy. And most of the non-final returns would probably be exceptions.

In short;

• Few returns are better than many returns
• More than one return is better than huge arrows, and guard clauses are generally ok.
• Exceptions could/should probably replace most 'guard clauses' whenever possible.