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I often talk to programmers who say "Don't put multiple return statements in the same method." When I ask them to tell me the reasons why, all I get is "The coding standard says so." or "It's confusing." When they show me solutions with a single return statement, the code looks uglier to me. For example:

if (condition)
   return 42;
else
   return 97;

"This is ugly, you have to use a local variable!"

int result;
if (condition)
   result = 42;
else
   result = 97;
return result;

How does this 50% code bloat make the program any easier to understand? Personally, I find it harder, because the state space has just increased by another variable that could easily have been prevented.

Of course, normally I would just write:

return (condition) ? 42 : 97;

But many programmers eschew the conditional operator and prefer the long form.

Where did this notion of "one return only" come from? Is there a historical reason why this convention came about?

share|improve this question
    
This is somewhat connected to Guard Clause refactoring. stackoverflow.com/a/8493256/679340 Guard Clause will add returns to the beginning of your methods. And it makes code a lot cleaner in my opinion. – Piotr Perak Dec 14 '13 at 9:14
    
    
and where is the discussion discussed? – Liviu May 13 at 8:58
    
It came from the notion of structured programming. Some may argue that having just one return allows you to easily modify the code to do something just before returning or to easily debug. – martinkunev May 14 at 11:39
up vote 605 down vote accepted

"Single Entry, Single Exit" was written when most programming was done in assembly language, FORTRAN, or COBOL. It has been widely misinterpreted, because modern languages do not support the practices Dijkstra was warning against.

"Single Entry" meant "do not create alternate entry points for functions". In assembly language, of course, it is possible to enter a function at any instruction. FORTRAN supported multiple entries to functions with the ENTRY statement:

      SUBROUTINE S(X, Y)
      R = SQRT(X*X + Y*Y)
C ALTERNATE ENTRY USED WHEN R IS ALREADY KNOWN
      ENTRY S2(R)
      ...
      RETURN
      END

C USAGE
      CALL S(3,4)
C ALTERNATE USAGE
      CALL S2(5)

"Single Exit" meant that a function should only return to one place: the statement immediately following the call. It did not mean that a function should only return from one place. When Structured Programming was written, it was common practice for a function to indicate an error by returning to an alternate location. FORTRAN supported this via "alternate return":

C SUBROUTINE WITH ALTERNATE RETURN.  THE '*' IS A PLACE HOLDER FOR THE ERROR RETURN
      SUBROUTINE QSOLVE(A, B, C, X1, X2, *)
      DISCR = B*B - 4*A*C
C NO SOLUTIONS, RETURN TO ERROR HANDLING LOCATION
      IF DISCR .LT. 0 RETURN 1
      SD = SQRT(DISCR)
      DENOM = 1.0 / (2*A)
      X1 = (-B + SD) / DENOM
      X2 = (-B - SD) / DENOM
      RETURN
      END

C USE OF ALTERNATE RETURN
      CALL QSOLVE(1, 0, 1, X1, X2, *99)
C SOLUTION FOUND
      ...
C QSOLVE RETURNS HERE IF NO SOLUTIONS
99    PRINT 'NO SOLUTIONS'

Both these techniques were highly error prone. Use of alternate entries often left some variable uninitialized. Use of alternate returns had all the problems of a GOTO statement, with the additional complication that the branch condition was not adjacent to the branch, but somewhere in the subroutine.

share|improve this answer
56  
+1, this answer needs some serious up voting. Hoping it'll get a chance to compete at the top. P.s. if it would, be prepared to add some references. ;p – Steven Jeuris Nov 9 '11 at 23:30
13  
That's indeed interesting histoy, +1 from me. How then got "Single Exit" perverted so badly? – sbi Nov 10 '11 at 15:13
22  
And don't forget spaghetti code. It was not unknown for subroutines to exit using a GOTO instead of a return, leaving the function call parameters and return address on the stack. Single exit was promoted as a way to at least funnel all the code paths to a RETURN statement. – TMN Nov 10 '11 at 15:52
4  
@kevin: Yeah, but according to you this doesn't even mean anymore what it was invented as. (BTW, I'm actually reasonably sure that Fred asked were the preference for the current interpretation of "Single Exit" comes from.) Also, C has had const since before many of the users here were born, so no need for capital constants anymore even in C. But Java preserved all those bad old C habits. – sbi Nov 10 '11 at 22:08
5  
@dodgy_coder: FORTRAN had to compete with assembly language, when memory and cycles where both quite expensive. Use of ENTRY instead of splitting the function saved the memory for the additional CALL and RETURN statements, and the tens of microseconds needed to execute them. – kevin cline Dec 2 '11 at 17:44

This notion of Single Entry, Single Exit (SESE) comes from languages with explicit resource management, like C and assembly. In C, code like this will leak resources:

void f()
{
  resource res = acquire_resource();  // think malloc()
  if( f1(res) )
    return; // leaks res
  f2(res);
  release_resource(res);  // think free()
}

In such languages, you basically have three options:

  • Replicate the cleanup code.
    Ugh. Redundancy is always bad.

  • Use a goto to jump to the cleanup code.
    This requires the cleanup code to be the last thing in the function. (And this is why some argue that goto has its place. And it has indeed – in C.)

  • Introduce a local variable and manipulate control flow through that.
    The disadvantage is that control flow manipulated through syntax (think break, return, if, while) is much easier to follow than control flow manipulated through the state of variables (because those variables have no state when you look at the algorithm).

In assembly it's even weirder, because you can jump to any address in a function when you call that function, which effectively means you have an almost unlimited number of entry points to any function. (Sometimes this is helpful. Such thunks are a common technique for compilers to implement the this pointer adjustment necessary for calling virtual functions in multiple-inheritance scenarios in C++.)

When you have to manage resources manually, exploiting the options of entering or exiting a function anywhere leads to more complex code, and thus to bugs. Therefore, a school of thought appeared that propagated SESE, in order to get cleaner code and less bugs.


However, when a language features exceptions, (almost) any function might be exited prematurely at (almost) any point, so you need to make provisions for premature return anyway. (I think finally is mainly used for that in Java and using (when implementing IDisposable... finally otherwise) in C#; C++ instead employs RAII.) Once you have done this, you cannot fail to clean up after yourself due to an early return statement, so what is probably the strongest argument in favor of SESE has vanished.

That leaves readability. Of course, a 200 LoC function with half a dozen return statements sprinkled randomly over it is not good programming style and does not make for readable code. But such a function wouldn't be easy to understand without those premature returns either.

So in languages where resources are not or should not be managed manually, there is little or no value in adhering to the old SESE convention. OTOH, as I have argued above, SESE often makes code more complex. So it is a dinosaur that (except for C) does not fit well into most of today's languages. Instead of helping the understandability of code, it hinders it.


So why do Java programmers stick to this? I don't know, but from my (outside) POV, Java took a lot of conventions from C (where they make sense) and applied them to its OO world (where they are useless or outrightly bad), where it now sticks to them, no matter what the costs. (Like the convention to define all your variables at the beginning of the scope.)

But programmers stick to all kinds of strange notations for irrational reasons. (Deeply nested structural statements – "arrowheads" – were, in languages like Pascal, once seen as beautiful code.) Applying pure logical reasoning to this seems to fail to convince the majority of them to deviate from their established ways. So the best way to change such habits is probably to teach them early on to do what's best, not what's conventional. You, being a programming teacher, have it in your hand. :)

share|improve this answer
46  
Right. In Java, cleanup code belongs in finally clauses where it gets executed regardless of early returns or exceptions. – dan04 Nov 9 '11 at 9:41
14  
@dan04 in Java 7 you don't even need the finally most of the time. – R. Martinho Fernandes Nov 9 '11 at 9:45
72  
@Steven: Of course you can demonstrate that! In fact, you can show convoluted and complex code with any feature that can also be shown to make code simpler and easier to understand. Everything can be abused. The point is to write code so that it is easier to understand, and when that involves throwing SESE out the window, so be it, and damn the old habits that applied to different languages. But I wouldn't hesitate to control execution by variables either if I'd think it made the code easier to read. It's just that I cannot remember having seen such code in almost two decades. – sbi Nov 9 '11 at 12:31
13  
@Karl: Indeed, it is a severe shortcoming of GC languages like Java that they relieve you from having to clean up one resource, but fail with all the others. (C++ solves this problem for all resources using RAII.) But I wasn't even talking of only memory (I only put malloc() and free() into a comment as an example), I was talking about resources in general. I also wasn't implying GC would solve these problems. (I did mention C++, which doesn't have GC out of the box.) From what I understand, in Java finally is used to solve this problem. – sbi Nov 9 '11 at 14:15
7  
@sbi: More important for a function (procedure, method, etc.) than being no more than a page long is for the function to have a clearly defined contract; if it's not doing something clear because it's been chopped up to satisfy an arbitrary length constraint, that's Bad. Programming is about playing off different, sometimes conflicting forces against each other. – Donal Fellows Nov 22 '11 at 14:20

On the one hand, single return statements make logging easier, as well as forms of debugging that rely on logging. I remember plenty of times I had to reduce the function into single return just to print out the return value at a single point.

  int function() {
     if (bidi) { print("return 1"); return 1; }
     for (int i = 0; i < n; i++) {
       if (vidi) { print("return 2"); return 2;}
     }
     print("return 3");
     return 3;
  }

On the other hand, you could refactor this into function() that calls _function() and logs the result.

share|improve this answer
23  
I would also add that it makes debugging easier because you only ever need to set one breakpoint to catch all exits* from the function. I beleive that some IDEs let you put a breakpoint on the close brace of the function to do the same thing. (* unless you call exit) – Skizz Nov 10 '11 at 11:03
2  
For a similar reason, it also makes it easier to extend (add to) the function, since your new functionality doesn't have to be inserted before each return. Say you needed to update a log with the result of the function call, for example. – JeffSahol Nov 10 '11 at 16:43
44  
Honestly, if I were maintaining that code, I'd rather have a sensibly-defined _function(), with returns at appropriate places, and a wrapper named function() that handles extraneous logging, than have a single function() with contorted logic to make all returns fit into a single exit-point just so I can insert an additional statement before that point. – ruakh Nov 13 '11 at 19:41
7  
In some debuggers (MSVS) you can put breakpoint on last closing brace – Abyx Nov 21 '11 at 21:56
1  
printing != debugging. That's not argument at all. – Piotr Perak Dec 14 '13 at 9:08

"Single Entry, Single Exit" originated with the Structured Programming revolution of the early 1970s, which was kicked off by Edsger W. Dijkstra's letter to the Editor "GOTO Statement Considered Harmful". The concepts behind structured programming were laid out in detail in the classic book "Structured Programming" by Ole Johan-Dahl, Edsger W. Dijkstra, and Charles Anthony Richard Hoare.

"GOTO Statement Considered Harmful" is required reading, even today. "Structured Programming" is dated, but still very, very rewarding, and should be at the top of any developer's "Must Read" list, far above anything from e.g. Steve McConnell. (Dahl's section lays out the basics of classes in Simula 67, which are the technical foundation for classes in C++ and all of object-oriented programming.)

share|improve this answer
10  
I bet you must be one of those guys that also litters the code with boolean flags instead of using break and continue in loops. – hugomg Nov 9 '11 at 13:53
20  
The article was also written in the days when goto could literally go anywhere, like right into some random point in another function, bypassing any notion of procedures, functions, a call stack, etc. No sane language permits that these days with a straight goto. C's setjmp/longjmp is the only semi-exceptional case i'm aware of, and even that requires cooperation from both ends. (Semi-ironic that i used the word "exceptional" there, though, considering that exceptions do almost the same thing...) Basically, the article discourages a practice that's long dead. – cHao Nov 9 '11 at 15:41
5  
From the last paragraph of "Goto Statement considered harmful": "in [2] Guiseppe Jacopini seems to have proved the (logical) superfluousness of the go to statement. The exercise to translate an arbitrary flow diagram more or less mechanically into a jump-less one, however, is not to be recommended. Then the resulting flow diagram cannot be expected to be more transparent than the original one." – hugomg Nov 9 '11 at 18:22
8  
What does this have to do with the question? Yes, Dijkstra's work eventually led to SESE languages, and so what? So did Babbage's work. And perhaps you should re-read the paper if you think it says anything about having multiple exit points in a function. Because it doesn't. – jalf Nov 9 '11 at 18:35
8  
@John, you seem to be trying to answer the question without actually answering it. It's a fine reading list, but you've neither quoted nor paraphrased anything to justify your claim that this essay and book have anything to say about the asker's concern. Indeed, outside of comments you've said nothing substantial about the question whatsoever. Consider expanding this answer. – Shog9 Nov 9 '11 at 22:05

It's always easy to link Fowler.

One of the main examples that go against SESE are guard clauses:

Replace Nested Conditional with Guard Clauses

Use Guard Clauses for all the special cases

double getPayAmount() {
    double result;
    if (_isDead) result = deadAmount();
    else {
        if (_isSeparated) result = separatedAmount();
        else {
            if (_isRetired) result = retiredAmount();
            else result = normalPayAmount();
        };
    }
return result;
};  

                                                                                                         http://www.refactoring.com/catalog/arrow.gif

double getPayAmount() {
    if (_isDead) return deadAmount();
    if (_isSeparated) return separatedAmount();
    if (_isRetired) return retiredAmount();
    return normalPayAmount();
};  

For more inforamtion see page 250 of Refactoring...

share|improve this answer
    
If your conditional check decides which code path to execute, yes. For simple assignments, no. – DanMan May 24 '14 at 11:16
    
Another bad example: it could just as easily be fixed with else-ifs. – Jack Jul 6 '15 at 17:58
    
Your example is not fair, how about this: double getPayAmount() { double ret = normalPayAmount(); if (_isDead) ret = deadAmount(); if (_isSeparated) ret = separatedAmount(); if (_isRetired) ret = retiredAmount(); return ret; }; – Charbel Apr 22 at 6:55

I wrote a blog post on this topic a while back.

The bottom line is that this rule comes from the age of languages that don't have garbage collection or exception handling. There is no formal study that shows that this rule leads to better code in modern languages. Feel free to ignore it whenever this will lead to shorter or more readable code. The Java guys insisting on this are blindly and unquestioning following a outdated, pointless rule.

This question has also been asked on Stackoverflow

share|improve this answer

Consider the fact that multiple return statements are equivalent to having GOTO's to a single return statement. This is the same case with break statements. As thus, some, like me, consider them GOTO's for all intents and purposes.

However, I don't consider these types of GOTO's harmful and will not hesitate to use an actual GOTO in my code if I find a good reason for it.

My general rule is that GOTO's are for flow control only. They should never be used for any looping, and you should never GOTO 'upwards' or 'backwards'. (which is how breaks/returns work)

As others have mentioned, the following is a must read GOTO Statement Considered Harmful
However, keep in mind that this was written in 1970 when GOTO's were way overused. Not every GOTO is harmful and I would not discourage their use as long as you don't use them instead of normal constructs, but rather in the odd case that using normal constructs would be highly inconvenient.

I find that using them in error cases where you need to escape an area because of a failure that should never occur in normal cases useful at times. But you should also consider putting this code into a separate function so that you can just return early instead of using a GOTO... but sometimes that's also inconvenient.

share|improve this answer
1  
All structured constructs that replace gotos are implemented in terms of goto. E.g. loops, "if" and "case". This does not make them bad - in fact the opposite. Also, it is "intents and purposes". – Anthony Nov 10 '11 at 15:05
    
Touche, but this doesn't differ my point... It just makes my explanation slightly wrong. oh well. – user606723 Nov 10 '11 at 16:33
    
GOTO should be always okay as long as (1) target is within the same method or function and (2) the direction is forward in the code (skip some code) and (3) the target is not inside some another nested structure (e.g. GOTO from the middle of if-case to the middle of else-case). If you follow these rules, all misuses of GOTO have a really strong code smell both visually and logically. – Mikko Rantalainen Dec 11 '12 at 11:29

protected by MichaelT Oct 29 '14 at 3:47

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