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Posted 23 Apr 2010

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The goto-less goto!

Ajay Vijayvargiya

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3.34/5 (23 votes)

24 Jan 2011CPOL

The goto-less goto!

You have non-looped, sequenced code, like below:

  bool bFailed = true;
  if(condition1_fails)
     goto Exit;
  ...
  if(condition2_fails)
     goto Exit;
  ...
  ...
  if(conditionN_fails)
     goto Exit;

  bFailed=false;
  PerformActionOnAllSuccess();

Exit:
  if(bFailed)
     DoFailedCleanup();
  else
     DoNormalCleanup()

Here is the goto-less goto, that you can use:

bool bFailed=true;
do
{
  if(condition1_fails)
     break;
  ...
  if(condition2_fails)
     break;
  ...
  ...
  if(conditionN_fails)
     break;

   bFailed = false;
   PerformActionOnAllSuccess();

 } while(false); // Ignore or disable warning at function-level

 if(bFailed)
    DoFailedCleanup();
 else
    DoNormalCleanup();

It runs a do-while loop, which would run only once. As soon as some failed-condition encounters, it exits (break) from loop.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Written By

Ajay Vijayvargiya

Software Developer (Senior)

India

Started programming with GwBasic back in 1996 (Those lovely days!). Found the hidden talent!

Touched COBOL and Quick Basic for a while.

Finally learned C and C++ entirely on my own, and fell in love with C++, still in love! Began with Turbo C 2.0/3.0, then to VC6 for 4 years! Finally on VC2008/2010.

I enjoy programming, mostly the system programming, but the UI is always on top of MFC! Quite experienced on other environments and platforms, but I prefer Visual C++. Zeal to learn, and to share!

Comments and Discussions

Manikandan106-Jun-14 20:52

6-Jun-14 20:52

Expecting more.

Krzysztof Kalinowski12-Mar-13 21:25

Krzysztof Kalinowski

12-Mar-13 21:25

goto isn't as bad as idea, but it encourage people to write really bad code. Your solution isn't any better, do while loop is very confusing

i0021-Oct-12 16:01

21-Oct-12 16:01

Great ... you replaced a poor use of goto (goto is not bad unless used poorly in this case you would be better using else if)... with a poor use of a do loop ...

Reason for my vote of 2 No reason at all to use loop stateme...

Albert Holguin14-Oct-11 5:01

14-Oct-11 5:01

Reason for my vote of 2
No reason at all to use loop statements... this would create confusion for a developer in the future that's looking at your code.

Reason for my vote of 5 This is a very good alternate to usi...

EFEaglehouse5-Apr-11 4:06

5-Apr-11 4:06

Reason for my vote of 5
This is a very good alternate to using goto. It allows the code to be readable, yet gives a single exit point to the function. I have an aversion to do...while loops, but this is excellent for those who have an aversion to goto. Since C and C++ do not allow exiting a block, this or goto are the next best things.

For those who are mindlessly parroting their academic conditioning by saying all code that uses a goto is bad, you need to rethink your programming objective. Is it to produce readable and well-performing code or to create unnecessary complexity to avoid a very useful language feature - that is, do you want your code to be real-world practical or theoretically ideal? The answer is: it's your choice.

Reason for my vote of 1 Misused gotos has been replaced by m...

Mass Nerder8-Feb-11 3:18

8-Feb-11 3:18

Reason for my vote of 1
Misused gotos has been replaced by misused breaks. And readability has also been reduced.

Reason for my vote of 5 I've looked at the many alternates, ...

Carroll Arbogast7-Feb-11 13:01

Carroll Arbogast

7-Feb-11 13:01

Reason for my vote of 5
I've looked at the many alternates, and this is the most elegant expression of a common situation.

Reason for my vote of 3 Not very efficient, too many breaks.

BillW332-Feb-11 8:18

2-Feb-11 8:18

Reason for my vote of 3
Not very efficient, too many breaks.

Reason for my vote of 1 IF you find yourself in this positio...

Kirk Wood31-Jan-11 15:10

31-Jan-11 15:10

Reason for my vote of 1
IF you find yourself in this position, rethink your entire situation. Clearly your logic needs some work and perhaps you need to make use of polymorphism.

Re: Now what Polymorphism has to do with this?

Ajay Vijayvargiya2-Feb-11 15:57

Ajay Vijayvargiya

2-Feb-11 15:57

Now what Polymorphism has to do with this?

Goto are not always bad... There are very specific cases whe...

Valery Possoz31-Jan-11 12:46

31-Jan-11 12:46

Goto are not always bad... There are very specific cases where they are actually very useful.
you can read this this chapter from Steve McConell book "Code Complete" about using or not using Gotos.
http://www.stevemcconnell.com/ccgoto.htm
The world isn't black and white.

Re: You are absolutely correct. An inspection of Dijkstra's pap...

Ravi Bhavnani6-Apr-11 8:25

6-Apr-11 8:25

You are absolutely correct. An inspection of Dijkstra's paper will reveal his reasoning of how the goto can (but not necessarily always) lead to code that's difficult to maintain and impossible to verify. goto's to the *end* of a block are always permitted. In C, C++ and C# they're called "break" and "continue". A goto to a target *inside* a block leads to code whose correctness is mathematically impossible to verify.

I don't use gotos either. My point was "how to avoid goto, i...

Ajay Vijayvargiya24-Jan-11 16:15

Ajay Vijayvargiya

24-Jan-11 16:15

I don't use gotos either. My point was "how to avoid goto, if used"

Re: If you feel you need to use goto, I'd say you need to re-eva...

fjdiewornncalwe26-Jan-11 14:54

fjdiewornncalwe

26-Jan-11 14:54

If you feel you need to use goto, I'd say you need to re-evaluate how the method is written.

Why not use if - else if - else if ...? You won't need to br...

Indivara24-Jan-11 13:24

24-Jan-11 13:24

Why not use if - else if - else if ...? You won't need to break that way. I generally dislike unconditional loops like that. What's more, if the method is long enough to require this sort if thing, it can generally be broken up more (unless performance is sacrificed)

Goto / multiple returns rules and alternatives

Philippe Mori7-Apr-11 2:53

7-Apr-11 2:53

In some case, some alternatives are as bad as having multiple returns or goto.

For example, any alternative based on a loop and break, will have a similar complexity. Thus a function 3 early returns or a function with 3 break in a loop or a function with 3 gotos... will essentially have the same complexity as it will have the same number of path.

However the multiple returns alternative have the advantage of moving thye complex condition out of the main logic... thus at the end, it would typically be easier to understand as we would deal with 2 functions somehow less complex.

For debugging, someone said it was easier to debug with a single return. Often, this is not the case. It is often easier to debug a function with multiple returns simply because we can put breakpoint on each returns and immediatly knows which condition was selected.

I prefer to thing in term of "what we should normally do" instead of "strict rules".

Finally sometime the extra code required to follow the rules (extra conditions, variables, functions) is not justified for a marginal benefit.

Philippe Mori

[My vote of 1] Pointless

balab31-Jan-11 8:58

31-Jan-11 8:58

What exactly is your objection to goto? All of your alternatives look obfuscated to me.

Re: [My vote of 1] Pointless

wim ton31-Jan-11 21:30

31-Jan-11 21:30

In some organizations goto is forbidden by the coding standard, no matter how messy the alternatives are.

Re: [My vote of 1] Pointless

mier7-Mar-11 22:43

7-Mar-11 22:43

That's just ridiculous. I agree that using goto is generally not good practice, but it's far more preferable than using a do...while or for loop.

SledgeHammer0124-Apr-10 8:23

24-Apr-10 8:23

Thumbs down... the goto version is cleaner and easier to read.

Re: My vote of 1

Ajay Vijayvargiya25-Apr-10 9:37

Ajay Vijayvargiya

25-Apr-10 9:37

But that wouldn't handle destructors, and is less efficient.

Re: My vote of 1 [modified]

SledgeHammer0125-Apr-10 10:13

25-Apr-10 10:13

The goto version handles destructors just fine. I'm not even sure what you mean by that since you aren't using any C++ objects here.

Your do/while is actually less efficient since it requires extra code vs. the goto method. Also, jmp instructions in the goto version are static vs. yours which are dynamic which again requires more asm instructions.

modified on Sunday, April 25, 2010 7:11 PM

Re: My vote of 1

supercat926-Apr-10 6:31

26-Apr-10 6:31

I would expect that a C++ compiler would be required to call destructors when leaving a block via any means, including via "goto". Any decent compiler should generate code for the do-while-false loop which is no worse than code using 'goto'. If anything, it allows for easier static analysis (using 'goto' in certain ways can make data-flow analysis intractable in ways that 'break' cannot; a compiler could examine the use of 'goto' and determine that it wasn't a 'problem case', but that's more complicated).

BTW, another GOTO-avoiding pattern which can come up, though not as often:

do
{
  iterate();
  check_for_problem_1();
  if (problem1_still_exists)
    continue;
  check_for_problem_2();
  if (problem2_still_exists)
    continue;
  break;
} while(1);

If the checks for problems won't readily fit in rvalues (so as to allow the "||" operator) the above approach can allow the loop to iterate until everything is good. Note that if repeated iteration won't solve all the problems, the code will loop forever.

Re: My vote of 1

Ajay Vijayvargiya26-Apr-10 18:37

Ajay Vijayvargiya

26-Apr-10 18:37

Hello all,

My whole point to give alternative to misused goto usage. It is true that smart compiler would optimize the goto usage, and would make sure that destructor also gets called. Though I dont use goto at all, I dont detest the usage either. But when anyone sees goto in your code, he/she doesnt appreciate it, and would always argue about it.

I tried looking at generated code for goto and goto-less-goto, and I see that goto forces the compiler to generate duplicate code. NO I did not understood those asm instructions, but I just saw duplicate code generated. Yes, I did that in non-optimized Debug build. Did not have time to explore it more.

For throwing the program-control down, with goto, it may be more efficient to use goto, rather than break, and makes the code more readable. But, does that make code acceptable? No!

Abhinav S23-Apr-10 22:27

23-Apr-10 22:27

....you need to include more explanations (like what what part of the code is doing what etc.) in this tip.

Nothing very detailed (after all, its only a tip), but some information would always be helpful to the reader.

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