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GOTOphobia considered harmful in C

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Re: GOTOphobia considered harmful in C

#251

Post by pjmlp »

moregrist wrote: post #32
kmeisthax wrote: post #19

In RAII languages[0], you obviously don't need unrestricted gotos. However, I always find myself missing it when writing nested loops. Labeled break and continue[1] ought to be considered standard structure programming primitives. These are restricted gotos and allowing them to break or continue a parent loop doesn't unrestrict them much. But it does significantly improve the expressive power of your looping construc…

This is one of those half-true claims that C++ aficionados make, probably because they can play a bit fast and loose with resource cleanup in their particular situations. RAII simplifies some of the resource cleanup, but at a cost: if the resource cleanup fails, there’s essentially no way to convey this. So yes, you can write a destructor that tries to clean up your resources regardless of how the code exits its curr…

> RAII simplifies some of the resource cleanup, but at a cost: if the resource cleanup fails, there’s essentially no way to convey this.

Exceptions.

Re: GOTOphobia considered harmful in C

#252

Post by pjmlp »

jeroenhd wrote: post #58
echelon wrote: post #10

> Bad code is the product of bad programmers This person is living in a pretend bubble that isn't grounded in the reality of large projects, multiple team members, deadlines, changing requirements, etc. No programmer is perfect. And when your tool can cut your arm off, you should be careful or route around the dangerous bits when possible.

I see this "if you're good then you don't need safety" mentality in a lot of conversations with C programmers about programming languages. Maybe it's some kind of defencive response against newer programing languages slowly eating up spaces C used to be dominant in (like command line tools and system daemons), maybe it's just the programmer saying this thinking they're that special. Either way, most people saying thi…

Nah, it is like that since C exists, they used to call those of us that rather use languages from ALGOL systems programming linage as coding with straighjacket, or nany languages.

I learned to appreciate C++ RAII already on Turbo C++ for MS-DOS around 1993, and using raw C has always been because I was required to deliver C code for some university projects or work.

Re: GOTOphobia considered harmful in C

#253

Post by int_19h »

bdowling wrote: post #231
JohnBooty wrote: post #124

Earlier quoted context omitted.

Not too many things make me shake my head harder than folks who consider continue/break to be GOTO equivalents. For the reasons you eloquently said. Additionally, far more often than not, continue/break allow you to avoid another form of complexity, bugs, and low comprehensibility: deeply nested conditionals.

CONTINUE and BREAK are simply jumps to the beginning of or just past the end of the current loop context. They are equivalent to GOTOs to particular program offsets without the programmer needing to create labels for those offsets. They do not have any magical meaning beyond that. You could even call them syntactic sugar.

Structured if/then/else is also merely syntactic sugar over if/goto, but that doesn't make it any less useful.

What makes break/continue (including labelled variants a la Java) useful is the fact that the restriction on where they can jump means that the control flow graph is guaranteed to be reducible. That is not the case with free-form goto.

Re: GOTOphobia considered harmful in C

#254

Post by spacechild1 »

bee_rider wrote: post #185
spacechild1 wrote: post #173

Earlier quoted context omitted.

Ah, right, I misread your post. DO WHILE is indeed a "normal" while loop. There doesn't seem to be an equivalent to "do { ... } while", as found in most C-style languages. > There seems to be a unfortunate inconsistency in the naming of this thing. Well, Fortran is older than C, so you cannot really blame them :-)

The funny thing is, I mostly program in Fortran (thus the interest in this construct). It is nice for expressing “this iterative method must be run at least once.” Unfortunately at some point I absorbed the name that comes from the C-ism, haha.

May I ask what field you're working in? Physics?

Re: GOTOphobia considered harmful in C

#255

Post by mort96 »

dheera wrote: post #211
mort96 wrote: post #140

Earlier quoted context omitted.

To give people some kind of an idea of what it was created in response to: Imagine writing an entire program in one single main function. The only thing you're allowed to do for flow control is goto. You can do 'goto somelabel;' for an unconditional goto, or you can do 'if (somecondition) goto somelabel;' for a conditional goto. Here's some examples of how it would look if if translated to something C-like: Loops wou…

Not too far from assembly ...

Almost, except in assembly, we always have a call stack. So even assembly is a procedural language, even though it doesn't otherwise have structured control flow.

Re: GOTOphobia considered harmful in C

#256

Post by int_19h »

mort96 wrote: post #140
arp242 wrote: post #77

Earlier quoted context omitted.

The thing is that many people today have never encountered the sort of spaghetti code that Dijkstra was talking about in 1968. There's plenty of confusing and messy code around, but true spaghetti code that GOTOs all over the place and is nigh-impossible to follow has been extremely rare for a long time. I can't recall encountering it in the last 30 years. It easy to misunderstand what he was even talking about becau…

To give people some kind of an idea of what it was created in response to: Imagine writing an entire program in one single main function. The only thing you're allowed to do for flow control is goto. You can do 'goto somelabel;' for an unconditional goto, or you can do 'if (somecondition) goto somelabel;' for a conditional goto. Here's some examples of how it would look if if translated to something C-like: Loops wou…

Procedural or not was more of a spectrum. If you look at early BASICs, for example, they had GOSUB, and it could recurse, so there was a return-address stack. But GOSUB did not have any provisions to pass arguments or return values - it was just a GOTO that remembered where it came from; you had to use globals to pass data around. So if you wanted a data stack (i.e. locals), you had to rig your own with arrays.

OTOH early FORTRAN had procedures with arguments and results, but no recursion.

Structural or not was also not necessarily all-in. FORTRAN and BASIC both had for-loops before they had structured conditionals.

Re: GOTOphobia considered harmful in C

#257

Post by int_19h »

bee_rider wrote: post #185
spacechild1 wrote: post #173

Earlier quoted context omitted.

Ah, right, I misread your post. DO WHILE is indeed a "normal" while loop. There doesn't seem to be an equivalent to "do { ... } while", as found in most C-style languages. > There seems to be a unfortunate inconsistency in the naming of this thing. Well, Fortran is older than C, so you cannot really blame them :-)

The funny thing is, I mostly program in Fortran (thus the interest in this construct). It is nice for expressing “this iterative method must be run at least once.” Unfortunately at some point I absorbed the name that comes from the C-ism, haha.

A less ambiguous name for those is repeat/until, as seen in Pascal and its descendants - I don't recall any language that uses that syntax for anything other than a postcondition loop.

Re: GOTOphobia considered harmful in C

#258

Post by int_19h »

throwaway2037 wrote: post #186
cesarb wrote: post #161

Earlier quoted context omitted.

> If the idea of throwing away a secondary exception makes you uncomfortable, then another possible solution might have been to allow secondary exceptions to be "attached" to the primary exception, like `std::exception::secondary()` could return an array of secondary exceptions that were caught. Java has that for its pseudo-RAII "try-with-resources" statement: when an exception happens during cleanup of a try-with-re…

I agree with your points about Java. I have direct experience with suppressed exceptions in Java 6 -- it was painful to debug ("where did my exception go???"). However, this works because Java forces everything thrown to be a sub-class of Throwable. (Please correct me if wrong.) C++ allows you to throw anything, including (bizarrely) null. I learned recently that C# allows the same -- you can throw null(!). How does…

In C#, if the finally-block of a try-finally throws, it replaces the current exception altogether; and using-statement desugars into try-finally.

And C# does not actually allow you to throw null. It does allow you to write "throw x" where x may be null, but that will just cause an immediate NullReferenceException at runtime.

Re: GOTOphobia considered harmful in C

#259

Post by owisd »

tester756 wrote: post #88
owisd wrote: post #26

> On the other hand, today we have the very opposite situation: programmers not using goto when it's appropriate and abusing other constructs, what ironically makes code only less readable. One I see all the time from beginners is creating a finite state machine using one method per state and jumping between states by calling the next state’s method from within the current state’s method. Essentially just emulating g…

Where in the world beginners use state machines instead of switch statement or if ladder? this is genuine question, because I do believe that people do not use enough state machines to model their systems.

Usually as a simple menu system, see e.g. https://github.com/sokrateshayraniyim/Your-Own-Library/blob/...

Re: GOTOphobia considered harmful in C

#260

Post by mFixman »

bazoom42 wrote: post #228
ufo wrote: post #50

I always encourage people to go read Dijkstra's GOTO paper, instead of just its title. It's a short and easy read, almost like a blog post. If you pay attention, you can see that the he was talking about spaghetti code vs structured code. It's better when the lexical structure of the source code maps to the execution structure. That is, if you know what is the current line being executed, you have a good idea of what…

Interestingly, many seem to want to project their own opinion upon Dijkstra. Dijkstra is clearly arguing for a “single entry single exit” style. But modern consensus seem to uphold single entry but accept multiple exits from a block. Break, continue, early returns, exceptions - all are example of multiple exit. These are more constrained than gotos but nevertheless Dijkstras argument applies to them also. I personall…

IIRC multiple entry points were common in Dijkstra's times.

Break, continue, and early returns always return to the end of the block, unlike goto in the '70s. Exceptions are more complicated, and the cause of a lot of inunderstandable programs.

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