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The Singleton Pattern: To be or not to be [used]?


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#61 Hodgman   Moderators   -  Reputation: 29493

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Posted 09 November 2012 - 11:18 PM

If you take Khatharr's statement, and think, "how can namespaces be used to provide single global systems", then there's lots of ways you can answer that question, rather than thinking of ways in which it's a nonsense statement. e.g.
namespace MySystem
{
  extern int someGlobalValueModifyWithCare;
  void SomeGlobalProcedureWithHiddenSideEffects();
  float GetSomeHiddenGlobalState();
}
^^That's basically a singleton, without putting any of the implementation details regarding the singleton into your public interface.

Edited by Hodgman, 09 November 2012 - 11:20 PM.


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#62 Khatharr   Crossbones+   -  Reputation: 2962

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Posted 09 November 2012 - 11:28 PM

If you take Khatharr's statement, and think, "how can namespaces be used to provide single global systems", then there's lots of ways you can answer that question, rather than thinking of ways in which it's a nonsense statement. e.g.

namespace MySystem
{
  extern int someGlobalValueModifyWithCare;
  void SomeGlobalProcedureWithHiddenSideEffects();
  float GetSomeHiddenGlobalState();
}
^^That's basically a singleton, without putting any of the implementation details regarding the singleton into your public interface.


That's basically what I mean. I'm not talking about nested types in a namespace. That would just be an extrapolation of a type in the global space.

I have a header that declares the public interface as a namespace and then the private "members" are statics within the code file that implements the namespace.

Edited by Khatharr, 09 November 2012 - 11:32 PM.

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#63 larspensjo   Members   -  Reputation: 1526

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Posted 10 November 2012 - 12:55 AM

That's basically what I mean. I'm not talking about nested types in a namespace. That would just be an extrapolation of a type in the global space.

I have a header that declares the public interface as a namespace and then the private "members" are statics within the code file that implements the namespace.

The singleton pattern is meant to be used for types that are global. Your solution can be a good one, if you simply don't make the type global. It is a solution you can frequently see with "opaque data types".
Current project: Ephenation.
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#64 Hodgman   Moderators   -  Reputation: 29493

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Posted 10 November 2012 - 01:14 AM

Your solution can be a good one, if you simply don't make the type global. It is a solution you can frequently see with "opaque data types".

Indeed, any global system can also be implemented as a non-global, e.g. for the above system:
namespace MyOtherSystem
{
  struct System { int somePublicValueModifyWithCare; };
  System* Create();
  void Destroy(System*);
  void SomeProcedureWithHiddenSideEffects(System*);
  float GetSomeHiddenState(System*);
}
And then if the user wants to, they can create a global instance of it, or a wrapper API that makes it look global (e.g. the namespace MySystem from my last post could be wrapper around this one). This way your code is clean and reusable, but the application author still has the choice of being lazy with globals if they want to.

I've worked on a few engines that used templates that allowed you to use any type as a singleton -- e.g. Singleton<FooSystem>::Get() would return a shared global instance of a FooSystem object.
This wasn't a "true" singleton, because FooSystem would be written in such a way where it would work with many instances -- however, if a game only needed/wanted 1 global instance, they could use this singleton template to ask for one (and every part of the game code using the template would get the same, single, shared instance).
If you're going to use globals, then IMHO that's a good way to design things -- keep the code sensible and allow the user to make use of it in whatever way they want (that's reusability, right?).

Edited by Hodgman, 10 November 2012 - 01:17 AM.


#65 kunos   Crossbones+   -  Reputation: 2205

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Posted 10 November 2012 - 01:30 AM

I don't see that singletons or globals are particularly bad. I've worked on commercial games where singletons and globals have been used. One used them wisely and was the easiest codebase that i've ever worked on and one used it badly and it was extremely complicated and hard to ramp up on.


in a way, globals are always "easier" to use, because you don't have to deal with the problems of "how do I get to this data from here? Do I really need more stuff passed into my constructor so I can get in there? Can I solve it differently? Can I make my class more independent " .. these questions are all GOOD questions that promote better quality code, but they are hitting your instantaneous productivity because they have a big impact on the entire software. If you want to add occlusions to your audio subsystem you'll have to pass in an interface to raycast into the scene.. this might have HUGE impacts on the way the system is built and constructed... and the lazy solution of "ok let's make the RayCastProvider a global singleton" is a quick fix, but it's not really a solution to the problem isnt it?
Having said that, good programmers will make pretty much everything work.. software has been made in asm and C for ages.
But any decent programmer should agree that locality is a good thing because it avoids unwanted or unexpected side effect.. so, if you don't have to ship your software TONIGHT and it's 10pm, there is always a better solution other than making everything global.
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#66 Dave   Members   -  Reputation: 1499

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Posted 10 November 2012 - 02:15 AM


Writing tight OOP code in studios where iteration time is important and year on year you're rewriting large portions of code is common because of new platforms and new titles is a complete waste of time and it is important that you make simple architectural decisions that:


Except every time I hear the 'new code because of new platforms' arguement is I can't help but think it is garbage.

Consoles have been stable for many years now and it's only recently that a nod towards the new ones has happened. We've had ~7 years of stability in that domain.
Windows is an even bigger pile of fun as DX9 was stable for the better part of a decade and again it is only in the last few years that DX11 has become 'a thing'.
Other platforms come along at a much lower frequency and, with correct abstraction, are not a case of 'rewrite large portions of code' but rework layers which are needed to get them up and running.

I would argue that by NOT taking time to design things properly up front people are wasting more time with rewrites and rebuilds of systems which don't need it. The 'get it done' mindset has its uses but hacking on things is not, in itself, a good way to go unless you want to waste time over the course of years rebuilding the problem.

a) Allow new programmers to get on with things easily.
b) Make it easy to add features to.


Except none of this is an arguement against designing things properly nor, more importantly when it comes to this thread, is it an arguement in favour of singletons which, in my experiance, have only served to cause more harm than good.

A correctly designed system can allow for BOTH of those things IFF it is correctly designed.
And by that I don't mean the classic 'just out of university' OOP method of 'zomg! classes for everyone!' but a sensible division of code and responcibility. People get too hung up on 'OOP = class' and lose track of the real meaning of the design methodology. More to the point they focus purely on 'put everything in a class and give it methods' than correctly splitting things up into classes, structures and free functions as required and, more importantly, ignore data flows and how information needs to move around the system.

In short; most people in the world write utterly shit OOP but that doesn't mean it isn't useful to design larger subsystems for it.

It is argued that you can screw things up if you start touching global variables that you shouldn't. If you hire programmers with an ounce of common sense they generally won't do that and no matter what "barriers" you put in place to protect programmers from themselves, if a shortcut is really desired then it will be found. The friend keyword comes out in no time.


The problem is that most people AREN'T good enough to not make mistakes and do it wrong.
I have seen some wonderful things where people fresh out of university have commited crimes against code so bad it makes me wonder why they are allowed to even write code in the first place. (My favorite being the guy who decided to, in his branch without telling anyone, change the interface to our lua subsystem from function(luaObject, ....) syntax to luaObject->function(....) syntax while at the same time weaving his work around the changes. It took me 4 HOURS to get his code back into mainline for a merge and I no longer trust fresh faced programmers not to fuck up in some massive manner)

So, yes, people make mistakes and yes there are ways to hack around the 'protection' BUT with a properly designed system the immediate solution is often cleaner, easier to maintain and, more importantly, does NOT lead them down the path of thinking 'hey, others have done it so it's probably ok for me to do so...' because they learn by example.

Truth of the matter is that most people in the industry can't design for crap - unfortunately if we tried to staff an industry with people who were good at it we'd soon run out of people so in order to save time later its best to discourage bad code to start with.

And to that end... a story!

In our engine we were lacking a sane method of the game team to transfer values from the logic thread to the renderer thread (and thus the GPU).
We took a day to hash out WHAT we wanted the system to do (I'll spare you the details, because frankly they are a tad confusing at times), roughly how we wanted it to interact and then I got cracking on the design and implementation details.
The two senior members of the team feedback into the process every few days as we discussed things and by the end of a two month period we had a working solution (with some of the most complicated multi-pass binding/patch table setup code I've written in my life BUT...) which was created when the renderer started up and cleanly passed, by reference, to the subsystems which required it.

However we weren't finished there because the aim for this system was to replace ALL the variables which could be passed to shaders; this included built in tokens for things like shadow map textures as well as the post-processing system's frame buffer textures and variables.

The 'global texture cache' (as I believe it was called) had been written partly by a senior and finished off by a junior. Frankly the code was overly complex for what it did BUT more importantly it had been implemented as a global system and because of this it had inflitrated multiple levels of code with no clarity as to the flow. It took LONGER for me to remove the old code and clean up the mess it had made than it took me to write the addition to the Parameter System which had been written; Clean OO intergration 1 v Global Code 0.
(The new system also expanded the abilities of the GTC functionality bring it in line with the general parameters affording the game team more control).

Post Processing was the same story; the functionality added to the Parameter System took maybe 2 days to add with no major interface changes aside from some special casing due to the stupid way the Post Processing was implemented.

Finally a few months back we needed a feature adding to the Parameter System at the request of the game team. The conversation as to if I should even add the feature took LONGER than the time it took me to write the feature, integrate it, test it and push it into the publish queue, the system was that extendable.

The system itself was built on both OOD and DOD principles to use classes when it made sense and pay attention to the data when it didn't.
The system was written in Jan/Feb of this year with a couple of weeks in March to integrate the extra functionality and remove the old.

This system is a lynch pin in the game F1 All Stars which is coming out soon; if it breaks then we lose logic->renderer data flow so it's one of the most critical systems in the whole engine right now.

The system has had ZERO time spent on bug fixing; not a single bug has been filed against it and when suspicion has been cast it has been easy to prove (despite the mad binding code which while it looks complex does work flawlessly) it wasn't a problem.

Now, we could have hacked together a system in a month, thrown it in as a global and watched as everyone abused it and dealt with bugs left, right and centre BUT instead we decided to do it right and because of that the integration is clean, the code is robust and its freed me up to work on other bits of genius instead.


Appreciate the detailed reply.

You're right, people in the games industry can't design code, generally, but more importantly they don't. There is good reason for this. If you're writing software that gets men to the moon then:

- Get the best programmers available.
- Define a rock solid coding standard.
- Design every last operation detail of the code before writing a single line.
- Code review, code review, code review.

Unfortunately very often none of these rules apply in the games industry. The only time i've ever seen code designed to any kind of level in the industry was when a new boss of mine tried to enforce it but ended up having to drop it. There simply isn't time in the games industry because:

- You're usually working through shorter development iterations.
- Management and investors these days want to see progress on your project more frequently than they used to. (related to the above)
- Your title is out in a year and no time can be afford to be "wasted" on writing design documentation that, as soon as the game hits the shelf, is irrelevant.

I will say, however, that designing code and having good OOP has its merits. I'm not trying to say that noone in the world should be writing OOP, but in the games industry and to be honest, even more importantly for the hobbyist, don't waste all your time wondering how to add something to your perfectly architected system.

The more time you spend working out how the hell you're going to integrate physics into your engine the more time you're not spending integrating physics into your engine. You have to find a good balance between writing a codebase that will be productive to work on and also decently written and unfortunately for C++ and OOP allow you to code yourself into a corner too often.

I'm not disagreeing with you here, i understand the necessity of OOP in software engineering. I'm just not convinced the games industry is a suitable domain for it. I've been full circle with writing game engines in my personal time and my current one is my most productive. Gradually i'm migrating it over to plain C with a few C++ niceties like templates here and there.

#67 larspensjo   Members   -  Reputation: 1526

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Posted 10 November 2012 - 02:40 AM

The classical three project objectives: time, quality and money. Only two can get higher priority.
Current project: Ephenation.
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#68 Khatharr   Crossbones+   -  Reputation: 2962

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Posted 10 November 2012 - 05:01 AM

Hey, I was about to go to bed and my subconscious kicked up something from years back, so I jumped back on the comp, lol.

Are you Lars Pensjo as in Mr. LPMUD Lars Pensjo? If so that's awesome. Posted Image
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#69 larspensjo   Members   -  Reputation: 1526

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Posted 10 November 2012 - 11:02 AM

Are you Lars Pensjo as in Mr. LPMUD Lars Pensjo?

That was old adventures. I got bored working as a SW project manager, so I had to go back to games business and have some fun.
Current project: Ephenation.
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#70 Khatharr   Crossbones+   -  Reputation: 2962

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Posted 10 November 2012 - 12:43 PM

I used to kick around Genesis when I was a pup. You rock dude. :D
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#71 Josh Petrie   Moderators   -  Reputation: 3111

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Posted 10 November 2012 - 01:46 PM

You're right, people in the games industry can't design code, generally, but more importantly they don't. There is good reason for this. If you're writing software that gets men to the moon then:

- Get the best programmers available.
- Define a rock solid coding standard.
- Design every last operation detail of the code before writing a single line.
- Code review, code review, code review.

Unfortunately very often none of these rules apply in the games industry. The only time i've ever seen code designed to any kind of level in the industry was when a new boss of mine tried to enforce it but ended up having to drop it. There simply isn't time in the games industry because:

- You're usually working through shorter development iterations.
- Management and investors these days want to see progress on your project more frequently than they used to. (related to the above)
- Your title is out in a year and no time can be afford to be "wasted" on writing design documentation that, as soon as the game hits the shelf, is irrelevant.

I will say, however, that designing code and having good OOP has its merits. I'm not trying to say that noone in the world should be writing OOP, but in the games industry and to be honest, even more importantly for the hobbyist, don't waste all your time wondering how to add something to your perfectly architected system.

The more time you spend working out how the hell you're going to integrate physics into your engine the more time you're not spending integrating physics into your engine. You have to find a good balance between writing a codebase that will be productive to work on and also decently written and unfortunately for C++ and OOP allow you to code yourself into a corner too often.

I'm not disagreeing with you here, i understand the necessity of OOP in software engineering. I'm just not convinced the games industry is a suitable domain for it. I've been full circle with writing game engines in my personal time and my current one is my most productive. Gradually i'm migrating it over to plain C with a few C++ niceties like templates here and there.


While historically this is certainly true, I think our industry is maturing to the point where we can start to evolve out of the "cowboy coding" mentality that marked our formative years. Consider MMOs, for example -- you can't expect to build a successful MMO with poor engineering practices. You are building a service more than you are building a product you can ship and forget, a service you'll need to maintain and extend for a decade (or more, hopefully). Sure, MMOs as we know them today will eventually become passe, but some of their core elements, such as massively-connected (or "social," as the kids like to call it days) systems are probably going to embed themselves in the culture of our products for a long time. Even primarily single-player games are starting to integrate those kinds of features.


At ArenaNet, for example, we do a very good job of hitting three out of four of your bullet points. Our automated testing and test tools can be greatly improved (and we're working on it actively), but we take hiring extremely seriously, do up-front design of new systems and RFCs for changes to old ones, and have a very rigorous set of coding standards. We've developed these polices because we have to actively maintain two services -- Guild Wars 1 and now Guild Wars 2 -- for the foreseeable future and now, post-launch, we don't have the luxury of a five-year development cycle to get updates and patches out in to the live environment, so we've had to make sure these practices are followed reasonably during development so that they become habitual and we still employ them even when scrambling to patch an exploit or bug. If we don't, we're going to be building a house of cards for the next seven years -- the failure of which could basically cost the company its existence.

To your last point about writing more "C-like:" I don't think that's actually counter to the idea of writing code that espouses good (possibly OO) design principles. Such code tends to be simpler and consequently easier to read and maintain, and can still involve things like clear responsibility segregation, implementation hiding, et cetera. Much of our code at work is, for example, very C-like in its straightforwardness, and I do the same in many of my own hobby projects. Clever, complicated template metaprogramming hackery and overzealous design-pattern boilerplate implementations that adhere dogmatically to the Rules of Proper Design are, practically speaking, the domain of the academic, the "I wonder if I could..." masturbatory excursions that serve mostly to stretch the language and boost the ego of the author. Certainly, interesting innovations come from such endeavors, but increasingly I find must of them inappropriate for production code on any kind of scale.

Josh Petrie | Core Tools Engineer, 343i | Microsoft C++ MVP


#72 Khatharr   Crossbones+   -  Reputation: 2962

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Posted 10 November 2012 - 02:27 PM

These are some things that I've been mulling over myself as I learn more and more about 'proper' coding. I can definitely see the benefits of all the rules and customs developed over the years, but it also strikes me that these things should be measured carefully. If I'm going to write something that needs to last for years to come then I had darn well better "do it right" because in the long run the amount of time saved will be enormous, but on the other hand I see things like "don't use C style things in C++" and in many cases I find myself asking "why the hell not?". It strikes me that rules or customs should be measured based on their effectiveness for the task at hand. In many (even most) cases there's good reasons, but sometimes an array is just a gorram array and using a vector is a waste of time both for the developer and for the CPU at runtime. If I need STL container functionality later it's not difficult to add.

A somewhat related issue, since you mentioned cowboy'ing, if I'm going to write something complex I'll usually sit down and bash it out as quickly as possible and then once its in place I can spot all the structural problems and go back over it (immediately) and write a stable system with all the bells and whistles. I don't want to get stuck spending extra hours writing something that's going to get scrapped or re-written anyway because a better design idea occurred or a structural problem emerged while the code was being written.

I guess what I mean is that I support the K.I.S.S. rule above all others. If something looks and feels complicated then I can't shake the feeling that I'm doing it wrong. I don't understand making something more complex than it needs to be, especially since it seems like complexity is easy to add and hard to remove. Sometimes I feel like the rules and culture are almost a language in and of themselves. It's like C extends to C++ and then C++ extends to 'proper' C++. I've seen criticism about using C++ as if it were C, but the fact is that vanilla C is an effective language as well. In terms of readability C-style is great, but I also find that it seems like the program likes it as well, if that makes any sense. Simple code just usually seems to run better. Algorithms, for instance. I ws looking into algorithms while familiarizing myself with the musty corners of STL and I saw that I could have used an algorithm in place of a loop for iterating through the values in a container and modifying them. Then, by chance, I looked at the code of the appropriate algorithm and saw that it was more or less exactly what I had already written. So all I would gain by implementing the loop with the algorithm is some excess stack framing. On the other hand, there are some situations where algorithms are more or less '****ing rad', to use the technical term, and in those places I gladly use them.

I don't really know where I'm going with this rambling. Like I said, it's just something that's been on my mind. Glad to see other people thinking about it as well.

Edited by Khatharr, 10 November 2012 - 02:46 PM.

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#73 Telastyn   Crossbones+   -  Reputation: 3726

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Posted 10 November 2012 - 03:13 PM

It strikes me that rules or customs should be measured based on their effectiveness for the task at hand.


**They are.**

"Don't use C style things in C++" is one of those things that has been done and found to cause issues due to odd interactions in the mixed codebase. It's one thing to question things, but many of these best practices are best practices for just this reason.

#74 SiCrane   Moderators   -  Reputation: 9560

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Posted 10 November 2012 - 04:32 PM

Probably the biggest issue with doing things the C way in C++ code is that C++ has exceptions. Some perfectly good C code compiled by a C++ compiler can become unsafe because all of the sudden you can't be sure that call to foo() won't unwind the stack on you (and even if it can't throw an exception now that doesn't mean someone won't change things later). Hence the recommendation to use things like RAII types. Following closely on that is the fact that many of those straight forward C style techniques are actually less secure than the C++ style equivalents.

#75 davepermen   Members   -  Reputation: 1007

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Posted 11 November 2012 - 08:56 AM

METADATA:

There are many times when a program needs to store and reference data that is a result of processing and that can be used again. How will you store metadata? I like to use singletons for structural/constant data/flags/metadata(temporary) instances, when the complexity of creating new instances and having it loading its data from something/somewhere doesn't worth as simply acessing it straight like cache of processors is used and Windows Registry (always in memory).

I just use an ordinary class for this. And if at some point i need multiple different resources, I can just do that. If i don't, I don't. That's the point: LIMITING YOURSELF doesn't help you later. It will BLOCK you later. Without ever having given you any gain. You want just one: Instanciate just one. You want global access? Make it a global. Don't try to hide it behind some fancy terms. Be honest to yourself.
If that's not the help you're after then you're going to have to explain the problem better than what you have. - joanusdmentia

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#76 LorenzoGatti   Crossbones+   -  Reputation: 2663

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Posted 14 November 2012 - 02:38 AM

You want just one: Instanciate just one. You want global access? Make it a global.

This is the right spirit. Avoiding useless extra instances of objects and getting the useful objects where are needed are two interlinked rather difficult problems; rather tragically, progressing on the former makes the latter harder.
Good solutions require good design and forethought, and often tedious code, for example lots of parameters and variables to pass objects around or checks of the presence and validity of objects that might or might not have been initialized, not to mention (in many cases) understanding and accepting the price of global variables: obviously the average mediocre programmer is attracted by singletons because
  • Singletons look clever and legitimate (a tie-wearing design pattern!) without entailing actual design effort. If it shouldn't have been a singleton, you'll find out when it's too late to change your mind cheaply, because you didn't consider different options and contingencies at the proper time.
  • Accessing singletons allows a strong subconscious denial (or intentional camouflage) that you are using a global variable, and a delusion that what you are doing is technically better (or at least more enterprisey).

Edited by LorenzoGatti, 14 November 2012 - 04:21 PM.

Produci, consuma, crepa

#77 Khatharr   Crossbones+   -  Reputation: 2962

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Posted 14 November 2012 - 03:10 AM

enterprisey


Increasing visibility is a good thing, and it will surely make us more... visible. To do that we should, uh... develop our strategy and strategize our development. Implement solutions and solutionize implementations. Aggressively.

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#78 Dave   Members   -  Reputation: 1499

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Posted 14 November 2012 - 04:03 AM

You want just one: Instanciate just one. You want global access? Make it a global.

This is the right spirit. Avoiding useless extra instances of objects and getting the useful objects where are needed are two interlinked rather difficult problems; rather tragically, progressing on the former makes the latter harder.
Good solutions require good design and forethought, and often tedious code, for example lots of parameters and variables to pass objects around or checks of the presence and validity of objects that might or might not have been initialized, not to mention (in many cases) understanding and accepting the price of global variables: obviously the average mediocre programmer is attracted by singletons because
  • Singletons look clever and legitimate (a tie-wearing design pattern!) without entailing actual design effort. If it shouldn't have been a singleton, you'll find out when it's too late to change your mind cheaply, because you didn't think consider different options and contingencies at the proper time.
  • Accessing singletons allows a strong subconscious denial (or intentional camouflage) that you are using a global variable, and a delusion that what you are doing is technically better (or at least more enterprisey).


What's the price of a global variable?

#79 Hodgman   Moderators   -  Reputation: 29493

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Posted 14 November 2012 - 04:27 AM

Welcome to 1973.
http://c2.com/cgi/wiki?GlobalVariablesConsideredHarmful
http://c2.com/cgi/wiki?GlobalVariablesAreBad

#80 Khatharr   Crossbones+   -  Reputation: 2962

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Posted 14 November 2012 - 05:16 AM

What's the price of a global variable?


Loss of access control. (Also violation of dogma.... j/k)

Globals can be reached by parts of the program that have no business reaching them and can result in serious design problems later on. It's a matter of whether or not you're working with people that can't handle the responsibility. Sadly this is a lot of people, from what I hear. Once things get complicated it can become nearly impossible to have reasonable state expectations for something with global accessibility.

That's a good article you got there, Hodge.

Edited by Khatharr, 14 November 2012 - 05:21 AM.

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There are ten kinds of people in this world: those who understand binary and those who don't.




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