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I'm wondering what I should be using instead of goto statements?
Should I be using nested if/while/do-while statements?
They say that using goto creates 'spaghetti code', but surely if someone is writing a large console application and they have if statement after if statement in an attempt to control the flow, that's going to make a mess?
I'm asking as many people have asked why the goto statement is bad, but not what to replace it with. I'm sure a lot of beginners could do with this.
This is for C++.
You are much better off using functions, loops, and conditional statements. Use break and continue as necessary.
I can nearly guarantee you any situation in which you are utilizing goto there is a better alternative. There is one notable exception to this: multi-level break.
while(1){
while(1){
goto breakOut;
}
//more code here?
}
breakOut:
In this one (relatively) rare situation, goto can be used in place of a typical "break" to make it clear we are actually getting out of a nested loop. The other way to approach this is with a "done" variable:
while(!done){
while(!done){
done = true;
break;
}
if(done){break;}
//More code here? If so, the above line is important!
}
As you can see, the done variable is more verbose when you have additional processing in outer loops, so a goto is a cleaner way of breaking free!
However, in 99% of cases you really, really, don't want to start writing a bunch of goto statements. Really think through each one.
With functions the above could also be written like so:
bool innerLoop(){
while(1){
return false;
}
return true;
}
...
while(innerLoop()){ //can only be written this way if the inner loop is the first thing that should run.
//More code here?
}
...
Sometimes breaking an inner loop out in this way can be messy if there are a lot of dependencies on the outer one. But it remains a viable way of breaking out of code early with return statements instead of goto or break.
If you can write your logic using a clean, modern construct, then use it. Otherwise, if goto makes sense, use it.
In general, goto can make your code harder to read and follow the flow of the code. For that reason, newbies are told to avoid goto altogether. This encourages them to think in terms of other constructs.
But some people just get religious about it. Coding is not a religion. And if a goto makes sense, C++ has a perfectly valid goto statement and you should use it when it makes good sense.
It doesn't make sense to me to ask what you should use instead of goto. You can generally use some type of loop or other construct, depending on what you are doing. But where goto makes sense, use it.
Don’t listen to people who say "never use goto". You're quite right that there are cases where nesting scoped blocks will make a heck of a lot more mess than a goto. It has its place, just as a switch/case does. However, with functions you can often refactor away the entire argument and keep people happy.
Simply forget that there is the goto statement in C++ and there will not be questions about how to replace the goto.:)
As for me I never see a code with goto statement that I could call it as a good code. Usually a goto statement is the source of some kind of errors and difficulties. Especially it is difficult to modify such code. Moreover one goto statement usually starts to produce other goto statements in the code.:)
The goto is bad because it allows you to jump from context to context. The context is a vector of all your variables (with their values) at a particular point of your program. A program execution graph shows you how your program jumps from context to context. It's in your best interests to keep the execution graph as simple as possible. The simplest one is a chain, the next step is an execution tree. A loop adds complexity, but it's a managed complexity. If you have a node in your execution graph, which is reachable via more than one execution path, and you need to understand a context at this node, then you'll need to follow more than one execution path back. These "merge" nodes add a lot to the complexity of your program. Each labeled statement (the goto target) is a potential merge node.
So, try don't use the goto operator at all - the language itself will force you to find a manageable solution using loops, boolean variables, classes, functions etc. and your program will be more clear and understandable. C++ exceptions is another manageable way to jump between contexts.
There are computing geniuses, who can keep in their minds and process very complex execution graphs, so they don't care much about complexity of their programs or a next programmer, who will be assigned to support or take over their code in the future. I guess, we have some of them here :-)
This question already has answers here:
Examples of good gotos in C or C++ [closed]
(16 answers)
Closed 9 years ago.
I know lots of questions about why not use goto, why goto is bad practice, why goto was create by devil, why the fingers of those who type goto should be ripped off, etc...
And in many answers to this questions , this question, and even at wikipedia it's possible to find reasons pro / against goto.
Even Stackoverflow has a goto tag :)
But, obviously, goto is accepted as a valid command, otherwise your compiler / interpreter wouldn't know what to do with it. Even in C / C++ it's possible to use goto, and that language even has a formal, ISO specification.
So, probably, there should be some case where only by using goto it's possible to accomplish some task. Then, why and when should a goto be used in C / C++ ?
Edit:
Just to make sure:
1 - this question doesn't require debate. It should be answerable: giving one or more cases where goto is the only solution, or stating no, it's always possible to avoid a goto. Of course, I'm not the only who can decide whether it should be closed or not, but I don't think that "too broad, asks for debate, etc" are reasons to close it.
2 - There might be some cases where goto make the code cleaner, faster, etc. I don't want those. I want to know if there exists one case where goto is the only solution (no, it's not a dup of that "give good examples of goto" question)
3 - I'll accept any downvote, with a smile on my face, if the downvote is justified. But I'm pretty sure that this question has an answer and has done previous research about the subject. Downvoting simple because those 4-letters taboo word that starts with a "G" was used... sorry...
There is no circumstance under which a goto is strictly necessary: you can always avoid it if you really want to, but to do that for purely idealogical reasons is a mistake.
For example, a friend of mine wrote a wavelet transform function that had (something like) 15 nested loops. In the event of an error in those loops, he had a goto to a cleanup block just before the function's return statement. To achieve the same effect without a goto would have involved setting a flag and testing it at every loop level, which would have made his code far less readable. In these circumstances, goto was the right choice.
The latest MISRA standard now allows gotos.
A good example where gotos make sense is when you have a large routine with a lot of exits points. You can have many return statements (not good) convolute the code with 'structured programming' conditionals (also not good) or a "goto Done; which sends the program to a set of ending statements before returning.
The MISRA standard basically allows gotos for these sort of circumstances. I think 'only downward' is one of their criteria.
The only reason I use a goto is when it is for an error return condition from a function that needs some common cleanup. The target of the goto is near the end of the function and the "normal" return returns before the label.
Here is an example where only goto will work:
https://stackoverflow.com/a/245801/193848
Basically if you have multiple nested for loops, the only way to break out of all the loops from an inner loop is with a goto statement (since unlike some other languages the C break keyword doesn't support a parameter for the number of nesting levels to break out).
I've been working on a foundational c++ library for some time now, and there are a variety of ideas I've had that could really simplify the code writing and managing process. One of these is the concept of introducing some macros to help simplify statements that appear very often, but are a bit more complicated than should be necessary.
For example, I've come up with this basic macro to simplify the most common type of for loop:
#define loop(v,n) for(unsigned long v=0; v<n; ++v)
This would enable you to replace those clunky for loops you see so much of:
for (int i = 0; i < max_things; i++)
With something much easier to write, and even slightly more efficient:
loop (i, max_things)
Is it a good idea to use conventions like this? Are there any problems you might run into with different types of compilers? Would it just be too confusing for someone unfamiliar with the macro(s)?
IMHO this is generally a bad idea. You are essentially changing well known and understood syntax to something of your own invention. Before long you may find that you have re-invented the language. :)
No, not a good idea.
int max = 23;
loop(i, ++max)...
It is, however, a good idea to refactor commonly used code into reusable components and then reuse instead of copy. You should do this through writing functions similar to the standard algorithms like std::find(). For instance:
template < typename Function >
void loop(size_t count, Function f)
{
for (size_t i = 0; i < count, ++i) f();
}
This is a much safer approach:
int max = 23;
loop(++max, boost::bind(....));
I think you've provided one strong argument against this macro with your example usage. You changed the loop iterator type from int to unsigned long. That has nothing to do with how much typing you want to do, so why change it?
That cumbersome for loop specifies the start value, end value, type and name of the iterator. Even if we assume the final part will always be ++name, and we're happy to stick to that, you have two choices - remove some of the flexibility or type it all out every time. You've opted to remove flexibility, but you also seem to be using that flexibility in your code base.
I would say it depends upon whether you expect anyone else to ever have to make sense of your code. If it's only ever going to be you in there, then I don't see a problem with the macros.
If anyone else is ever going to have to look at this code, then the macros are going to cause problems. The other person won't know what they are or what they do (no matter how readable and obvious they seem to you) and will have to go hunting for them when they first run across them. The result will be to make your code unreadable to anyone but yourself - anyone using it will essentially have to learn a new language and program at the same time.
And since the chances of it just being you dealing with the code are pretty much nil if you hope the code to be a library that will be for more than just your personal use - then I'd go with don't.
In Unix, I find that by the time I want to create an alias for a command I use all the time, the command is on my fingers, and I'd have a harder time remembering the syntax of my alias than the original command.
The same applies here -- by the time you use an idiom so much that you want to create a macro for it, the idiom will be on you fingers and cause you more pain than just typing out the code.
Getting rid of the for loops is generally a good idea -- but replacing them with macros is not. I'd take a long, hard look at the standard library algorithms instead.
Apart from the maintenance/comprehension problems mentionned by others, you'll also have a hard time breaking and single-stepping through macro code.
One area where I think macros might be acceptable would be for populating large data structures with constants/litterals (when it can save an excessive amount of typing). You normally would not single-step through such code.
Steve Jessop makes a good point. Macros have their uses. If I may expound upon his statements, I would go so far as to say that the argument for or against macros comes down to "It depends". If you make your macros without careful thought, you risk making future maintaners' lives harder. On the other hand, using the wxWidgets library requires using library provided macros to connect your code with the gui library. In this case, the macros lower the barrier of entry for using the library, as magic whose innards are irrelevant to understanding how to work with the library are hidden away from the user. In this case, the user is saved from having to understand things they really don't need to know about, and can be argued that this is a "Good" use of macros. Also, wxWidgets clearly documents how these macros are supposed to be used. So make sure that what you hide isn't something that is going to need to be understood by someone else coming in.
Or, if its just for your use, knock yourself out.
It's a question of where you're getting your value. Is typing those 15 extra characters in your loops really what's slowing your development down? Probably not. If you've got multiple lines of confusing, unavoidable boilerplate popping up all over the place, then you can and should look for ways to avoid repeating yourself, such as creating useful functions, cleaning up your class hierarchies, or using templates.
But the same optimization rules apply to writing code as to running it: optimizing small things with little effect is not really a good use of time or energy.
I don't mean external tools. I think of architectural patterns, language constructs, habits. I am mostly interested in C++
Automated Unit Testing .
There's an oft-unappreciated technique that I like to call The QA Team that can do wonders for weeding out bugs before they reach production.
It's been my experience (and is often quoted in textbooks) that programmers don't make the best testers, despite what they may think, because they tend to test to behaviour they already know to be true from their coding. On top of that, they're often not very good at putting themelves in the shoes of the end user (if it's that kind of app), and so are likely to neglect UI formatting/alignment/usability issues.
Yes, unit testing is immensely important and I'm sure others can give you better tips than I on that, but don't neglect your system/integration testing. :)
..and hey, it's a language independent technique!
Code Review, Unit Testing, and Continuous Integration may all help.
I find the following rather handy.
1) ASSERTs.
2) A debug logger that can output to the debug spew, console or file.
3) Memory tracking tools.
4) Unit testing.
5) Smart pointers.
Im sure there are tonnes of others but I can't think of them off the top of my head :)
RAII to avoid resource leakage errors.
Strive for simplicity and conciseness.
Never leave cases where your code behavior is undefined.
Look for opportunities to leverage the type system and have the compiler check as much as possible at compile time. Templates and code generation are your friends as long as you keep your common sense.
Minimize the number of singletons and global variables.
Use RAII !
Use assertions !
Automatic testing of some nominal and all corner cases.
Avoid last minute changes like the plague.
I use thinking.
Reducing variables scope to as narrow as possible. Less variables in outer scope - less chances to plant and hide an error.
I found that, the more is done and checked at compile time, the less can possibly go wrong at run-time. So I try to leverage techniques that allow stricter checking at compile-time. That's one of the reason I went into template-meta programming. If you do something wrong, it doesn't compile and thus never leaves your desk (and thus never arrives at the customer's).
I find many problems before i start testing at all using
asserts
Testing it with actual, realistic data from the start. And testing is necessary not only while writing the code, but it should start early in the design phase. Find out what your worst use cases will be like, and make sure your design can handle it. If your design feels good and elegant even against these use cases, it might actually be good.
Automated tests are great for making sure the code you write is correct. However, before you get to writing code, you have to make sure you're building the right things.
Learning functional programming helps somehow.
HERE
Learn you a haskell for great good.
Model-View-Controller, and in general anything with contracts and interfaces that can be unit-tested automatically.
I agree with many of the other answers here.
Specific to C++, the use of 'const' and avoiding raw pointers (in favor of references and smart pointers) when possible has helped me find errors at compile time.
Also, having a "no warnings" policy helps find errors.
Requirements.
From my experience, having full and complete requirements is the number one step in creating bug-free software. You can't write complete and correct software if you don't know what it's supposed to do. You can't write proper tests for software if you don't know what it's supposed to do; you'll miss a fair amount of stuff you should test. Also, the simple process of writing the requirements helps you to flesh them out. You find so many issues and problems before you ever write the first line of code.
I find peer progamming tends to help avoid a lot of the silly mistakes, and al ot of the time generates discussions which uncover flaws. Plus with someone free to think about the why you are doing something, it tends to make everything cleaner.
Code reviews; I've personally found lots of bugs in my colleagues' code and they have found bugs in mine.
Code reviews, early and often, will help you to both understand each others' code (which helps for maintenance), and spot bugs.
The sooner you spot a bug the easier it is to fix. So do them as soon as you can.
Of course pair programming takes this to an extreme.
Using an IDE like IntelliJ that inspects my code as I write it and flags dodgy code as I write it.
Unit Testing followed by Continious Integration.
Book suggestions: "Code Complete" and "Release it" are two must-read books on this topic.
In addition to the already mentioned things I believe that some features introduced with C++0x will help avoiding certain bugs. Features like strongly-typed enums, for-in loops and deleteing standard functions of objects come to mind.
In general strong typing is the way to go imho
Coding style consistency across a project.
Not just spaces vs. tab issues, but the way that code is used. There is always more than one way to do things. When the same thing gets done differently in different places, it makes catching common errors more difficult.
It's already been mentioned here, but I'll say it again because I believe this cannot be said enough:
Unnecessary complexity is the arch nemesis of good engineering.
Keep it simple. If things start looking complicated, stop and ask yourself why and what you can do to break the problem down into smaller, simpler chunks.
Hire someone that test/validate your software.
We have a guy that use our software before any of our customer. He finds bugs that our automated tests processes do not find, because he thinks as a customer not as a software developper. This guy also gives support to our customers, because he knows very well the software from the customer point of view. INVALUABLE.
all kinds of 'trace'.
Something not mentioned yet - when there's even semi-complex logic going on, name your variables and functions as accurately as you can (but not too long). This will make incongruencies in their interactions with each other, and with what they're supposed to be doing stand out better. The 'meaning', or language-parsing part of your brain will have more to grab on to. I find that with vaguely named things, your brain sort of glosses over what's really there and sees what is /supposed to/ be happening rather than what actually is.
Also, make code clean, it helps to keep your brain from getting fuzzy.
Test-driven development combined with pair programming seems to work quite well on keeping some bugs down. Getting the tests created early helps work out some of the design as well as giving some confidence should someone else have to work with the code.
Creating a string representation of class state, and printing those out to console.
Note that in some cases single line-string won't be enough, you will have to code small printing loop, that would create multi-line representation of class state.
Once you have "visualized" your program in such a way you can start to search errors in it. When you know which variable contained wrong value in the end, it's easy to place asserts everywhere where this variable is assigned or modified. This way you can pin point the exact place of error, and fix it without using the step-by-step debugging (which is rather slow way to find bugs imo).
Just yesterday found a really nasty bug without debugging a single line:
vector<string> vec;
vec.push_back("test1");
vec.push_back(vec[0]); // second element is not "test1" after this, it's empty string
I just kept placing assert-statements and restarting the program, until multi-line representation of program's state was correct.
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What guidelines do you follow to improve the general quality of your code? Many people have rules about how to write C++ code that (supposedly) make it harder to make mistakes. I've seen people insist that every if statement is followed by a brace block ({...}).
I'm interested in what guidelines other people follow, and the reasons behind them. I'm also interested in guidelines that you think are rubbish, but are commonly held. Can anyone suggest a few?
To get the ball rolling, I'll mention a few to start with:
Always use braces after every if / else statement (mentioned above). The rationale behind this is that it's not always easy to tell if a single statement is actually one statement, or a preprocessor macro that expands to more than one statement, so this code would break:
// top of file:
#define statement doSomething(); doSomethingElse
// in implementation:
if (somecondition)
doSomething();
but if you use braces then it will work as expected.
Use preprocessor macros for conditional compilation ONLY. preprocessor macros can cause all sorts of hell, since they don't allow C++ scoping rules. I've run aground many times due to preprocessor macros with common names in header files. If you're not careful you can cause all sorts of havoc!
Now over to you.
A few of my personal favorites:
Strive to write code that is const correct. You will enlist the compiler to help weed out easy to fix but sometimes painful bugs. Your code will also tell a story of what you had in mind at the time you wrote it -- valuable for newcomers or maintainers once you're gone.
Get out of the memory management business. Learn to use smart pointers: std::auto_ptr, std::tr1::shared_ptr (or boost::shared_ptr) and boost::scoped_ptr. Learn the differences between them and when to use one vs. another.
You're probably going to be using the Standard Template Library. Read the Josuttis book. Don't just stop after the first few chapters on containers thinking that you know the STL. Push through to the good stuff: algorithms and function objects.
Delete unnecessary code.
That is all.
Use and enforce a common coding style and guidelines. Rationale: Every developer on the team or in the firm is able to read the code without distractions that may occur due to different brace styles or similar.
Regularly do a full rebuild of your entire source base (i.e. do daily builds or builds after each checkin) and report any errors! Rationale: The source is almost always in a usable state, and problems are detected shortly after they are "implemented", where problem solving is cheap.
Turn on all the warnings you can stand in your compiler (gcc: -Wall is a good start but doesn't include everything so check the docs), and make them errors so you have to fix them (gcc: -Werror).
Google's style guide, mentioned in one of these answers, is pretty solid. There's some pointless stuff in it, but it's more good than bad.
Sutter and Alexandrescu wrote a decent book on this subject, called C++ Coding Standards.
Here's some general tips from lil' ole me:
Your indentation and bracketing style are both wrong. So are everyone else's. So follow the project's standards for this. Swallow your pride and setup your editor so that everything is as consistent as possible with the rest of the codebase. It's really really annoying having to read code that's indented inconsistently. That said, bracketing and indenting have nothing whatsoever to do with "improving your code." It's more about improving your ability to work with others.
Comment well. This is extremely subjective, but in general it's always good to write comments about why code works the way it does, rather than explaining what it does. Of course for complex code it's also good for programmers who may not be familiar with the algorithm or code to have an idea of what it's doing as well. Links to descriptions of the algorithms employed are very welcome.
Express logic in as straightforward a manner as possible. Ironically suggestions like "put constants on the left side of comparisons" have gone wrong here, I think. They're very popular, but for English speakers, they often break the logical flow of the program to those reading. If you can't trust yourself (or your compiler) to write equality compares correctly, then by all means use tricks like this. But you're sacrificing clarity when you do it. Also falling under this category are things like ... "Does my logic have 3 levels of indentation? Could it be simpler?" and rolling similar code into functions. Maybe even splitting up functions. It takes experience to write code that elegantly expresses the underlying logic, but it's worth working at it.
Those were pretty general. For specific tips I can't do a much better job than Sutter and Alexandrescu.
In if statements put the constant on the left i.e.
if( 12 == var )
not
if( var == 12 )
Beacause if you miss typing a '=' then it becomes assignment. In the top version the compiler says this isn't possible, in the latter it runs and the if is always true.
I use braces for if's whenever they are not on the same line.
if( a == b ) something();
if( b == d )
{
bigLongStringOfStuffThatWontFitOnASingleLineNeatly();
}
Open and close braces always get their own lines. But that is of course personal convention.
Only comment when it's only necessary to explain what the code is doing, where reading the code couldn't tell you the same.
Don't comment out code that you aren't using any more. If you want to recover old code, use your source control system. Commenting out code just makes things look messy, and makes your comments that actually are important fade into the background mess of commented code.
Use consistent formatting.
When working on legacy code employ the existing style of formatting, esp. brace style.
Get a copy of Scott Meyer's book Effective C++
Get a copy of Steve MConnell's book Code Complete.
There is also a nice C++ Style Guide used internally by Google, which includes most of the rules mentioned here.
Start to write a lot of comments -- but use that as an opportunity to refactor the code so that it's self explanatory.
ie:
for(int i=0; i<=arr.length; i++) {
arr[i].conf() //confirm that every username doesn't contain invalid characters
}
Should've been something more like
for(int i=0; i<=activeusers.length; i++) {
activeusers[i].UsernameStripInvalidChars()
}
Use tabs for indentations, but align data with spaces
This means people can decide how much to indent by changing the tab size, but also that things stay aligned (eg you might want all the '=' in a vertical line when assign values to a struct)
Allways use constants or inline functions instead of macros where posible
Never use 'using' in header files, because everything that includes that heafer will also be affected, even if the person includeing your header doesn't want all of std (for example) in their global namespace.
If something is longer than about 80 columes, break it up into multiple lines eg
if(SomeVeryLongVaribleName != LongFunction(AnotherVarible, AString) &&
BigVaribleIsValid(SomeVeryLongVaribleName))
{
DoSomething();
}
Only overload operators to make them do what the user expects, eg overloading the + and - operators for a 2dVector is fine
Always comment your code, even if its just to say what the next block is doing (eg "delete all textures that are not needed for this level"). Someone may need to work with it later, posibly after you have left and they don't want to find 1000's of lines of code with no comments to indicate whats doing what.
setup coding convention and make everyone involved follow the convention (you wouldn't want reading code that require you to figure out where is the next statement/expression because it is not indented properly)
constantly refactoring your code (get a copy of Refactoring, by Martin Fowler, pros and cons are detailed in the book)
write loosely coupled code (avoid writing comment by writing self-explanatory code, loosely coupled code tends to be easier to manage/adapt to change)
if possible, unit test your code (or if you are macho enough, TDD.)
release early, release often
avoid premature optimization (profiling helps in optimizing)
In a similar vein you might find some useful suggestions here: How do you make wrong code look wrong? What patterns do you use to avoid semantic errors?
Where you can, use pre-increment instead of post-increment.
I use PC-Lint on my C++ projects and especially like how it references existing publications such as the MISRA guidelines or Scott Meyers' "Effective C++" and "More Effective C++". Even if you are planning on writing very detailed justifications for each rule your static analysis tool checks, it is a good idea to point to established publications that your user trusts.
Here is the most important piece of advice I was given by a C++ guru, and it helped me in a few critical occasions to find bugs in my code:
Use const methods when a method is not supposed to modify the object.
Use const references and pointers in parameters when the object is not supposed to modify the object.
With these 2 rules, the compiler will tell you for free where in your code the logic is flawed!
Also, for some good techniques you might follow Google's blog "Testing on the Toilet".
Look at it six months later
make sure you indent properly
Hmm - I probably should have been a bit more specific.
I'm not so much looking for advice for myself - I'm writing a static code analysis tool (the current commercial offerings just aren't good enough for what I want), and I'm looking for ideas for plugins to highlight possible errors in the code.
Several people have mentioned things like const correctness and using smart pointers - that's the kind of think I can check for. Checking for indentation and commenting is a bit harder to do (from a programming point of view anyway).
Smart pointers have a nice way of indicating ownership very clearly. If you're a class or a function:
if you get a raw pointer, you don't own anything. You're allowed to use the pointee, courtesy of your caller, who guarantees that the pointee will stay alive longer than you.
if you get a weak_ptr, you don't own the pointee, and on top of that the pointee can disappear at any time.
if you get a shared_ptr, you own the object along with others, so you don't need to worry. Less stress, but also less control.
if you get an auto_ptr, you are the sole owner of the object. It's yours, you're the king. You have the power to destroy that object, or give it to someone else (thereby losing ownership).
I find the case for auto_ptr particularly strong: in a design, if I see an auto_ptr, I immediately know that that object is going to "wander" from one part of the system to the other.
This is at least the logic I use on my pet project. I'm not sure how many variations there can be on the topic, but until now this ruleset has served me well.