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Currently I am working on a project where goto statements are heavely used. The main purpose of goto statements is to have one cleanup section in a routine rather than multiple return statements.
Like below:
BOOL foo()
{
BOOL bRetVal = FALSE;
int *p = NULL;
p = new int;
if (p == NULL)
{
cout<<" OOM \n";
goto Exit;
}
// Lot of code...
Exit:
if(p)
{
delete p;
p = NULL;
}
return bRetVal;
}
This makes it much easier as we can track our clean up code at one section in code, that is, after the Exit label.
However, I have read many places it's bad practice to have goto statements.
Currently I am reading the Code Complete book, and it says that we need to use variables close to their declarations. If we use goto then we need to declare/initialize all variables before first use of goto otherwise the compiler will give errors that initialization of xx variable is skipped by the goto statement.
Which way is right?
From Scott's comment:
It looks like using goto to jump from one section to another is bad as it makes the code hard to read and understand.
But if we use goto just to go forward and to one label then it should be fine(?).
I am not sure what do you mean by clean up code but in C++ there is a concept called "resource acquisition is initialization" and it should be the responsibility of your destructors to clean up stuff.
(Note that in C# and Java, this is usually solved by try/finally)
For more info check out this page:
http://www.research.att.com/~bs/bs_faq2.html#finally
EDIT: Let me clear this up a little bit.
Consider the following code:
void MyMethod()
{
MyClass *myInstance = new MyClass("myParameter");
/* Your code here */
delete myInstance;
}
The problem: What happens if you have multiple exits from the function? You have to keep track of each exit and delete your objects at all possible exits! Otherwise, you will have memory leaks and zombie resources, right?
The solution: Use object references instead, as they get cleaned up automatically when the control leaves the scope.
void MyMethod()
{
MyClass myInstance("myParameter");
/* Your code here */
/* You don't need delete - myInstance will be destructed and deleted
* automatically on function exit */
}
Oh yes, and use std::unique_ptr or something similar because the example above as it is is obviously imperfect.
I've never had to use a goto in C++. Ever. EVER. If there is a situation it should be used, it's incredibly rare. If you are actually considering making goto a standard part of your logic, something has flown off the tracks.
There are basically two points people are making in regards to gotos and your code:
Goto is bad. It's very rare to encounter a place where you need gotos, but I wouldn't suggest striking it completely. Though C++ has smart enough control flow to make goto rarely appropriate.
Your mechanism for cleanup is wrong: This point is far more important. In C, using memory management on your own is not only OK, but often the best way to do things. In C++, your goal should be to avoid memory management as much as possible. You should avoid memory management as much as possible. Let the compiler do it for you. Rather than using new, just declare variables. The only time you'll really need memory management is when you don't know the size of your data in advance. Even then, you should try to just use some of the STL collections instead.
In the event that you legitimately need memory management (you have not really provided any evidence of this), then you should encapsulate your memory management within a class via constructors to allocate memory and deconstructors to deallocate memory.
Your response that your way of doing things is much easier is not really true in the long run. Firstly, once you get a strong feel for C++ making such constructors will be 2nd nature. Personally, I find using constructors easier than using cleanup code, since I have no need to pay careful attention to make sure I am deallocating properly. Instead, I can just let the object leave scope and the language handles it for me. Also, maintaining them is MUCH easier than maintaining a cleanup section and much less prone to problems.
In short, goto may be a good choice in some situations but not in this one. Here it's just short term laziness.
Your code is extremely non-idiomatic and you should never write it. You're basically emulating C in C++ there. But others have remarked on that, and pointed to RAII as the alternative.
However, your code won't work as you expect, because this:
p = new int;
if(p==NULL) { … }
won't ever evaluate to true (except if you've overloaded operator new in a weird way). If operator new is unable to allocate enough memory, it throws an exception, it never, ever returns 0, at least not with this set of parameters; there's a special placement-new overload that takes an instance of type std::nothrow and that indeed returns 0 instead of throwing an exception. But this version is rarely used in normal code. Some low-level codes or embedded device applications could benefit from it in contexts where dealing with exceptions is too expensive.
Something similar is true for your delete block, as Harald as said: if (p) is unnecessary in front of delete p.
Additionally, I'm not sure if your example was chose intentionally because this code can be rewritten as follows:
bool foo() // prefer native types to BOOL, if possible
{
bool ret = false;
int i;
// Lots of code.
return ret;
}
Probably not a good idea.
In general, and on the surface, there isn't any thing wrong with your approach, provided that you only have one label, and that the gotos always go forward. For example, this code:
int foo()
{
int *pWhatEver = ...;
if (something(pWhatEver))
{
delete pWhatEver;
return 1;
}
else
{
delete pWhatEver;
return 5;
}
}
And this code:
int foo()
{
int ret;
int *pWhatEver = ...;
if (something(pWhatEver))
{
ret = 1;
goto exit;
}
else
{
ret = 5;
goto exit;
}
exit:
delete pWhatEver;
return ret;
}
really aren't all that different from each other. If you can accept one, you should be able to accept the other.
However, in many cases the RAII (resource acquisition is initialization) pattern can make the code much cleaner and more maintainable. For example, this code:
int foo()
{
Auto<int> pWhatEver = ...;
if (something(pWhatEver))
{
return 1;
}
else
{
return 5;
}
}
is shorter, easier to read, and easier to maintain than both of the previous examples.
So, I would recommend using the RAII approach if you can.
Your example is not exception safe.
If you are using goto to clean up the code then, if an exception happens before the cleanup code, it is completely missed. If you claim that you do not use exceptions then you are mistaken because the new will throw bad_alloc when it does not have enough memory.
Also at this point (when bad_alloc is thrown), your stack will be unwound, missing all the cleanup code in every function on the way up the call stack thus not cleaning up your code.
You need to look to do some research into smart pointers. In the situation above you could just use a std::auto_ptr<>.
Also note in C++ code there is no need to check if a pointer is NULL (usually because you never have RAW pointers), but because new will not return NULL (it throws).
Also in C++ unlike (C) it is common to see early returns in the code. This is because RAII will do the cleanup automatically, while in C code you need to make sure that you add special cleanup code at the end of the function (a bit like your code).
I think other answers (and their comments) have covered all the important points, but here's one thing that hasn't been done properly yet:
What your code should look like instead:
bool foo() //lowercase bool is a built-in C++ type. Use it if you're writing C++.
{
try {
std::unique_ptr<int> p(new int);
// lots of code, and just return true or false directly when you're done
}
catch (std::bad_alloc){ // new throws an exception on OOM, it doesn't return NULL
cout<<" OOM \n";
return false;
}
}
Well, it's shorter, and as far as I can see, more correct (handles the OOM case properly), and most importantly, I didn't need to write any cleanup code or do anything special to "make sure my return value is initialized".
One problem with your code I only really noticed when I wrote this, is "what the hell is bRetVal's value at this point?". I don't know because, it was declared waaaaay above, and it was last assigned to when? At some point above this. I have to read through the entire function to make sure I understand what's going to be returned.
And how do I convince myself that the memory gets freed?
How do I know that we never forget to jump to the cleanup label? I have to work backwards from the cleanup label, finding every goto that points to it, and more importantly, find the ones that aren't there. I need to trace through all paths of the function just to be sure that the function gets cleaned up properly. That reads like spaghetti code to me.
Very fragile code, because every time a resource has to be cleaned up you have to remember to duplicate your cleanup code. Why not write it once, in the type that needs to be cleaned up? And then rely on it being executed automatically, every time we need it?
In the eight years I've been programming I've used goto a lot, most of that was in the first year when I was using a version of GW-BASIC and a book from 1980 that didn't make it clear goto should only be used in certain cases. The only time I've used goto in C++ is when I had code like the following, and I'm not sure if there was a better way.
for (int i=0; i<10; i++) {
for (int j=0; j<10; j++)
{
if (somecondition==true)
{
goto finish;
}
//Some code
}
//Some code
}
finish:
The only situation I know of where goto is still used heavily is mainframe assembly language, and the programmers I know make sure to document where code is jumping and why.
As used in the Linux kernel, goto's used for cleanup work well when a single function must perform 2 or more steps that may need to be undone. Steps need not be memory allocation. It might be a configuration change to a piece of code or in a register of an I/O chipset. Goto's should only be needed in a small number of cases, but often when used correctly, they may be the best solution. They are not evil. They are a tool.
Instead of...
do_step1;
if (failed)
{
undo_step1;
return failure;
}
do_step2;
if (failed)
{
undo_step2;
undo_step1;
return failure;
}
do_step3;
if (failed)
{
undo_step3;
undo_step2;
undo_step1;
return failure;
}
return success;
you can do the same with goto statements like this:
do_step1;
if (failed) goto unwind_step1;
do_step2;
if (failed) goto unwind_step2;
do_step3;
if (failed) goto unwind_step3;
return success;
unwind_step3:
undo_step3;
unwind_step2:
undo_step2;
unwind_step1:
undo_step1;
return failure;
It should be clear that given these two examples, one is preferable to the other. As to the RAII crowd... There is nothing wrong with that approach as long as they can guarantee that the unwinding will always occur in exactly reverse order: 3, 2, 1. And lastly, some people do not use exceptions in their code and instruct the compilers to disable them. Thus not all code must be exception safe.
You should read this thread summary from the Linux kernel mailing lists (paying special attention to the responses from Linus Torvalds) before you form a policy for goto:
http://kerneltrap.org/node/553/2131
In general, you should design your programs to limit the need for gotos. Use OO techniques for "cleanup" of your return values. There are ways to do this that don't require the use of gotos or complicating the code. There are cases where gotos are very useful (for example, deeply nested scopes), but if possible should be avoided.
The downside of GOTO is pretty well discussed. I would just add that 1) sometimes you have to use them and should know how to minimize the problems, and 2) some accepted programming techniques are GOTO-in-disguise, so be careful.
1) When you have to use GOTO, such as in ASM or in .bat files, think like a compiler. If you want to code
if (some_test){
... the body ...
}
do what a compiler does. Generate a label whose purpose is to skip over the body, not to do whatever follows. i.e.
if (not some_test) GOTO label_at_end_of_body
... the body ...
label_at_end_of_body:
Not
if (not some_test) GOTO the_label_named_for_whatever_gets_done_next
... the body ...
the_label_named_for_whatever_gets_done_next:
In otherwords, the purpose of the label is not to do something, but to skip over something.
2) What I call GOTO-in-disguise is anything that could be turned into GOTO+LABELS code by just defining a couple macros. An example is the technique of implementing finite-state-automata by having a state variable, and a while-switch statement.
while (not_done){
switch(state){
case S1:
... do stuff 1 ...
state = S2;
break;
case S2:
... do stuff 2 ...
state = S1;
break;
.........
}
}
can turn into:
while (not_done){
switch(state){
LABEL(S1):
... do stuff 1 ...
GOTO(S2);
LABEL(S2):
... do stuff 2 ...
GOTO(S1);
.........
}
}
just by defining a couple macros. Just about any FSA can be turned into structured goto-less code. I prefer to stay away from GOTO-in-disguise code because it can get into the same spaghetti-code issues as undisguised gotos.
Added: Just to reassure: I think one mark of a good programmer is recognizing when the common rules don't apply.
Using goto to go to a cleanup section is going to cause a lot of problems.
First, cleanup sections are prone to problems. They have low cohesion (no real role that can be described in terms of what the program is trying to do ), high coupling (correctness depends very heavily on other sections of code), and are not at all exception-safe. See if you can use destructors for cleanup. For example, if int *p is changed to auto_ptr<int> p, what p points to will be automatically released.
Second, as you point out, it's going to force you to declare variables long before use, which will make it harder to understand the code.
Third, while you're proposing a fairly disciplined use of goto, there's going to be the temptation to use them in a looser manner, and then the code will become difficult to understand.
There are very few situations where a goto is appropriate. Most of the time, when you are tempted to use them, it's a signal that you're doing things wrong.
The entire purpose of the every-function-has-a-single-exit-point idiom in C was to put all the cleanup stuff in a single place. If you use C++ destructors to handle cleanup, that's no longer necessary -- cleanup will be done regardless of how many exit points a function has. So in properly-designed C++ code, there's no longer any need for this kind of thing.
Since this is a classic topic, I will reply with Dijkstra's Go-to statement considered harmful (originally published in ACM).
Goto provides better don't repeat yourself (DRY) when "tail-end-logic" is common to some-but-not-all-cases. Especially within a "switch" statement I often use goto's when some of the switch-branches have tail-end-commonality.
switch(){
case a: ... goto L_abTail;
case b: ... goto L_abTail;
L_abTail: <commmon stuff>
break://end of case b
case c:
.....
}//switch
You have probably noticed than introducing additional curly-braces is enough to satisfy the compiler when you need such tail-end-merging in-the-middle of a routine. In other words, you don't need to declare everything way up at the top; that's inferior readability indeed.
...
goto L_skipMiddle;
{
int declInMiddleVar = 0;
....
}
L_skipMiddle: ;
With the later versions of Visual Studio detecting the use of uninitialized variables, I find myself always initializing most variables even though I think they may be assigned in all branches - it's easy to code a "tracing" statement which refs a variable that was never assigned because your mind doesn't think of the tracing statement as "real code", but of course Visual Studio will still detect an error.
Besides don't repeat yourself, assigning label-names to such tail-end-logic even seems to help my mind keep things straight by choosing nice label names. Without a meaningful label your comments might end up saying the same thing.
Of course, if you are actually allocating resources then if auto-ptr doesn't fit, you really must use a try-catch, but tail-end-merge-don't-repeat-yourself happens quite often when exception-safety is not an issue.
In summary, while goto can be used to code spaghetti-like structures, in the case of a tail-end-sequence which is common to some-but-not-all-cases then the goto IMPROVES the readability of the code and even maintainability if you would otherwise be copy/pasting stuff so that much later on someone might update one-and-not-the-other. So it's another case where being fanatic about a dogma can be counterproductive.
The only two reasons I use goto in my C++ code are:
Breaking a level 2+ nested loops
Complicated flows like this one (a comment in my program):
/* Analysis algorithm:
1. if classData [exporter] [classDef with name 'className'] exists, return it,
else
2. if project/target_codename/temp/classmeta/className.xml exist, parse it and go back to 1 as it will succeed.
3. if that file don't exists, generate it via haxe -xml, and go back to 1 as it will succeed.
*/
For code readability here, after this comment, I defined the step1 label and used it in step 2 and 3. Actually, in 60+ source files, only this situation and one 4-levels nested for are the places I used goto. Only two places.
A lot of people freak out with gotos are evil; they are not. That said, you will never need one; there is just about always a better way.
When I find myself "needing" a goto to do this type of thing, I almost always find that my code is too complex and can be easily broken up into a few method calls that are easier to read and deal with. Your calling code can do something like:
// Setup
if(
methodA() &&
methodB() &&
methodC()
)
// Cleanup
Not that this is perfect, but it's much easier to follow since all your methods will be named to clearly indicate what the problem might be.
Reading through the comments, however, should indicate that your team has more pressing issues than goto handling.
The code you're giving us is (almost) C code written inside a C++ file.
The kind of memory cleaning you're using would be OK in a C program not using C++ code/libraries.
In C++, your code is simply unsafe and unreliable. In C++ the kind of management you're asking for is done differently. Use constructors/destructors. Use smart pointers. Use the stack. In a word, use RAII.
Your code could (i.e., in C++, SHOULD) be written as:
BOOL foo()
{
BOOL bRetVal = FALSE;
std::auto_ptr<int> p = new int;
// Lot of code...
return bRetVal ;
}
(Note that new-ing an int is somewhat silly in real code, but you can replace int by any kind of object, and then, it makes more sense). Let's imagine we have an object of type T (T could be an int, some C++ class, etc.). Then the code becomes:
BOOL foo()
{
BOOL bRetVal = FALSE;
std::auto_ptr<T> p = new T;
// Lot of code...
return bRetVal ;
}
Or even better, using the stack:
BOOL foo()
{
BOOL bRetVal = FALSE;
T p ;
// Lot of code...
return bRetVal;
}
Anyway, any of the above examples are magnitudes more easy to read and secure than your example.
RAII has many facets (i.e. using smart pointers, the stack, using vectors instead of variable length arrays, etc.), but all in all is about writing as little code as possible, letting the compiler clean up the stuff at the right moment.
All of the above is valid, you might also want to look at whether you might be able to reduce the complexity of your code and alleviate the need for goto's by reducing the amout of code that is in the section marked as "lot of code" in your example. Additionaly delete 0 is a valid C++ statement
Using GOTO labels in C++ is a bad way to program, you can reduce the need by doing OO programming (deconstructors!) and trying to keep procedures as small as possible.
Your example looks a bit weird, there is no need to delete a NULL pointer. And nowadays an exception is thrown when a pointer can't get allocated.
Your procedure could just be wrote like:
bool foo()
{
bool bRetVal = false;
int p = 0;
// Calls to various methods that do algorithms on the p integer
// and give a return value back to this procedure.
return bRetVal;
}
You should place a try catch block in the main program handling out of memory problems that informs the user about the lack of memory, which is very rare... (Doesn't the OS itself inform about this too?)
Also note that there is not always the need to use a pointer, they are only useful for dynamic things. (Creating one thing inside a method not depending on input from anywhere isn't really dynamic)
I am not going to say that goto is always bad, but your use of it most certainly is. That kind of "cleanup sections" was pretty common in early 1990's, but using it for new code is pure evil.
The easiest way to avoid what you are doing here is to put all of this cleanup into some kind of simple structure and create an instance of it. For example instead of:
void MyClass::myFunction()
{
A* a = new A;
B* b = new B;
C* c = new C;
StartSomeBackgroundTask();
MaybeBeginAnUndoBlockToo();
if ( ... )
{
goto Exit;
}
if ( ... ) { .. }
else
{
... // what happens if this throws an exception??? too bad...
goto Exit;
}
Exit:
delete a;
delete b;
delete c;
StopMyBackgroundTask();
EndMyUndoBlock();
}
you should rather do this cleanup in some way like:
struct MyFunctionResourceGuard
{
MyFunctionResourceGuard( MyClass& owner )
: m_owner( owner )
, _a( new A )
, _b( new B )
, _c( new C )
{
m_owner.StartSomeBackgroundTask();
m_owner.MaybeBeginAnUndoBlockToo();
}
~MyFunctionResourceGuard()
{
m_owner.StopMyBackgroundTask();
m_owner.EndMyUndoBlock();
}
std::auto_ptr<A> _a;
std::auto_ptr<B> _b;
std::auto_ptr<C> _c;
};
void MyClass::myFunction()
{
MyFunctionResourceGuard guard( *this );
if ( ... )
{
return;
}
if ( ... ) { .. }
else
{
...
}
}
A few years ago I came up with a pseudo-idiom that avoids goto, and is vaguely similar to doing exception handling in C. It has been probably already invented by someone else so I guess I "discovered it independently" :)
BOOL foo()
{
BOOL bRetVal = FALSE;
int *p=NULL;
do
{
p = new int;
if(p==NULL)
{
cout<<" OOM \n";
break;
}
// Lot of code...
bRetVal = TRUE;
} while (false);
if(p)
{
delete p;
p= NULL;
}
return bRetVal;
}
I think using the goto for exit code is bad since there's a lot of other solutions with low overhead such as having an exit function and returning the exit function value when needed. Typically in member functions though, this shouldn't be needed, otherwise this could be indication that there's a bit too much code bloat happening.
Typically, the only exception I make of the "no goto" rule when programming is when breaking out of nested loops to a specific level, which I've only ran into the need to do when working on mathematical programming.
For example:
for(int i_index = start_index; i_index >= 0; --i_index)
{
for(int j_index = start_index; j_index >=0; --j_index)
for(int k_index = start_index; k_index >= 0; --k_index)
if(my_condition)
goto BREAK_NESTED_LOOP_j_index;
BREAK_NESTED_LOOP_j_index:;
}
That code has a bunch of problems, most of which were pointed out already, for example:
The function is too long; refactoring out some code into separate functions might help.
Using pointers when normal instances will probably work just fine.
Not taking advantage of STL types such as auto_ptr
Incorrectly checking for errors, and not catching exceptions. (I would argue that checking for OOM is pointless on the vast majority of platforms, since if you run out of memory you have bigger problems than your software can fix, unless you are writing the OS itself)
I have never needed a goto, and I've always found that using goto is a symptom of a bigger set of problems. Your case appears to be no exception.
Using "GOTO" will change the "logics" of a program and how you enterpret or how you would imagine it would work.
Avoiding GOTO-commands have always worked for me so guess when you think you might need it, all you maybe need is a re-design.
However, if we look at this on an Assmebly-level, jusing "jump" is like using GOTO and that's used all the time, BUT, in Assembly you can clear out, what you know you have on the stack and other registers before you pass on.
So, when using GOTO, i'd make sure the software would "appear" as the co-coders would enterpret, GOTO will have an "bad" effect on your software imho.
So this is more an explenation to why not to use GOTO and not a solution for a replacement, because that is VERY much up to how everything else is built.
I may have missed something: you jump to the label Exit if P is null, then test to see if it's not null (which it's not) to see if you need to delete it (which isn't necessary because it was never allocated in the first place).
The if/goto won't, and doesn't need to delete p. Replacing the goto with a return false would have the same effect (and then you could remove the Exit label).
The only places I know where goto's are useful are buried deep in nasty parsers (or lexical analyzers), and in faking out state machines (buried in a mass of CPP macros). In those two cases they've been used to make very twisted logic simpler, but that is very rare.
Functions (A calls A'), Try/Catches and setjmp/longjmps are all nicer ways of avoiding a difficult syntax problem.
Paul.
Ignoring the fact that new will never return NULL, take your code:
BOOL foo()
{
BOOL bRetVal = FALSE;
int *p=NULL;
p = new int;
if(p==NULL)
{
cout<<" OOM \n";
goto Exit;
}
// Lot of code...
Exit:
if(p)
{
delete p;
p= NULL;
}
return bRetVal;
}
and write it like this:
BOOL foo()
{
BOOL bRetVal = FALSE;
int *p = new int;
if (p!=NULL)
{
// Lot of code...
delete p;
}
else
{
cout<<" OOM \n";
}
return bRetVal;
}
Related
Technically, in C++ we have the possibility to use curly-braces to declare a new scope. For example in this function, which swaps two numbers
void swap_int(int& first, int& second)
{
int temp = first;
first = second;
second = temp;
}
we could also declare temp inside its own block:
void swap_int(int& first, int& second)
{
// Do stuf...
{
int temp = first;
first = second;
second = temp;
}
// Do other stuff...
}
This obviously has the advantage that temp is deleted directly when it is not needed anymore.
However, in the code I write I never use it. Also, in code from 3rd-party libraries I almost never see it at all.
Why is it not used publically? Does it bring any performance increase at all, or does it just mean additional typing-work?
I don't see anything wrong per-se with naked brackets. They're a part of the language, and they're well defined. Historically, one place I have found them useful is when working with code that uses status codes instead of exceptions, while keeping const goodness:
const StatusCode statusCode = DoThing();
if (statusCode == STATUS_SUCCESS)
Foo();
else
Bar();
const StatusCode statusCode2 = DoAnotherThing(); // Eww variable name.
...
The alternative would be:
{
const StatusCode statusCode = DoThing();
if (statusCode == STATUS_SUCCESS)
Foo();
else
Bar();
}
{
// Same variable name, used for same purpose, easy to
// find/replace, and has const guarantees. Great success.
const StatusCode statusCode = DoAnotherThing();
...
}
The same applies to objects such as thread lockers that use RAII as well (Mutex objects, semaphores, etc), or generally any kind of resource that you may want to have an extremely short lifetime (file handles, for example).
Personally, I think the reason it's rare is that it can be indicative of a code smell (although not always). Where there are naked brackets, there may be an opportunity to factor out a function.
To take your example, if there is more than one job for swap_int, then the function is doing more than one thing. By extracting the actual swap code into another function, you can encourage reuse! For example:
template <typename T>
void swap_anything(T &first, T& second)
{
T temp = first;
first = second;
second = temp;
}
// -------------------------------------------
void swap_int(int& first, int& second)
{
// Do stuff...
swap_anything(first, second);
// Do other stuff...
}
Sometimes it's good practice (although I dislike that term, as it's subjective and context-specific), but like many things in life, taking it to the extreme (on either end of the spectrum) is a bad idea.
You'll see new scopes introduced sometimes in C++ where RAII is important, like when dealing with thread locks. Sometimes the precise moment of when an object is created and destroyed is very important and needs to be controlled. In those situations, introducing a new scope is a very useful and often-used way of accomplishing this.
But that's not frequently the case. Most of the objects we (the broad programming community) use don't have strict lifetimes that need to be so carefully managed. As such, it's not worth arbitrarily introducing new scopes to manage lifetimes of objects whose lifetimes aren't worth micromanaging.
If you do, you'll decrease the signal-to-noise ratio, and people will have a hard time telling which scopes are introduced to carefully control important resources, and which scopes are not. This can make it easy to introduce bugs when refactoring or developing code across teams. At the very least, you'll make programming a whole lot more tedious, which sucks, and you should generally avoid that if you can or else a violent psychopath may take it out on you.
This question already has answers here:
GOTO still considered harmful? [closed]
(49 answers)
Closed 8 years ago.
I had a code review recently and a debate began. Much of my code looks like this:
for (i = 1; i <= 3; i++)
{
DoubleValue = tODBCX->getDouble(KeyFieldCount + i, IsNULL, IsSuccess);
if (IsNULL)
{
LoggerDrillHole::LogToDB(LOG_ERROR, L"Survey depth, dip and azimuth values can not be NULL.", __FUNCTIONW__);
IsSuccess = false;
goto EXIT;
}
else
{
if (i == 1)
Depth = DoubleValue;
else if(i == 2)
DipDegrees = DoubleValue;
else if (i == 3)
AzimuthDegrees = DoubleValue;
}
}
The contentious goto statement sparked debate. This code is contained within a function that begins life by initializing a local boolean variable IsSuccess = true and the function finally returns IsSuccess. The EXIT strategy looks after essential tidy up code;
EXIT:
tODBCX->Close();
if (Key != 0) free(Key);
Key = 0;
if (PriorKey != 0) free(PriorKey);
PriorKey = 0;
return IsSuccess;
There were several such goto EXIT statements adjacent to setting IsSuccess = false and logging to database and so on. The comment was made that this is disruptive to the flow of the code. One should instead use a do loop (infinite loop) and break out of that loop and then process all of the required instead of using the goto statement.
I intensely disliked the infinite loop strategy, but I could get used to it if it truly improves the readability. Is there a better way?
I wanted to mark this as a duplicate of this question. Still, it is not exactly the same, even though the solution is the same:
In C++, the best solution is to use RAII and transactional code. In C, the best solution is to use goto, with following a few rules (only use for return/cleanup, do not use goto to simulate loops, etc).
See my answer in the question mentioned above (basically, the solution is to use RAII and transactional code); this will eliminate completely the need for a goto cleanup/error handling block.
There is nothing wrong with using goto here. This is one of the few situations where it is the cleanest solution. (Another example would be breaking out of an inner loop.)
Using a do { ... } while (false) loop is an artificial solution that actually reduces the readability of the code.
In C, consider breaking your code into two functions... an outer function that does common intialisation and passes down the variables the inner function needs, such that the inner function can simply return the success status knowing the outer function will clean up.
In C++ it's usually a good idea to use scope guards so destructors ensure proper clean up. Consider your:
tODBCX->Close();
If tODBCX needs to live longer than the function call - so a Close() in the destructor doesn't help - then create a helper:
struct Odbc_Access_Guard
{
Odbc_Access_Guard(ODBC& o) : odbc_(o) { }
~Odbc_Access_Guard() { odbc_.close(); }
operator ODBC&() { return odbc_; }
operator const ODBC&() const { return odbc_; }
ODBC& odbc_;
};
Then inside your function:
Odbc_Access_Guard odbc(tODBC);
odbc.xyz();
if (whatever)
return ...success expression...;
The same thing goes for your pointers: they should probably be shared pointers or guards using the logic above. Then you can return any time without having to even think about where to go for the clean up code, and wondering if it's up to date with the current variable use.
Recently, I was reviewing some code I maintain and I noticed a practice different than what I am used to. As a result, I'm wondering which method to use when performing an early return in a function.
Here's some example:
Version 1:
int MyFunction(int* ptr)
{
if(!ptr) { // oh no, NULL pointer!
return -1; // what was the caller doing? :(
}
// other code goes here to do work on the pointer
// ...
return 0; // we did it!
}
Version 2:
int MyFunction(int* ptr)
{
if(!ptr) { // oh no, NULL pointer!
return -1; // what was the caller doing? :(
} else { // explicitly show that this only gets call when if statement fails
// other code goes here to do work on the pointer
// ...
return 0; // hooray!
}
}
As a result, I'm wondering which is considered the "best practice" for those of you who have endured (and survived) many code reviews. I know each effectively does the same thing, but does the "else" add much in terms of readability and clarity? Thanks for the help.
The else would only add clarity if the else clause is short, a few lines of code at best. And if you have several initial conditions you want to check, the source gets cluttered very quickly.
The only time I would use an else if it is a small function with a small else, meaning less than about 10 source lines, and there are no other initial checks to make.
In some cases I have used a single loop so that a series of initial checks can use a break to leave.
do {
...
} while (0);
I am loathe to use a goto which is practically guaranteed to get at least one true believer of goto less programming up in arms.
So much would depend on any code standards of your organization. I tend to like minimalism so I use the first version you provide without the else.
I might also do something like the following in a smaller function say less than 20 or 30 lines:
int MyFunction(int* ptr)
{
int iRetStatus = -1; // we have an error condition
if (ptr) { // good pointer
// stuff to do in this function
iRetStatus = 0;
}
return iRetStatus; // we did it!
}
The only problem with returns in the body of the function is that sometimes people scanning the function do not realize that there is a return. In small functions where everything can be pretty much seen on a single screen, the chance of missing a return is pretty small. However for large functions, returns in the middle can be missed especially large complex functions that have gone through several maintenance cycles and had a lot of cruft and work arounds put into them.
I have a setup that looks like this.
class Checker
{ // member data
Results m_results; // see below
public:
bool Check();
private:
bool Check1();
bool Check2();
// .. so on
};
Checker is a class that performs lengthy check computations for engineering analysis. Each type of check has a resultant double that the checker stores. (see below)
bool Checker::Check()
{ // initilisations etc.
Check1();
Check2();
// ... so on
}
A typical Check function would look like this:
bool Checker::Check1()
{ double result;
// lots of code
m_results.SetCheck1Result(result);
}
And the results class looks something like this:
class Results
{ double m_check1Result;
double m_check2Result;
// ...
public:
void SetCheck1Result(double d);
double GetOverallResult()
{ return max(m_check1Result, m_check2Result, ...); }
};
Note: all code is oversimplified.
The Checker and Result classes were initially written to perform all checks and return an overall double result. There is now a new requirement where I only need to know if any of the results exceeds 1. If it does, subsequent checks need not be carried out(it's an optimisation). To achieve this, I could either:
Modify every CheckN function to keep check for result and return. The parent Check function would keep checking m_results. OR
In the Results::SetCheckNResults(), throw an exception if the value exceeds 1 and catch it at the end of Checker::Check().
The first is tedious, error prone and sub-optimal because every CheckN function further branches out into sub-checks etc.
The second is non-intrusive and quick. One disadvantage is I can think of is that the Checker code may not necessarily be exception-safe(although there is no other exception being thrown anywhere else). Is there anything else that's obvious that I'm overlooking? What about the cost of throwing exceptions and stack unwinding?
Is there a better 3rd option?
I don't think this is a good idea. Exceptions should be limited to, well, exceptional situations. Yours is a question of normal control flow.
It seems you could very well move all the redundant code dealing with the result out of the checks and into the calling function. The resulting code would be cleaner and probably much easier to understand than non-exceptional exceptions.
Change your CheckX() functions to return the double they produce and leave dealing with the result to the caller. The caller can more easily do this in a way that doesn't involve redundancy.
If you want to be really fancy, put those functions into an array of function pointers and iterate over that. Then the code for dealing with the results would all be in a loop. Something like:
bool Checker::Check()
{
for( std::size_t id=0; idx<sizeof(check_tbl)/sizeof(check_tbl[0]); ++idx ) {
double result = check_tbl[idx]();
if( result > 1 )
return false; // or whichever way your logic is (an enum might be better)
}
return true;
}
Edit: I had overlooked that you need to call any of N SetCheckResultX() functions, too, which would be impossible to incorporate into my sample code. So either you can shoehorn this into an array, too, (change them to SetCheckResult(std::size_t idx, double result)) or you would have to have two function pointers in each table entry:
struct check_tbl_entry {
check_fnc_t checker;
set_result_fnc_t setter;
};
check_tbl_entry check_tbl[] = { { &Checker::Check1, &Checker::SetCheck1Result }
, { &Checker::Check2, &Checker::SetCheck2Result }
// ...
};
bool Checker::Check()
{
for( std::size_t id=0; idx<sizeof(check_tbl)/sizeof(check_tbl[0]); ++idx ) {
double result = check_tbl[idx].checker();
check_tbl[idx].setter(result);
if( result > 1 )
return false; // or whichever way your logic is (an enum might be better)
}
return true;
}
(And, no, I'm not going to attempt to write down the correct syntax for a member function pointer's type. I've always had to look this up and still never ot this right the first time... But I know it's doable.)
Exceptions are meant for cases that shouldn't happen during normal operation. They're hardly non-intrusive; their very nature involves unwinding the call stack, calling destructors all over the place, yanking the control to a whole other section of code, etc. That stuff can be expensive, depending on how much of it you end up doing.
Even if it were free, though, using exceptions as a normal flow control mechanism is a bad idea for one other, very big reason: exceptions aren't meant to be used that way, so people don't use them that way, so they'll be looking at your code and scratching their heads trying to figure out why you're throwing what looks to them like an error. Head-scratching usually means you're doing something more "clever" than you should be.
In C++ you can initialize a variable in an if statement, like so:
if (CThing* pThing = GetThing())
{
}
Why would one consider this bad or good style? What are the benefits and disadvantages?
Personally i like this style because it limits the scope of the pThing variable, so it can never be used accidentally when it is NULL. However, i don't like that you can't do this:
if (CThing* pThing = GetThing() && pThing->IsReallySomeThing())
{
}
If there's a way to make the above work, please post. But if that's just not possible, i'd still like to know why.
Question borrowed from here, similar topic but PHP.
The important thing is that a declaration in C++ is not an expression.
bool a = (CThing* pThing = GetThing()); // not legit!!
You can't do both a declaration and boolean logic in an if statement, C++ language spec specifically allows either an expression or a declaration.
if(A *a = new A)
{
// this is legit and a is scoped here
}
How can we know whether a is defined between one term and another in an expression?
if((A *a = new A) && a->test())
{
// was a really declared before a->test?
}
Bite the bullet and use an internal if. The scope rules are useful and your logic is explicit:
if (CThing* pThing = GetThing())
{
if(pThing->IsReallySomeThing())
{
}
}
About the advantages:
It's always recommended to define variables when you first need them, not a line before. This is for improved readability of your code, since one can tell what CThing is without scrolling and searching where it was defined.
Also reducing scope to a loop/if block, causes the variable to be unreferenced after the execution of the code block, which makes it a candidate for Garbage Collection (if the language supports this feature).
if (CThing* pThing = GetThing())
It is bad style, because inside the if you are not providing a boolean expression. You are providing a CThing*.
CThing* pThing = GetThing();
if (pThing != NULL)
This is good style.
You can have initialization statements inside if and switch since C++17.
Your code would now be:
if (CThing* pThing = GetThing(); pThing->IsReallySomeThing())
{
// use pThing here
}
// pThing is out of scope here
One reason I don't normally do that is because of the common bug from a missed '=' in a conditional test. I use lint with the error/warnings set to catch those. It will then yell about all assignments inside conditionals.
Just an FYI some of the older Microsoft C++ compliers(Visual Studios 6, and .NET 2003 I think) don't quite follow the scoping rule in some instances.
for(int i = 0; i > 20; i++) {
// some code
}
cout << i << endl;
I should be out of scope, but that was/is valid code. I believe it was played off as a feature, but in my opinion it's just non compliance. Not adhering to the standards is bad. Just as a web developer about IE and Firefox.
Can someone with VS check and see if that's still valid?
This shoulddoesn't work in C++ sinceeven though it supports short circuiting evaluation. MaybeDon't try the following:
if ((CThing* pThing = GetThing()) && (pThing->IsReallySomeThing()))
{
}
err.. see Wesley Tarle's answer
So many things. First of all, bare pointers. Please avoid them by all means. Use references, optional, unique_ptr, shared_ptr. As the last resort, write your own class that deals with pointer ownership and nothing else.
Use uniform initialization if you can require C++11 (C++14 preferred to avoid C++11 defects): - it avoids = vs == confusion and it's stricter at checking the arguments if there are any.
if (CThing thing {})
{
}
Make sure to implement operator bool to get predictable conversion from CThing to bool. However, keep in mind that other people reading the code would not see operator bool right away. Explicit method calls are generally more readable and reassuring. If you can require C++17, use initializer syntax.
if (CThing thing {}; thing.is_good())
{
}
If C++17 is not an option, use a declaration above if as others have suggested.
{
CThing thing {};
if (thing.is_good())
{
}
}
You can also enclose the assignment in an extra set of ( ) to prevent the warning message.
I see that as kind of dangerous. The code below is much safer and the enclosing braces will still limit the scope of pThing in the way you want.
I'm assuming GetThing() sometimes returns NULL which is why I put that funny clause in the if() statement. It prevents IsReallySomething() being called on a NULL pointer.
{
CThing *pThing = GetThing();
if(pThing ? pThing->IsReallySomeThing() : false)
{
// Do whatever
}
}
also notice that if you're writing C++ code you want to make the compiler warning about "=" in a conditional statement (that isn't part of a declaration) an error.
It's acceptable and good coding practice. However, people who don't come from a low-level coding background would probably disagree.