I have code that does something like this:
//datareader.cpp
if (populateFoo(dataReader, foo))
else {
// Do other things with the reader.
}
//foo.cpp
bool populateFoo(const DataReader &dataReader, Foo &foo)
{
if (dataReader.name() == "bar") {
foo.bar() = dataReader.value();
return true;
} // More similar checks.
return false;
}
I feel like it's misleading to have an if statement with conditions that have side-effects. However, I can't move the body of the populateFoo function into datareader.cpp. Is there a good way to restructure this code so we get rid of this misleading if statement, without duplicating the body of populateFoo()?
Do you have a strong hatred of local variables? If not:
bool populated = populateFoo(dataReader, foo);
if (populated)
{
// Do things
}
else
{
// Do other things
}
The compiler will almost certainly emit exactly the same code, so performance shouldn't be an issue. It's a readability/style choice, ultimately.
The obvious solution seems like storing the result of populateFoo and using it for determining whether populateFoo was successful:
bool fooPopulated = populateFoo(dataReader, Foo);
if (!fooPopulated)
//Do other things with reader.
However, I don't find the original difficult to understand, and it's a fairly well-established practice to both modify values and test the success of the modification in the same line. However, I would change it to:
if (!populateFoo(dataReader, Foo)
//Do other things with reader.
How about:
if (!populateFoo(dataReader, foo)) {
// Do other things with the reader.
}
Edit: The title of the question suggests it is the fact the if statement is empty that bothers you but the body seems more that it is the side effect that is the concern. I think it's fine in C++ to have conditions in if statements that have side effects but this won't solve your issue if you want to avoid that.
Having conditions with side-effects is quite common - think about calling a C API and checking its return code for errors.
Usually, as long as it's not buried in a complicated expression where it may be missed by the casual bystander, I don't bother to do particular refactorings, but, in case you wanted to make it extra clear (or document what the return value is, which is particularly useful in case of booleans) just assign it to a variable before the branch - or even just a few comments may help.
You could split the populateFoo function into two, a const check function (shouldPopulateFoo) that checks the condition, and another non-const function that performs the actual modifications (populateFoo):
//datareader.cpp
if (shouldPopulateFoo(dataReader)) {
populateFoo(dataReader, foo);
}
else {
// Do other things with the reader.
}
//foo.cpp
bool shouldPopulateFoo(const DataReader &dataReader) /* const */
{
return (dataReader.name() == "bar");
}
void populateFoo(const DataReader &dataReader, Foo &foo) /* non-const */
{
assert(shouldPopulateFoo(dataReader));
foo.bar = dataReader.value();
}
Note that when using these functions as class methods, you could declare the check function const.
How about:
if (populateFoo(dataReader, foo) == false) {
// Do other things with the reader.
}
It is very readable, I often see code where the returned value from function is a signal to the caller for branching in the caller. The else block with empty if block bothers me more then the side effects inside the if (). There is a sense of reverse logic, which is alway less readable.
So I ran across this (IMHO) very nice idea of using a composite structure of a return value and an exception - Expected<T>. It overcomes many shortcomings of the traditional methods of error handling (exceptions, error codes).
See the Andrei Alexandrescu's talk (Systematic Error Handling in C++) and its slides.
The exceptions and error codes have basically the same usage scenarios with functions that return something and the ones that don't. Expected<T>, on the other hand, seems to be targeted only at functions that return values.
So, my questions are:
Have any of you tried Expected<T> in practice?
How would you apply this idiom to functions returning nothing (that is, void functions)?
Update:
I guess I should clarify my question. The Expected<void> specialization makes sense, but I'm more interested in how it would be used - the consistent usage idiom. The implementation itself is secondary (and easy).
For example, Alexandrescu gives this example (a bit edited):
string s = readline();
auto x = parseInt(s).get(); // throw on error
auto y = parseInt(s); // won’t throw
if (!y.valid()) {
// ...
}
This code is "clean" in a way that it just flows naturally. We need the value - we get it. However, with expected<void> one would have to capture the returned variable and perform some operation on it (like .throwIfError() or something), which is not as elegant. And obviously, .get() doesn't make sense with void.
So, what would your code look like if you had another function, say toUpper(s), which modifies the string in-place and has no return value?
Have any of you tried Expected; in practice?
It's quite natural, I used it even before I saw this talk.
How would you apply this idiom to functions returning nothing (that is, void functions)?
The form presented in the slides has some subtle implications:
The exception is bound to the value.
It's ok to handle the exception as you wish.
If the value ignored for some reasons, the exception is suppressed.
This does not hold if you have expected<void>, because since nobody is interested in the void value the exception is always ignored. I would force this as I would force reading from expected<T> in Alexandrescus class, with assertions and an explicit suppress member function. Rethrowing the exception from the destructor is not allowed for good reasons, so it has to be done with assertions.
template <typename T> struct expected;
#ifdef NDEBUG // no asserts
template <> class expected<void> {
std::exception_ptr spam;
public:
template <typename E>
expected(E const& e) : spam(std::make_exception_ptr(e)) {}
expected(expected&& o) : spam(std::move(o.spam)) {}
expected() : spam() {}
bool valid() const { return !spam; }
void get() const { if (!valid()) std::rethrow_exception(spam); }
void suppress() {}
};
#else // with asserts, check if return value is checked
// if all assertions do succeed, the other code is also correct
// note: do NOT write "assert(expected.valid());"
template <> class expected<void> {
std::exception_ptr spam;
mutable std::atomic_bool read; // threadsafe
public:
template <typename E>
expected(E const& e) : spam(std::make_exception_ptr(e)), read(false) {}
expected(expected&& o) : spam(std::move(o.spam)), read(o.read.load()) {}
expected() : spam(), read(false) {}
bool valid() const { read=true; return !spam; }
void get() const { if (!valid()) std::rethrow_exception(spam); }
void suppress() { read=true; }
~expected() { assert(read); }
};
#endif
expected<void> calculate(int i)
{
if (!i) return std::invalid_argument("i must be non-null");
return {};
}
int main()
{
calculate(0).suppress(); // suppressing must be explicit
if (!calculate(1).valid())
return 1;
calculate(5); // assert fails
}
Even though it might appear new for someone focused solely on C-ish languages, to those of us who had a taste of languages supporting sum-types, it's not.
For example, in Haskell you have:
data Maybe a = Nothing | Just a
data Either a b = Left a | Right b
Where the | reads or and the first element (Nothing, Just, Left, Right) is just a "tag". Essentially sum-types are just discriminating unions.
Here, you would have Expected<T> be something like: Either T Exception with a specialization for Expected<void> which is akin to Maybe Exception.
Like Matthieu M. said, this is something relatively new to C++, but nothing new for many functional languages.
I would like to add my 2 cents here: part of the difficulties and differences are can be found, in my opinion, in the "procedural vs. functional" approach. And I would like to use Scala (because I am familiar both with Scala and C++, and I feel it has a facility (Option) which is closer to Expected<T>) to illustrate this distinction.
In Scala you have Option[T], which is either Some(t) or None.
In particular, it is also possible to have Option[Unit], which is morally equivalent to Expected<void>.
In Scala, the usage pattern is very similar and built around 2 functions: isDefined() and get(). But it also have a "map()" function.
I like to think of "map" as the functional equivalent of "isDefined + get":
if (opt.isDefined)
opt.get.doSomething
becomes
val res = opt.map(t => t.doSomething)
"propagating" the option to the result
I think that here, in this functional style of using and composing options, lies the answer to your question:
So, what would your code look like if you had another function, say toUpper(s), which modifies the string in-place and has no return value?
Personally, I would NOT modify the string in place, or at least I will not return nothing. I see Expected<T> as a "functional" concept, that need a functional pattern to work well: toUpper(s) would need to either return a new string, or return itself after modification:
auto s = toUpper(s);
s.get(); ...
or, with a Scala-like map
val finalS = toUpper(s).map(upperS => upperS.someOtherManipulation)
if you don't want to follow a functional route, you can just use isDefined/valid and write your code in a more procedural way:
auto s = toUpper(s);
if (s.valid())
....
If you follow this route (maybe because you need to), there is a "void vs. unit" point to make: historically, void was not considered a type, but "no type" (void foo() was considered alike a Pascal procedure). Unit (as used in functional languages) is more seen as a type meaning "a computation". So returning a Option[Unit] does make more sense, being see as "a computation that optionally did something". And in Expected<void>, void assumes a similar meaning: a computation that, when it does work as intended (where there are no exceptional cases), just ends (returning nothing). At least, IMO!
So, using Expected or Option[Unit] could be seen as computations that maybe produced a result, or maybe not. Chaining them will prove it difficult:
auto c1 = doSomething(s); //do something on s, either succeed or fail
if (c1.valid()) {
auto c2 = doSomethingElse(s); //do something on s, either succeed or fail
if (c2.valid()) {
...
Not very clean.
Map in Scala makes it a little bit cleaner
doSomething(s) //do something on s, either succeed or fail
.map(_ => doSomethingElse(s) //do something on s, either succeed or fail
.map(_ => ...)
Which is better, but still far from ideal. Here, the Maybe monad clearly wins... but that's another story..
I've been pondering the same question since I've watched this video. And so far I didn't find any convincing argument for having Expected, for me it looks ridiculous and against clarity&cleanness. I have come up with the following so far:
Expected is good since it has either value or exceptions, we not forced to use try{}catch() for every function which is throwable. So use it for every throwing function which has return value
Every function that doesn't throw should be marked with noexcept. Every.
Every function that returns nothing and not marked as noexcept should be wrapped by try{}catch{}
If those statements hold then we have self-documented easy to use interfaces with only one drawback: we don't know what exceptions could be thrown without peeking into implementation details.
Expected impose some overheads to the code since if you have some exception in the guts of your class implementation(e.g. deep inside private methods) then you should catch it in your interface method and return Expected. While I think it is quite tolerable for the methods which have a notion for returning something I believe it brings mess and clutter to the methods which by design have no return value. Besides for me it is quite unnatural to return thing from something that is not supposed to return anything.
It should be handled with compiler diagnostics. Many compilers already emit warning diagnostics based on expected usages of certain standard library constructs. They should issue a warning for ignoring an expected<void>.
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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;
}