I have a savePotentiometerState(...) -function, which returns true if the there were changes to save, and false if nothing was done. Further, I know that on any single pass through my main loop, at most one of the potentiometers may have changed (due to the way they're read out).
This is on a very time-constrained embedded platform, so it's important (or at least matters) that I don't call savePotentiometerState more often than I have to. However, the code I come up with naturally seems silly, something likely to end up at thedailywtf:
if (!savePotentiometerState(pot1))
if (!savePotentiometerState(pot2))
...
if (!savePotentiometerState(potn));
Another way to do this would be to use short-circuit evaluation:
const bool retval = savePotentiometerState(pot1) || savePotentiometerState(pot2) || ... || savePotentiometerState(potn);
I suppose I could even drop the assignment here. But this doesn't feel like good style either, since I'm abusing the short circuiting of the || operator.
The various potn objects are member variables of the containing class, so there's no obvious way to write this as a loop.
I feel like I'm missing something obvious here, so my question is: is there an idiomatic/easy to read way to do this, which doesn't sacrifice efficiency? If it matters, I'm using C++17.
Loop seems the way to go:
for (auto& pot : {std::ref(pot1), std::ref(pot2), /*..,*/ std::ref(potn)}) {
if (savePotentiometerState(pot)) {
break;
}
}
Since you can use C++17 you can leverage fold expressions and write a helper function to do the evaluation for you.
template<typename... Args>
bool returtnPotentiometerState(Args&&... args)
{
return (... || savePotentiometerState(args));
}
and then you would call it like
if (returtnPotentiometerState(pot1, pot2, ..., potn))
This means you don't have a loop, and you get short circuiting.
Personally - I'd avoid the algorithm you're using.
I'd save the state for every pot all the time; and track the current and previous values; and then only call a given callback if if the value had changed.
This way, savePotState is always as fast as it needs to be for a given pot; and you'll never get into the state where pot1 to pot(n-1) can block potn from being read.
I need to convert an expression to its result before applying it on a preprocessor. This is probably a simple problem, but I couldn't figure out a way to do it.
My preprocessor is like this:
#define ABCD(BITPOS) \
if(BIT##BITPOS##_MASK & 0x01) { \
Do something; }
And BIT0_MASK to BIT100_MASK is defined at some place.
If I call ABCD(5), preprocessor converts it to BIT5_MASK and it works fine.
But, I want to call it like this:
ABCD(START_VAL+2),
it gives me compilation error saying BITSTART_VAL is not declared, )_MASK is not defined and whole bunch of related errors.
How can I make it work ? Appreciate your responses.
The preprocessor macro system cannot evaluate arithmetic operators. It can only splice tokens together and substitute identifiers.
You will need to find another solution.
If you really really must do this, the folks at Boost created macros to perform some basic arithmetic, using only splicing and substitution as a basis. That is not an appropriate tool for this job, though.
Looks like you need an inline function rather than a macro.
inline size_t ABCD(unsigned int bitmask)
{
if (bitmask & 0x01U)
{
something();
}
}
The inline keyword will hint to the compiler that you want the code to be pasted rather than called.
I have a function that I'm using to try to check whether a string is in the correct format. I'm trying to do that by looking at each character and determining if it is of the correct type. However no matter what I try I get an error that I cannot figure out. The code is below:
bool valid(string checkcode)
{
if(checkcode.length()!=6) return false;
else if(isalpha(checkcode.at(0)))&(isalpha(checkcode.at(1)))&(isdigit(checkcode.at(2)))&(isdigit(checkcode.at(3)))&(isalpha(checkcode.at(4)))&(isalpha(checkcode.at(5))) return true;
else return false;
}
The error I'm getting is at the first '&' and it says "Error: expression must be an Ivalue or function designator" I'm really stuck here, any help is appreciated.
isalpha(checkcode.at(0)))&(isalpha(checkcode.at(1)))
//bit and
should be
isalpha(checkcode.at(0)))&&(isalpha(checkcode.at(1)))
//^^logical and
You need to use logical and in this case.
You also need to make sure that your parentheses match.
//better to format multiple conditions and make sure () match
if(
(isalpha(checkcode.at(0)))
&&(isalpha(checkcode.at(1)))
&&(isdigit(checkcode.at(2)))
&&(isdigit(checkcode.at(3)))
&&(isalpha(checkcode.at(4)))
&&(isalpha(checkcode.at(5)))
)
return true;
bool valid(string checkcode)
{
return checkcode.length() == 6
&& (isalpha(checkcode.at(0)))
&& (isalpha(checkcode.at(1)))
&& (isdigit(checkcode.at(2)))
&& (isdigit(checkcode.at(3)))
&& (isalpha(checkcode.at(4)))
&& (isalpha(checkcode.at(5)));
}
The reason for your error is that your parentheses are in the wrong places. The & that the compiler complains about is outside the conditional expression of the if statement.
The compiler sees this as the condition to be tested:
if(isalpha(checkcode.at(0)))
The remaining part is considered the statement to execute when the condition is true:
&(isalpha(checkcode.at(1)))...
Thus, the compiler is correct. When it sees a unary & operator, it expects the operand to be something that it can take the address of, such as an lvalue or a function.
Careful reading of error messages and code will help you find this kind of error next time. (That is, when the compiler complains about a missing "lvalue or function designator," ask yourself what would make it expect such a thing in the first place. It wants those when it's taking the address of something, so consider why it thinks it should be taking the address of anything. It does that with a unary & operator, but you intended to have a binary operator, so look closely at the code to determine why it's not interpreted as a binary operator. You know C++ syntax, so you know that if statements need to be entirely surrounded in parentheses, and so you know that only the first expression is part of the condition. That's not what you intended, so you'll realize that the closing parenthesis is in the wrong place.)
As it is, the single-ampersand & operator isn't what you should use for Boolean expressions. You should use && instead. That difference would not lead to the error you saw, and it would have no discernible effect on the run-time behavior of your code, either.
There are places where I check for valid pointers before I perform an operation with them; these checks can be nested pretty deeply sometimes.
For example, I have
if (a)
{
if (a->b())
{
if (a->b()->c())
{
a->b()->c()->DoSomething();
}
}
}
I really don't like the look of this. Is there a way to turn this into something more readable? Ideally,
if (a && a->b() && a->b()->c() )
{
...
}
would be great, but obviously would not work.
EDIT - nvm the example that I put up DOES work as everybody has pointed out. I did test it out to see if this works, but there was a bug in my code in my test. duh!
Why would the latter not work?
In C, && is a short-circuit operator, so it is evaluated from left to right, and if any evaluation is false, evaluation stops.
In fact, you could write:
a && a->b() && a->b()->c() && a->b()->c()->DoSomething();
Quoting from K&R1:
Expressions connected by && or || are evaluated from left to right, and it is guaranteed that evaluation will stop as soon as the truth or falsehood is known.
Therefore the latter example will work perfectly, as WhirlWind has noted.
1 The C Programming Language, Second Edition, Page 21.
Your second example does work in C/C++. It short circuits when the first FALSE value is hit.
You've seen from the other answers that using && will work, and will short-circuit the evaluation when a null pointer is encountered.
The uneasy programmer in me likes to avoid repeating method calls for tests like this since it avoids worrying if they are idempotent or not. One option is to rewrite like this
A* a;
B* b;
C* c;
if ((a=a()) && (b=a->b()) && (c=b->c())) {
c->doSomething();
}
Admittedly verbose and a bit clunky, but at least you know each method is called just once.
Why 'obviously would not work'? Since the && operator only evaluates the right term if the left is valid, the rewrite is perfectly safe.
Since you've already received the direct answer to your question I'll just mention that long chains of calls like you've got there are a code smell and you might consider a better design. Such a design might, in this case, include use of the null object pattern so that your call might just boil down to:
a->CDoSomething();
Chaining works, but it's not necessarily the best general-case answer, particularly because it obscures the failure point. I would instead suggest flattening the tests by inverting the logic so that it exits on failure.
if (!pa)
return Fail("No pa");
B* pb = pa->b();
if (!pb)
return Fail("No pb");
C* pc = b->c();
if (!pc)
return Fail("No pc");
pc->DoSomething();
Same thing, but flat and easy to read. Also, because it immediately handles the failure case, that doesn't get relegated to an else that you might never get around to writing.
In this example, I assumed you didn't want to just silently fail, so I added Fail as a helper that logs the text and returns false. You could also just throw an exception. In fact, if the various methods signaled their failure by throwing an appropriate exception instead of returning null, then all this would be unnecessary. If silent failure was desirable, then a null object pattern would be appropriate.
if (a && a->b() && a->b()->c()) { a->b()->c()->DoSomething(); }
C++ performs lazy evaluation so this will work. First, a would be evaluated and if it's 0 the whole condition is false so there will be no evaluation of the other parts.
This works for the || operator as well. If you write if (a || b), b won't be evaluated if a is true.
Second one will work provided you want to call only DoSomething().
I was looking at some code by an individual and noticed he seems to have a pattern in his functions:
<return-type> function(<params>)
{
<initialization>
do
{
<main code for function>
}
while(false);
<tidy-up & return>
}
It's not bad, more peculiar (the actual code is fairly neat and unsurprising). It's not something I've seen before and I wondered if anyone can think of any logic behind it - background in a different language perhaps?
You can break out of do{...}while(false).
A lot of people point out that it's often used with break as an awkward way of writing "goto". That's probably true if it's written directly in the function.
In a macro, OTOH, do { something; } while (false) is a convenient way to FORCE a semicolon after the macro invocation, absolutely no other token is allowed to follow.
And another possibility is that there either once was a loop there or iteration is anticipated to be added in the future (e.g. in test-driven development, iteration wasn't needed to pass the tests, but logically it would make sense to loop there if the function needed to be somewhat more general than currently required)
The break as goto is probably the answer, but I will put forward one other idea.
Maybe he wanted to have a locally defined variables and used this construct to get a new scope.
Remember while recent C++ allows for {...} anywhere, this was not always the case.
I've seen it used as a useful pattern when there are many potential exit points for the function, but the same cleanup code is always required regardless of how the function exits.
It can make a tiresome if/else-if tree a lot easier to read, by just having to break whenever an exit point is reached, with the rest of the logic inline afterwards.
This pattern is also useful in languages that don't have a goto statement. Perhaps that's where the original programmer learnt the pattern.
I've seen code like that so you can use break as a goto of sorts.
I think it's more convenient to write break instead of goto end. You don't even have to think up a name for the label which makes the intention clearer: You don't want to jump to a label with a specific name. You want to get out of here.
Also chances are you would need the braces anyway. So this is the do{...}while(false); version:
do {
// code
if (condition) break; // or continue
// more code
} while(false);
And this is the way you would have to express it if you wanted to use goto:
{
// code
if (condition) goto end;
// more code
}
end:
I think the meaning of the first version is much easier to grasp. Also it's easier to write, easier to extend, easier to translate to a language that doesn't support goto, etc.
The most frequently mentioned concern about the use of break is that it's a badly disguised goto. But actually break has more resemblance to return: Both instructions jump out of a block of code which is pretty much structured in comparison to goto. Nevertheless both instructions allow multiple exit points in a block of code which can be confusing sometimes. After all I would try to go for the most clear solution, whatever that is in the specific situation.
This is just a perversion of while to get the sematics of goto tidy-up without using the word goto.
It's bad form because when you use other loops inside the outer while the breaks become ambiguous to the reader. "Is this supposed to goto exit? or is this intended only to break out of the inner loop?"
This trick is used by programmers that are too shy to use an explicit goto in their code. The author of the above code wanted to have the ability to jump directly to the "cleanup and return" point from the middle of the code. But they didn't want to use a label and explicit goto. Instead, they can use a break inside the body of the above "fake" cycle to achieve the same effect.
Several explanations. The first one is general, the second one is specific to C preprocessor macros with parameters:
Flow control
I've seen this used in plain C code. Basically, it's a safer version of goto, as you can break out of it and all memory gets cleaned up properly.
Why would something goto-like be good? Well, if you have code where pretty much every line can return an error, but you need to react to all of them the same way (e.g. by handing the error to your caller after cleaning up), it's usually more readable to avoid an if( error ) { /* cleanup and error string generation and return here */ } as it avoids duplication of clean-up code.
However, in C++ you have exceptions + RAII for exactly this purpose, so I would consider it bad coding style.
Semicolon checking
If you forget the semicolon after a function-like macro invocation, arguments might contract in an undesired way and compile into valid syntax. Imagine the macro
#define PRINT_IF_DEBUGMODE_ON(msg) if( gDebugModeOn ) printf("foo");
That is accidentally called as
if( foo )
PRINT_IF_DEBUGMODE_ON("Hullo\n")
else
doSomethingElse();
The "else" will be considered to be associated with the gDebugModeOn, so when foo is false, the exact reverse of what was intended will happen.
Providing a scope for temporary variables.
Since the do/while has curly braces, temporary variables have a clearly defined scope they can't escape.
Avoiding "possibly unwanted semicolon" warnings
Some macros are only activated in debug builds. You define them like:
#if DEBUG
#define DBG_PRINT_NUM(n) printf("%d\n",n);
#else
#define DBG_PRINT_NUM(n)
#endif
Now if you use this in a release build inside a conditional, it compiles to
if( foo )
;
Many compilers see this as the same as
if( foo );
Which is often written accidentally. So you get a warning. The do{}while(false) hides this from the compiler, and is accepted by it as an indication that you really want to do nothing here.
Avoiding capturing of lines by conditionals
Macro from previous example:
if( foo )
DBG_PRINT_NUM(42)
doSomething();
Now, in a debug build, since we also habitually included the semicolon, this compiles just fine. However, in the release build this suddenly turns into:
if( foo )
doSomething();
Or more clearly formatted
if( foo )
doSomething();
Which is not at all what was intended. Adding a do{ ... }while(false) around the macro turns the missing semicolon into a compile error.
What's that mean for the OP?
In general, you want to use exceptions in C++ for error handling, and templates instead of macros. However, in the very rare case where you still need macros (e.g. when generating class names using token pasting) or are restricted to plain C, this is a useful pattern.
It looks like a C programmer. In C++, automatic variables have destructors which you use to clean up, so there should not be anything needed tidying up before the return. In C, you didn't have this RAII idiom, so if you have common clean up code, you either goto it, or use a once-through loop as above.
Its main disadvantage compared with the C++ idiom is that it will not tidy up if an exception is thrown in the body. C didn't have exceptions, so this wasn't a problem, but it does make it a bad habit in C++.
It is a very common practice. In C. I try to think of it as if you want to lie to yourself in a way "I'm not using a goto". Thinking about it, there would be nothing wrong with a goto used similarly. In fact it would also reduce indentation level.
That said, though, I noticed, very often this do..while loops tend to grow. And then they get ifs and elses inside, rendering the code actually not very readable, let alone testable.
Those do..while are normally intended to do a clean-up. By all means possible I would prefer to use RAII and return early from a short function. On the other hand, C doesn't provide you as much conveniences as C++ does, making a do..while one of the best approaches to do a cleanup.
Maybe it’s used so that break can be used inside to abort the execution of further code at any point:
do {
if (!condition1) break;
some_code;
if (!condition2) break;
some_further_code;
// …
} while(false);
I think this is done to use break or continue statements. Some kind of "goto" code logic.
It's simple: Apparently you can jump out of the fake loop at any time using the break statement. Furthermore, the do block is a separate scope (which could also be achieved with { ... } only).
In such a situation, it might be a better idea to use RAII (objects automatically destructing correctly when the function ends). Another similar construct is the use of goto - yes, I know it's evil, but it can be used to have common cleanup code like so:
<return-type> function(<params>)
{
<initialization>
<main code for function using "goto error;" if something goes wrong>
<tidy-up in success case & return>
error:
<commmon tidy-up actions for error case & return error code or throw exception>
}
(As an aside: The do-while-false construct is used in Lua to come up for the missing continue statement.)
How old was the author?
I ask because I once came across some real-time Fortran code that did that, back in the late 80's. It turns out that is a really good way to simulate threads on an OS that doesn't have them. You just put the entire program (your scheduler) in a loop, and call your "thread" routines" one by one. The thread routines themselves are loops that iterate until one of a number of conditions happen (often one being a certain amount of time has passed). It is "cooperative multitasking", in that it is up to the individual threads to give up the CPU every now and then so the others don't get starved. You can nest the looping subprogram calls to simulate thread priority bands.
Many answerers gave the reason for do{(...)break;}while(false). I would like to complement the picture by yet another real-life example.
In the following code I had to set enumerator operation based on the address pointed to by data pointer. Because a switch-case can be used only on scalar types first I did it inefficiently this way
if (data == &array[o1])
operation = O1;
else if (data == &array[o2])
operation = O2;
else if (data == &array[on])
operation = ON;
Log("operation:",operation);
But since Log() and the rest of code repeats for any chosen value of operation I was wandering how to skip the rest of comparisons when the address has been already discovered. And this is where do{(...)break;}while(false) comes in handy.
do {
if (data == &array[o1]) {
operation = O1;
break;
}
if (data == &array[o2]) {
operation = O2;
break;
}
if (data == &array[on]) {
operation = ON;
break;
}
} while (false);
Log("operation:",operation);
One may wonder why he couldn't do the same with break in an if statement, like:
if (data == &array[o1])
{
operation = O1;
break;
}
else if (...)
break interacts solely with the closest enclosing loop or switch, whether it be a for, while or do .. while type, so unfortunately that won't work.
In addition to the already mentioned 'goto examples', the do ... while (0) idiom is sometimes used in a macro definition to provide for brackets in the definition and still have the compiler work with adding a semi colon to the end of a macro call.
http://groups.google.com/group/comp.soft-sys.ace/browse_thread/thread/52f670f1292f30a4?tvc=2&q=while+(0)
I agree with most posters about the usage as a thinly disguised goto. Macros have also been mentioned as a potential motivation for writing code in the style.
I have also seen this construct used in mixed C/C++ environments as a poor man's exception. The "do {} while(false)" with a "break" can be used to skip to the end of the code block should something that would normally warrant an exception be encountered in the loop.
I have also sen this construct used in shops where the "single return per function" ideology is enforced. Again, this is in lieu of an explicit "goto" - but the motivation is to avoid multiple return points, not to "skip over" code and continue actual execution within that function.
I work with Adobe InDesign SDK, and the InDesign SDK examples have almost every function written like this. It is due to fact that the function are usually really long. Where you need to do QueryInterface(...) to get anything from the application object model. So usually every QueryInterface is followed by if not went well, break.
Many have already stated the similarity between this construct and a goto, and expressed a preference for the goto. Perhaps this person's background included an environment where goto's were strictly forbidden by coding guidelines?
The other reason I can think of is that it decorates the braces, whereas I believe in a newer C++ standard naked braces are not okay (ISO C doesn't like them). Otherwise to quiet a static analyzer like lint.
Not sure why you'd want them, maybe variable scope, or advantage with a debugger.
See Trivial Do While loop, and Braces are Good from C2.
To clarify my terminology (which I believe follows standard usage):
Naked braces:
init();
...
{
c = NULL;
mkwidget(&c);
finishwidget(&c);
}
shutdown();
Empty braces (NOP):
{}
e.g.
while (1)
{} /* Do nothing, endless loop */
Block:
if (finished)
{
closewindows(&windows);
freememory(&cache);
}
which would become
if (finished)
closewindows(&windows);
freememory(&cache);
if the braces are removed, thus altering the flow of execution, not just the scope of local variables. Thus not 'freestanding' or 'naked'.
Naked braces or a block may be used to signify any section of code that might be a potential for an (inline) function that you wish to mark, but not refactor at that time.
It's a contrived way to emulate a GOTO as these two are practically identical:
// NOTE: This is discouraged!
do {
if (someCondition) break;
// some code be here
} while (false);
// more code be here
and:
// NOTE: This is discouraged, too!
if (someCondition) goto marker;
// some code be here
marker:
// more code be here
On the other hand, both of these should really be done with ifs:
if (!someCondition) {
// some code be here
}
// more code be here
Although the nesting can get a bit ugly if you just turn a long string of forward-GOTOs into nested ifs. The real answer is proper refactoring, though, not imitating archaic language constructs.
If you were desperately trying to transliterate an algorithm with GOTOs in it, you could probably do it with this idiom. It's certainly non-standard and a good indicator that you're not adhering closely to the expected idioms of the language, though.
I'm not aware of any C-like language where do/while is an idiomatic solution for anything, actually.
You could probably refactor the whole mess into something more sensible to make it more idiomatic and much more readable.
Some coders prefer to only have a single exit/return from their functions. The use of a dummy do { .... } while(false); allows you to "break out" of the dummy loop once you've finished and still have a single return.
I'm a java coder, so my example would be something like
import java.util.Arrays;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;
import java.util.stream.Stream;
public class p45
{
static List<String> cakeNames = Arrays.asList("schwarzwald torte", "princess", "icecream");
static Set<Integer> forbidden = Stream.of(0, 2).collect(Collectors.toSet());
public static void main(String[] argv)
{
for (int i = 0; i < 4; i++)
{
System.out.println(String.format("cake(%d)=\"%s\"", i, describeCake(i)));
}
}
static String describeCake(int typeOfCake)
{
String result = "unknown";
do {
// ensure type of cake is valid
if (typeOfCake < 0 || typeOfCake >= cakeNames.size()) break;
if (forbidden.contains(typeOfCake)) {
result = "not for you!!";
break;
}
result = cakeNames.get(typeOfCake);
} while (false);
return result;
}
}
In such cases I use
switch(true) {
case condution1:
...
break;
case condution2:
...
break;
}
This is amusing. There are probably breaks inside the loop as others have said. I would have done it this way :
while(true)
{
<main code for function>
break; // at the end.
}