1 Votes - Reported May 30, 2018 9:19 PM In the following code, the C4100 warning should not occurs. C++ : [[maybe_unused]] on member variable, GCC warns (incorrectly?) Making statements based on opinion; back them up with references or personal experience. Syntax [[maybe_unused]] Explanation This attribute can appear in the declaration of the following entities: class/struct/union: struct [[maybe_unused]] S; , typedef, including those declared by alias declaration: [[maybe_unused]] typedef S* PS;, using PS [[maybe_unused]] = S*; , Missing a break or return in a switch case is a very common programmer error, so compilers usually warn about it, but sometimes a fallthrough can result in some very terse code. To learn more, see our tips on writing great answers. However, neither GCC nor Clang warns for the case above, and afaict there are no warnings related to "unused public field of local class or class hidden with internal linkage" neither for GCC nor Clang. that attribute is ignored. by the comments in the GCC bug report(s) I link to. 2. 576), AI/ML Tool examples part 3 - Title-Drafting Assistant, We are graduating the updated button styling for vote arrows. Citing my unpublished master's thesis in the article that builds on top of it, Ways to find a safe route on flooded roads. If the return value is ignored, the compiler should give a warning on this. . . The attribute-token maybe_unused indicates that a name or entity is possibly intentionally unused. C++17 adds three new attributes for programmers to better express their intent to the compiler and readers of the code: maybe_unused, fallthrough, and nodiscard. 2. The attribute may be applied to the declaration of a class, a typedef-name, a variable, a non-static data member, a function, an enumeration, or an enumerator. 6.7.11.3 Maybe_unused attribute Constraints 1 The attribute-token maybe_unused indicates that a name or entity is possibly intentionally unused. Syntax [ [ maybe_unused ]] [ [ __maybe_unused__ ]] Explanation This attribute can appear in the declaration of the following entities: struct / union: struct [[maybe_unused]] S; , typedef name: [[maybe_unused]] typedef S* PS; , object: [[maybe_unused]] int x; , struct/union member: union U { [[maybe_unused]] int n; }; , Asking for help, clarification, or responding to other answers. The [[nodiscard]] attribute can be used to indicate that the return value of a function shouldn't be ignored when you do a function call. Without the fallthrough in the Alert::Red case, it is not obvious whether a red alert is supposed to trigger the alarm and be recorded, or just evacuate everyone. Why is it "Gaudeamus igitur, *iuvenes dum* sumus!" @VTT Nope, it shouldn't. What are some symptoms that could tell me that my simulation is not running properly? Weird compiler warning about unused variable, is this a compiler bug? In CMake, how can I test if the compiler is Clang? Description Matt Whitlock 2019-06-17 07:07:33 UTC Description Barry Revzin 2019-07-31 00:35:06 UTC Can I also say: 'ich tut mir leid' instead of 'es tut mir leid'? In case you arent familiar with the concept, attributes in C++ allow you mark functions, variables, and other entities with compiler-specific or standard properties. C++17 attributes - maybe_unused, fallthrough and nodiscard. Connect and share knowledge within a single location that is structured and easy to search. This option controls warnings when an attribute is ignored. Only the more extensive option -Wno-attributes menage to suppress this warning. On the other hand, implementation-defined "bug" reports (not bugs, per se, but more of feature/change requests) are naturally prioritized lower than actual bug reports, meaning a bug such as the one above is not likely to be implemented unless someone from the open source community jumps in to drive and support with the change. // Compile with cl /std:c++latest /W4 void f ( [ [maybe_unused]] int a) {} // no warnings auto g = [] ( [ [maybe_unused]] int a) {}; // C4100 int main () { } One workaround is to use for gcc the flag -Wno-ignored-attributes. rev2023.6.2.43474. For a structured binding declaration not marked maybe_unused, implementations should not emit such a warning unless all of its structured bindings are unused. there is no strict requirement on implementations on how to apply this attribute, only a recommendation as for how implementations should apply it, as per [dcl.attr.unused]/4 [emphasis mine]: Recommended practice: For an entity marked maybe_unused, implementations should not emit a warning that the entity or its structured bindings (if any) are used or unused. Any attribute can be "ignored by the compiler" for any reason, except where the standard says otherwise (such as using an attribute in a location where it is expressly forbidden). Note You need to log in before you can comment on or make changes to this bug. Can I trust my bikes frame after I was hit by a car if there's no visible cracking? rather than "Gaudeamus igitur, *dum iuvenes* sumus!"? Find centralized, trusted content and collaborate around the technologies you use most. Is there anything called Shallow Learning? The most important function of fallthrough is as documentation for maintainers. in @0x5453: self-answer, to attempt to align the implementation-defined behaviours of the selected compilers. Note, the following, even if apparently correct, doesn't actually work. GCC emits a warning where Clang and MSVC do not: As far as I can tell, this should be legal (and not ignored by the compiler). Is it possible for rockets to exist in a world that is only in the early stages of developing jet aircraft? This means as long as implementations allow it to be applied to a declaration of a non-static data member, they are standard compliant, and it is not a compiler bug that the attribute is non implemented using the recommended practice, even if we can argue that a compiler should be able to diagnose an unused non-static data member of a class defined with internal linkage within a single translation unit. Why are mountain bike tires rated for so much lower pressure than road bikes? Not the answer you're looking for? I hate to swing around the "compiler bug" hammer, but I'm not sure what else it could be in this case. How to make a HUE colour node with cycling colours. This warning is enabled by default. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Does a knockout punch always carry the risk of killing the receiver? How can an accidental cat scratch break skin but not damage clothes? Why doesnt SpaceX sell Raptor engines commercially? It is the correct placement syntax according to the grammar production: The optional attribute-specifier-seq comes after the identifier and before the =. If I had more time on my hands, it would probably be interesting and instructive to help out sorting out a few GCC/Clang minor bug reports such as this one. It's only a warning, your code is still legal. As far as I can tell, this should be legal (and not ignored by the compiler). I feel like this attribute is more useful to supress compiler warnings than to document your code for others, but at least we now have a standard way to do so. This seems strange since it is normally placed infront. Did you find a way to remove the warning for gcc, while keeping the. Not the answer you're looking for? Making statements based on opinion; back them up with references or personal experience. These behaviours are both correct, as it lies in the domain of implementation-defined behaviour. How common is it to take off from a taxiway? Korbanot only at Beis Hamikdash ? Syntax for [[maybe_unused]] attribute applied for type alias declaration, Building a safer community: Announcing our new Code of Conduct, Balancing a PhD program with a startup career (Ep. Don't have to recite korbanot at mincha? 91304 - maybe_unused attribute ignored on variable declared in if declaration Last modified: 2020-05-28 08:51:16 UTC - Attachments Add an attachment (proposed patch, testcase, etc.) Does the policy change for AI-generated content affect users who (want to) How do I best silence a warning about unused variables? Functions declared with [[nodiscard]] should not have their return values ignored by the caller. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. @BaummitAugen: For local types (function or file scope), compiler might know. Using [[maybe_unused]] attribute on unused class *tors, [[maybe_unused]] applied to static data members, [[maybe_unused]] on member variable, GCC warns (incorrectly?) which one to use in this conversation? Use of Stein's maximal principle in Bourgain's paper on Besicovitch sets. int main ( [ [maybe_unused]] int argc, char** argv) { //. } This can be useful if you want to ensure that callers check a return value, or that some scope guard object has a reasonable lifetime. The presence of it in the code above shows anyone looking at the code that an orange alert is absolutely supposed to be recorded. Why does the bool tool remove entire object? To learn more, see our tips on writing great answers. Functions declared with [[nodiscard]] should not have their return values ignored by the caller. shall not be ignored: no, it's up to the implementation to decide whether or not and how to use this attribute. GCC won't warn you for unused member variables in the first place so that attribute has no purpose, that's why it's warning you for ignoring it. Those warnings will help the user notice that do_something_else might be given a bad object, or the critical section wont be locked. be to use implementation-specific pragmas, as shown e.g. Attempt to figure out correct syntax for [[maybe_unused]] attribute resulted in the following code: All of it sudden it only works if written after the target type name. If you are using CMake, this will look like this: Detecting compiler in CMake Complexity of |a| < |b| for ordinal notations? . This can be useful if you want to ensure that callers check a return value, or that some scope guard object has a reasonable lifetime. 10 I am trying to ignore the unused parameter warning using the new c++17 attribute [ [maybe_unused]], as below. The standard attributes prior to C++17 are noreturn (function doesnt return), deprecated (entity is going to be removed in a later version) and carries_dependency (used for optimizing atomic operations). To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Noise cancels but variance sums - contradiction? It shall appear at most once in each attribute-list and no attribute-argument-clause shall be present. You mark an entity with an attribute like this: With that out of the way, on to the new attributes! GCC isn't saying you can't put one there; it's saying that putting one there won't do anything, because they probably don't warn about maybe-unused member variables. Connect and share knowledge within a single location that is structured and easy to search. 04/14/2023 5 contributors Feedback In this article C++ standard attributes Microsoft-specific attributes The C++ Standard defines a common set of attributes. Why does the bool tool remove entire object? Don't have to recite korbanot at mincha? How can an accidental cat scratch break skin but not damage clothes? on variable, C++ Leave a comment if you have ideas for other attributes you would like to be added to the language! @Antonio One upside, however, is that GCC and Clang (in my experience) mutually tries to have similar behaviour in the implementation-defined domain, as can be seen e.g. Read the grammar again. As there are no compiler bugs to hammer down on here, a "portable" (for the specific compilers chosen) implementation of the Foo class (if it it were to have, say, (maybe) non-used private data members), w.r.t. [[fallthrough]] indicates that a fallthrough in a switch statement is intentional. If its green, we do nothing; if its yellow, we record the alert; if its orange we record and trigger the alarm; if its red, we record, trigger the alarm and evacuate. C++ : [[maybe_unused]] on member variable, GCC warns (incorrectly?) Which comes first: CI/CD or microservices? unused warnings, would e.g. Warning about not used value does not always appear, GCC: "__unused__" vs just "unused" in variable attributes, [[maybe_unused]] applied to static data members. E.g. Korbanot only at Beis Hamikdash ? I am not fully satisfied with this answer, because still my problem is not solved. Suppress unused variable warning in C++ => Compiler bug or code bug? Adding compiler flags in CMake. By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. But I won't do anything to prevent the auto-awarding of the bounty :), It gets a little interesting when supporting both compilers and when using, > Clang warns on unused members < As far as I can tell, it warns only if, @Youw I've encountered the case described with clang warning for unused private field. The attribute may be applied to the declaration of a class, a typedef-name, a variable, a non-static data member, a function, an enumeration, or an enumerator. For some more examples of using these attributes, see posts by Kenneth Benzie, Bartomiej Filipek, and Arne Mertz. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. but sometimes they can be intentional, such as variables which are only used in release mode, or functions only called when logging is enabled. Types can be marked [[nodiscard]] to implicitly mark all functions returning that type as the same. . do I put [[maybe unused]] on function declarations or definitions? C++ attribute: maybe_unused (since C++17) Suppresses warnings on unused entities. Syntax [[maybe_unused]] // Warning suppressed by [[maybe_unused]], // Warning emitted if LOGGING_ENABLED is not defined, // Warning: this statement may fall through. 1 Answer. Can the logo of TSR help identifying the production time of old Products? However, compilers are only required to recognize the attributes defined in the standard. Also according to the linked page. Is this really a correct syntax or just come compiler glitch? The reason is that Clang does emit a -Wunused-private-field warning for non-used private static data members: Whereas GCC does not, which would also include why GCC (correctly) warns the maybe_unused attribute will be ignored by it for non-static data members (even private ones), as it will simply not diagnose non-used private data members (whereas Clang does). Semantics of the `:` (colon) function in Bash when used in a pipe? An alternative approach would be to consider entirely removing Clang's -Wunused-private-field warning globally (-Wno-unused-private-field), leaving these kind of diagnostics to a static analysis tool instead. 90894 - maybe_unused attribute is ignored on function parameters in explicitly instantiated templates Attachments Add an attachment (proposed patch, testcase, etc.) How do I add a linker or compile flag in a CMake file? Is Spider-Man the only Marvel character that has been represented as multiple non-human characters? I hate to swing around the "compiler bug" hammer, but I'm not sure what else it could be in this case. Why does bunched up aluminum foil become so extremely hard to compress? It also allows compiler vendors to define their own attributes within a vendor-specific namespace. Is it OK to pray any five decades of the Rosary or do they have to be in the specific set of mysteries? 2 The attribute shall be applied to the declaration of a struct, a union, a typedef-name, a variable, a . [[maybe_unused]] suppresses compiler warnings about unused entities. Now that's unfortunate. Compilers have shipped non-standard extensions to express these concepts for years, but its great that we now have standard methods to do the same. Say we want to process an alert message. Then we may as ourselves why Clang does not emit an implementation-defined warning that the attribute will be ignored for the case of non-static data members? Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. in the following example: The non-static data member member of Foo is not used; this is diagnosable and the maybe_unused attribute could arguably be used to suppress such an implementation-defined unused warning. [[maybe_unused]] in if statement doesn't seem to work in GCC, [[maybe_unused]] on label defined but not used, Unused variable warning even when explicitly using it inside IF statement, Clang++ Fails To Detect Use of Uninitialized Member Variable. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, Now that's unfortunate. By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. donnez-moi or me donner? Why does a rope attached to a block move when pulled? What happens if you've already found the item an old map leads to? Complexity of |a| < |b| for ordinal notations? It shall appear at most once in each attribute-list and no attribute-argument-clause shall be present. that attribute is ignored\rLet me know if you have more specific questions or concerns by leaving a comment or starting a chat.\rYour thoughts and contributions are welcome, so please leave a comment below if you have an answer or insights to the answer.\rI will 'heart' your answer to show my gratitude for your contribution.\rignored GCC attribute warns that [[maybe_unused]] : is member (incorrectly?) Does anyone have any insight? Now that I'm looking at this again I'm not able to get Clang to warn about the unused member, so you may be able to remove the attribute to satisfy all compilers. Would a revenue share voucher be a "security"? Thanks for contributing an answer to Stack Overflow! According to the standard: 10.6.7 Maybe unused attribute [dcl.attr.unused] MTG: Who is responsible for applying triggered ability effects, and what is the limit in time to claim that effect? Asking for help, clarification, or responding to other answers. Does the policy change for AI-generated content affect users who (want to) G++: Can __attribute__((__may_alias__)) be used for pointer to class instance rather than to class definition itself? Note You need to log in before you can comment on or make changes to this bug. It is the correct placement syntax according to the grammar production: alias-declaration: using identifier attribute-specifier-seq = defining-type-id ; The optional attribute-specifier-seq comes after the identifier and before the =. Movie in which a group of friends are driven to an abandoned warehouse full of vampires. This is different from the -Wattributes option in that it warns whenever the compiler decides to drop an attribute, not that the attribute is either unknown, used in a wrong place, etc. What is the first science fiction work to use the determination of sapience as a plot point? Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. If you want to disable the warning entirely in GCC, you can use this compile flag: And here's an example of selectively disabling the warning, but it's not pretty: The #ifdef __GNUC__s are required because MSVC emits warnings upon seeing #pragma GCC. Why shouldnt I be a skeptic about the Necessitation Rule for alethic modal logics? Attributes Suppresses warnings on unused entities. We may note that there exists a GCC bug report from 2016 that asks for the feature which Clang implements: In a duplicate-marked bug report, Bug 87409 - Implement -Wunused-private-field, Jonathan Wakely comments that if this feature were to be implemented in GCC, they would also need to implement the suppression of it for the (maybe) unused attribute: Clang suppresses the warning if the member declaration has attribute unused, which we would need to do too. How does TeX know whether to eat this space if its catcode is about to change? Why does a rope attached to a block move when pulled? I think you are not 100% correct, but you put me on the right path, [[maybe_unused]] on member variable, GCC warns (incorrectly?) Thanks for contributing an answer to Stack Overflow! But I still get warning: unused parameter 'argc' [-Wunused-parameter] with the following additional warning. This is a quick post to outline what they do and why they are useful. c++ c++17 compiler-warnings compiler-bug Share How would a compiler know whether some public member is unused anyway? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Usually unused functions or variables indicates a programmer error if you never use it, why is it there? that attribute is ignoredTo Access My Live Chat Page, On Google, Search for "hows tech de. that attribute is ignored, Building a safer community: Announcing our new Code of Conduct, Balancing a PhD program with a startup career (Ep. What maths knowledge is required for a lab-based (molecular and cell biology) PhD? rev2023.6.2.43474. Find centralized, trusted content and collaborate around the technologies you use most. on July 20, 2017 under c++ . Do we decide the output of a sequental circuit based on its present state or next state? Whilst [dcl.attr.unused]/2 specified that the maybe_unused attribute may be applied to a declaration of a non-static data member [emphasis mine]: The attribute may be applied to the declaration of a class, a typedef-name, a variable (including a structured binding declaration), a non-static data member, a function, an enumeration, or an enumerator. that attribute is ignored\rTo Access My Live Chat Page, \rOn Google, Search for \"hows tech developer connect\"\r\rI promised to share a hidden feature with you, and here it is.\rThis is a YouTube's feature which works on Desktop.\rFirst, Make sure this video is playing.\rThen, type the letters 'awesome' on your keyboard as the next step.\rYour YouTube progress indicator will turn into a shimmering rainbow.\r\rHere's a brief description of who I am,\rGreetings, my name is Delphi.\rI am here to assist you in getting the answers you need.\rC++ : [[maybe_unused]] on member variable, GCC warns (incorrectly?) 576), AI/ML Tool examples part 3 - Title-Drafting Assistant, We are graduating the updated button styling for vote arrows.
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