In July 2022, did China have more nuclear weapons than Domino's Pizza locations? Thanks for contributing an answer to Stack Overflow! CSS codes are the only stabilizer codes with transversal CNOT? What one-octave set of notes is most comfortable for an SATB choir to sing in unison/octaves? Is the RobertsonSeymour theorem equivalent to the compactness of some topological space? rev2023.6.2.43473. How can an accidental cat scratch break skin but not damage clothes? Return IEnumerable from a private method; use Dictionary or anon. For every use case of a C macro the C++ language provides a language alternative (apart from maybe conditional compilation). How to correctly use LazySubsets from Wolfram's Lazy package? How does a government that uses undead labor avoid perverse incentives? Why should I declare a private static const variable in header (and initialize it in cpp) instead of just defining + declaring it in the cpp? Can I also say: 'ich tut mir leid' instead of 'es tut mir leid'? Your first assumption is that it takes up space. Is this stupid? (assuming I never. Making statements based on opinion; back them up with references or personal experience. Recently I have been using more macros in my C++ code. e.g. Please explain the intended purpose of this code, though, as per the. CSS codes are the only stabilizer codes with transversal CNOT? Find centralized, trusted content and collaborate around the technologies you use most. By "won't change" I meant the value is the same for all objects of the class. How appropriate is it to post a tweet saying that I am looking for postdoc positions? How can an accidental cat scratch break skin but not damage clothes? Can you be arrested for not paying a vendor like a taxi driver or gas station? In Portrait of the Artist as a Young Man, how can the reader intuit the meaning of "champagne" in the first chapter? A defined macro to copy selected values of std::vector to an array using std::copy. C dynamic array implementation using preprocessor, Creating a structural type array wrapper to be qualified as non-type template parameter C++, State-machine class based upon arduino toolchain. To learn more, see our tips on writing great answers. One reason would be that regular const members will take up more memory i.e. Does it have any impact on the memory compared to a macro? And I am not so sure it would be useful anyway, as I fail to see where the, @dribeas, i was attempting to illustrate that sometimes mixing the, Private members: Static const vs. just const, Building a safer community: Announcing our new Code of Conduct, Balancing a PhD program with a startup career (Ep. @oddRaven, I guess it does, but I'd argue that it's quite an ugly way of doing that whereas a constant declared within the class will make it obvious that, Using C preprocessor macro instead of private static const [closed], Building a safer community: Announcing our new Code of Conduct, Balancing a PhD program with a startup career (Ep. 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. In C++ there is no need for macros (for programming). Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Is this a good solution or should I stick with the static const way? Learn more about Stack Overflow the company, and our products. Assuming common c++ conventions are followed (1 header & cpp is only associated with 1 class, never #include .cpp file), in both cases, the variable is private to the class, both are initialized before the constructor is called, both provide the function of being a "static class local constant". 9 Answers Sorted by: 16 static double m = 30000; double foo (double x, double y) { return x/m + y; } This doesn't win you anything. Therefore, in the interests of not unnecessarily polluting your class definition, I'd be inclined to adopt the second approach. 576), AI/ML Tool examples part 3 - Title-Drafting Assistant, We are graduating the updated button styling for vote arrows. Two attempts of an if with an "and" are failing: if [ ] -a [ ] , if [[ && ]] Why? Asking for help, clarification, or responding to other answers. I'm trying to decide what the best option would be for when an object has some traits that won't change, and are needed throughout its functions. Can I infer that Schrdinger's cat is dead without opening the box, if I wait a thousand years? Is it possible to write unit tests in Applesoft BASIC? type? Also, why is this question tagged with, @200_success The constant in the code makes sure I won't have to edit every place I use the. The best answers are voted up and rise to the top, Not the answer you're looking for? C++ makes use of const as well as constexpr (c++11) to declare constants, thus discarding the need for #define. Why is a static const in the header defined as const in the code file, c++ - difference between const and static keyword in function header, Class scope constants: const vs static const, Inline static const vs static const variable. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. The main question here is whether different objects of the class need to have different values of these const members (even if they don't change during the object's lifetime) or all objects should use (share) the same value. See the C++98 standard section 3.5.3. You could go one step further and define it in an anonymous namespace: in that way, it is certainly localised to a single translation unit. It only takes a minute to sign up. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Thanks for contributing an answer to Stack Overflow! By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. for each object of the class you create, one const member object will be included in that object and initialized. Why is Bb8 better than Bc7 in this position? Should convert 'k' and 't' sounds to 'g' and 'd' sounds when they follow 's' in a word for pronunciation? In C, the only way to declare compile time constants is not by using const, but rather with the use of macros ( #define ). If the value is the same for all objects of the class, then, of course, it should be a static const member of the class. How to fix this loose spoke (and why/how is it broken)? In that case you have no reason not to make it, @AnT You contradicted yourself in the comment, If the value is meant to be the same for all objects of the class, it has to be 'static' regardless of the member being 'const'. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Static members obey the class member access rules (private, protected, public). How appropriate is it to post a tweet saying that I am looking for postdoc positions? Static const integer class member in header-only file - the proper way? Add details and clarify the problem by editing this post. Not that it matters much, but does the 4 bytes of memory gets freed up after the class has been initialized? It is a nice habit to get into though. private static const member variable in header vs const variable in cpp, Building a safer community: Announcing our new Code of Conduct, Balancing a PhD program with a startup career (Ep. Not the answer you're looking for? "Won't change" is not precise enough. I'll wiki this so anyone can contribute. 5 Answers Sorted by: 17 "Won't change" is not precise enough. If it's a static const member, on the other hand, then only one object will be created and initialized, no matter how many objects of the class you create, and all class objects will share the same object. Connect and share knowledge within a single location that is structured and easy to search. You should post the code from which you got that output, else it is useless. 576), AI/ML Tool examples part 3 - Title-Drafting Assistant, We are graduating the updated button styling for vote arrows. Browse other questions tagged, Start here for a quick overview of the site, Detailed answers to any questions you might have, Discuss the workings and policies of this site. Making statements based on opinion; back them up with references or personal experience. To learn more, see our tips on writing great answers. Faster algorithm for max(ctz(x), ctz(y))? The main question here is whether different objects of the class need to have different values of these const members (even if they don't change during the object's lifetime) or all objects should use (share) the same value. As @danio points out, if the header is reused in different compilation units, there is only one compilation unit where the compiler could know that the value is shared, as opposed to many that must define a compatible memory layout. Modern compilers will probably use zero space (as long as you don't take it's address) if the value is available from the header file. Is there any difference between the above two approaches? Is it possible to raise the frequency of command input to the processor in this way? How to join two one dimension lists as columns in a matrix, I was wondering how I should interpret the results of my molecular dynamics simulation, Splitting fields of degree 4 irreducible polynomials containing a fixed quadratic extension, Noisy output of 22 V to 5 V buck integrated into a PCB. Why does bunched up aluminum foil become so extremely hard to compress? Most of C++ is derived from C, but C++ is still C++. 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 the language does not guarantee that. To demonstrate, try compiling it both ways, creating several objects, and doing a printf("%p", &theConstMemberObject) when it's static, you'll see they all print the same pointer value; when it's not static, they each have their own object and thus they each print a different pointer value. Short story (possibly by Hal Clement) about an alien ship stuck on Earth. Can I also say: 'ich tut mir leid' instead of 'es tut mir leid'? The one that knows, cannot change the memory layout as it would break code in all the other compilation units. As opposed to each instance having its own (identical) copy of the constant value. Declaring a static member function const or virtual, Difference between private static members and having them in the cpp file, Class scope constants: const vs static const. How could a nonprofit obtain consent to message relevant individuals at a company on LinkedIn under the ePrivacy Directive? How does the damage from Artificer Armorer's Lightning Launcher work? A const member should be used when that member doesn't change on a per-instance basis. Are non-string non-aerophone instruments suitable for chordal playing? First some background: Translation unit: A source file after the pre-processor (recursively) included all its include files. Want to improve this question? Since it's a private member, only the implementation of the class can access it. When called, they have no this pointer.. Static member functions cannot be virtual, const, volatile, or ref-qualified.. Therefore, in the interests of not unnecessarily polluting your class definition, I'd be inclined to adopt the second approach. Then I thought, why use a private static const if a macro could solve the same problem, without taking up 4 bytes of memory? Connect and share knowledge within a single location that is structured and easy to search. Note however that your using const implies internal linkage by default. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. my preference is not to use static const members as it always seem to create more coupling than const members; sometimes confusion w.r.t diamond inheritance hierarchy where the final class inherits from 2 super classes (1 class is inherited public; the other is inherited virtual public) then defining a static const member that points to a dynamic memory region via new, malloc, calloc etc would result in a double free error. Solving the problem of using directives in a header file with a macro. The address of a static member function may be stored in a regular pointer to function, but not in a pointer to . Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, Aren't they local to the single translation unit anyway (without anonymous namespace)? 576), AI/ML Tool examples part 3 - Title-Drafting Assistant, We are graduating the updated button styling for vote arrows. Not the answer you're looking for? Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. rev2023.6.2.43473. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Aside from the memory usage, it also takes up two lines in the code, just like the macro (#define and #undef). Why does bunched up aluminum foil become so extremely hard to compress? You could go one step further and define it in an anonymous namespace: namespace { const MyType some_constant = . What control inputs to make if a wing falls off? 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. How to show a contourplot within a region? I'm not sure if that's what you're looking for since that's merely an explanation of how they behave, but I hope it helps somewhat. Why is Bb8 better than Bc7 in this position? Stack Exchange network consists of 181 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. A static const member should be used when that member doesn't change on a per-class basis. A const member variable may be initialised in the constructor so may vary for different instances of the class. When is a non-static const data member more useful than a const static one? What is the name of the oscilloscope-like software shown in this screenshot? Seems that a deeper answer ought to emerge. What one-octave set of notes is most comfortable for an SATB choir to sing in unison/octaves? Enabling a user to revert a hacked change in their email, Efficiently match all values of a vector in another vector. A copy of m has to be made to do the computation. (And which one is preferable)? It seems to me like the real reason for a static member is to have a variable that can be changed, and thus affect all other objects of the same class. How much of the power drawn by a chip turns into heat? How could a nonprofit obtain consent to message relevant individuals at a company on LinkedIn under the ePrivacy Directive? A lot of people gave the basic answer but nobody pointed out that in C++ const defaults to static at namespace level (and some gave wrong information). In other words, no matter how many instances you create, the static const member remains fixed between all instances whereas the const member is constant only for a specific instance. @KonstantinT. private static const member variable in header vs const variable in cpp, Inline static const vs static const variable, Word to describe someone who is ignorant of societal problems. Use that instead. If different objects might require different values, then it should be just a non-static const member. Is there any philosophical theory behind the concept of object in computer science? Does the policy change for AI-generated content affect users who (want to) Why does "extern const int n;" not work as expected? Connect and share knowledge within a single location that is structured and easy to search. If you want your constant to be public or private, declare it in your class as such and initialise it. A float is 4 bytes in size. Is Spider-Man the only Marvel character that has been represented as multiple non-human characters? In this movie I see a strange cable for terminal connection, what kind of connection is this? You would make a member variable static const because, with it being unchanging, you can reuse the same copy of it, shared by all instances of your class. In C, the only way to declare compile time constants is not by using const, but rather with the use of macros (#define). Can you be arrested for not paying a vendor like a taxi driver or gas station? You would expect the compiler to be smart enough to do this optimisation for you though, and I suspect it does. C++ makes use of const as well as constexpr (c++11) to declare constants, thus discarding the need for #define. I wrote a class with a private static const float because it is useful to prevent making mistakes. Citing my unpublished master's thesis in the article that builds on top of it. Why are radicals so intolerant of slight deviations in doctrine? Code Review Stack Exchange is a question and answer site for peer programmer code reviews. What task does this code accomplish? Did an AI-enabled drone attack the human operator in a simulation environment? Short story (possibly by Hal Clement) about an alien ship stuck on Earth. Please explain. Asking for help, clarification, or responding to other answers. Unless SomeProgrammerDude is still watching this - they are an expert. Noise cancels but variance sums - contradiction? How to deal with "online" status competition at work? ; } (Without const, the variable would be accessible by others using extern). @oddRaven: A float may be 4 bytes on your current configuration (Hardware/OS/Compiler). Is there a reason beyond protection from potential corruption to restrict a minister's ability to personally relieve and appoint civil servants? First story of aliens pretending to be humans especially a "human" family (like Coneheads) that is trying to fit in, maybe for a long time? Private members: Static const vs. just const, c++ private member declared in header vs static variable declared in cpp file. Why use static const (int/string/..) if you can put it in private? TOTAL_TIME being a macro pollutes the whole namespace with its mess. How to show a contourplot within a region? nonetheless I agree with the answer (but not your comment). Macros are public to whomever "includes" the file they are declared in, but private members of a class are only . Why do front gears become harder when the cassette becomes larger but opposite for the rear ones? However, I've had people recommend class "invariants" to be static const members. In C++11 the ability to initialize these values in the class definition was introduced. here's the output of a simple diamond inheritance situation. Pythonic way for validating and categorizing user input. Macros are public to whomever "includes" the file they are declared in, but private members of a class are only visible to the object of that class. You must be careful with what you can assume the compiler to do for you. Should I contact arxiv if the status "on hold" is pending for a week? Does the policy change for AI-generated content affect users who (want to) C++: access const member vars through class or an instance? Where should I declare my private class level static constant in the header file? You don't need a macro for that. I'm looking for some insight regarding the recommended approach to establishing class constants, and reasons why. 1 Answer Sorted by: 5 Since it's a private member, only the implementation of the class can access it. : Interesting, I'll need to check that one. Find centralized, trusted content and collaborate around the technologies you use most. Static linkage: A symbol is only available within its translation unit. rev2023.6.2.43473. [] Static member functionStatic member functions are not associated with any object. A regular pointer to constructor so may vary for different instances of the class to restrict a minister 's to! Licensed under CC BY-SA a taxi driver or gas station the constant value to function but... Not unnecessarily polluting your class definition, I 'll need to check that.. Or private, protected, public ) bunched up aluminum foil become extremely! Inc ; user contributions licensed under CC BY-SA function, but does the from! You create, one const member can not change the memory compared to a macro theory the... Careful with what you can put it in an anonymous namespace: namespace { const some_constant. ; back them up with references or personal experience a header file with a macro the one knows... Class definition was introduced a C macro the C++ language provides a language (... The whole namespace with its mess be arrested for not paying a like... Maybe conditional compilation ) such and initialise it deviations in doctrine using directives in regular! Well as constexpr ( c++11 ) to declare constants, and our products difference the. Case of a static const vs. just const, volatile, or ref-qualified compiler to be smart enough do... Member function may be stored in a header file with a private member declared in header vs variable... Url into your RSS reader the power drawn by a chip turns into heat with you! Vote arrows the intended purpose of this code, though, as per the write unit tests in Applesoft?! Keyvaluepair > from a private method ; use Dictionary or anon memory compared to a.! Sing in unison/octaves arxiv if the status `` on hold '' is not precise enough my C++ code careful... Human operator in a pointer to function, but not damage clothes const integer class member in header-only file the..., though, as per the collaborate around the technologies you use most any object and reasons why saying I! Change & quot ; Won & # x27 ; t change & quot ; is not precise.! More, see our tips on writing great answers in C++ there no! Do this optimisation for you though, and our products answers are voted up rise. Potential corruption to restrict a minister 's ability to initialize these values in the that. ; use Dictionary or anon on the memory compared to a macro code which... - Title-Drafting Assistant, We are graduating the updated button styling for vote arrows however that your using const internal. Codes with transversal CNOT is this I 've had people recommend class invariants! ; Won & # x27 ; t change & quot ; is not precise enough are graduating the updated styling! With coworkers, Reach developers & technologists worldwide a float may be 4 bytes of memory gets freed up the... '' I meant the value is the name of the oscilloscope-like software in... An alien ship stuck on Earth to do this optimisation for you though, as per.. Of m has to be made to do for you though, and reasons why not associated any. More useful than a const static one a user to revert a hacked change in their email Efficiently! Called, they have no this pointer.. static member functionStatic member functions are not associated any! However that your using const implies internal linkage by default a simulation environment members obey the class #! Not paying a vendor like a taxi driver or gas station opinion back... Object will be included in that object and initialized in header-only file - the proper way got that output else. To other answers prevent making mistakes a nice habit to get into though Artificer Armorer 's Lightning work... Competition at work must be careful with what you can put it in?. Be that regular const members will take up more memory i.e private member, only the implementation the! Their email, Efficiently match all values of a C macro the language! Private, protected, public ) of notes is most comfortable for SATB! Per-Instance basis linkage by default strange cable for terminal connection, what kind of connection is a... Declare my private class level static constant in the constructor so may vary for different of. Memory i.e Tool examples part 3 - Title-Drafting Assistant, We are graduating updated... When that member does n't change '' I meant the value is the RobertsonSeymour theorem equivalent to processor. Have more nuclear weapons than Domino 's Pizza locations that has been initialized the name of the c++ private static const! User to revert a hacked change in their email, Efficiently match all values of std::copy (. To sing in unison/octaves connection is this a good solution or should I declare my class! Polluting your class definition, I 've had people recommend class `` invariants '' to be public private. Opposite for the rear ones code reviews constructor so may vary for different of! Of memory gets freed up after the class you create, one const member all its include.. Css codes are the only stabilizer codes with transversal CNOT you be arrested for not a. Input to the compactness of some topological space ; back them up with references or experience! Appoint civil servants a minister 's ability to personally relieve and appoint civil servants thus discarding need! Const data member more useful than a const static one personal experience kind of is... Then it should be just a non-static const c++ private static const member more useful than a const should. Why are radicals so intolerant of slight deviations in doctrine you should post the code which! Members: static const float because it is useless sing in unison/octaves can the! Processor in this way ePrivacy Directive using extern ) to revert a hacked change in their email, match... Nice habit to get into though unit: a float may be 4 bytes on your current configuration ( )! In private the interests of not unnecessarily polluting your class as such and it! Box, if I wait a thousand years a chip turns into heat regarding... You though, as per the restrict a minister 's ability to these. Chip turns into heat variable may be stored in a header file object and initialized drawn by a chip into... But C++ is still watching this - they are an expert C++ is still C++ put c++ private static const in anonymous! Header vs static variable declared in cpp file personal experience drawn by chip... Url into your RSS reader, copy and paste this URL into your RSS reader would code! 'S Lazy package opinion ; back them up with references or personal experience more about Stack Overflow company... Macro pollutes the whole namespace with its mess reason beyond protection from potential corruption to restrict a 's! This code, though, and I suspect it does and clarify the problem of using directives a! C++ code corruption to restrict a minister 's c++ private static const to personally relieve appoint... Are not associated with any object correctly use LazySubsets from Wolfram 's package! I 'm looking for some insight regarding the recommended approach to establishing class constants, and suspect! Uses undead labor avoid perverse incentives discarding the need for # define ( ctz ( x ), AI/ML examples... Equivalent to the processor in this movie I see a strange cable for terminal connection, what kind of is. Output of a vector in another vector control inputs to make if a wing off... Minister 's ability to initialize these values in the constructor so may vary for different instances the. Up after the pre-processor ( recursively ) included all its include files unit... Efficiently match all values of a simple diamond inheritance situation total_time being a macro become so extremely to! Avoid perverse incentives above two approaches vs static variable declared in header vs static variable declared in header vs variable! Code from which you got that output, else it is useless others using extern.... Function, but does the damage from Artificer Armorer 's Lightning Launcher work tweet saying that I looking! In unison/octaves 2023 Stack Exchange Inc ; user contributions licensed under CC BY-SA such and it! That member does n't change on a per-class basis ( Hardware/OS/Compiler ) constants, I... Though, as per the it 's a private member, only implementation...: 17 & quot ; Won & # x27 ; t change quot... Language provides a language alternative ( apart from maybe conditional compilation ) that builds on of. Applesoft BASIC can I also say: 'ich tut mir leid ' instead 'es. I wrote a class with a private method ; use Dictionary or anon RSS reader habit to into! Some insight regarding the recommended approach to establishing class constants, thus discarding the need for (! Company, and I suspect it does file after the pre-processor ( recursively included. Has to be public or private, declare it in an anonymous namespace: namespace { const some_constant! Design / logo 2023 Stack Exchange Inc ; user contributions licensed under CC BY-SA to adopt second. Not damage clothes ctz ( x ), ctz ( x ), AI/ML Tool examples part 3 - Assistant... Need to check that one for not paying a vendor like a taxi driver or gas station to... Therefore, in the class can access it topological space to deal with `` online '' status competition work... The rear ones find centralized, trusted content and collaborate around the technologies you use most part. Most of C++ is still watching this - they are an expert, only implementation... Efficiently match all values of c++ private static const::copy simple diamond inheritance situation do computation...
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