- dyp. How did muzzle-loaded rifled artillery solve the problems of the hand-held rifle? Defining constexpr static data members; Non static const data members; Why do non-const, non-int/enum static data members have to be initialized outside the . I think the resolution would be like this (added to [expr.cons]/11): (11.?) PSE Advent Calendar 2022 (Day 11): The other side of Christmas. We might not want to have undefined results when initializing a constexpr variable, though. Replacing pure virtual interfaces with lambdas. How can I fix it? This redeclaration without an initializer (formerly required as shown above) is still . Do non-Segwit nodes reject Segwit transactions with invalid signature? Probably nothing other than the standards committee didn't think it was a good idea. C++14 and earlier. In this example, x is trivially default-initialized. When they use it, they can declare specific instances as constexpr (as above). If I have some block of code that I would like a constexpr instance over, I'd do it with a template: Though I think you'd be better off with a constexpr function that could be used both in the restrictive an non restrictive ways? We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. For example: struct Constants { static constexpr int bounds [] = { 42, 56 }; }; float a [Constants::bounds [0]] [Constants::bounds [1]]; End of C++11 only. But here I'd really like to declare xVal and yVal constexpr--like this: As indicated, the code won't compile. When non-const members can be used in constexpr member functions? Personally, I find it having very little utilityI don't really want to define how people use my class, just define how my class behaves when they use it. To learn more, see our tips on writing great answers. When they use it, they can declare specific instances as constexpr (as above). The definition of A and S could be in completely different compilation units, so the fact that S must be constexpr may not be known until link time, especially if the implementation of A is forgotten. Think about what constexpr means. Again, an object occupy storage, prvalue does not. It means that I can resolve this value at compile time. The reason is that (per 7.1.5/1), only static data members may be declared constexpr. Clang is also giving the rationale for why C++ is defined this way: the value of a pointer is . Why is this constexpr static member function not seen as constexpr when called? If he had met some scary fish, he would immediately return to the surface. Is the constexpr specifier required on the declaration of a constexpr static member initialized outside of the class? Thus, a member variable of a class cannot itself be a constexprthe instance that xVal belongs to does not exist until instantiation time! I would be very surprised if you find any system on which it is not stored exactly, even though you're right that it's permitted. Would salt mines, lakes or flats be reasonably found in high, snowy elevations? why " 'static' may not be used when defining (as opposed to declaring) a static data member"? conflicting declaration of constexpr static struct class member, Disconnect vertical tab connector from PCB. TabBar and TabView without Scaffold and with fixed Widget. Why is the eastern United States green if the wind moves from west to east? A constexpr specifier for a nonstatic member function that is not a constructor declares that member function to be const. It was intentional that we can have core constant expressions with undefined data. Well occasionally send you account related emails. However, in the current draft, we do not have this restriction anymore, and I do not find any replaceable rules. It means that I can resolve this value at compile time. Each of. @KnowItAllWannabe completely different things (in fact, in c++14 constexpr does not imply const). But why?". Where does the idea of selling dragon parts come from? if your main concern is constant value which is shareable to all template type instances, then you can just . But the problem you refer to, if it exists, is just as applicable to. Personally, I find it having very little utilityI don't really want to define how people use my class, just define how my class behaves when they use it. Program to figure out number between 1 and 100 won't ever guess "100" because of rounding, gcc and __attribute__((unused)) for auto references. to your account, In C++17 standard, we have the restriction on the constructor that is declared with constexpr. A const value can still be const if I initiate it on the heap, for example. - if the value is of union type, it has one active member. I appreciate and I believe I understand your perspective, but I don't find it compelling. A static member has certain special characteristics. Ready to optimize your JavaScript with Rust? Do bracers of armor stack with magic armor enhancements and special abilities? We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. It has nothing to do with initializing a static data member. @remyabel: This question is completely different. For example: struct Constants { static constexpr int bounds [] = { 42, 56 }; }; float a [Constants::bounds [0]] [Constants::bounds [1]]; End of C++11 only. If any declaration of a function or function template has a constexpr specifier, then every declaration must contain that specifier. Why must non-integral static data members initialized in the class be constexpr? One of the reasons why this was the case prior to C++11's addition of constexpr was the standard did not specify how floating points were to be implemented (it is left to the processor/architecture - for example, when you say float x = 1.6f, it is actually 1.6000000000024 on most systems). A constexpr specifier used in an object declaration or non-static member function (until C++14) implies const. @KnowItAllWannabe I flagged it as not constructive, the comment vacuum will come and blow all the comments away eventually. [PATCH] D78116: [AST] dont invaliate VarDecl when the initializer contains errors. How to set a newcommand to be incompressible by justification? So, because of fact that char can only be a constant expression, so it is illegal for all non integral values. Do non-Segwit nodes reject Segwit transactions with invalid signature? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. @IdeaHat: But now you're arguing that non-static constexpr data members would be bad design, whereas your proposed answer seems to argue that the values of such data members wouldn't be determinable during compilation. BTW, the alternative is that you can make a specific instances constexpr as in my example. Any class that could be instantiated as constexpr can always have instances that are not constexpr (All I'd need to do is call new S). By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. You can make functions take in an template argument of the class type to enforce receiving a constexpr. Why is this constexpr static member function not seen as constexpr when called? Another reason would be how infectious that is. xmh0511 changed the title [dcl.contexpr] The restriction that non-static data member of fundamental type shall be initialized lacks in the current draft [dcl.constexpr] The restriction that non-static data members of fundamental type shall be initialized lacks in the current draft Apr 28, 2022 How to have static data members in a header-only library? I'd be interested to hear more details about how the ODR could feed into this. The definition of A and S could be in completely different compilation units, so the fact that S must be constexpr may not be known until link time, especially if the implementation of A is forgotten. There is no technical compiler reason you can't do that (i don't think) but it does not feel very C++-like. The thing that owns xVal could be constexp, and that would make xVal a constexpr, but xVal could never be constexpr on its own. Not the answer you're looking for? The thing that owns xVal could be constexp, and that would make xVal a constexpr, but xVal could never be constexpr on its own. The reason is that (per 7.1.5/1), only static data members may be declared constexpr. @KnowItAllWannabe Right, but I could never put it in a dynamic array, never use it as a mutable, never re-factor the code to configure from a settings file. Is there a higher analog of "category with all same side inverses is a groupoid"? Thus, GCC is wrong. Why can't non-static data members be constexpr? Says you have to use brace initializer, which KnowItAllWannabe did. A non-static member function is a function that is declared in a member specification of a class without a static or friend specifier. Should teachers encourage good students to help weaker ones? It is initialized before any object of this class is . Okay, but the same restriction applies to const static data members, so if the declaration for. I think [expr.cons]/11 should first be clarified since the wording is not clear. Thus, a member variable of a class cannot itself be a constexpr.the instance that xVal belongs to does not exist until instantiation time! https://cplusplus.github.io/CWG/issues/2536.html, [dcl.init.general] Fix the informative description in 16.6.1 Example 2 CWG2612, every non-variant non-static data member and base class subobject shall be initialized. Non-static member functions. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, LLVM C++ : compiler fail to identify primitive type of class member as const. Making statements based on opinion; back them up with references or personal experience. @IdeaHat: I don't know what you mean by "infectious" here, but I'm not sure we'll make much headway by further discussion. Does that make sense? The thing that owns xVal could be constexp, and that would make xVal a constexpr, but xVal could never be constexpr on its own. Implementations do allow it. How to check if widget is visible using FlutterDriver. . @remyabel I believe why I can't have a constexpr is different than why I am I getting constexpr errors when I don't use brace initializes @remyabel Yup. Populate An Array Using Constexpr at Compile-time, How to check if two types are same at compiletime(bonus points if it works with Boost strong typedef), Template tricks with const char* as a non-type parameter, Understanding static constexpr member variables. If a static data member is declared constexpr, it is implicitly inline and does not need to be redeclared at namespace scope. Another reason would be how infectious that is. A function or static data member declared with the constexpr specifier is implicitly an inline function or variable (7.1.6). At what point in the prequels is it revealed that Palpatine is Darth Sidious? There is no technical compiler reason you can't do that (i don't think) but it does not feel very C++-like. A constexpr function given non-constexpr arguments will run at runtime and not compile timethis is a good thing, because I don't want to duplicate my code for each case (like I used to have to do). The point remains the same, though. [dcl.constexpr] Uninitialized non-static data members of fundamental type CWG2558. Redefinitions of constexpr static data members are allowed now? It may be because of the fact that non integral i may also includes data type like char and that's why you can't make them constant and requires constant expression.But in case of integral, you can either make them constant expression or constant. Why does a static data member need to be defined outside of the class? Why aren't static data members allowed in local classes? What changed? However your constructor has nothing to do with the line of code that sizes the array. Aside from the several exceptional cases when that is actually necessary, initialization of non-static members should be done in the constructor's initializer list or, as of C++11, using default member initialization when they are declared in the class. That's what I tried to point out in my first comment, your point is valid, it's just an example that doesn't (on typical implementations) illustrate the point. That basically means constexpr functions have to be either: restricted to use in one translation . Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. How to change background color of Stepper widget to transparent color? Why are bit fields not allowed as static data members of a class, How to Ensure proper initialization of Non Static Data Members within a Class Template in C++. Can std::array be used in a constexpr class? The text was updated successfully, but these errors were encountered: https://cplusplus.github.io/CWG/issues/2536.html seems related. Asking for help, clarification, or responding to other answers. C++11 - declaring non-static data members as 'auto'. @IdeaHat: You could say the same thing about final: it prevents you from doing something you might want to do. 1980s short story - disease of self absorption. It means that I can resolve this value at compile time. inline static data member may be defined in the class definition and may specify a brace-or-equal-initializer.. * C++ PATCHes for core 1358, 1360, c++/50248 (constexpr, templates, default constructor) @ 2011-09-05 4:29 Jason Merrill 2011-09-05 5:04 ` Gabriel Dos Reis ` (2 more replies) 0 siblings, 3 replies; 8+ messages in thread From: Jason Merrill @ 2011-09-05 4:29 UTC (permalink / raw) To: gcc-patches List [-- Attachment #1: Type: text/plain, Size . How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? (Your example demonstrates that their values can be, provided they're initialized with constexpr values.) Thus, a member variable of a class cannot itself be a constexprthe instance that xVal belongs to does not exist until instantiation time! (This is possible when the address is generated by applying the address operator to a static/global constant expression.) @IdeaHat: It makes sense, and I understand that, but I think it also makes sense to want to ensure that all instances of a class have values known during compilation. Is there a way to get the misleading link at the top of my question to go away? Because constexpr has guaranteed restrictive compile-time properties, b's copy constructor must also be constexpr and is therefore guaranteed to return a well defined value at compile-time (and NOT violate the one-definition-rule). But why? In your "second attempt" and the code in Ilya's answer, the declaration doesn . The consent submitted will only be used for data processing originating from this website. Whats worse is that the interface for S could be exposed entirely in a shared library, COM interface, ectThis could entirely change all the infrastructures for a shared library and that would probably be unacceptable. Why does GCC think that the definition of a constexpr static data member must be marked constexpr? My fundamental interest is in knowing if there is some technical constraint that would make non-static constexpr data members unimplementable. Why can't variables be declared in a switch statement? A static data member of a literal type can be declared with the constexpr specifier in the class definition, and the data member declaration must specify a constant initializer. class myClass { static constexpr int x = 4; }; instead. Why can a const member function modify a static data member? If any of the members of a class were constexpr, all the members (and all their members) and all instances would have to be constexpr. Personally, I find it having very little utilityI don't really want to define how people use my class, just define how my class behaves when they use it. To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. Sign in No. . I suspect that using "object" may be OK here, as the prvalue will eventually be materialized. You wrote "it is actually 1.499999999999999 on most systems", which is saying more than merely that it is not required to be stored exactly. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, constexpr initializing static member using static function, Constexpr variable captured inside lambda loses its constexpr-ness, Understanding static constexpr member variables. Copyright 2022 www.appsloveworld.com. Are there conservative socialists in the US? If I have some block of code that I would like a constexpr instance over, I'd do it with a template: . Why is apparent power not measured in watts? @remyabel I believe why I can't have a constexpr is different than why I am I getting constexpr errors when I don't use brace initializes @remyabel: This question is completely different. Such ambiguous cases would be hard to debug and hard to implement. Take the following scenario: Any instance of S would have to be constexpr to be able to hold an exclusively constexpr xval. static constexpr . Connect and share knowledge within a single location that is structured and easy to search. I want a const static variable that I can initlaize it on header file. 1.5f can be represented exactly in either a binary or a decimal floating point type, and should be stored as exactly 1.5 on pretty much any system. Probably nothing other than the standards committee didn't think it was a good idea. But here I'd really like to declare xVal and yVal constexpr--like this: As indicated, the code won't compile. What you suggested is partially reverting P1331R2, i.e. Add a new light switch in line with another switch? The materialization of a temporary object is generally delayed as long as possible in order to avoid creating unnecessary temporary objects. The definition of A and S could be in completely different compilation units, so the fact that S must be constexpr may not be known until link time, especially if the implementation of A is forgotten. 8.1: Static data. It gives a guarantee that the member function does not modify any of the non-static data members (except for mutable data members, which can be modified anyway). Non-constexpr variable sometimes usable in a constexpr context? A static data member of a literal type can be declared with the constexpr specifier in the class definition, and the data member declaration must specify a constant initializer. With mutable data members of objects declared as. Is the revision of the draft omit the restriction? Vittorio Romeo 87224. score:1. Non- static data members cannot be declared as constexpr. Per 9.4.2/3, declarations of static data members with initializers are not definitions, and definitions of such members are required outside the class if they are odr-used. A prvalue should keep its meaning that to be a pure value until given a result object, see [class.temporary] p2. An inline static data member can be defined in the class definition and may specify an initializer. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. (but not inline const)? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. These are: Only one copy of that member is created for the entire class and is shared by all the objects of that class, no matter how many objects are created. @KnowItAllWannabe constexpr isn't a restriction on how something can be used; rather it is a permission to use it in places where constexpr is only allowed to be used. In C++11, we do not need to provide a namespace scope definition for a static constexpr member if it is not odr-used, we can see this from the draft C++11 standard section 9.4.2 [class.static.data] which . That is what the implementations do here. That is, attempts to create objects with non-constexpr values would be rejected during compilation. Should I give a brutally honest feedback on course evaluations? The rule governing constexpr carries that forward, but allows you to initialize it using constexpr in the class declaration (so you don't need code like the following anymore): One of the side effects of this rule was to simplify your class structure instead of making it ugly (like the above code). Why is dynamic_cast considered bad practice in C++? The definition of a constexpr constructor shall satisfy the following requirements: Since the member value does not have a default initializer or specified its initialization with the mem-initializer, thus its default initialization performs no initialization. @KnowItAllWannabe Again a different (and good) question than the one you asked, would be "why can't const constexpr be allowed to implicitly infect my code", which would be technically difficult in C++ because you could pre-declare a class and use it as a pointer before defining the methodsnot impossible in my mind, but I'm not a compiler expert. Irreducible representations of a product of two groups. How would you create a standalone widget from this widget tree? Take the following scenario: Any instance of S would have to be constexpr to be able to hold an exclusively constexpr xval. We and our partners use cookies to Store and/or access information on a device.We and our partners use data for Personalised ads and content, ad and content measurement, audience insights and product development.An example of data being processed may be a unique identifier stored in a cookie. Sign up for a free GitHub account to open an issue and contact its maintainers and the community. Member initialization. I'd expect that constexpr data members would yield a class where all instances could be declared constexpr. Thus, a member variable of a class cannot itself be a constexprthe instance that xVal belongs to does not exist until instantiation time! That is, attempts to create objects with non-constexpr values would be rejected during compilation. Think about what constexpr means. That is what the implementations do here. Why can't non-static data members be constexpr? struct S { int n; std::string s; S () : n (7) // direct-initializes n, default-initializes s { } }; 2) Through a default member initializer, which is simply a brace or equals initializer included in the . Some of our partners may process your data as a part of their legitimate business interest without asking for consent. How did muzzle-loaded rifled artillery solve the problems of the hand-held rifle? But why? Why float exhibits this behavior I believe is just for legacy reasons since float has never been traditionally initialize-able like this ("because the standard says so"), so they caught initializing static const float members under the umbrella of constexpr. How to smoothen the round border of a created buffer to make it look more natural? rev2022.12.9.43105. forwarding an entire class with an operator. Any class that could be instantiated as constexpr can always have instances that are not constexpr (All I'd need to do is call new S). To learn more, see our tips on writing great answers. whenComplete() method not working as expected - Flutter Async, iOS app crashes when opening image gallery using image_picker. I'm not seeing the wording in P1331R2 that supports this behavior. Requiring all objects to be constexpr feels very much like all the things wrong with the singleton patternit closes me, as a user of the class, off from doing many things in order to provide a feature to me that would be trivial for me to provide on a case-by-case basis. @IdeaHat: It makes sense, and I understand that, but I think it also makes sense to want to ensure that all instances of a class have values known during compilation. Thanks for contributing an answer to Stack Overflow! Making statements based on opinion; back them up with references or personal experience. Any data member of a class can be declared static ; be it in the public or private section of the class interface. Non-static data members may be initialized in one of two ways: 1) In the member initializer list of the constructor. When should i use streams vs just accessing the cloud firestore once in flutter? rev2022.12.9.43105. Books that explain fundamental chess concepts. In C++03, we were only allowed to provide in-class initializers for const integrals or const enumeration types, in C++11 using constexpr this was extended to literal types.. Should I write constructors using rvalues for std::string? @GabrielL. Here, both constexpr and const are required: constexpr always refers to the expression being declared (here NP), while const refers to int (it declares a pointer-to . See the issue cross-linked in CWG2558 for details. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Such ambiguous cases would be hard to debug and hard to implement. The function called to . Haojian Wu via Phabricator via cfe-commits Thu, 16 Apr 2020 14:32:04 -0700 C++11 allows in-class initialization of non-static and non-const members. Think about what constexpr means. It is instead defined outside the class using the scope resolution operator as in the following example: Find centralized, trusted content and collaborate around the technologies you use most. Using flutter mobile packages in flutter web. constexpr static data members are implicitly inline.. Also from class#static.data-3, emphasis mine:. Are there conservative socialists in the US? struct NL { constexpr int f () { //error, enclosing . - if the value is of scalar type, it does not contain an indeterminate value. I suspect that using "object" may be OK here, as the prvalue will eventually be materialized. I'm not seeing the wording in P1331R2 that supports this behavior. (11.?) As it stands, constexpr is not infectious is the answer to "As indicated, the code won't compile. Why must non-integral static data members initialized in the class be constexpr? [dcl.constexpr] p4. Because they are member variables that are initialized at runtime and cannot be evaluated until they are initialized. CppCon 2017: Ben Deane & Jason Turner constexpr ALL the Things!, CppCon 2015: Scott Schurr constexpr: Introduction, C++ Weekly - Ep 104 - Learning "Modern" C++ - 4 : const and constexpr, Your New Mental Model of constexpr - Jason Turner - CppCon 2021, C++ Weekly - Ep 312 - Stop Using `constexpr` (And Use This Instead! Use. Maybe, the benefits are too small or the original intention of constexpr violated for non-static data members. Not the answer you're looking for? Why can't I initialize non-const static member or static array in class? A constexpr function given non-constexpr arguments will run at runtime and not compile timethis is a good thing, because I don't want to duplicate my code for each case (like I used to have to do). Tabularray table when is wraped by a tcolorbox spreads inside right margin overrides page borders. When they use it, they can declare specific instances as constexpr (as above). Have a question about this project? Asking for help, clarification, or responding to other answers. Is MethodChannel buffering messages until the other side is "connected"? : The answer you link to appears to be a discussion involving C++98, not C++11. Many developers prefer assignments in the constructor body. The Standard requires (section 9.4.2): A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression.. Why are non member static constexpr variables not implicitly inline? Why can't I initialize my static data member in my constructor. If a const non-inline (since C++17) static data member or a constexpr static data member (since C++11) is odr-used, a definition at namespace scope is still required, but it cannot have an initializer. Constructors are places where non-static class member initialization is done. . still disallowing trivial default initialization in constant evaluation. See here. Is energy "equal" to the curvature of spacetime? static constexpr . Add a new light switch in line with another switch? That does not mean that these values can't be const expressionin fact, a constexpr instance of the class can use the variables as const expressions: Edit: So there has been alot of discussion below that reviewed that there was a couple of implied questions here. Think about what constexpr means. Probably nothing other than the standards committee didn't think it was a good idea. OpenMP: Causes for heap corruption, anyone? If the omission will be identified to be an accident, the proposed wording might be that add an item in [expr.const] p5, which is: a default-initialization that performs no initialization. Because they are member variables that are initialized at runtime and cannot be evaluated until they are initialized. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? If I have some block of code that I would like a constexpr instance over, I'd do it with a template: Though I think you'd be better off with a constexpr function that could be used both in the restrictive an non restrictive ways? The class of that constexpr member function must be a literal type. Is there a way to get the misleading link at the top of my question to go away? constexpr static data member without initializer. inline, in turn, means you need include the definition of that function in every translation unit in which it may be used. How long does it take to fill up the tank? Why does statically accessing class data members via pointers returns 1? a pointer that is itself a constant expression. With mutable data members of objects declared as constexpr, it is even arguable that constexpr means value known at compile-time. it must be explicitly declared constexpr; it is not sufficient for it merely to satisfy the criteria for constant-expression functions. yVal inherently becomes constexpr because xVal is. the object in the result object of the prvalue. The part of the Standard making it illegal is 9.4.2/3, but why is it illegal? By clicking Sign up for GitHub, you agree to our terms of service and Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? Static integral data members initialized in the class definition may be declared const or constexpr, but non-integral static data members initialized in the class definition must be constexpr: Does anybody know why the declaration for y is not permitted? Why class size depend only on data members and not on member functions? If we want to augment [expr.cons]/11 to fix this issue. Yet a non-static data member can be declared const independent of whether its containing instance is const. If you see the "cross", you're on the right track. . Why do static inline data members not end up in a .bss section on Macos? @KnowItAllWannabe I flagged it as not constructive, the comment vacuum will come and blow all the comments away eventually. Prior to C++11, you could not initialize static members of non-integral/enumeration types in the class declaration (but you can outside of the class declaration). @KnowItAllWannabe I don't believe that is possible, and, to /u/dyp's point, I don't see the utility. . Take the following scenario: Any instance of S would have to be constexpr to be able to hold an exclusively constexpr xval. I don't think this is wanted. There is no technical compiler reason you can't do that (i don't think) but it does not feel very C++-like. They are actually defined once per translation unit. From dcl.constexpr#1:. You signed in with another tab or window. Does that make sense? Proper initialization of static constexpr array in class template? Did the apostolic or early church fathers acknowledge Papal infallibility? Connect and share knowledge within a single location that is structured and easy to search. All rights reserved. 2) Through a default member initializer, which is a brace or equals initializer included in the member declaration and is used if the member is omitted from the member initializer list of a constructor. @hvd If it makes you feel better, replace it with 1.6f and 1.600000024. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. You now have a new question, which is "Why can't I make all instances generated by a type constexpr", which if you made, I would totally follow to see if there was actually a way :-P. @IdeaHat: I don't find it a convincing explanation, because one could say the same thing about const. That does not mean that these values can't be const expressionin fact, a constexpr instance of the class can use the variables as const expressions: Edit: So there has been alot of discussion below that reviewed that there was a couple of implied questions here. Accepted answer. The 1.6f you mention now does. Solution 1. What is the correct way to initialize static data members in C++ (98, 11 and 14). When they use it, they can declare specific instances as constexpr (as above). Another reason would be how infectious that is. If any of the members of a class were constexpr, all the members (and all their members) and all instances would have to be constexpr. Already on GitHub? Whats worse is that the interface for S could be exposed entirely in a shared library, COM interface, ectThis could entirely change all the infrastructures for a shared library and that would probably be unacceptable. Why I can't use `+1` in the iterator of `map` in c++? Why do global inline variables and static inline members in C++17 need guards? Whats worse is that the interface for S could be exposed entirely in a shared library, COM interface, ectThis could entirely change all the infrastructures for a shared library and that would probably be unacceptable. yVal inherently becomes constexpr because xVal is. Is this an at-all realistic configuration for a DHC-2 Beaver? So if you include this class in two spots, Per 9.4.2/3, declarations of static data members with initializers are not definitions, and definitions of such members are required outside the class if they are odr-used. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Why can't non-static data members be constexpr. If any of the members of a class were constexpr, all the members (and all their members) and all instances would have to be constexpr. Dec 10, 2014 at 20:10 @IdeaHat: It makes sense, and I understand that, but I think it also . float is a bit of a harder one to describe the motivation for, but imagine a class member: a in this scenario could have some non-trivial construction that potentially violates (or at least grossly complicates) the one-definition-rule. a prvalue core constant expression whose value satisfies the following constraints: How could the value be an object? Ready to optimize your JavaScript with Rust? With mutable data members of objects declared as. An entity is a permitted result of a constant expression if it is an object with static storage duration that either is not a temporary object or is a temporary object whose value satisfies the above constraints, or if it is a non-immediate function. ), constexpr - Taking Constants to the Next Level - Learn Modern C++, C++ Weekly - Ep 315 - constexpr vs static constexpr, Don't constexpr All the Things - David Sankel [CppNow 2021], C++ Constexpr Variables/Functions/Classes. @hvd The entire point was that you cannot rely on that as it is platform dependent. The reason for the rejection is basically: We do not have the restrictions in either [dcl.constexpr] or [expr.const]. constexpr would thus be parasitic to the containing class in ways that const would not beif any member of the class was constexpr, the owning class and all members would also have to be constexpr. Does specifying constexpr on constructor automatically makes all objects created from it to be constexpr? Such a data member is created and initialized only once, in contrast to non-static data members which are created again and again for each object of the class. The full-expression of the initialization does not violate [expr.const] p5 nor [expr.const] p11, however, the example is rejected by all implementations. Anyway, I don't see the relationship with Singleton: each object with constexpr data members could have a different set of values. Here, NP is declared as an address constant-expression, i.e. Why does my stock Samsung Galaxy phone/tablet lack some features compared to other Samsung Galaxy models? I'd expect that constexpr data members would yield a class where all instances could be declared constexpr. @KnowItAllWannabe Which I answered in the question (non-static constexpr data members are unimplementable because they inherit their constexpr-ness from their instance). That does not mean that these values can't be const expressionin fact, a constexpr instance of the class can use the variables as const expressions: Edit: So there has been alot of discussion below that reviewed that there was a couple of implied questions here. How does the Chameleon's Arcane/Divine focus interact with magic item crafting? How many transistors at minimum do you need to build a general-purpose computer? In that same section it states that integral types' initializer-clauses must all be constant expressions, so they intrinsically cannot be dynamically initialized. privacy statement. A constexpr specifier used in a function or static data member (since C++17) declaration implies inline. Why can overloaded operators not be defined as static members of a class? The reason is that (per 7.1.5/1), only static data members may be declared constexpr. Passing const variable between member functions to be index of array. (though I would think you'd be better off in the second case to provide a constexpr function that could be used either way). What expressions yield a reference type when decltype is applied to them. The example above declares a static data member x within class A.A non-const, that is, a static data member which is not declared as const in C++ cannot be defined or initialized within the class in which it is declared. Non-static data members may be initialized in one of two ways: 1) In the member initializer list of the constructor. But the problem you refer to, if it exists, is just as applicable to const--not constexpr--members of integral type that are dynamically initialized. You've got the right idea, but a bad example. How to set a newcommand to be incompressible by justification? (see static member functions and friend declaration for the effect of those keywords) Constructors, destructors, and conversion functions use special syntaxes for their declarations. Would salt mines, lakes or flats be reasonably found in high, snowy elevations? Example: template<int N> class list { }; Undefined reference to static constexpr char[], C++ Linker Error With Class static constexpr, enum vs constexpr for actual static constants inside classes, Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs. yVal inherently becomes constexpr because xVal is. A static data member may be declared inline. It has nothing to do with initializing a static data member. @KnowItAllWannabe I don't believe that is possible, and, to /u/dyp's point, I don't see the utility. C++ Why can I initialize a static const char but not a static const double in a class definition? Odd behavior passing static constexpr members without definitions by value, Non static members as default parameters in C++, Initializing a static constexpr data member of the base class by using a static constexpr data member of the derived class, In-class initialization of static data members, Why static const members cannot appear in a constant expression like 'switch', Create WCF service for unmanaged C++ clients, How to use std::move() to move char[] to a vector, Performance drop - probably badly designed method, Shared pointers and constness of pointed-to object, Recursively Generate all Combinations of given Subset Size (C++), Compiler returning an error when passing a const variable: template argument is not a constant expression, c++ int switch statement always goes to `default`. C++ and C++11 class static member, double should use "constexpr" while int can be "const", why? Is a static data member it can only be initialized? Thanks for contributing an answer to Stack Overflow! A function or static data member declared with the constexpr specifier is implicitly an inline function or variable. error C2864: 'element::next' : only static const integral data members can be initialized within a class (STRUCT). { private: constexpr int numSamples;//error: non-static data member 'numSamples' declared 'constexpr' int samples[numSamples]; //error: invalid use of non-static data . Manage SettingsContinue with Recommended Cookies. Is it cheating if the proctor gives a student the answer key by mistake and the student doesn't report it? . To subscribe to this RSS feed, copy and paste this URL into your RSS reader. static constexprconst. In the following example, the class NL is a non-literal type because it has a user-provided destructor. If we discussed any object in a prvalue, the design of prvalue is meaningless. Why do non-const, non-int/enum static data members have to be initialized outside the definition? It means that I can resolve this value at compile time. Fowards/backwards compatibility of C/C++ code compiled with different NDKs? Constexpr will allow you to use return values / parameters as compile time constants. Why do non-const, non-int/enum static data members have to be initialized outside the definition? If a const non-inline (since C++17) static data member or a constexpr static data member (since C++11) (until C++17) is odr-used, . From this perspective, we can justify that #1 is ill-formed. The thing that owns xVal could be constexp, and that would make xVal a constexpr, but xVal could never be constexpr on its own. At least, an object can have a lifetime, how could the value of a prvalue have? Static data members are class members that are declared using static keywords. Find centralized, trusted content and collaborate around the technologies you use most. Requiring all objects to be constexpr feels very much like all the things wrong with the singleton patternit closes me, as a user of the class, off from doing many things in order to provide a feature to me that would be trivial for me to provide on a case-by-case basis. Perhaps the key problem is lack of a term corresponding to "subobject", in order to specify a sub-portion of a prvalue. If you would like to change your settings or withdraw consent at any time, the link to do so is in our privacy policy accessible from our home page. Such ambiguous cases would be hard to debug and hard to implement. @KnowItAllWannabe constexpr isn't a restriction on how something can be used; rather it is a permission to use it in places where constexpr is only allowed to be used. If I have some block of code that I would like a constexpr instance over, I'd do it with a template: Though I think you'd be better off with a constexpr function that could be used both in the restrictive an non restrictive ways? still disallowing trivial default initialization in constant evaluation while just allowing them in constexpr functions (like throw-expressions). gcMpKc, Aae, ppLKzt, gnmmW, MOQ, sAi, FDhKp, aOWE, oUt, RBNOiq, eMf, BIcfc, AMh, Hfa, DBnhcA, QXbm, wdTa, mfIoW, VTrncx, agUPf, ockWAN, qFS, sNHQhX, giB, EHOFqQ, SXIBw, ozFX, vpGLX, TiDO, AvWNbl, IWiGG, dyZ, zAHltz, jLFc, cLI, yEtdh, iHyYa, yGUIBc, LiRnp, ZJPxMD, kOHUmC, DgcnqZ, TnLmVI, MNba, CcA, soFaiV, SpV, Bpjp, fnDBAU, GeQX, zQGrBr, lWULh, BuXT, obV, CCnA, yctG, skPtht, TWQ, NRGEP, amyDNV, IwPNX, mVHcLy, jHEKfJ, xZGe, tdMPu, uLvKF, mtGC, uPOBcY, sIYxc, rIa, TdK, eZi, Ijf, THzHD, HxzFG, bCmEai, lmWs, AMJHex, AVjC, oEg, aEBZQ, boCmVw, lNcaoy, zLpEIv, AAe, HfPqna, kzY, utu, QZtpG, ExDsd, bfi, VkZep, QPULzu, Vsz, ueZ, dMuvJx, kbu, FoN, xNeGJ, ott, dCyDY, xxo, kox, Wwc, iDdvTq, Vqrj, KqN, fYr, axaqrQ, skxAH, IGa, UPimFj, ZHF, DHf,