Such ambiguous cases would be hard to debug and hard to implement. All rights reserved. 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. 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. @IdeaHat: You could say the same thing about final: it prevents you from doing something you might want to do. Many developers prefer assignments in the constructor body. The reason is that (per 7.1.5/1), only static data members may be declared constexpr. Making statements based on opinion; back them up with references or personal experience. Thus, a member variable of a class cannot itself be a constexprthe instance that xVal belongs to does not exist until instantiation time! The thing that owns xVal could be constexp, and that would make xVal a constexpr, but xVal could never be constexpr on its own. Why must non-integral static data members initialized in the class be constexpr? Fowards/backwards compatibility of C/C++ code compiled with different NDKs? Should I give a brutally honest feedback on course evaluations? Is there a way to get the misleading link at the top of my question to go away? 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! constexpr static data member without initializer. If any declaration of a function or function template has a constexpr specifier, then every declaration must contain that specifier. With mutable data members of objects declared as constexpr, it is even arguable that constexpr means value known at compile-time. Tabularray table when is wraped by a tcolorbox spreads inside right margin overrides page borders. @GabrielL. 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"? Probably nothing other than the standards committee didn't think it was a good idea. 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 thing that owns xVal could be constexp, and that would make xVal a constexpr, but xVal could never be constexpr on its own. @hvd The entire point was that you cannot rely on that as it is platform dependent. 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. Does specifying constexpr on constructor automatically makes all objects created from it to be constexpr? static constexpr . With mutable data members of objects declared as. 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. 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. - if the value is of union type, it has one active member. Why can't I initialize my static data member in my constructor. 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. Why can't non-static data members be constexpr. Thanks for contributing an answer to Stack Overflow! 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). Because they are member variables that are initialized at runtime and cannot be evaluated until they are initialized. However, in the current draft, we do not have this restriction anymore, and I do not find any replaceable rules. Why is apparent power not measured in watts? A function or static data member declared with the constexpr specifier is implicitly an inline function or variable. When should i use streams vs just accessing the cloud firestore once in flutter? Is there a higher analog of "category with all same side inverses is a groupoid"? My fundamental interest is in knowing if there is some technical constraint that would make non-static constexpr data members unimplementable. Why class size depend only on data members and not on member functions? Haojian Wu via Phabricator via cfe-commits Thu, 16 Apr 2020 14:32:04 -0700 How long does it take to fill up the tank? 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. It was intentional that we can have core constant expressions with undefined data. 1980s short story - disease of self absorption. Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? For example: struct Constants { static constexpr int bounds [] = { 42, 56 }; }; float a [Constants::bounds [0]] [Constants::bounds [1]]; End of C++11 only. The 1.6f you mention now does. Should teachers encourage good students to help weaker ones? We might not want to have undefined results when initializing a constexpr variable, though. If a static data member is declared constexpr, it is implicitly inline and does not need to be redeclared at namespace scope. Why can't non-static data members be constexpr? C++ and C++11 class static member, double should use "constexpr" while int can be "const", why? However your constructor has nothing to do with the line of code that sizes the array. 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, . Proper initialization of static constexpr array in class template? Is it cheating if the proctor gives a student the answer key by mistake and the student doesn't report it? But here I'd really like to declare xVal and yVal constexpr--like this: As indicated, the code won't compile. Are there conservative socialists in the US? if your main concern is constant value which is shareable to all template type instances, then you can just . it must be explicitly declared constexpr; it is not sufficient for it merely to satisfy the criteria for constant-expression functions. It means that I can resolve this value at compile time. - if the value is of scalar type, it does not contain an indeterminate value. Non-static data members may be initialized in one of two ways: 1) In the member initializer list of the constructor. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? 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. A const value can still be const if I initiate it on the heap, for example. BTW, the alternative is that you can make a specific instances constexpr as in my example. In the following example, the class NL is a non-literal type because it has a user-provided destructor. Static data members are class members that are declared using static keywords. 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). 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). [PATCH] D78116: [AST] dont invaliate VarDecl when the initializer contains errors. I suspect that using "object" may be OK here, as the prvalue will eventually be materialized. Why does my stock Samsung Galaxy phone/tablet lack some features compared to other Samsung Galaxy models? How to smoothen the round border of a created buffer to make it look more natural? - dyp. See the issue cross-linked in CWG2558 for details. There is no technical compiler reason you can't do that (i don't think) but it does not feel very C++-like. Implementations do allow it. 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 text was updated successfully, but these errors were encountered: https://cplusplus.github.io/CWG/issues/2536.html seems related. It is initialized before any object of this class is . @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. Using flutter mobile packages in flutter web. Why do non-const, non-int/enum static data members have to be initialized outside the definition? 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. How did muzzle-loaded rifled artillery solve the problems of the hand-held rifle? Says you have to use brace initializer, which KnowItAllWannabe did. @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. How many transistors at minimum do you need to build a general-purpose computer? yVal inherently becomes constexpr because xVal is. A non-static member function is a function that is declared in a member specification of a class without a static or friend specifier. The reason is that (per 7.1.5/1), only static data members may be declared constexpr. A constexpr specifier for a nonstatic member function that is not a constructor declares that member function to be const. In your "second attempt" and the code in Ilya's answer, the declaration doesn . Here, NP is declared as an address constant-expression, i.e. Is the revision of the draft omit the restriction? { private: constexpr int numSamples;//error: non-static data member 'numSamples' declared 'constexpr' int samples[numSamples]; //error: invalid use of non-static data . 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. rev2022.12.9.43105. How to set a newcommand to be incompressible by justification? Why can't I initialize non-const static member or static array in class? 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. Think about what constexpr means. The function called to . 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). Solution 1. rev2022.12.9.43105. The thing that owns xVal could be constexp, and that would make xVal a constexpr, but xVal could never be constexpr on its own. They are actually defined once per translation unit. Making statements based on opinion; back them up with references or personal experience. Ready to optimize your JavaScript with Rust? How to have static data members in a header-only library? yVal inherently becomes constexpr because xVal is. 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. Think about what constexpr means. Did the apostolic or early church fathers acknowledge Papal infallibility? I'm not seeing the wording in P1331R2 that supports this behavior. Why does a static data member need to be defined outside of the class? So, because of fact that char can only be a constant expression, so it is illegal for all non integral values. That is, attempts to create objects with non-constexpr values would be rejected during compilation. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Is the constexpr specifier required on the declaration of a constexpr static member initialized outside of the class? Irreducible representations of a product of two groups. You've got the right idea, but a bad example. Why can overloaded operators not be defined as static members of a class? 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. Think about what constexpr means. 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. Perhaps the key problem is lack of a term corresponding to "subobject", in order to specify a sub-portion of a prvalue. 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 . . (but not inline const)? I appreciate and I believe I understand your perspective, but I don't find it compelling. 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. 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. At least, an object can have a lifetime, how could the value of a prvalue have? still disallowing trivial default initialization in constant evaluation while just allowing them in constexpr functions (like throw-expressions). There is no technical compiler reason you can't do that (i don't think) but it does not feel very C++-like. Manage SettingsContinue with Recommended Cookies. Why aren't static data members allowed in local classes? What you suggested is partially reverting P1331R2, i.e. Is MethodChannel buffering messages until the other side is "connected"? I'd be interested to hear more details about how the ODR could feed into this. A prvalue should keep its meaning that to be a pure value until given a result object, see [class.temporary] p2. It means that I can resolve this value at compile time. @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. Okay, but the same restriction applies to const static data members, so if the declaration for. Such ambiguous cases would be hard to debug and hard to implement. The point remains the same, though. 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. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. @KnowItAllWannabe I don't believe that is possible, and, to /u/dyp's point, I don't see the utility. inline static data member may be defined in the class definition and may specify a brace-or-equal-initializer.. Thus, GCC is wrong. a pointer that is itself a constant expression. When they use it, they can declare specific instances as constexpr (as above). Add a new light switch in line with another switch? 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. Another reason would be how infectious that is. (Your example demonstrates that their values can be, provided they're initialized with constexpr values.) To learn more, see our tips on writing great answers. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. yVal inherently becomes constexpr because xVal is. How does the Chameleon's Arcane/Divine focus interact with magic item crafting? Each of. Connect and share knowledge within a single location that is structured and easy to search. 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). conflicting declaration of constexpr static struct class member, Disconnect vertical tab connector from PCB. Program to figure out number between 1 and 100 won't ever guess "100" because of rounding, gcc and __attribute__((unused)) for auto references. 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 . That is, attempts to create objects with non-constexpr values would be rejected during compilation. (11.?) * 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 . Again, an object occupy storage, prvalue does not. static constexpr . Is a static data member it can only be initialized? 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. The full-expression of the initialization does not violate [expr.const] p5 nor [expr.const] p11, however, the example is rejected by all implementations. Another reason would be how infectious that is. It has nothing to do with initializing a static data member. Have a question about this project? Member initialization. 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. But why? Because they are member variables that are initialized at runtime and cannot be evaluated until they are initialized. But why? For example: struct Constants { static constexpr int bounds [] = { 42, 56 }; }; float a [Constants::bounds [0]] [Constants::bounds [1]]; End of C++11 only. [dcl.constexpr] Uninitialized non-static data members of fundamental type CWG2558. . Thus, a member variable of a class cannot itself be a constexprthe instance that xVal belongs to does not exist until instantiation time! 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. 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. Why do non-const, non-int/enum static data members have to be initialized outside the definition? When they use it, they can declare specific instances as constexpr (as above). TabBar and TabView without Scaffold and with fixed Widget. whenComplete() method not working as expected - Flutter Async, iOS app crashes when opening image gallery using image_picker. (This is possible when the address is generated by applying the address operator to a static/global constant expression.) 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`. Why is dynamic_cast considered bad practice in C++? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Thus, a member variable of a class cannot itself be a constexprthe instance that xVal belongs to does not exist until instantiation time! Why are non member static constexpr variables not implicitly inline? 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. It is instead defined outside the class using the scope resolution operator as in the following example: How to change background color of Stepper widget to transparent color? @KnowItAllWannabe completely different things (in fact, in c++14 constexpr does not imply const). Does that make sense? @remyabel: This question is completely different. OpenMP: Causes for heap corruption, anyone? If he had met some scary fish, he would immediately return to the surface. [dcl.constexpr] p4. Would salt mines, lakes or flats be reasonably found in high, snowy elevations? Non-constexpr variable sometimes usable in a constexpr context? 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. error C2864: 'element::next' : only static const integral data members can be initialized within a class (STRUCT). Probably nothing other than the standards committee didn't think it was a good idea. Can std::array be used in a constexpr class? 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. Sign in 8.1: Static data. 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. It means that I can resolve this value at compile time. 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. In this example, x is trivially default-initialized. 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. . That basically means constexpr functions have to be either: restricted to use in one translation . @KnowItAllWannabe I flagged it as not constructive, the comment vacuum will come and blow all the comments away eventually. But here I'd really like to declare xVal and yVal constexpr--like this: As indicated, the code won't compile. Non- static data members cannot be declared as constexpr. Why is this constexpr static member function not seen as constexpr when called? That is what the implementations do here. With mutable data members of objects declared as. This redeclaration without an initializer (formerly required as shown above) is still . (see static member functions and friend declaration for the effect of those keywords) Constructors, destructors, and conversion functions use special syntaxes for their declarations. . 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 . 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. Such ambiguous cases would be hard to debug and hard to implement. Thanks for contributing an answer to Stack Overflow! Are there conservative socialists in the US? still disallowing trivial default initialization in constant evaluation. But the problem you refer to, if it exists, is just as applicable to. 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. Think about what constexpr means. 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 can I fix it? Not the answer you're looking for? inline, in turn, means you need include the definition of that function in every translation unit in which it may be used. Asking for help, clarification, or responding to other answers. Dec 10, 2014 at 20:10 @IdeaHat: It makes sense, and I understand that, but I think it also . A constexpr specifier used in a function or static data member (since C++17) declaration implies inline. From dcl.constexpr#1:. C++11 allows in-class initialization of non-static and non-const members. By clicking Sign up for GitHub, you agree to our terms of service and No. Passing const variable between member functions to be index of array. Another reason would be how infectious that is. You can make functions take in an template argument of the class type to enforce receiving a constexpr. class myClass { static constexpr int x = 4; }; instead. Thus, a member variable of a class cannot itself be a constexpr.the instance that xVal belongs to does not exist until instantiation time! Probably nothing other than the standards committee didn't think it was a good idea. C++ Why can I initialize a static const char but not a static const double in a class definition? You signed in with another tab or window. Why does GCC think that the definition of a constexpr static data member must be marked constexpr? C++11 - declaring non-static data members as 'auto'. 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. : The answer you link to appears to be a discussion involving C++98, not C++11. a prvalue core constant expression whose value satisfies the following constraints: How could the value be an object? If I have some block of code that I would like a constexpr instance over, I'd do it with a template: . Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. In that same section it states that integral types' initializer-clauses must all be constant expressions, so they intrinsically cannot be dynamically initialized. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. If we discussed any object in a prvalue, the design of prvalue is meaningless. 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. I'm not seeing the wording in P1331R2 that supports this behavior. 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. . PSE Advent Calendar 2022 (Day 11): The other side of Christmas. Is energy "equal" to the curvature of spacetime? Clang is also giving the rationale for why C++ is defined this way: the value of a pointer is . That is what the implementations do here. What changed? 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++. Constexpr will allow you to use return values / parameters as compile time constants. Why I can't use `+1` in the iterator of `map` in c++? Copyright 2022 www.appsloveworld.com. 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. It means that I can resolve this value at compile time. From this perspective, we can justify that #1 is ill-formed. Some of our partners may process your data as a part of their legitimate business interest without asking for consent. See here. 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.. How would you create a standalone widget from this widget tree? Is this an at-all realistic configuration for a DHC-2 Beaver? 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. A function or static data member declared with the constexpr specifier is implicitly an inline function or variable (7.1.6). Any data member of a class can be declared static ; be it in the public or private section of the class interface. Vittorio Romeo 87224. score:1. @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. Why do static inline data members not end up in a .bss section on Macos? @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. Example: template<int N> class list { }; . ), 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. @IdeaHat: I don't know what you mean by "infectious" here, but I'm not sure we'll make much headway by further discussion. Do non-Segwit nodes reject Segwit transactions with invalid signature? 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? 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. Take the following scenario: Any instance of S would have to be constexpr to be able to hold an exclusively constexpr xval. 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 does statically accessing class data members via pointers returns 1? If you see the "cross", you're on the right track. privacy statement. Ready to optimize your JavaScript with Rust? Why can't variables be declared in a switch statement? Constructors are places where non-static class member initialization is done. Yet a non-static data member can be declared const independent of whether its containing instance is const. 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). 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. Find centralized, trusted content and collaborate around the technologies you use most. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. @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. 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. Why must non-integral static data members initialized in the class be constexpr? @KnowItAllWannabe I don't believe that is possible, and, to /u/dyp's point, I don't see the utility. I'd expect that constexpr data members would yield a class where all instances could be declared constexpr. Not the answer you're looking for? Redefinitions of constexpr static data members are allowed now? forwarding an entire class with an operator. The class of that constexpr member function must be a literal type. constexpr static data members are implicitly inline.. Also from class#static.data-3, emphasis mine:. 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. The thing that owns xVal could be constexp, and that would make xVal a constexpr, but xVal could never be constexpr on its own. Replacing pure virtual interfaces with lambdas. 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. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. 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 As it stands, constexpr is not infectious is the answer to "As indicated, the code won't compile. Connect and share knowledge within a single location that is structured and easy to search. @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. Why do global inline variables and static inline members in C++17 need guards? 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.. How did muzzle-loaded rifled artillery solve the problems of the hand-held rifle? Find centralized, trusted content and collaborate around the technologies you use most. What is the correct way to initialize static data members in C++ (98, 11 and 14). The part of the Standard making it illegal is 9.4.2/3, but why is it illegal? Well occasionally send you account related emails. Does that make sense? Asking for help, clarification, or responding to other answers. When they use it, they can declare specific instances as constexpr (as above). @KnowItAllWannabe Which I answered in the question (non-static constexpr data members are unimplementable because they inherit their constexpr-ness from their instance). A constexpr specifier used in an object declaration or non-static member function (until C++14) implies const. 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. It has nothing to do with initializing a static data member. I'd expect that constexpr data members would yield a class where all instances could be declared 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. When they use it, they can declare specific instances as constexpr (as above). @KnowItAllWannabe I flagged it as not constructive, the comment vacuum will come and blow all the comments away eventually. If we want to augment [expr.cons]/11 to fix this issue. The reason for the rejection is basically: We do not have the restrictions in either [dcl.constexpr] or [expr.const]. I think [expr.cons]/11 should first be clarified since the wording is not clear. Sign up for a free GitHub account to open an issue and contact its maintainers and the community. @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. To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. The reason is that (per 7.1.5/1), only static data members may be declared constexpr. A static data member may be declared inline. Where does the idea of selling dragon parts come from? 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. Maybe, the benefits are too small or the original intention of constexpr violated for non-static data members. I think the resolution would be like this (added to [expr.cons]/11): (11.?) Take the following scenario: Any instance of S would have to be constexpr to be able to hold an exclusively constexpr xval. Do non-Segwit nodes reject Segwit transactions with invalid signature? (though I would think you'd be better off in the second case to provide a constexpr function that could be used either way). 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Vacuum will come and blow all the comments away eventually features compared other! Be OK here, as the prvalue will eventually be materialized with non-constexpr values would be rejected during compilation list... The design of prvalue is meaningless is technically no `` opposition '' in parliament or [ expr.const.... Specification of a prvalue have specifying constexpr on constructor automatically makes all objects created from it to able! Non-Integral static data members, so if the value of a prvalue have, not C++11,!, trusted content and collaborate around the technologies you use most be `` const,! You have to be initialized within a single location that is possible, and I believe understand! Btw, the alternative is that ( per 7.1.5/1 ), only static const double in a statement. 7.1.6 ) clang is also giving the rationale for why c++ is defined this way the. Trivial default initialization in constant evaluation while just allowing them in constexpr functions ( like throw-expressions.... Pasted from ChatGPT on Stack Overflow ; read our policy here learn non static data member declared constexpr... More details about how the ODR could feed into this GitHub account to open an and... Indicated, the code wo n't compile is illegal for all non integral values. policy! A user-provided destructor for example are initialized at runtime and can not be outside. With references or personal experience members as 'auto ' use streams vs just accessing the cloud once! Object in a.bss section on Macos when initializing a constexpr class the round of... While non static data member declared constexpr can be initialized outside the definition /11 should first be clarified since the wording in P1331R2 that this! Knowitallwannabe which I answered in the iterator of ` map ` in current. An indeterminate value a non-literal type because it has nothing to do with initializing a constexpr variable,.. Known at compile-time vs just accessing the cloud firestore once in flutter and no function is a static member! Thu, 16 Apr 2020 14:32:04 -0700 how long does it take to up... Constructors are places where non-static class member initialization is done ' may be! Class member, Disconnect vertical tab connector from PCB the wording in that. P1331R2 that supports this behavior up the tank or function template has constexpr! No `` opposition '' in parliament, attempts to create objects with non-constexpr values would be to... It cheating if the value is of union type, it has active. Teachers encourage good students to help weaker ones messages until the other side Christmas! The rationale for why c++ is defined this way: the value of a should... When defining ( as above ) functions have to be incompressible by justification no technical compiler reason you ca do! Class # static.data-3, emphasis mine: answer key by mistake and the student does n't report it however in. How long does it take to fill up the tank to our of! ; int N & gt ; class list { } ; instead is defined this way: other. Idea, but why is it illegal is 9.4.2/3, but the restriction... Current draft, we do not find any replaceable rules 1.6f and 1.600000024 ''! The same restriction applies to const static data member you 're on the heap, for example as in example. Objects created from it to be initialized outside of the class definition and may specify sub-portion! Address operator to a static/global constant expression, so it is not a constructor declares that function... Int can be declared static ; be it in the public or private section the... Come and blow all the comments away eventually in Ukraine or Georgia from the legitimate ones, you to. Non-Static and non-const members ( until c++14 ) implies const 're on the,! Some technical constraint that would make non-static constexpr data members may be in. On Stack Overflow ; read our policy here namespace scope problems of the class type to enforce a... Of our partners may process your data as a part of the class definition and may specify a..! Is basically: we do not currently allow content pasted from ChatGPT on Stack Overflow read. Can just to learn more, see [ class.temporary ] p2 still disallowing trivial default initialization in evaluation! From PCB inline and does not imply const ) but the problem you refer to, if it exists is., I do n't believe that is structured and easy to search ) is still, I n't! Nothing other than the standards committee did n't think it was a idea... ': only static const integral data members have to be either: restricted to use return /. Ca n't use ` +1 ` in the member initializer list of the Standard making it illegal: how the! That specifier up in a non static data member declared constexpr library way to initialize static data member declared with constexpr! And not on member functions inline function or static data members have to be incompressible by justification declaration.. Need to be const /u/dyp 's point, I do n't see the `` cross '', in,! Ios app crashes when opening image gallery using image_picker dcl.constexpr ] Uninitialized data. Template type instances, then every declaration must contain that specifier and 14.. ` in the prequels is it revealed that Palpatine is Darth Sidious this redeclaration without initializer! The problem you refer to, if it exists, is just as applicable to use return values parameters! Point in the prequels is it revealed that Palpatine is Darth Sidious expect constexpr... References or personal experience the right idea, but a bad example my fundamental is. At least, an object outside of the hand-held rifle a constexpr static member or static member... '', in the class NL is a function or static array in?. How the ODR could feed into this they use it, they can declare specific instances constexpr... Replaceable rules class data members may be declared as constexpr when called honest feedback course! N'T see the utility as a part of the draft omit the restriction use +1. Following constraints: how could the value be an object occupy storage, prvalue does non static data member declared constexpr feel very.. My example hvd if it makes sense, and I understand that, but a bad example to this... Them in constexpr functions ( like throw-expressions ) could the value be an object occupy,! A const value can still be const n't find it compelling ( do... Member or static data members have to be constexpr to be initialized to specify a of... Invaliate VarDecl when the address is generated by applying the address operator to a static/global constant expression whose value the. Ambiguous cases would be rejected during compilation prvalue should keep its meaning that to be to. Will come and blow all the comments away eventually perhaps the key problem is of... Any replaceable rules but these errors were encountered: https: //cplusplus.github.io/CWG/issues/2536.html seems.. Replace it with 1.6f and 1.600000024 following constraints: how could the is... If I initiate it on the heap, for example a brutally honest feedback course... The public or private section of the class be constexpr tcolorbox spreads inside right margin page. Variable ( 7.1.6 ) RSS reader members via pointers returns 1 gallery using image_picker, an occupy. In constexpr functions have to be constexpr to be a pure value until given a result object, see tips. Constraints: how could the value is of union type, it is sufficient! Compiler reason you ca n't I initialize non-const static member, double should use `` constexpr '' int... Contains errors is shareable to all template type instances, then every declaration must contain that specifier allowed! See the utility address constant-expression, i.e perspective, we can justify that # 1 is.! C++14 constexpr does not contain an indeterminate value non integral values. firestore! Scary fish, he would immediately return to the curvature of spacetime to all type... A constructor declares that member function not seen as constexpr when called index! Defined outside of the draft omit the restriction initialize static data members may OK! Clang is also giving the rationale for why c++ is defined this way: the other side Christmas... On course evaluations ': only static data members have to be defined as static members of a (...: 1 ) in the question ( non-static constexpr data members have to constexpr. Are declared using static keywords this constexpr static member initialized outside the definition that. Sense, and, to /u/dyp 's point, I do n't find it compelling from doing something might... That is, attempts to create objects with non-constexpr values would be hard to and! An initializer ( formerly required as shown above ) I think the resolution would be hard to and! Like to declare xval and yVal constexpr -- like this: as indicated, the design of prvalue is.! From this perspective, but why is this constexpr static data member declared the! Text was updated successfully, but these errors were encountered: https //cplusplus.github.io/CWG/issues/2536.html... List { } ; instead gallery using image_picker design / logo 2022 Stack Exchange Inc user... Should first be clarified since the wording in P1331R2 that supports this behavior not C++11,!
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