Find centralized, trusted content and collaborate around the technologies you use most. In C++11 the template std::addressof, in the <memory> header, was added to solve this problem. Sign in. If you put the reinterpret_cast in a place that will always get executed, then when the code is actually executed in the compiler then it will throw errors even with VC. Finally, as a bit of playing around, how does this following code work in Clang? object been initialized precedingly, nor did we define any such object with constexpr. You are probably aware of the fact that your code causes undefined behavior, since you dereference a type punned pointer and thus break strict aliasing. pytorch bin2c >C++exebin.hC++bin2cexe On the other hand, it is clear that, for embedded users and specialized compilers/flags/environments, it could be useful to some degree. blob . Acorn 23513 What does the construct keyword do when added to a method? constexpr not working if the function is declared inside class scope. error\u FILE\u NOT\u FOUND. post. In C++14, the first step is impossible to accomplish in constant expressions. The default policy is to exit the spin loop. On the other hand, it is clear that, for embedded users and specialized compilers/flags/environments, it could be useful to some degree. Why Pointer Type Cast Does not Work on Template Non-type Parameters, C++: Why is this constexpr not a compile time constant. How can you assign an integer to a function? Getting around the reinterpret cast limitation with constexpr. What does the single ampersand after the parameter list of a member function declaration mean? Why does the cast operator to a private base not get used? Therefore I'd say that such aliasing isn't possible in constant expressions. . This is a signature. (unsigned*)&x therefore reduces to reinterpret_cast(&x) and doesn't work. I'm compiling the library with clang using both -std=C++11 and -std=C++14. Is MethodChannel buffering messages until the other side is "connected"? This is important because the capabilities of constexpr was vastly increased with the C++14 standard compared to C++11. [Solved]-Alternative to reinterpret_cast with constexpr functions-C++ [Solved]-Alternative to reinterpret_cast with constexpr functions-C++ score:9 Accepted answer Two's complement is not guaranteed by the standard; in clause 3.9.1: 7 - [.] In theory, I can't see how a reinterpret cast in this or similar cases can be any different from arithmetic operators, but the complier (and the standard) don't allow it. Replace constexpr (used to calculate constant at compile time) with template? whenComplete() method not working as expected - Flutter Async, iOS app crashes when opening image gallery using image_picker. I'm trying to trying to explore the boundaries of some c++11 features, this isn't really a necessity. Boost's addressof utility. But reinterpret_cast essentially ignores the type system, so checking for legality and possibilities gets really hard. fftw C++ inverse 2D FFT from magnitude and phase arrays, Returning reference to local temporary object, How to build a VS2010 C++ Project on a BuildServer. No need to describe all the reinterpret_castrestrictions N4567 contains restrictions on constant expressions, that make any UB code not a constant expression: [expr.const] "(2.5) an operation that would have So any attempt to make reinterpret_castthat results in UB will make the expression non-constant. Can the "main" function be declared with the "noexcept" specifier? Is it appropriate to ignore emails from a student asking obvious questions? The final sentence of thee last point "No diagnostic is required for a violation" also means that if the compiler detects a violation, but doesn't actually use the constexpr function for anything then the compiler can just ignore it. If Type is an lvalue reference type or an rvalue reference to a function type, reinterpret_cast<Type>(expression) is an lvalue. What's the right way to fix this template resolution ambiguity? What version of GCC did you test this on? constexpr int FooOrDie(int a) { if( a < 100 ) return a; else throw std:exception("Augh! constexpr allows reinterpret_cast in function body. A constexpr @DavidRodrguez-dribeas - indeed. Plus it is practically impossible to check if the use is valid. A simplified explanation of why this exists is performance. Type alias declaration (C++11) Casts. Feature-test macro Contribute to uselessgoddess/constexpr-reinterpret_cast development by creating an account on GitHub. Memory allocation. To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. So for instance, if one wanted to manipulate the bits in a floating point number, say to find the mantissa of the number: constexpr unsigned int mantissa (float x) { return ( (* (unsigned int*)&x << 9) >> 9); }; The above code would fail to be constexpr. A tag already exists with the provided branch name. constexpr To bit_cast (const From & from ) noexcept; (since C++20) . 95307 - Compiler accepts reinterpret_cast in constexpr Last modified: 2021-12-03 16:33:54 UTC Bug 95307 - Compiler accepts reinterpret_cast in constexpr Attachments Add an attachment (proposed patch, testcase, etc.) How do I create an array of function pointers of different prototypes? So VC isn't See the following code: Starting with C++20 there's a standard library solution: std::bit_cast (supported in GCC since GCC 11). Note You need to log in before you can comment on or make changes to this bug. At runtime this is a tradeoff (a compromise if you will), but it is unacceptable at compile time. As a profession, more and more of our code has to C Wrapper for C++: How to deal with C++ templates? They are meant to just illustrate a point. I may also give inefficient code or introduce some problems to discourage copy/paste coding. Browse Source Add core of c10::complex Summary: Step 0 of https://github.com/pytorch/pytorch/issues/35284 Reference: https://en.cppreference.com/w/cpp/numeric/complex . blob: 2711eac88e923f4b78f4d063d08882aa820ef951 . This is important . Hmm, then maybe there is something more subtle going on here. How to check if widget is visible using FlutterDriver. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Asking for help, clarification, or responding to other answers. So that code that does fail in Clang, try it with --std=c++14 or --std=c++17. in as a constant expression, then it is nothing more than a regular function. So testing - clang ignores the reinterpret_cast when do_stuff is used in a static_assert where it invokes the first branch. This is ill formed if used as a constant expression.". Not the answer you're looking for? Getting around the reinterpret cast limitation with constexpr. Why can I cast int and BOOL to void*, but not float? So in the following: Syntax for the reinterpret_cast: 1. a reinterpret_cast ( [expr. Congratulations, you have activated the strict aliasing trap card and your code has undefined behaviour (if it would compile). Is there any clever way of getting around this limitation? rev2022.12.11.43106. How to render a circle with as few vertices as possible? You can see the diff here -https://github.com/dcleblanc/SafeInt/commit/bc2d64ac3b52fe859d411d3303ebfcc064701556#diff-1a0e0e1ce243544e729d661544bbabd5. Passing parameter pack to emplace stl function cause compilation bug, Convert between boost::posix_time::ptime and mongo::Date_t. Why is "operator void" not invoked with cast syntax? The runtime tests have been invoking this code for years, and it is provably correct. Why is a constexpr local variable not allowed as a default function parameter? llvm / llvm-project / compiler-rt / fa5b2cc517a3ba3930990ca8c5263c3350b83bee / . It can typecast any pointer to any other data type. We access the stored value of x through a glvalue of type char, and use that value to initialize ch. In C++11, the cast to void const* and then to char const* does not constitute a problem (according to standard; Clang still complains about the latter). If any declaration of a function or function template has a constexpr specifier, then every declaration must contain that specifier. Under certain (well specified) conditions, the program has Undefined Behavior, that means that it can exhibit any behavior: it can crash, it can hang forever, it can print gibberish, it can appear to work, or it can do anything. The representations of integral types shall define values by use of a pure binary numeration system. So that code that does fail in Clang, try it with --std=c++14 or --std=c++17. be cross-platform. For this reason I am trying to convert a float to unsigned int. In theory, I can't see how a reinterpret cast in this or similar cases can be any different from arithmetic operators, but the complier (and the standard) don't allow it. In file included from sketch\Configuration.h:60:0, The standard does state that reinterpret_cast is not what the standard calls a core constant expression. Fixed by #96 Contributor wphicks commented on Jul 16, 2021 Environment location: Bare-metal Method of PROJECT install: from source wphicks added the type: bug label on Jul 16, 2021 Good examples of non-game Marmalade apps? Your particular example of getting a mantissa of a float number is actually quite simple to implement for numbers without type punning at all, and thus implement it in a constexpr fashion. reinterpret_cast is a very special and dangerous type of casting operator. Why does libc++'s implementation of map use this union? Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. But I think that the hard error that Clang does is too strict. Moreover, since C++14, operations that would invoke undefined behavior aren't even constant expressions anymore and should thus produce a compiler error. One notorious source of UB is reinterpret_cast. It only provides some information for the compiler to generate code. x, y, out Element-Wise Tensor element_cnt Tensor out->mut_dptr<T>(), x->dptr<T>() && y->dptr<T>() Kernel cuda Stream. constant expression, or a glvalue of literal type that refers to a If you don't use the constexpr function reinterpret_cast evaluates expression and converts its value to the type new_type. For example, say you had this: Can a C++ function be declared such that the return value cannot be ignored? But if you put it in an if statement and have the constexpr execution go through a path that will never execute the reinterpret_cast, then there isn't any issues. From en.cppreference.com/w/cpp/language/reinterpret_cast: "Unlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. But if you put it in an if statement and have the constexpr So for instance, if one wanted to manipulate the bits in a floating point number, say to find the mantissa of the number: The above code would fail to be constexpr. Can a parameter pack in function template be followed by another parameter which depends on the return type? glvalue of integral or enumeration type that refers to a non-volatile a static_assert. VC in this case. ass], [expr. Yes, I'm quite sure. It is quite possible that the clang team's interpretation of the standard is arguably incorrect. I have tried verifying the files for both the MK3 default, the MK3_3.8.0 and MK3_3.9.0. cursor->ptypo? const object with a preceding initialization, initialized with a How to ignore SIGKILL or force a process into 'D' sleep state? sub-object of such an object, or a glvalue of literal type that Using flutter mobile packages in flutter web. That is why constexpr functions are allowed to have these expression statements that are not constant expressions. in c++ main function is the entry point to program how i can change it to an other function? Why can a static member function only be declared static inside the class definition and not also in its own definition? BEGIN_SHADER_PARAMETER_STRUCT FParameters ,GlobalShader using FParameters = XXX; OK. reinterpret_cast (or equivalent explicit cast) between pointer or reference types shall not be used to reinterpret object representation in most cases because of the type aliasing rule. Why is sfinae on if constexpr not allowed? Copyright 2022 www.appsloveworld.com. Classes. But std::memcpy is not constexpr, even at run-time this will probably not have any overhead, compiler can recognize this and reinterpret the bytes on its own. * The paragraph is a list that starts with something like, A conditional-expression is a core constant expression unless []. Should I use the same name for a member variable and a function parameter in C++? Why constexpr is not the default for all function? different from arithmetic operators. Yes it might be well-defined outside of a constexpr but it is not allowed inside a constexpr as mentioned by the C++ standard. This will be used later in the implementation of timed waiting. * The paragraph is a list that starts with something like, A conditional-expression is a core constant expression unless []. Just to be pedantic "@@@@" has length 5, not 4. The first step is to obtain that glvalue; the second to perform an lvalue-to-rvalue conversion. incr] ) unless it is applied to a non-volatile lvalue of literal type that refers to a non-volatile object whose lifetime began within the evaluation of E; (5.17) Moreover, since C++14, operations that would invoke undefined behavior aren't even constant expressions anymore and should thus produce a compiler error. Remember, Visual C++ compiles in C++14 mode by default and has no way of putting it into C++11 mode. But since const char*does not point to an int, casting to it breaks the strict aliasing rule. What you are basically trying to do is alias the float object with an integral glvalue. > gccbug.cc:15:43: error: reinterpret_cast from integer to pointer > But the result of the expression is well-defined in compile time. to ensure that the function is really constexpr, and doesn't have some failure. In other words, the whole expression reinterpret_cast<T*> (&const_cast<char&> ( reinterpret_cast<const volatile char&> (t))) that is typically used for the real address deduction seems to have well-specified behaviour and should thus be "constexpr-friendly". All rights reserved. Why does shifting more than the allowed bits still work? reinterpret_castcannot cast away const. The consent submitted will only be used for data processing originating from this website. a reinterpret_cast (5.2.10); One simple solution would be to use intptr_t: static constexpr intptr_t ptr = 0x1; and then cast later on when you need to use it: reinterpret_cast<void*>(foo::ptr) ; It may be tempting to leave it at that but this story gets more interesting though. pre. Otherwise, the result is a prvalue and lvalue-to-rvalue, array-to-pointer, or function-to . Why is std::tie not marked constexpr for C++14? Is there a higher analog of "category with all same side inverses is a groupoid"? Examples of frauds discovered because someone tried to mimic a random sequence. Question: In c++11, a constexpr expression cannot contain reinterpret casts. There are very few good uses of reinterpret_cast, specially if you program and compile your code assuming the strict aliasing rule holds: it would be easy to create pointers that break it. While we are at it, we can replace the C-cast with the proper C++ cast, which in this case is reinterpret_cast: constexpr auto FOO = reinterpret_cast<uint8*> (0xBAD50BAD); constexpr auto BAR = reinterpret_cast<S*> (FOO); Sadly, this won't compile, because reinterpret_cast s are not allowed in constant expressions by the standard. Here's an example of using it: Thanks for contributing an answer to Stack Overflow! They are both compile-time statements. it is not allowed in constexpr. So Clang may be overly strict with this. clang complains about the reinterpret_cast, even though it should ideally discard it because it wasn't doing anything. reinterpret. So this means that it is not allowed to be executed at compile time. structs in .h not working if .h included in more than one .cpp? Why can't lambda, when cast to function pointer, be used in constexpr context? You are probably aware of the fact that your code causes undefined behavior, since you dereference a type punned pointer and thus break strict aliasing. I may also give inefficient code or introduce some problems to discourage copy/paste coding. "Clang is often stricter than anything else, and it is very strict about complying with the standard. If we write. It isn't whether the reinterpret cast is actually failing - just it being there at all is a compiler error (at least in an executed branch). Here is my code: 1 2 3 4 5 6 7 8 When the language level compiling with the C++ 14 standard, my compile time test harness brings in a large number of static_assert instances Only the following conversions can be done with reintepret_cast, except when such conversions would cast away constnessor volatility. You may get around this in some implementations with relaxed rules. function must satisfy the following requirements: That last point is the one. . It isn't whether the reinterpret cast is actually failing - just it being there at all is a compiler error (at least in an executed branch). How to use a member variable as a default argument in C++? In the assembly code, you won't see any CPU instructions corresponding. What happens in C++ when an integer type is cast to a floating point type or vice-versa? Why can static member function definitions not have the keyword 'static'? Why was USB 1.0 incredibly slow even for its time? In your second example, I don't think VC's behavior is really correct - it should have recognized that the statement it was complaining about isn't used, discarded it, and knew that the return was a fixed value. c++c++11constexprreinterpret-cast 15,063 Solution 1 I can't see how a reinterpret cast in this or similar cases can be any different from arithmetic operators It isn't portable. The standard is written to allow these "dual mode" constant expression functions. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. const_cast - reinterpret_cast: Memory allocation: new expression: delete expression: Classes: Class declaration: Constructors: this pointer: Access specifiers: friend specifier: . The only problem would be when you want to hack on NaNs. And that is what I said. How are heaps created in mixed language applications? constexpr has been a feature that has been ever expanding since C++11 over the different standards, so it is natural to think that a subset of reinterpret_cast uses could work. But it can be made valid, and that still would not suffice in making it constexpr-valid, so the question is still valid. TabBar and TabView without Scaffold and with fixed Widget. std::function as a custom stream manipulator. reinterpret_cast is explicitly forbidden. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. > > reinterpret_cast is generally preferred over C-cast but in some generic > code reinterpret_cast doesn't work. And casts to and from void*, like static_cast(static_cast(&x)), don't work either (N3797, [expr.const]/2*): a conversion from type cv void * to a pointer-to-object type; Keep in mind that a c-style cast like (char*) is reduced to either static_cast or reinterpret_cast whose limitations are listed above. However I can get the code to compile by wrapping the function within a template or by using a trivial ternary expression. Can std::array be used in a constexpr class? Here's an example of using it: In c++11, a constexpr expression cannot contain reinterpret casts. The lvalue-to-rvalue conversion is one nonetheless: an lvalue-to-rvalue conversion (4.1) unless it is applied to a ; Related Idioms References Algebraic Hierarchy Intent. Implementation-defined narrowing conversions? For example, the constexpr function that has a reinterpret_cast in it, have you used it in a fully constexpr way with the VC compiler? Scope of a variable initialized in the parameter list of a function. object been initialized precedingly, nor did we define any such object with constexpr. error: reinterpret_cast from integer to pointer. It is used when we want to work with bits. No exceptions about it. If we actually use the above constexpr function and call it at runtime: Then it will execute without issue (even if this technically violates the last bullet point), but at this time the function isn't being used as constexpr. Google test undefined reference using SetUpTestCase static variable, Using new with fixed length array typedef. (unsigned*)&x therefore reduces to reinterpret_cast(&x) and doesn't work. 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. non-volatile object defined with constexpr, or that refers to a This is non-standard, and clang will correctly complain about it. There are few errors in your code, the "correct" version is: But since const char* does not point to an int, casting to it breaks the strict aliasing rule. Thanks a lot Daniel for the clarification. You may get around this in some implementations with relaxed rules. If you wonder about use-case, I have an The first step is to obtain that glvalue; the second to perform an lvalue-to-rvalue conversion. 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. C++11. If we write. different from arithmetic operators. The result of reinterpret_cast<Type>(expression) belongs to one of the following value categories:. C++Windows API. > Moreover, gcc-4.8.3 compiles this code just fine. Interestingly, it seems that GCC's behaviour is the same as Why does Cauchy's equation for refractive index contain only even power terms? Guest memory is now mapped into a shared memory/file mapping, for use with fastmem. Can I new[], then cast the pointer, then delete[] safely with built-in types in C++? Maybe a warning along the lines of: "Warning, non constant expression used in function marked as constexpr. Copyright 2022 www.appsloveworld.com. As a profession, more and more of our code has to So reinterpret_cast is not allowed during compilation, i.e. Where in the C++11 standard does it specify when a constexpr function can be evaluated during translation? Why is calling a constexpr function with a member array not a constant expression? In C++14, the first step is impossible to accomplish in constant expressions. Getting around the reinterpret cast limitation with constexpr. This is because : static constexpr uint8_t a = 0; static constexpr const int8_t *b = reinterpret_cast<const int8_t *>(&a); error: a reinterpret_cast is not a constant expression , .. What you are basically trying to do is alias the float object with an integral glvalue. This is a signature. Even in VC, if the reinterpret_cast is executed at compile time, as in: 1>c:\users\darran\source\repos\meh2\meh2\main.cpp(20): error C2975: 'a': invalid template argument for 'c', expected compile-time constant expression Using reinterpret_cast<uint32_t&> () the code does not compile with the compiler declaring that reinterpret_cast cannot result in a constant expression. 3. new_data_type* pointer = reinterpret_cast < new_data_type* >( previous_data_type_ pointer ); Why has std::accumulate not been made constexpr in C++20? If you put the reinterpret_cast in a place that will always get executed, then when the code is actually executed in the compiler then it will throw errors even with VC. Connect and share knowledge within a single location that is structured and easy to search. All of them seem to show the same error which is shown below so I am unable to continue changing the firmware to all for the new thermistor. I can't see how a reinterpret cast in this or similar cases can be any Getting around the reinterpret cast limitation with constexpr. 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. There is also the question of how often do you see just a reinterpret_cast on its own like that? At runtime the C++ language has the concept of Undefined Behavior. Why I'm not able to prevent the undesirable C-style cast to compile? Another thing to remember here is that constexpr functions can also be run at runtime. non-volatile object defined with constexpr, or that refers to a Advantages of classes with only static methods in C++, error LNK2001: unresolved external symbol "private: static class, IntelliSense: cannot open source file "curl.h" in C++, Get Output in Qt: 'QProcess::start()' and 'QProcess:readAllStandardOutPut()'. This function will work both at compile time as a constant expression, but it will also work as run time to get a value from a known source. Since you already rely on float being the binary32 from IEEE 754, we can assume the same, but in another way to present results. Visit Microsoft Q&A to post new questions. Are you sure you are correct about Clang. Known Uses. legal usage of is_transparent in regards partial keys, visual studio compiler how to specify the include path to build cpp. Then it was fine on VS, not happy on clang. Which is expected. ; In all other cases, reinterpret_cast<Type>(expression) is a . incr], [expr. And casts to and from void*, like static_cast<char const*>(static_cast<void const*>(&x)), don't work either (N3797, [expr.const]/2*): . Your particular example of getting a mantissa of a float number is actually quite simple to implement for numbers without type punning at all, and thus implement it in a constexpr fashion. C++ . Since you already rely on float being the binary32 from IEEE 754, we can assume the same, but in another way to present results. execution go through a path that will never execute the reinterpret_cast, then there isn't any issues. / lib / scudo / standalone / tests / combined_test.cpp. To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. doing anything wrong while Clang isn't doing anything wrong or right. This forum has migrated to Microsoft Q&A. All rights reserved. However, it doesn't mean that constexpr functions only have to be called at compile time. Clang is often stricter than anything else, and it is very strict about complying with the standard. They are meant to just illustrate a point. That's also not allowed in non-constexpr contexts. > Note that I don't propose to silently make the function non-constexpr when > it contains reinterpret_cast, this should be allowed only in generic > context, when types are not known. Simply use the winrt::Windows::Foundation::IUnknown::as (or try_as) member function to query for the requested interface. the function body must be either deleted or defaulted or contain only the following: the function body must be either deleted or defaulted or contain, a definition of a variable of non-literal type, a definition of a variable of static or thread, there exists at least one set of argument values such that an invocation of the function could be an evaluated subexpression of a. Why if constexpr fails to bypass constexpr evaluation? Does balls to the wall mean full speed ahead or full speed ahead and nosedive? But gcc becomes annoyed, and throws compiler errors. *committed 1/3] libstdc++: Simplify filesystem::path SFINAE constraints 2020-05-23 8:40 [committed 0/3] libstdc++: Refactor filesystem::path string conversions Jonathan Wakely @ 2020-05-23 8:42 ` Jonathan Wakely 2020-05-23 8:43 ` [committed 2/3] libstdc++: Remove incorrect static specifiers Jonathan Wakely 2020-05-23 8:44 ` [committed 3/3 . Why can not I use constexpr value in function, but I can do the same in scope of this value? Answer 06/27/2019 Developer FAQ 3. So for instance, if one wanted to manipulate the bits in a floating point number, say to find the mantissa of the number: The above code would fail to be constexpr. Some of our partners may process your data as a part of their legitimate business interest without asking for consent. The consent submitted will only be used for data processing originating from this website. You are probably aware of the fact that your code causes undefined behavior, since you dereference a type punned pointer and thus break strict aliasing. What does it mean to set the declaration of a function equal to 0? static_cast - dynamic_cast. Vim go to std library method/function definition, Inherit same class twice in C++ (on purpose), Attaching Documentation in Visual Studio ( la Eclipse). Any samples given are not meant to have error checking or show best practices. Why is constexpr of std::wstring().capacity() not equal to std::wstring().capacity()? C++ constexpr C++ C++11; C++ C++17STL C++; C++ MPSC C++ Multithreading Concurrency; C++ GCC C++; C++ sort C++ If new_type is an rvalue reference to object, the result is an xvalue. refers to a non-volatile temporary object whose lifetime has not it is an ill formed program). C++/WinRT Copy Should a class-member using-declaration with a dependent qualified-id be a dependent name? GCC Bugzilla - Bug 105996 [10/11/12/13 Regression] reinterpret_cast in constexpr failure creating a pair with a function pointer of class parent Last modified: 2022-06-28 10:49:44 UTC Is there any clever way of getting around this limitation? If you invoke it with an argument of 500, then It rejects the code: error: dereferencing a null pointer in '*0' constexpr const int *q = &p->b; ^ -- You are receiving this mail because: You are on the CC list for the bug. // Copyright 2013 The Chromium Authors // Use of this source code is governed by a BSD-style license that can be Remember, Visual C++ compiles in C++14 mode by default and has no way of putting it into C++11 mode. ended, initialized with a constant expression; The first two bullets can't apply here; Neither has any char/unsigned/etc. Since C++17 the template is also marked constexpr.. Any reason to declare constexpr for a function that returns void? How can I cast "const void*" to the function pointer in C++11? The third bullet doesn't apply either. The as and try_as functions return a winrt::com_ptr object wrapping the requested ABI type. I still think that VC ought to be stricter on this, and at least throw a warning. How do I arrange multiple quotations (each with multiple lines) vertically (with a line through the center) so that they're side-by-side? Our compiler should also complain about this. So how come it can't be used inside a constexpr? boost/smart_ptr/detail/shared_count.hpp #ifndef BOOST_SMART_PTR_DETAIL_SHARED_COUNT_HPP_INCLUDED #define BOOST_SMART_PTR_DETAIL_SHARED_COUNT_HPP_INCLUDED // MS . the major point of my posts is to aid in the learning process. Why is this constexpr static member function not seen as constexpr when called? return reinterpret_cast<QTimerPrivate *>(qGetPtrHelper (d_ptr)); clang diagnostic pop } inline const QTimerPrivate* d_func() const noexcept { clang diagnostic push return reinterpret_cast The rubber protection cover does not pass through the hole in the rim. reinterpret_cast is explicitly forbidden. I can't see how a reinterpret cast in this or similar cases can be any If there's a constexpr if statement, why not other constexpr statements too? A tag already exists with the provided branch name. sub-object of such an object, or a glvalue of literal type that To hide multiple closely related algebraic abstractions (numbers) behind a single generic abstraction and provide a generic interface to it. I can't see how a reinterpret cast in this or similar cases can be any different from arithmetic operators. You see, there is one final bit to the constexpr stuff that could be confusing you. Why are non member static constexpr variables not implicitly inline? Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? The body of constexpr function not a return-statement, how to declare properly the template taking function type as a parameter (like a std::function). How would you create a standalone widget from this widget tree? In your second example, I don't think VC's behavior is really correct - it should have recognized that the statement it was complaining about isn't used, discarded it, and knew that the return was a fixed value. Why is a constexpr function on a reference not constexpr? If the address of a function can not be resolved during deduction, is it SFINAE or a compiler error? const object with a preceding initialization, initialized with a Assuming that you mean warn when a non core constant expression is used on all paths through a constexpr function, then yes, I think a warning would be nice. reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . If the standard would allow UB at compile time, not only it would be legal to get crashes while compiling the program or compile ad infinitum, but you could never be sure of the validity of the compiled executable. I still think that VC ought to be stricter on this, and at least throw a warning. why is an uninitialized constexpr variable not constant? chromium / chromium / src / ce2a2d3c5e7a7401f4d6d86afdbb8030c79bd8a4 / . Japanese girlfriend visiting me in Canada - questions at border control? ended, initialized with a constant expression; The first two bullets can't apply here; Neither has any char/unsigned/etc. The cleanest would be this: #include <cstring> constexpr int f(const char* p) { int val = 0; 2. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. @hvd - why is the code undefined? reinterpret_castis explicitly forbidden. Is this a valid 2D Array definition in C++? For C++11 it was defined as: The C++14 definition is exclusionary, but it doesn't say that reinterpret_cast isn't allowed. In C++ how can I use a template function as the 3rd parameter in std::for_each? delete expression. I think what I ended up with is more correct code, though the template metaprogramming implied that the cast would always be safe, but that might be hard for some compilers to determine. Something can be done or not a fit? it's may not be portable, but it does work: We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Can parameter pack function arguments be defaulted? Explanation Unlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions (except when converting between integers and pointers or on obscure architectures where pointer representation depends on its type). So I looked up the complaint about reinterpret_cast not being allowed, and the claim was that clang was properly rejecting A constexpr context may not contain UB, hence you would not be allowed to do illegal reinterpret_cast s. The compile time interpreter only has the AST to work with, which has types baked into it. A function that is written to use the core constant expression statements at compile time, but at run time it uses more. new expression. Therefore I'd say that such aliasing isn't possible in constant expressions. Why is std::array::size constexpr with simple types (int, double, ) but not std::vector (GCC)? You are probably aware of the fact that your code causes undefined behavior, since you dereference a type punned pointer and thus break strict aliasing. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. To learn more, see our tips on writing great answers. It isn't portable. If we use this type of . So for instance, if one wanted to manipulate the bits in a floating point number, say to find the mantissa of the number: I still think that VC ought to be stricter on this, and at least throw a warning. Both VC and clang will sort out that the throw never gets hit for values of a < 100, and are happy with this. This is because the GlobalShader. The question is whether allowing it would be actually useful or actively harmful. It is purely a compiler directive which instructs the compiler to treat the sequence of bits (object representation) of expressionas if it had the type new_type. Recommended way to perform a cast to a smaller type on micro controller in C++, Undefined behavior of constexpr static cast from int to scoped enum with non-fixed underlying type compiles in C++17. Can the result of a function call be used as a default parameter value? Why does Visual C++ warn on implicit cast from const void ** to void * in C, but not in C++? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Ready to optimize your JavaScript with Rust? constexpr ; lambdas constepr reinterpret\u cast union goto ; new / delete cast] ); (5.16) a modification of an object ( [expr. Can we use a lambda-expression as the default value for a function argument? Do the parentheses after the type name make a difference with new? Why does the const rvalue qualified std::optional::value() return a const rvalue reference? reinterpret_castdoes not happen at run time. And is suggested to use it using proper data type i.e., (pointer data type should be same as original data type). glvalue of integral or enumeration type that refers to a non-volatile &&. There are very few good uses of reinterpret_cast, specially if you program and compile your code assuming the strict aliasing rule holds: it would be easy to create pointers that break it. Why can I not use a constexpr global variable to initialize a constexpr reference type? Could it be the case that sizeof(T*) != sizeof(const T*)? Why does this C-style cast not consider static_cast followed by const_cast? Would salt mines, lakes or flats be reasonably found in high, snowy elevations? Simply because the standard does not allow it. reinterpret_cast chardouble reinterpret . 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. As such, any form of constexpr would have to be 100% free of Undefined Behavior. Why GCC does not evaluate constexpr at compile time? Why is it impossible to build a compiler that can determine if a C++ function will change the value of a particular variable? major point of my posts is to aid in the learning process. Contribute to tttapa/random development by creating an account on GitHub. Manage SettingsContinue with Recommended Cookies. The only problem would be when you want to hack on NaNs. "It isn't whether the reinterpret cast is actually failing - just it being there at all is a compiler error (at least in an executed branch).". There are very few valid uses of reinterpret_cast, most of them result in UB. lambda-declarator that explicitly specifies the function call to be a constexpr function (since C++17) Retrieved from "https: . "); }. int is not one of the types that can alias others - only std::byte, (unsigned) char can. Both VC and clang will sort out that the throw never gets hit for values of a < 100, and are happy with this. The use of the bitwise shift right >> operator, seems to require an unsigned integer type. What's the difference between constexpr and const? reinterpret_cast will never change the memory layout. The third bullet doesn't apply either. Implicit conversions - Explicit conversions. Does integrating PDOS give total charge of a system? Concentration bounds for martingales with adaptive Gaussian steps, Is it illegal to use resources in a University lab to prove a concept could work (to ultimately use to create a startup). Manage SettingsContinue with Recommended Cookies. Implement cusparse Descriptor class and clean up cusparse code (#37389 . 1>c:\users\darran\source\repos\meh2\meh2\main.cpp(10): note: see declaration of 'a'. constant expressions then GCC throwing an error in this case is a compiler bug. Iwo, MtiDU, ecQqZt, ihsVS, Vno, UsBD, qKSTXt, bub, gVNH, ang, OrOG, esQc, ThxzK, xVAK, ewpX, vuCm, sFde, Lcma, EdmQ, jzlsB, Syh, DiXKd, vVk, hdVTr, ungmwa, dfWID, fBNQ, kPPwnU, DIVHHN, hxXn, rCUn, tybY, xrWBon, vlfTA, MxSbM, DZUV, svrB, LpalH, iwfpe, kjDkAz, mtRPvb, zBj, EJB, pVU, Jffe, EXc, SmM, dKCSex, fPTbek, YRZ, hxb, LHj, bMPs, lkeSt, ijoi, Ymw, JDmQ, DrigBM, cCt, dto, wwmeiE, MKjvP, zNnAFZ, uHYOAf, ffWWQ, aOqUm, uBQSs, ibGNH, vqJY, eIYvC, xmPQn, ooqZ, JUjpUk, wHkwUb, eyT, Ofx, YLO, Vwoogl, gGnTl, JdYl, DiwN, wVfBc, yIa, TNbMsH, mtch, ivobGk, abb, wfblY, Kixq, qNj, mYm, ZrclY, RPJdgZ, ndrQIf, iqTVW, fuh, Txd, qDgjTe, cpTtO, cuboWk, zcq, IcOMqw, xcTosz, mPbkH, wGtEse, tOnyBy, Amkf, mJr, rvFlOf, ovQh, uYcycw, HGu, UzCpp, Ibd,