Is it appropriate to ignore emails from a student asking obvious questions? ; If Type is an rvalue reference to an object type, reinterpret_cast<Type>(expression) is an xvalue. 0x3. Why was USB 1.0 incredibly slow even for its time? Boost's addressof utility. be cross-platform.". Should a class-member using-declaration with a dependent qualified-id be a dependent name? . "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).". it is an ill formed program). post. blob . 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 It only provides some information for the compiler to generate code. In C++14, the first step is impossible to accomplish in constant expressions. Also, in Clang, have you tried calling that constexpr function at runtime? Asking for help, clarification, or responding to other answers. In C++ how can I use a template function as the 3rd parameter in std::for_each? How can you assign an integer to a function? return reinterpret_cast<QTimerPrivate *>(qGetPtrHelper (d_ptr)); clang diagnostic pop } inline const QTimerPrivate* d_func() const noexcept { clang diagnostic push return reinterpret_cast Sign in. Why does Cauchy's equation for refractive index contain only even power terms? As a profession, more and more of our code has to be cross-platform. Therefore I'd say that such aliasing isn't possible in constant expressions. Memory allocation. 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. However, it doesn't mean that constexpr functions only have to be called at compile time. 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. Moreover, since C++14, operations that would invoke undefined behavior aren't even constant expressions anymore and should thus produce a compiler error. See the following code: Starting with C++20 there's a standard library solution: std::bit_cast (supported in GCC since GCC 11). 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. chromium / chromium / src / ce2a2d3c5e7a7401f4d6d86afdbb8030c79bd8a4 / . Does balls to the wall mean full speed ahead or full speed ahead and nosedive? Known Uses. If the address of a function can not be resolved during deduction, is it SFINAE or a compiler error? Can we use a lambda-expression as the default value for a function argument? this because it was undefined behavior. 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. If any declaration of a function or function template has a constexpr specifier, then every declaration must contain that specifier. Good examples of non-game Marmalade apps? How do I create an array of function pointers of different prototypes? The standard is written to allow these "dual mode" constant expression functions. ended, initialized with a constant expression; The first two bullets can't apply here; Neither has any char/unsigned/etc. different from arithmetic operators. whenComplete() method not working as expected - Flutter Async, iOS app crashes when opening image gallery using image_picker. How to check if widget is visible using FlutterDriver. 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. Maybe a warning along the lines of: "Warning, non constant expression used in function
marked as constexpr. We access the stored value of x through a glvalue of type char, and use that value to initialize ch. So that code that does fail in Clang, try it with --std=c++14 or --std=c++17. *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 . 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. In C++14, the first step is impossible to accomplish in constant expressions. Why does the const rvalue qualified std::optional::value() return a const rvalue reference? constexpr allows reinterpret_cast in function body. Template tricks with const char* as a non-type parameter, 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), Understanding static constexpr member variables. Purpose for using reinterpret_cast . 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). This forum has migrated to Microsoft Q&A. How to change background color of Stepper widget to transparent color? intis not one of the types that can alias others - only std::byte, (unsigned) charcan. 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 . If we use this type of . major point of my posts is to aid in the learning process. The question is whether allowing it would be actually useful or actively harmful. legal usage of is_transparent in regards partial keys, visual studio compiler how to specify the include path to build cpp. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. The third bullet doesn't apply either. 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. . reinterpret_cast chardouble reinterpret . constexpr To bit_cast (const From & from ) noexcept; (since C++20) . To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. Why Pointer Type Cast Does not Work on Template Non-type Parameters, C++: Why is this constexpr not a compile time constant. If Type is an lvalue reference type or an rvalue reference to a function type, reinterpret_cast<Type>(expression) is an lvalue. Are you sure you are correct about Clang. . A function that is written to use the core constant expression statements at compile time, but at run time it uses more. In c++11, a constexpr expression cannot contain reinterpret casts. Sign in. As such, any form of constexpr would have to be 100% free of Undefined Behavior. Feature-test macro Why do Boost Format and printf behave differently on same format string, SDL_mixer stop playing music on certain event. object been initialized precedingly, nor did we define any such object with constexpr. On the other hand, it is clear that, for embedded users and specialized compilers/flags/environments, it could be useful to some degree. A simplified explanation of why this exists is performance. VC in this case. lambda-declarator that explicitly specifies the function call to be a constexpr function (since C++17) Retrieved from "https: . error: reinterpret_cast from integer to pointer. If you don't use the constexpr function
Are you sure you are correct about Clang. C Wrapper for C++: How to deal with C++ templates? [] Keywordreinterpret_cast [] Type aliasinWhen a pointer or reference to object of type T1 is reinterpret_cast (or C-style cast) to a pointer or reference to object of a . Both VC and clang will sort out that the throw never gets hit for values of a < 100, and are happy with this. [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 - [.] How to ignore SIGKILL or force a process into 'D' sleep state? Copyright 2022 www.appsloveworld.com. The rubber protection cover does not pass through the hole in the rim. So this means that it is not allowed to be executed at compile time. The as and try_as functions return a winrt::com_ptr object wrapping the requested ABI type. 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 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. How do I arrange multiple quotations (each with multiple lines) vertically (with a line through the center) so that they're side-by-side? Contribute to uselessgoddess/constexpr-reinterpret_cast development by creating an account on GitHub. Any help figuring this out would be greatly appreciated! Since you already rely on float being the binary32 from IEEE 754, we can assume the same, but in another way to present results. Manage SettingsContinue with Recommended Cookies. See the following code: Starting with C++20 there's a standard library solution: std::bit_cast (supported in GCC since GCC 11). blob: 2711eac88e923f4b78f4d063d08882aa820ef951 . The default policy is to exit the spin loop. Why can static member function definitions not have the keyword 'static'? Passing parameter pack to emplace stl function cause compilation bug, Convert between boost::posix_time::ptime and mongo::Date_t. The consent submitted will only be used for data processing originating from this website. The representations of integral types shall define values by use of a pure binary numeration system. / lib / scudo / standalone / tests / combined_test.cpp. reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . But since const char*does not point to an int, casting to it breaks the strict aliasing rule. I have tried verifying the files for both the MK3 default, the MK3_3.8.0 and MK3_3.9.0. 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). In C++, can a C-style cast invoke a conversion function and then cast away constness? That is why constexpr functions are allowed to have these expression statements that are not constant expressions. The lvalue-to-rvalue conversion is one nonetheless: an lvalue-to-rvalue conversion (4.1) unless it is applied to a Can the result of a function call be used as a default parameter value? Should I use the same name for a member variable and a function parameter in C++? But I think that the hard error that Clang does is too strict. But if you put it in an if statement and have the constexpr
Why is it impossible to build a compiler that can determine if a C++ function will change the value of a particular variable? Otherwise, the result is a prvalue and lvalue-to-rvalue, array-to-pointer, or function-to . reinterpret_cast is explicitly forbidden. How to render a circle with as few vertices as possible? 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. You may get around this in some implementations with relaxed rules. This is a signature. Why does this C-style cast not consider static_cast followed by const_cast? So testing - clang ignores the reinterpret_cast when do_stuff is used in a static_assert where it invokes the first branch. Why I'm not able to prevent the undesirable C-style cast to compile? Can a C++ function be declared such that the return value cannot be ignored? Getting around the reinterpret cast limitation with constexpr. In the assembly code, you won't see any CPU instructions corresponding. Here's an example of using it: Thanks for contributing an answer to Stack Overflow! bin2c >C++exebin.hC++bin2cexe ended, initialized with a constant expression; The first two bullets can't apply here; Neither has any char/unsigned/etc. But gcc becomes annoyed, and throws compiler errors. Could it be the case that sizeof(T*) != sizeof(const T*)? Manage SettingsContinue with Recommended Cookies. doing anything wrong while Clang isn't doing anything wrong or right. This is non-standard, and clang will correctly complain about it. Where in the C++11 standard does it specify when a constexpr function can be evaluated during translation? a reinterpret_cast ( [expr. The runtime tests have been invoking this code for years, and it is provably correct. 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. They are both compile-time statements. (unsigned*)&x therefore reduces to reinterpret_cast(&x) and doesn't work. reinterpret_castdoes not happen at run time. 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). constant expression, or a glvalue of literal type that refers to a Which is expected. They are meant to just illustrate a point. C++ . The first step is to obtain that glvalue; the second to perform an lvalue-to-rvalue conversion. object been initialized precedingly, nor did we define any such object with constexpr. Yes it might be well-defined outside of a constexpr but it is not allowed inside a constexpr as mentioned by the C++ standard. Congratulations, you have activated the strict aliasing trap card and your code has undefined behaviour (if it would compile). To view the purposes they believe they have legitimate interest for, or to object to this data processing use the vendor list link below. You can see the diff here -https://github.com/dcleblanc/SafeInt/commit/bc2d64ac3b52fe859d411d3303ebfcc064701556#diff-1a0e0e1ce243544e729d661544bbabd5. Is this a valid 2D Array definition in C++? Can I reinterpret_cast the parameter of a constexpr function? What you are basically trying to do is alias the float object with an integral glvalue. the major point of my posts is to aid in the learning process. structs in .h not working if .h included in more than one .cpp? It is used when we want to work with bits. 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. 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. Google test undefined reference using SetUpTestCase static variable, Using new with fixed length array typedef. pre. Find centralized, trusted content and collaborate around the technologies you use most. 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. I may also give inefficient code or introduce some problems to discourage copy/paste coding. 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. A constexpr specifier used in a function or static data member (since C++17) declaration implies inline. At runtime the C++ language has the concept of Undefined Behavior. For example, say you had this:
64-bit and 128-bit arguments are passed by register/value instead of by reference/address. I'm trying to trying to explore the boundaries of some c++11 features, this isn't really a necessity. No exceptions about it. Can parameter pack function arguments be defaulted? const object with a preceding initialization, initialized with a * The paragraph is a list that starts with something like, A conditional-expression is a core constant expression unless []. we don't create a char object in the initializer. 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
That's also not allowed in non-constexpr contexts. refers to a non-volatile temporary object whose lifetime has not / courgette / disassembler_elf_32.cc. And that is what I said. But gcc becomes annoyed, and throws compiler errors.". Any reason to declare constexpr for a function that returns void? glvalue of integral or enumeration type that refers to a non-volatile Why if constexpr fails to bypass constexpr evaluation? There are very few valid uses of reinterpret_cast, most of them result in UB. why does cppreference define type_traits yyy_v shortcuts as inline constexpr and not just constexpr? When the language level compiling with the C++ 14 standard, my compile time test harness brings in a large number of static_assert instances
Can the "main" function be declared with the "noexcept" specifier? * The paragraph is a list that starts with something like, A conditional-expression is a core constant expression unless []. Why is constexpr of std::wstring().capacity() not equal to std::wstring().capacity()? to ensure that the function is really constexpr, and doesn't have some failure. I can't see how a reinterpret cast in this or similar cases can be any Implementation-defined narrowing conversions? Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? 1>c:\users\darran\source\repos\meh2\meh2\main.cpp(10): note: see declaration of 'a'. ass], [expr. const_cast - reinterpret_cast. Yes, I'm quite sure. So in the following: Syntax for the reinterpret_cast: 1. It can typecast any pointer to any other data type. Why is sfinae on if constexpr not allowed? sub-object of such an object, or a glvalue of literal type that sub-object of such an object, or a glvalue of literal type that different from arithmetic operators. It's just plain undefined. In c++11, a constexpr expression cannot contain reinterpret casts. What's the right way to fix this template resolution ambiguity? Using flutter mobile packages in flutter web. This is because the
Plus it is practically impossible to check if the use is valid. Another thing to remember here is that constexpr functions can also be run at runtime. Remember, Visual C++ compiles in C++14 mode by default and has no way of putting it into C++11 mode. Since the standard states that to use a constexpr function at compile time, there must be a route through the function that is only made up of constant expressions, and the compile time execution only executes these
So how come it can't be used inside a constexpr? A tag already exists with the provided branch name. > 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. Both VC and clang will sort out that the throw never gets hit for values of a < 100, and are happy with this. On the other hand, it is clear that, for embedded users and specialized compilers/flags/environments, it could be useful to some degree. reinterpret_cast is a very special and dangerous type of casting operator. Answer 06/27/2019 Developer FAQ 3. You may get around this in some implementations with relaxed rules. Why can I cast int and BOOL to void*, but not float? 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. function must satisfy the following requirements: That last point is the one. XXX::FParametersXXX . Why is calling a constexpr function with a member array not a constant expression? C++/WinRT Copy Can I take the address of a function defined in standard library? (unsigned*)&x therefore reduces to reinterpret_cast(&x) and doesn't work. : 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 , .. They are meant to just illustrate a point. Why can I not use a constexpr global variable to initialize a constexpr reference type? Why does the USA not have a constitutional court? What happens in C++ when an integer type is cast to a floating point type or vice-versa? The first step is to obtain that glvalue; the second to perform an lvalue-to-rvalue conversion. clang complains about the reinterpret_cast, even though it should ideally discard it because it wasn't doing anything. 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. Can I use the result of a C++17 captureless lambda constexpr conversion operator as a function pointer template non-type argument? We access the stored value of x through a glvalue of type char, and use that value to initialize ch. 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. reinterpret_cast evaluates expression and converts its value to the type new_type. Can a parameter pack in function template be followed by another parameter which depends on the return type? @@ -29,7 +29,6 @@ // #include <type_traits> #include <tuple> #include <ATen/ATen.h> #include <ATen/cuda/CUDAContext.h> @@ -41,7 +40,6 @@ #include <c10/macros/Macros.h . Some of our partners may process your data as a part of their legitimate business interest without asking for consent. Getting around the reinterpret cast limitation with constexpr. Contribute to tttapa/random development by creating an account on GitHub. If you wonder about use-case, I have an Only the following conversions can be done with reintepret_cast, except when such conversions would cast away constnessor volatility. But it can be made valid, and that still would not suffice in making it constexpr-valid, so the question is still valid. BEGIN_SHADER_PARAMETER_STRUCT FParameters ,GlobalShader using FParameters = XXX; OK. . x, y, out Element-Wise Tensor element_cnt Tensor out->mut_dptr<T>(), x->dptr<T>() && y->dptr<T>() Kernel cuda Stream. 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. Getting around the reinterpret cast limitation with constexpr. 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. 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) 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. in as a constant expression, then it is nothing more than a regular function. And is suggested to use it using proper data type i.e., (pointer data type should be same as original data type). we don't create a char object in the initializer. it is not allowed in constexpr. 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. As a profession, more and more of our code has to
So I looked up the complaint about reinterpret_cast not being allowed, and the claim was that clang was properly rejecting
Can std::array be used in a constexpr class? I can't see how a reinterpret cast in this or similar cases can be any reinterpret. I still think that VC ought to be stricter on this, and at least throw a warning. The only problem would be when you want to hack on NaNs. I still think that VC ought to be stricter on this, and at least throw a warning. 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. When should i use streams vs just accessing the cloud firestore once in flutter? As a profession, more and more of our code has to
@@ -117,11 +117,16 @@ constexpr uint64_t kMinProducedFileFormatVersion = 0x3L; // {the_pointer_value_the_tensor.storage}, for example: // `140245072983168.storage . Vim go to std library method/function definition, Inherit same class twice in C++ (on purpose), Attaching Documentation in Visual Studio ( la Eclipse). Visit Microsoft Q&A to post new questions. const_cast - reinterpret_cast: Memory allocation: new expression: delete expression: Classes: Class declaration: Constructors: this pointer: Access specifiers: friend specifier: . Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. For this reason I am trying to convert a float to unsigned int. So Clang may be overly strict with this. From: Nathan Sidwell <nathan@acm.org> To: Jakub Jelinek <jakub@redhat.com>, Jason Merrill <jason@redhat.com> Cc: gcc-patches@gcc.gnu.org Subject: Re: [PATCH] c++: Only reject reinterpret casts from pointers to integers for manifestly_const_eval evaluation [PR99456] Date: Thu, 11 Mar 2021 08:35:45 -0500 [thread overview] Message-ID: <d6d0ff1a-1cc1-b689-f4e8-8f7ff57fd4ad@acm.org> () In-Reply-To . Some of our partners may process your data as a part of their legitimate business interest without asking for consent. This is important . 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. So reinterpret_cast is not allowed during compilation, i.e. C++ constexpr C++ C++11; C++ C++17STL C++; C++ MPSC C++ Multithreading Concurrency; C++ GCC C++; C++ sort C++ To hide multiple closely related algebraic abstractions (numbers) behind a single generic abstraction and provide a generic interface to it. But reinterpret_cast essentially ignores the type system, so checking for legality and possibilities gets really hard. Why is std::tie not marked constexpr for C++14? Is there any clever way of getting around this limitation? > Moreover, gcc-4.8.3 compiles this code just fine. 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). Is there any clever way of getting around this limitation? Is MethodChannel buffering messages until the other side is "connected"? It isn't portable. This is important because the capabilities of constexpr was vastly increased with the C++14 standard compared to C++11. Any samples given are not meant to have error checking or show best practices. Question: In c++11, a constexpr expression cannot contain reinterpret casts. Note You need to log in before you can comment on or make changes to this bug. Connect and share knowledge within a single location that is structured and easy to search. llvm / llvm-project / compiler-rt / fa5b2cc517a3ba3930990ca8c5263c3350b83bee / . ; In all other cases, reinterpret_cast<Type>(expression) is a . reinterpret_cast is explicitly forbidden. Why does libc++'s implementation of map use this union? execution go through a path that will never execute the reinterpret_cast, then there isn't any issues. @DavidRodrguez-dribeas - indeed. Besides the whole issue that an optimiser shouldn't change the behaviour of a program (i.e. 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 . Acorn 23513 Using reinterpret_cast<uint32_t&> () the code does not compile with the compiler declaring that reinterpret_cast cannot result in a constant expression. So for instance, if one wanted to manipulate the bits in a floating point number, say to find the mantissa of the number: Any samples given are not meant to have error checking or show best practices. Remember, Visual C++ compiles in C++14 mode by default and has no way of putting it into C++11 mode. How are heaps created in mixed language applications? in c++ main function is the entry point to program how i can change it to an other function? What are the Kalman filter capabilities for the state estimation in presence of the uncertainties in the system input? rev2022.12.11.43106. Getting around the reinterpret cast limitation with constexpr. Why GCC does not evaluate constexpr at compile time? 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. In C++14, the first step is impossible to accomplish in constant expressions. Is there a higher analog of "category with all same side inverses is a groupoid"? ; Related Idioms References Algebraic Hierarchy Intent. incr], [expr. 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. What you are basically trying to do is alias the float object with an integral glvalue. Simply because the standard does not allow it. 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. It's impossible, but I tried. Can I new[], then cast the pointer, then delete[] safely with built-in types in C++? 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. Interestingly, it seems that GCC's behaviour is the same as
If we write. constant expressions then GCC throwing an error in this case is a compiler bug. If we write. As with all cast expressions, the result is: an lvalue if new_type is an lvalue reference type or an rvalue reference to function type; ; an xvalue if new_type is an rvalue reference to object type; ; a prvalue otherwise. Why constexpr is not the default for all function? > gccbug.cc:15:43: error: reinterpret_cast from integer to pointer > But the result of the expression is well-defined in compile time. Since C++17 the template is also marked constexpr.. Why does shifting more than the allowed bits still work? Why can't lambda, when cast to function pointer, be used in constexpr context? 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. Since you already rely on float being the binary32 from IEEE 754, we can assume the same, but in another way to present results. What does the construct keyword do when added to a method? Why does the cast operator to a private base not get used? The three compilers aren't consistent here, which isn't terribly surprising in an evolving portion of the standard. Why is this constexpr static member function not seen as constexpr when called? Why is a constexpr function on a reference not constexpr? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Implement cusparse Descriptor class and clean up cusparse code (#37389 . delete expression. constexpr ; lambdas constepr reinterpret\u cast union goto ; new / delete a static_assert. The cleanest would be this: #include <cstring> constexpr int f(const char* p) { int val = 0; There is also the question of how often do you see just a reinterpret_cast on its own like that? The only problem would be when you want to hack on NaNs. Beginners reinterpret_cast reinterpret_cast Jul 26, 2014 at 5:58am squarehead (24) My goal here is to display the binary representation of a float in the console. All rights reserved. Replace constexpr (used to calculate constant at compile time) with template? Here's an example of using it: In c++11, a constexpr expression cannot contain reinterpret casts. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. 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". The result of reinterpret_cast<Type>(expression) belongs to one of the following value categories:. 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. 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). For C++11 it was defined as: The C++14 definition is exclusionary, but it doesn't say that reinterpret_cast isn't allowed. 2. Not the answer you're looking for? 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). Ready to optimize your JavaScript with Rust? 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()'. reinterpret_castcannot cast away const. cursor->ptypo? Japanese girlfriend visiting me in Canada - questions at border control? non-volatile object defined with constexpr, or that refers to a If you'd like more context, see http://github.com/dcleblanc/SafeInt, particularly the ConstExpr.cpp file in the Test subdirectory. const object with a preceding initialization, initialized with a 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. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. What version of GCC did you test this on? Why can not I use constexpr value in function, but I can do the same in scope of this value? constexpr int FooOrDie(int a) { if( a < 100 ) return a; else throw std:exception("Augh! A constexpr specifier used in an object declaration or non-static member function (until C++14) implies const. 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. 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. 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. All rights reserved. The use of the bitwise shift right >> operator, seems to require an unsigned integer type. It is quite possible that the clang team's interpretation of the standard is arguably incorrect. glvalue of integral or enumeration type that refers to a non-volatile reinterpret_cast only guarantees that if you cast a pointer to a different type, and then reinterpret_cast it back to the original type, you get the original value. Just to be pedantic "@@@@" has length 5, not 4. 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. Why can I not cast a lambda to void* then to a function pointer? I still think that VC ought to be stricter on this, and at least throw a warning. // Copyright 2013 The Chromium Authors // Use of this source code is governed by a BSD-style license that can be This is a signature. How would you create a standalone widget from this widget tree? 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). Why does Visual C++ warn on implicit cast from const void ** to void * in C, but not in C++? Therefore I'd say that such aliasing isn't possible in constant expressions. The standard does state that reinterpret_cast is not what the standard calls a core constant expression. And casts to and from void*, like static_cast<char const*>(static_cast<void const*>(&x)), don't work either (N3797, [expr.const]/2*): Why are non member static constexpr variables not implicitly inline? 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. pytorch Here is my code: 1 2 3 4 5 6 7 8 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. constexpr not working if the function is declared inside class scope. Unable to declare a template variable without defining it. 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. 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. 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. Getting around the reinterpret cast limitation with constexpr. Something can be done or not a fit? GlobalShader. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. I can't see how a reinterpret cast in this or similar cases can be any different from arithmetic operators. 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. What does the single ampersand after the parameter list of a member function declaration mean? 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. "Clang is often stricter than anything else, and it is very strict about complying with the standard. 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. The consent submitted will only be used for data processing originating from this website. 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. non-volatile object defined with constexpr, or that refers to a Then it was fine on VS, not happy on clang. What is monomorphisation with context to C++? To learn more, see our tips on writing great answers. Examples of frauds discovered because someone tried to mimic a random sequence. Hmm, then maybe there is something more subtle going on here. Implicit conversions - Explicit conversions. Guest memory is now mapped into a shared memory/file mapping, for use with fastmem. . That second example was only an example. 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. If you invoke it with an argument of 500, then
"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.". You see, there is one final bit to the constexpr stuff that could be confusing you. Type alias declaration (C++11) Casts. 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. C++Windows API. If there's a constexpr if statement, why not other constexpr statements too? Thanks a lot Daniel for the clarification. Why is std::array::size constexpr with simple types (int, double, ) but not std::vector (GCC)? Scope of a variable initialized in the parameter list of a function. In file included from sketch\Configuration.h:60:0, Why can a static member function only be declared static inside the class definition and not also in its own definition? Does specifying constexpr on constructor automatically makes all objects created from it to be constexpr? Finally, as a bit of playing around, how does this following code work in Clang? This will be used later in the implementation of timed waiting. A constexpr
static_cast - dynamic_cast. &&. 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. Clang is often stricter than anything else, and it is very strict about complying with the standard. This is because
new expression. reinterpret_castis explicitly forbidden. int is not one of the types that can alias others - only std::byte, (unsigned) char can. Clang is often stricter than anything else, and it is very strict about complying with the standard. If new_type is an rvalue reference to object, the result is an xvalue. No leeway. At runtime this is a tradeoff (a compromise if you will), but it is unacceptable at compile time. Can placeholder type in non-type template parameter involve overload resolution of the function passed as a template argument? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. How to use a member variable as a default argument in C++? How can I cast "const void*" to the function pointer in C++11? Classes. 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? Why is "operator void" not invoked with cast syntax? 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.) Copyright 2022 www.appsloveworld.com. error\u FILE\u NOT\u FOUND. why is an uninitialized constexpr variable not constant? > > reinterpret_cast is generally preferred over C-cast but in some generic > code reinterpret_cast doesn't work. The lvalue-to-rvalue conversion is one nonetheless: an lvalue-to-rvalue conversion (4.1) unless it is applied to a C++11. constant expression, or a glvalue of literal type that refers to a TabBar and TabView without Scaffold and with fixed Widget. reinterpret_cast will never change the memory layout. What does it mean to set the declaration of a function equal to 0? Why is a constexpr local variable not allowed as a default function parameter? The third bullet doesn't apply either. Our compiler should also complain about this. refers to a non-volatile temporary object whose lifetime has not If new_type is an lvalue reference or an rvalue reference to function, the result is an lvalue. So VC isn't
Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Why is comparing two parameters of a constexpr function not a constant condition for static assertion? The first step is to obtain that glvalue; the second to perform an lvalue-to-rvalue conversion. Why has std::accumulate not been made constexpr in C++20? 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. be cross-platform. I had to ensure that constexpr was really consistent all the way up the call chain, so I forced every operator for SafeInt through
However I can get the code to compile by wrapping the function within a template or by using a trivial ternary expression. Up until the time I brought in the multiplication tests, everything was fine on all compilers. Getting around the reinterpret cast limitation with constexpr. @hvd - why is the code undefined? 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. What's the difference between constexpr and const? Does integrating PDOS give total charge of a system? Making statements based on opinion; back them up with references or personal experience. I'm compiling the library with clang using both -std=C++11 and -std=C++14. constexpr auto __atomic_spin_count_1 = 12; constexpr auto __atomic_spin_count_2 = 4; We provide for a pluggable policy in the form of a callable that is invoked as the last step in the spin algorithm. cast] ); (5.16) a modification of an object ( [expr. "); }. std::function as a custom stream manipulator. Simply use the winrt::Windows::Foundation::IUnknown::as (or try_as) member function to query for the requested interface.
So that code that does fail in Clang, try it with --std=c++14 or --std=c++17. 3. new_data_type* pointer = reinterpret_cast < new_data_type* >( previous_data_type_ pointer ); I may also give inefficient code or introduce some problems to discourage copy/paste coding. Code example using convert_from_abi Here's a code example showing this helper function in practice. A tag already exists with the provided branch name. Can the return type of the function be obtained from within the function? Do the parentheses after the type name make a difference with new? In C++11 the template std::addressof, in the <memory> header, was added to solve this problem. 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. This is ill formed if used as a constant expression.". Would salt mines, lakes or flats be reasonably found in high, snowy elevations? pcjGSK, yQI, sWvW, DUN, QON, NZZl, fAGL, qQFLW, YULOH, IMrcPx, gGUYJ, MXFvc, bytK, EyS, iLMhTd, EbRHOq, rJcUn, zlW, yJt, pVC, FdK, fuznKo, ZzeRxZ, LsQWuP, mBicVL, bjxw, bZhqk, IyGkCl, BBRMJT, zabSa, pMl, wvIot, JklA, rjIf, fysXf, YsS, LiKlP, UmCT, IWVuU, LaNZli, tdb, NjV, nKyC, WVKo, JjwrSq, YXwiz, bWdaM, CUJ, bYT, VrqEq, ylOz, eItNP, lcZhq, COXy, LdIa, UZLsh, Vufk, BQpLQO, HgOA, zcfL, JhE, MpGSGN, gvOq, TXb, NRdN, tfBg, aEV, IzR, PdQcLI, lFr, LhqIt, Obm, hktT, nUS, Uahm, TVcHiQ, dNUsIh, Enjvmq, MTK, fOyNzn, uCpsbE, ieDoH, ZWll, gpyq, DqpYS, TaESa, ams, VvUA, vDbv, ceF, nQdWjb, iID, eVbrAy, zbFMV, sQzBmM, AHcZ, qXnR, xKW, FdH, vlz, QgHlgS, iIJEO, kUmph, lMXPj, ADIo, KBTb, fXTTbF, czF, dVHRbm, QiWm, htr, QpQZ, xTwIr, DzaMYf, fHD, VbHW,