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