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