reinterpret_cast can't be used to cast a pointer to function to a void*. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Does balls to the wall mean full speed ahead or full speed ahead and nosedive? Books that explain fundamental chess concepts. 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. It is efficient because it does not copy the value. 8 vscodewindows. Would salt mines, lakes or flats be reasonably found in high, snowy elevations? Making statements based on opinion; back them up with references or personal experience. Losing bytes like this is called 'truncation', and that's what the first warning is telling you. Is it correct to say "The glue on the back of the sticker is dying down so I can not stick the sticker to the wall"? The reinterpret_cast operator should not be used to convert between pointers to different classes that are in the same class hierarchy; use a static or dynamic cast for that purpose. Can several CRTs be wired in parallel to one oscilloscope circuit? Don't write a binary file "because it's faster". For a conversion between different function type pointers or between different object type pointers you need to use reinterpret_cast. Rather, reinterpret_cast has a number of meanings, for all of which holds that the mapping performed by reinterpret_cast is implementation-defined. [5.2.10.3]. reinterpret_cast casts away const qualifier? For example: However , make sure that the Dialog is actually not a const object; attempting to modify a const object (presumably setValue does this) causes undefined behaviour. It does not mean they are the only type you can use with reinterpret_cast. (175) QT0-5qimageqpainter . even round trip isn't guaranteed to work). How can I use a VPN to access a Russian website that is banned in the EU? For example, you can use reinterpret_cast to convert from a void * to an int, which will work correctly if your system happens to have sizeof (void*) sizeof (int). size as regular pointers, since on CbDrawIndexed *drawCmd = reinterpret_cast<CbDrawIndexed*>(mSwIndirectBufferPtr + (size_t)cmd->indirectBufferOffset ); bufferCONST_SLOT_STARTVES_POSITION I want to reinterpret cast a function pointer into a void* variable. is it better than static_cast? I don't see any rule in the standard allowing this conversion. How do I arrange multiple quotations (each with multiple lines) vertically (with a line through the center) so that they're side-by-side? - reinterpret_cast is a keyword. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? Asking for help, clarification, or responding to other answers. Understanding reinterpret_cast. It does not check if the pointer type and data pointed by the pointer is same or not. reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . To learn more, see our tips on writing great answers. This is the object: Expand | Select | Wrap | Line Numbers template<class T> class Coordinates { public: T *x; T *y; int size; public: Coordinates (); Coordinates (int s, T data); Coordinates (const Coordinates &c); ~Coordinates ();; void zeros (void); }; reinterpret_cast is a tricky beast. How many transistors at minimum do you need to build a general-purpose computer? It is used to convert a pointer of some data type into a pointer of another data type, even if the data types before and after conversion are different. Error: #694: reinterpret_cast cannot I suggest using the weakest possible cast always. What happens if you score more than 99 points in volleyball? Japanese girlfriend visiting me in Canada - questions at border control? static_cast is the cast of choice when there is a natural, intuitive conversion between two types that isn't necessarily guaranteed to work at runtime. But with ARM compiler, it gives compilation error. When casting back to the original type, AliasedType and DynamicType are the same, so they are similar, which is the first case listed by the aliasing rules where it is legal to dereference the result of reinterpret_cast : Whenever an attempt is made to read or modify the stored value of an object of type DynamicType through a glvalue of type AliasedType, the behavior is undefined unless one of the following is true: An object pointer can be explicitly converted to an object pointer of a different type. In computer science, a pointer is an object in many programming languages that stores a memory address.This can be that of another value located in computer memory, or in some cases, that of memory-mapped computer hardware.A pointer references a location in memory, and obtaining the value stored at that location is known as dereferencing the pointer. How to do such conversions from void (*)(void*) -> void* effectively so that atleast it compiles almost the same in most of the compilers ? static _ cas t const _ cas tre interp ret_ cas t dynamic _ cas t. kingsfar . In C++11 through C++17, it is implementation defined if conversions between function pointers and void * are allowed. "it's the cast from the void* to the int** that requires a reinterpret_cast" is wrong. - type is a pointer reinterpreted as. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. 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). Scenario 3: Forward and backward transitions between voids Errors can occur if no one pointer can be converted to void, and void can be converted backward to any pointer (for static_cast<> and reinterpret_cast<> conversions). Why should I use a pointer rather than the object itself? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. You can also use reinterpret_cast to convert a float* to an int* or vice-versa, which is platform-specific because the particular representations of floats and ints aren't guaranteed to have anything in common with one another. (*)(void) to a void*). qualifiers, I read the explanation in Casting a function pointer to another type. So, when you cast a (void*) to (long), you are losing 32 bits of data in the conversion. It also allows. Asking for help, clarification, or responding to other answers. Fixed by #403 yurivict commented on Oct 16, 2018 cryos on Oct 18, 2018 #403 cryos completed in #403 on Oct 18, 2018 Sign up for free to join this conversation on GitHub . Every object pointer type whose pointed type is cv-unqualified is implicitly convertible to void*, and the inverse can be done by static_cast. remember that it's undefined behavior. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. reinterpret_cast to void* not working with function pointers, error: passing xxx as 'this' argument of xxx discards qualifiers, Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs. Here: Ready to optimize your JavaScript with Rust? So why have reinterpret_cast<>? I've reopened the question because I didn't see only the top answer applies to this. How can I use a VPN to access a Russian website that is banned in the EU? Tim Roberts [MVP] wrote: There do exist processors on which a "char *" and an "int *" are different sizes. This rule bans (T)expression only when used to perform an unsafe cast. How should I cast the result of malloc in C++? It's my understanding that C-style cast is defined in terms of the new cast styles. Is this an oversight? You likely obtained that void* with implicit conversion, so you should use static_cast because it is closest to the implicit conversion. Generally reinterpret_cast is much less restrictive than other C++ style casts in that it will allow you to cast most types to most other types which is both it's strength and weakness. Only in the rarest of rare cases when there is no other way use reinterpret_cast. Also, does the direction of the conversion matter. Thanks for contributing an answer to Stack Overflow! You claimed that it was possible to cast a pointer to, @BenVoigt you offered that code in response to someone asking "How do I cast", and then when someone said that the code casts between pointers (which it does), you said "Nope". By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Putting a space in ". c++ reinterpret_cast unique_ptr * unique_ptr * c++ Using reinterpret_cast to cast unique_ptr * to unique_ptr * for creating a transformable tree structure reinterpret_cast does NOT guarantee that the same address is used. C++ static _ cas t dynamic _ cas t const _ cas tre interp ret_ cas t. Just be so kind as to only use it for comparison, as key in a hash map or similarly innocent things. Syntax : reinterpret_cast <type> (Expression) reinterpret_cast performs a low level reinterpretation of the bit pattern of its operands. Should teachers encourage good students to help weaker ones? The type of the function pointer will be of type Class* (*) (void*). In the case of casting an object pointer to another object pointer type, failing to meet the requirements of strict aliasing rules means you cannot safely dereference the result. (void*) &d); } Since the arrays are reference types and hold their own metadata about their type you cannot reinterpret them without overwriting the . some architectures they might contain C++. target-type is the target of the cast whereas expr is being cast into the new target-type. My use case requires a reinterpret_cast because I'm casting from an int** to a void*. Something can be done or not a fit? reinterpret_cast can't cast away cv-qualifiers So you can use reinterpret_castand const_casttogether. When would I give a checkpoint to my D&D party that they can return to if they die? Once a pointer has degenerated into a void* you can static_cast it to any type of pointer. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. The compiler can then check that the cast is between related types, reporting a compile-time error if this isn't the case. I use reinterpret_cast to convert integer memory addresses to pointers to overlay structs. However, this doesnt actually describe the effect of a reinterpret_cast. 10 QGuiApplication::allWindows () 11 QSharedPointer. rev2022.12.11.43106. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Is it Legal to reinterpret_cast to a void*, https://en.cppreference.com/w/cpp/language/reinterpret_cast, stackoverflow.com/questions/573294/when-to-use-reinterpret-cast. The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. should I use it or static_cast then static_cast to avoid reinterpret_cast? This type of cast reinterprets the value of a variable of one type as another variable of a different type. Keyboard shortcuts: Use 'j/k' keys for keyboard navigation; Use 'Shift+S' to show/hide relevant lines; Use '?' to toggle this window The reinterpret_cast operator can be used for conversions such as char* to int*, or One_class* to Unrelated_class*, which are inherently unsafe. like int to pointer and pointer to int etc. Both static_cast and reinterpret_cast seem to work fine for casting void* to another pointer type. void is not included here because you can never dereference a void *. Fixes the following MSVC 2005 warning when doing tests like ClassName *p = 0; REQUIRE( p != 0 ); warning C4312: 'reinterpret_cast' : conversion from 'int' to 'ClassName *' of greater size int needs to be cast to intptr_t apparently: old:. Why use static_cast(x) instead of (int)x? Der C++ FAQ Lite dies erklrt in einigen Details. When you refer to byte and char being the only legal types, it is just that it is legal to dereference the converted pointer only for those types. reinterpret_cast is very much standard C++. Using reinterpret_cast to do this with pointer conversions completely bypasses the compile-time safety check. */ /* */ /* $Source: src/runtime/rt_vfs.C . The rubber protection cover does not pass through the hole in the rim. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. Asking for help, clarification, or responding to other answers. Are the S&P 500 and Dow Jones Industrial Average securities? At what point in the prequels is it revealed that Palpatine is Darth Sidious? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. confusion between a half wave and a centre tapped full wave rectifier. As we learnt in the generic types example, static_cast<> will fail if you try to cast an object to another unrelated class, while reinterpret_cast<> will always succeed by "cheating" the compiler to believe that the object is really that unrelated class. Not the answer you're looking for? For example: Note that if that make sense or not will depend on the target more than the compiler: a portable compiler like gcc will have a behavior imposed by the target architecture and possibly ABI. reinterpret_cast, on the other hand, is a casting operator designed to do conversions that are fundamentally not safe or not portable. reninterpret_cast does not check if the pointer type and data pointed by the pointer is same or not. #include<iostream> float fastInvSqrt( float x ) { const int INV_SQRT_N = 1597292357; const float MULT = 1.000363245811462f; float const mx = 0.5f * MULT * x; i.e. by two? Ready to optimize your JavaScript with Rust? To learn more, see our tips on writing great answers. The rubber protection cover does not pass through the hole in the rim. You should use static_cast so that the pointer is correctly manipulated to point at the correct location. .. I'm casting from an int** to a void*. Did neanderthals need vitamin C from the diet? How do I arrange multiple quotations (each with multiple lines) vertically (with a line through the center) so that they're side-by-side? As for which one is preferred by the spec, neither is overly mentioned as "the right one to use" (or at least, I don't remember one of them being mentioned this way.) It doesn't guarantee any safety and your program might crash as the underlying object could be anything. All you need is a single static_cast: We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Japanese girlfriend visiting me in Canada - questions at border control? reinterpret_cast<char*>(&x)int 10910 (in practice it will typically contain the same address as a and c, but that's not specified in the standard, and it may not be true on machines with more complex memory systems.) Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Additionally, and arguably more important, is the fact that every use of reinterpret_cast is downright dangerous because it converts anything to anything else really (for pointers), while static_cast is much more restrictive, thus providing a better level of protection. Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? It is always legal to convert from a pointer to a type to a pointer to a different type including void, so if T is a type this is legal C++: In real world it is never used because void * is a special case, and you obtain the same value with static_cast: (in fact above conversion is implicit and can be simply written void *y = x; - thank to Michael Kenzel for noticing it), To be more explicit the standard even says in draft n4659 for C++17 8.2.10 Reinterpret cast [expr.reinterpret.cast], 7. Was Sie wahrscheinlich tun mssen, ist, wickeln Sie Ihre member-Funktion in einem regulren Funktion. Why does the USA not have a constitutional court? This means in particular that a cast from a pointer to function to void * is not possible, but you can cast it to void(*)(). 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. rev2022.12.11.43106. When casting from a void* there is not type information for the cast to work with. If you have a function, and want a void* pointing to it, you can do it all in one line, but that's a bit cumbersome on the syntax. Raw memory access like this is not type-safe and can only be done under a full trust security environment. pointers aren't necessarily the same Some POSIX functions need the conversion to be well useful. The more structure-breaking the cast is, the more attention using it requires. ReInterpret Cast ( reinterpret_cast) is a cast operator that converts a pointer of some data type into a pointer of another data type, even if the the data types before and after conversion are different. to cv void. This will allow you to cast it to void*. For example, when using a C-style cast, as in. reinterpret_cast will forcefully convert the void* to the target data type. -P.S. So if your converting from Void* to Type* or from Type* to Void* should you use: To me static_cast seems the more correct but I've seen both used for the same purpose. MOSFET is getting very hot at high frequency PWM. I was concerned about the below explanation. To learn more, see our tips on writing great answers. When is casting void pointer needed in C? The type of a pointer to cv void or a pointer to an object type is called an object pointer type. Did neanderthals need vitamin C from the diet? The Boost Serialization library has a good. I want to reinterpret cast a function pointer into a void* variable. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, casting via void* instead of using reinterpret_cast. Was the ZX Spectrum used for number crunching? The trick to start being able to do the type but is to transform the temporary function pointer to an lvalue reference to const, which you can take the address of, and then proceed like above. Why is processing a sorted array faster than processing an unsorted array? Das Hauptproblem ist, dass die Daten bentigt um einen Zeiger-auf . Counterexamples to differentiation under integral sign, revisited, i2c_arm bus initialization and device-tree overlay. reinterpret_cast, on the other hand, is a casting operator designed to do conversions that are fundamentally not safe or not portable. example how; so does the tinyxml project. Thanks to Richard which makes me revisit the issue more in depth (for the record, I was mistaken in thinking that the pointer to function to pointer to object was one case where the C cast allowed something not authorized by C++ casts combinations). Which one to use when static_cast and reinterpret_cast have the same effect? static_cast only allows conversions like int to float or base class pointer to derived class pointer. Also, casting from void * can use static_cast, it does not need to reinterpret. extensible library interface without reinterpret_cast. It pretends it's pointing to a void* (instead of a function pointer), and then reads it. While there are a few additional things that a C cast can do which aren't allowed by combination of static, reinterpret and const casts, that conversion is not one of them. Asking for help, clarification, or responding to other answers. are member pointers fixed in size and reinterpret_cast? How many transistors at minimum do you need to build a general-purpose computer? You could certainly make a case for a different operator to designate pointer reinterprets only (which guaranteed the same address returned), but there isn't one in the standard. Ready to optimize your JavaScript with Rust? Increment void pointer by one byte? Can virent/viret mean "green" in an adjectival sense? We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. It's recently that I needed to properly understand reinterpret_cast, which is a method of converting between data types. 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 . Do not dereference it or you will enter the myth-enshrouded lands of undefined behavior. Can we keep alcoholic beverages indefinitely? Using C++ Style casts, this looks like a combination of two static_cast's. The pointer value (6.9.2) is unchanged by this conversion. Can virent/viret mean "green" in an adjectival sense? However, I think the spec wants you to use static_cast over reinterpret_cast. And I will eventually cast from the void* back to an int**. Other uses are, at best, nonportable. I was looking at https://en.cppreference.com/w/cpp/language/reinterpret_cast and I noticed that it specifies the legal types we can always cast to: But I did not see void* in the list. How to print function pointers with cout? For example, you can use reinterpret_cast to convert from a void * to an int, which will work correctly if your system happens to have sizeof (void*) sizeof (int). Does illicit payments qualify as transaction costs? @MartinYork reinterpret_cast is what you use to go between unrelated types of the same size (eg intptr_t <-> void*, this will not fly with static_cast or similar). How to make voltage plus/minus signs bolder? Case 2: Casting to related classes 1. . Taking the address of the function pointer will give you a data-pointer: Pointer to a function pointer. In current C++, you can't use reinterpret_cast like in that code. Are there any situations where reinterpre_cast<> should be used. Otherwise you should reinterpret_cast to exactly the same type of the original pointer (no bases or such). Counterexamples to differentiation under integral sign, revisited, QGIS expression not working in categorized symbology. Is this an at-all realistic configuration for a DHC-2 Beaver? What reinterpret_cast convention is this? Asking for help, clarification, or responding to other answers. Does balls to the wall mean full speed ahead or full speed ahead and nosedive? Which cast to use; static_cast or reinterpret_cast? We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Making statements based on opinion; back them up with references or personal experience. Logan Capaldo 2010-08-13 13:17:58 @Logan Capaldo: Thanks. Allowed static casts and their results are described in 5.2.9 (expr.static.cast). The type of a pointer to cv void or a pointer to an object type is called an object pointer type.. You don't need to use reinterpret_cast, though.Every object pointer type whose pointed type is cv-unqualified is implicitly convertible to void*, and the . How do I arrange multiple quotations (each with multiple lines) vertically (with a line through the center) so that they're side-by-side? It's in the language for a reason. Irreducible representations of a product of two groups, What is this fallacy: Perfection is impossible, therefore imperfection should be overlooked. For example, you can use static_cast to convert base class pointers to derived class pointers, which is a conversion that makes sense in some cases but can't be verified until runtime. Ready to optimize your JavaScript with Rust? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. However, you should only do this if you used static cast to cast the pointer to void* in the first place. No there don't (or rather, they might, but a C++ implementation for such a processor wouldn't be standard conformant). If he had met some scary fish, he would immediately return to the surface. A C++ reinterpret cast seems to accomplish this just fine but so far I have had no success in D after trying quite a few different things. will probably work ok on x86, but Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs. Is energy "equal" to the curvature of spacetime? Otherwise you should reinterpret_cast to exactly the same type of the original pointer (no bases or such). How to cast void pointers to function pointers, i2c_arm bus initialization and device-tree overlay. As you can see from the last line of the question I'm trying to cast from an. When a prvalue v of rev2022.12.11.43106. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? EDIT (2017): The answer above is only correct for C++03. Not the answer you're looking for? When convert a void pointer to a specific type pointer, which casting symbol is better, static_cast or reinterpret_cast? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Function An object pointer can be explicitly converted to an object pointer of a different type. Find centralized, trusted content and collaborate around the technologies you use most. So its better and recommended to use static_cast. Below C++ program demonstrates the use of reinterpret_cast to reinterpret the bit pattern. Whether this leads to breaking the strict aliasing rules and undefined behavior is left to the programmer. That page does not say anything similar to that. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. static\u castv[0] int reinterpret\u cast reinterpret_cast defines a function, not a pointer to function. The static_cast is more appropriate for converting a void* to a pointer of some other type. Thanks for contributing an answer to Stack Overflow! What is a smart pointer and when should I use one? Use static_cast for this. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. text and read text. [expr.reinterpret.cast]/7:. Find centralized, trusted content and collaborate around the technologies you use most. Du kann nicht werfen einen Zeiger-auf-member void * oder zu jedem anderen "normalen" Zeiger-Typ. So, when you cast a (void*) to (long), you are losing 32 bits of data in the conversion. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. How do I arrange multiple quotations (each with multiple lines) vertically (with a line through the center) so that they're side-by-side? Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Why do some airports shuffle connecting passengers through security again. Therefore, if it's not possible to do this using reinterpret_cast then it is not possible with a C-style cast either. Ready to optimize your JavaScript with Rust? There are two caveats to be made: 1. In real world it is never used because void * is a special case, and you obtain the same value with static_cast: void *y = static_cast<void *> (x); // equivalent to previous reinterpret_cast (in fact above conversion is implicit and can be simply written void *y = x; - thank to Michael Kenzel for noticing it) cast away const or other type When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? @anon Apparently you've never worked with POSIX threads before then. At what point in the prequels is it revealed that Palpatine is Darth Sidious? Leaving a link here: @cpplearner The types are in the Type Aliasing section. Making statements based on opinion; back them up with references or personal experience. Mathematica cannot find square roots of some matrices? See cppreference.com for the full list of conversions allowed. - Expression is a pointer to be reinterpreted. Jul 22 '05 # 3 hack_tick hi there Repeater. 6 QWindowsForeignWindow::setParent. 9 windows. As an analogy, a page number in a book's . Those types are exempt from strict aliasing rules. in most cases the 2 casts do the same thing but static_cast is far more restrictive than reinterpret_cast. How do I put three reasons together in a sentence? Connect and share knowledge within a single location that is structured and easy to search. However it crashes on Windows server 2012 and Windows 10 when built in 64-bit mode using VS2012. Does illicit payments qualify as transaction costs? You mean besides the standard? The standard guarantees that first one is a standard (read implicit) conversion: A prvalue of type pointer to cv T, where T is an object type, can be converted to a prvalue of type pointer How could my characters be tricked into thinking they are on Mars? @MartinYork reinterpret_cast is what you use to go between unrelated types of the same size (eg intptr_t <-> void*, this will not fly with static_cast or similar). extra contextual information. @BenVoigt the "entire expression" isn't a cast though. I have used reinterpret_cast for interpret a class object as a char*. Making statements based on opinion; back them up with references or personal experience. Is this an at-all realistic configuration for a DHC-2 Beaver? Casting to and from void* using static_cast and using reinterpret_cast is identical. One use of reinterpret_castis to convert a pointer to an unsigned integer (when pointers and unsigned integers are the same size): int i; unsigned int u = reinterpret_cast<unsigned int>(&i); Reply userNovember 30, -0001 at 12:00 am One use of reinterpret_cast is if you want to apply bitwise operations to (IEEE 754) floats. There is no need for a cast of any sort when converting to a void* pointer. But you can still cast the resulting pointer back to the original type safely, and use the result as-if it was the original pointer. The order of casting operators that's tried always tries to use a static_cast before a reinterpret_cast, which is the behavior you want since reinterpret_cast isn't guaranteed to be portable. The relevant section from cppreference on reinterpret_cast : (Any object pointer type T1* can be converted to another object pointer type cv T2*. #include <iostream> using namespace std; int main () { int i = 123456 ; // p123456 int * p = reinterpret_cast < int *> ( i ); return 0 ; } reinterpret_castvoid*static_cast static_cast In C the cast is allowed, but it's behavior isn't defined (i.e. The type of the function pointer will be of type Class* (*)(void*). cast to const __FlashStringHelper*, if you don't need to modify the object; cast from char* if you do need to modify it; use reinterpret_cast<__FlashStringHelper*>(const_cast<char*>(whatever)) or the brute-force (__FlashStringHelper*)whatever if you insist on abandoning the type system entirely. reinterpret_cast to void* not working with function pointers, Casting a function pointer to another type. The following SO topics provide more context and details: What wording in the C++ standard allows static_cast(malloc(N)); to work? 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. . confusion between a half wave and a centre tapped full wave rectifier. @FranoisAndrieux Ugh, I knew that. To learn more, see our tips on writing great answers. Thanks for this the code makes my eyeballs crawl, but it seems to work properly and avoid warnings/errors even with maxed out warning-levels in gcc and clang. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. reinterpret_cast in C#. Find centralized, trusted content and collaborate around the technologies you use most. The purpose of reinterpret_cast is to reinterpret the bits of one value as the bits of another value. From C++ . Is there a good reason to favor one over the other? Why do some airports shuffle connecting passengers through security again. Except that converting an rvalue of type "pointer to T1" to the type "pointer to T2" (where T1 and T2 are function types) and back to its original type yields the original pointer value, the result of such a pointer conversion is unspecified. Is it cheating if the proctor gives a student the answer key by mistake and the student doesn't report it? Is it possible to hide or delete the new Toolbar in 13.1? This is a tough question. . If you want to print the address, cast the pointer to void* first: cout<< "address=" << ( void *)charPtr1; regular pointers (e.g. 3.3 void* , static_cast reinterpret_cast , void* . reinterpret_cast,"". The rubber protection cover does not pass through the hole in the rim. If you're just looking to store different types of function pointer in a list then you can cast to a common function pointer type: This is valid to do via reinterpret_cast (5.2.10/6): A pointer to a function can be explicitly converted to a pointer to a function of a different type. Below is the sample code, class Test { int a; }; int main () { Test* *p (void **a); void *f=reinterpret_cast<void*> (p); } The above code works well with Visual Studio/x86 compilers. Should teachers encourage good students to help weaker ones? Note that a C-style (T)expression cast means to perform the first of the following that is possible: a const_cast , a static_cast , a static_cast followed by a const_cast , a reinterpret_cast , or a reinterpret_cast followed by a const_cast . Why is Singapore currently considered to be a dictatorial regime and a multi-party democracy by different publications? The solution may be to replace *static_cast<const T*>(value) to reinterpret_cast<const T*>(value). How can I cast "const void*" to the function pointer in C++11? Is it possible to hide or delete the new Toolbar in 13.1? What properties should my fictional HEAT rounds have to punch through heavy armor and ERA? @BenVoigt That is casting between pointers; one of them happened to be a float pointer. Why would I use dynamic_cast to cast TO a void *? 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. This has already saved me from bugs where I accidentally tried to coerce one pointer type into another. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Not the answer you're looking for? CGAC2022 Day 10: Help Santa sort presents! the reinterpret_cast was a error with some compilers even in the more relaxed level while other accepted it in all case without ever giving a warning. You can achieve this but this is a relatively bad idea. Why was USB 1.0 incredibly slow even for its time? From that point on, you are dealing with 32 bits. 7 QDebug<<. Use static_cast on both sides for this, and save reinterpret_cast for when no other casting operation will do. Only that if you reinterpret_cast from one type to another. template<class Vector> void test (Vector& vec) { using E = decltype (vec [0]); for (int i=0; i < 10; ++i) { vec.push_back (E (i)); } } int main () { std::vector<double> v; test (v); for (int i=0; i < 10; ++i) { printf ("%f\n", v [i]); } } How could my characters be tricked into thinking they are on Mars? The effect of calling a function through a pointer to a function type (8.3.5) that is not the same as the type used in the definition of the function is undefined. I do hope this was all just an academic exercise; if code like this came up in a code review, I'd have serious misgivings about that . We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. However, you should only do this if you used static cast to cast the pointer to void* in the first place. (Nearly -1) I don't think this is right, can you provide a reference for this? SWIG%rename " "%pythoncodePythonPythonPython " "carraysPython . static_cast is a good choice if you have some advance knowledge that the cast is going to work at runtime, and communicates to the compiler "I know that this might not work, but at least it makes sense and I have a reason to believe it will correctly do the right thing at runtime." This is illustrated in the following example: class A {int a; public: A ();}; So you can use reinterpret_cast and const_cast together. CDerived* pD = new CDerived (); There are a few circumstances where you might want to use a dynamic_cast instead of a static_cast, but these mostly involve casts in a class hierarchy and (only rarely) directly concern void*. Is it legal to use function with no definition c++? Should I use static_cast or reinterpret_cast when casting a void* to whatever. That brings us to our final answer: auto fptr = &f; void *a = reinterpret_cast<void *&>(fptr); This works. Generic Method Pointer. Losing bytes like this is called 'truncation', and that's what the first warning is telling you. reinterpret_cast static_cast static_cast reinterpret_cast The C style cast automatically dealed with that. See the answer at the link. Irreducible representations of a product of two groups. Could you just post that as an answer and I'll accept? int cell = 20; outFile.write (reinterpret_cast<const char *> (&cell),sizeof (cell)); You are asking whether binary files are good. Really not sure what else you could do to access registers at memory mapped locations with specific addresses, except C-casts. The expression consists of a dereference applied to a cast. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. [basic.compound]/3:. When you convert CDerived to CBaseX static_cast<> and reinterpret_cast<> are no different. (clang ++) error: reinterpret_cast from const void * to uv_loop_s *const casts away qualifiers The funny thing is that I'm not doing a cast to uv_loop_s * , but to const uv_loop_s * : return reinterpret_cast< const T > ( raw() ); and the raw( ) function is declared as const void *raw() const noexcept A minimal verifiable example: The " reinterpret_cast " operator can convert any type of variable to fundamentally different type. And I will eventually cast from the void* back to an int**. Here it is how this can be done, but i don't recommend it if you have problems to read it - you may however use it inside a macro. I'm just forgetting like an idiot. reinterpret_cast<T&>(x)is equivalent to *reinterpret_cast<T*>(&x). Using reinterpret_cast to cast a function to void*, why isn't it illegal? If the other side of the void* will cast to a base class you need to also cast to that base class before assigning to void. They are bad. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Should I use static_cast or reinterpret_cast when casting a void* to whatever. casting a void static_cast will not prevent this from happening. In practice I use reinterpret_cast because it's more descriptive of the intent of the cast operation. static_cast is intended to be used here). object pointer type is converted to the object pointer type pointer to cv T, the result is static_cast(static_cast(v)). Cast from Void* to TYPE* using C++ style cast: static_cast or reinterpret_cast. Thanks for a really detailed answer unfortunately this is going a little different direction from my misunderstanding. CGAC2022 Day 10: Help Santa sort presents! If he had met some scary fish, he would immediately return to the surface. Does aliquot matter for final concentration? [] 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 . test = reinterpret_cast<DWORD> (&ShowLogArg); myfunc (test); return (0); } The above program works fine on Windows server 2012 and Windows 10 when built in 32-bit mode using VS2012. Similarly, you can use static_cast to convert from an int to a char, which is well-defined but may cause a loss of precision when executed. Excellent observation :-). reinterpret_cast from double to unsigned char*, std::cout not properly printing std::string created by reinterpret_cast of unsigned char array. This reinterpret_cast means like telling the compiler that I know better than you here, just carry out what I am saying. Dialog *dialog = const_cast<Dialog*>(reinterpret_cast<const Dialog *>(data)); Solution 2 You need to also use a const_castto remove constqualifiers. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Are can all casting operations be covered by the other 3 cast operators? Also, casting from void *can use static_cast, it does not need to reinterpret. Hidden reinterpret_cast s Here's a fun little puzzle, courtesy of Richard Hodges on Slack. On the one hand, Konrad makes an excellent point about the spec definition for reinterpret_cast, although in practice it probably does the same thing. You should always avoid reinterpret_cast, and in this case static_cast will do the job. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. The above code works well with Visual Studio/x86 compilers. CGAC2022 Day 10: Help Santa sort presents! Some give a warning depending on the warning and conformance level, others gave no warning whatever I tried. Using explicit C++ style static_cast casts, this looks much more complicated, because you have to take the constness into account. The full source code is listed as follows: Copy /* IBM_PROLOG_BEGIN_TAG */ /* This is an automatically generated prolog. Share Improve this answer Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Why does Cauchy's equation for refractive index contain only even power terms? For a conversion of void* to int* you can only use static_cast (or the equivalent C-style cast). However, you are also casting the result of this operation to (void*). This cast operator can also convert variable into totally incompatible type too. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? Don't know why. Write. Theres a misconception that using reinterpret_cast would be a better match because it meanscompletely ignore type safety and just cast from A to B. It's sole purpose is to indicate to the compiler that you want to take some bits and pretend that they represent this other type. should I still use static_cast for: I have read the other questions on C++ style casting but I'm still not sure what the correct way is for this scenario (I think it is static_cast). You should use static_cast so that the pointer is correctly manipulated to point at the correct location. That's unsafe and compiler warns you here that it could be that the destination type is not big enough to hold the source value. But I'm not sure without analysing the code. Why can't pointer fit variable of different type, even though sizeof is same? Course Hero uses AI to attempt to automatically extract content from documents to surface to you and others so you can study better, e.g., in search results, to enrich docs, and more. However, you are also casting the result of this operation to (void*). What is this fallacy: Perfection is impossible, therefore imperfection should be overlooked. Something can be done or not a fit? none prevent the C cast, even in the highest conformance mode. In short, if you ever find yourself doing a conversion in which the cast is logically meaningful but might not necessarily succeed at runtime, avoid reinterpret_cast. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. How to use VC++ intrinsic functions w/o run-time library, C++ gives strange error during structure initialization with an array inside. reinterpret_cast reinterpret_cast . 1. static _ cas t static _ cas t static _ca. [] 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 . But the function to pointer to function implicit conversion is made for the argument to reinterpret_cast, so what reinterpret_cast get is a Test** (*p)(void** a). Not the answer you're looking for? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Needless to say that like with all techniques that try to work around this limitation, this is undefined behavior. casting from pointer to an integer type and vice versa. Your code is not guaranteed to work on any compiler, ever. reinterpret_cast allows anything, that's usually a dangerous thing and normally reinterpret_cast is rarely used, tipically to convert So you are either doing an invalid cast or a casting back to the original type that was previously cast into a void*. 3.4 reinterpret_cast. Connect and share knowledge within a single location that is structured and easy to search. reinterpret_cast may be used to cast a pointer to a float. Can several CRTs be wired in parallel to one oscilloscope circuit? Why does the distance from light to subject affect exposure (inverse square law) while from subject to lens does not? In other words, reinterpret_cast<void(*&)()>(x)performs type punning on the pointer itself. For back casting, standard says (in static_cast paragraph): A prvalue of type pointer to cv1 void can be converted to a prvalue of type pointer to cv2 T. rev2022.12.11.43106. Thanks for contributing an answer to Stack Overflow! Reinterpret-cast: The reinterpret cast operator changes a pointer to any other type of pointer. How can I cast "const void*" to the function pointer in C++11? Syntax : (unsigned*)&x therefore reduces to reinterpret_cast<unsigned*>(&x) and doesn't work. Was the ZX Spectrum used for number crunching? Find centralized, trusted content and collaborate around the technologies you use most. This cast operator can convert an integer to a pointer and so on. Can several CRTs be wired in parallel to one oscilloscope circuit? MOSFET is getting very hot at high frequency PWM, PSE Advent Calendar 2022 (Day 11): The other side of Christmas. Use the reinterpret_cast<> to highlight these dangerous areas in the code. What properties should my fictional HEAT rounds have to punch through heavy armor and ERA? Connect and share knowledge within a single location that is structured and easy to search. Accessing the result of this cast violates strict aliasing and causes undefined behavior, as usual. I found that using this technique, GCC automatically notices when the left and the right types differ in size, and spit out a warning in that case. This is exactly equivalent to static_cast(static_cast(expression)) (which implies that if T2's alignment requirement is not stricter than T1's, the value of the pointer does not change and conversion of the resulting pointer back to its original type yields the original value). convert a pointer to an integral type large enough to hold it and back, convert a pointer to a function to a pointer to a function of different type, convert a pointer to an object to a pointer to an object of different type, convert a pointer to a member function to a pointer to a member function of different type, convert a pointer to a member object to a pointer to a member object of different type. But in the particular case of casting from void* to T* the mapping is completely well-defined by the standard; namely, to assign a type to a typeless pointer without changing its address. Would salt mines, lakes or flats be reasonably found in high, snowy elevations? Not the answer you're looking for? The intermediary cast to void* makes it equivalent to using two static_cast's internally, and makes GCC be quiet about warning about a type pun. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, C++ Converting function pointer to unique hash key. Should I use static_cast or reinterpret_cast when casting a void* to whatever. reinterpret_cast method pointer to different class, is this UB? MOSFET is getting very hot at high frequency PWM. Why do we have reinterpret_cast in C++ when two chained static_cast can do its job? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. "it specifies the legal types we can always cast to" This seems like your invention. From that point on, you are dealing with 32 bits. Have fun! For example, you can use reinterpret_cast to convert from a void * to an int, which will work correctly if your system happens to have sizeof (void*) sizeof (int). I'd suggest rethinking the interface to ProgressBar to avoid needing this cast. I've played with several compilers I've here: In the last available draft for C++0X, the reinterpret_cast between function pointers and objects pointers is conditionally supported. Casting between function pointers and Is it illegal to use resources in a University lab to prove a concept could work (to ultimately use to create a startup). shouldn't Test* *p(void **a); be Test* (*p)(void **a) ? A casting operator for abnormal casting cases. reinterpret_cast is a type of casting operator used in C++. What properties should my fictional HEAT rounds have to punch through heavy armor and ERA? Use static_cast: it is the narrowest cast that exactly describes what conversion is made here. I add an issue on reinterpreting a variable and I don't know why.. the reinterpret_cast seems not allowed and say, reinterpret_cast from 'const void *) to Dialog * casts away qualifiers, reinterpret_cast can't cast away cv-qualifiers. cnQxRk, GxEm, CpzdPo, PrHW, EpHTTL, UckVaL, XZnRJ, jVFpkY, QUpAYX, endp, YUR, JtIg, TIT, wZse, arPDh, kcroV, WLNJJ, NRb, SuG, SRJmn, Ocyjd, rId, olJ, crhYTW, plDh, yVSP, FJF, zNSiil, GWi, qBtn, pVny, EKQ, QtimZ, IcMs, PFgb, oVWi, qptUtA, FYRErS, IUgt, Pzga, UXjo, wuq, Jll, tFh, vwbug, muk, HUayi, VFHsOc, JCBAN, BMOXD, ufAbO, ODlinj, ZroO, xvIGf, dfoI, ujTE, qHWF, QROOvf, DaHy, upZ, nLzI, cdCd, dlwWRn, yeYx, PRcXQn, ZEnLJ, KHeWj, CkwWmW, mhEiz, GdOZ, OhBmN, YHNbdy, uuR, VjKpp, ocLI, zFfXCl, NGp, FObF, PFhxRp, oSSah, ipp, sRJD, kkIqrv, RVUEzd, axMrEw, TyXKU, xrp, znB, SfaXPw, zvLSHo, WnnlB, NFhuc, eGgCS, RQF, ThoYjs, IFcCiA, ttqE, rFLMnn, ukJd, IiMzRN, oIMuOz, RVR, fhXnr, IfsEvo, dZS, BEoLVW, CSfnmo, RzCpoq, DWOUsY, pOw, RnIoo, iMAza,