Raw memory access like this is not type-safe and can only be done under a full trust security environment. 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). 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 . Find centralized, trusted content and collaborate around the technologies you use most. Asking for help, clarification, or responding to other answers. Not the answer you're looking for? Can virent/viret mean "green" in an adjectival sense? How can I use a VPN to access a Russian website that is banned in the EU? MOSFET is getting very hot at high frequency PWM, PSE Advent Calendar 2022 (Day 11): The other side of Christmas. How many transistors at minimum do you need to build a general-purpose computer? Why do some airports shuffle connecting passengers through security again. The type of the function pointer will be of type Class* (*) (void*). The paragraphs about void* are 4 and 10. I was concerned about the below explanation. extensible library interface without reinterpret_cast. A casting operator for abnormal casting cases. However, you should only do this if you used static cast to cast the pointer to void* in the first place. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. So its better and recommended to use static_cast. */ /* */ /* $Source: src/runtime/rt_vfs.C . cast away const or other type When would I give a checkpoint to my D&D party that they can return to if they die? 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. Could you just post that as an answer and I'll accept? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. static_cast only allows conversions like int to float or base class pointer to derived class pointer. void(*println_ptr)(Printer*const, const char*) = reinterpret_cast<void(*)(Printer*const, const char*)>(&Printer::println); Last edited on . - type is a pointer reinterpreted as. 9 windows. Did neanderthals need vitamin C from the diet? Use the reinterpret_cast<> to highlight these dangerous areas in the code. Every object pointer type whose pointed type is cv-unqualified is implicitly convertible to void*, and the inverse can be done by static_cast. reinterpret_cast, on the other hand, is a casting operator designed to do conversions that are fundamentally not safe or not portable. Casting to and from void* using static_cast and using reinterpret_cast is identical. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. 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. Irreducible representations of a product of two groups. Should I use static_cast or reinterpret_cast when casting a void* to whatever. To learn more, see our tips on writing great answers. My use case requires a reinterpret_cast because I'm casting from an int** to a void*. 3.5 int reinterpret_cast. . SWIG%rename " "%pythoncodePythonPythonPython " "carraysPython . Connect and share knowledge within a single location that is structured and easy to search. reinterpret_cast allows anything, that's usually a dangerous thing and normally reinterpret_cast is rarely used, tipically to convert Connect and share knowledge within a single location that is structured and easy to search. 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); }; You need to also use a const_cast to remove const qualifiers. Const_cast: The const_cast operator is used to explicitly override const and/or . If you want to print the address, cast the pointer to void* first: cout<< "address=" << ( void *)charPtr1; What is a smart pointer and when should I use one? When convert a void pointer to a specific type pointer, which casting symbol is better, static_cast or reinterpret_cast? Using C++ Style casts, this looks like a combination of two static_cast's. How can I cast "const void*" to the function pointer in C++11? rev2022.12.11.43106. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Why would I use dynamic_cast to cast TO a void *? reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . reinterpret_cast<char*>(&x)int 10910 reinterpret_cast reinterpret_cast . Not the answer you're looking for? "it's the cast from the void* to the int** that requires a reinterpret_cast" is wrong. You can achieve this but this is a relatively bad idea. Add a new light switch in line with another switch? Otherwise you should reinterpret_cast to exactly the same type of the original pointer (no bases or such). Ready to optimize your JavaScript with Rust? This is usually the case on POSIX compatible systems because dlsym() is declared to return void *, and clients are expected to reinterpret_cast it to the correct function pointer type. I want to reinterpret cast a function pointer into a void* variable. will probably work ok on x86, but How to make voltage plus/minus signs bolder? I've reopened the question because I didn't see only the top answer applies to this. Is it possible to hide or delete the new Toolbar in 13.1? Should I use static_cast or reinterpret_cast when casting a void* to whatever. What is this fallacy: Perfection is impossible, therefore imperfection should be overlooked. EDIT (2017): The answer above is only correct for C++03. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? The following SO topics provide more context and details: What wording in the C++ standard allows static_cast(malloc(N)); to work? Repeater. Something can be done or not a fit? Was Sie wahrscheinlich tun mssen, ist, wickeln Sie Ihre member-Funktion in einem regulren Funktion. reinterpret_cast can't cast away cv-qualifiers So you can use reinterpret_castand const_casttogether. 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 . CGAC2022 Day 10: Help Santa sort presents! 7 QDebug<<. So you are either doing an invalid cast or a casting back to the original type that was previously cast into a void*. 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*. [] 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 . 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. 3.4 reinterpret_cast. Only in the rarest of rare cases when there is no other way use reinterpret_cast. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? Books that explain fundamental chess concepts. It's my understanding that C-style cast is defined in terms of the new cast styles. @BenVoigt That is casting between pointers; one of them happened to be a float pointer. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. If he had met some scary fish, he would immediately return to the surface. 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. reninterpret_cast does not check if the pointer type and data pointed by the pointer is same or not. From C++ . Your code is not guaranteed to work on any compiler, ever. When you convert CDerived to CBaseX static_cast<> and reinterpret_cast<> are no different. How to print function pointers with cout? 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. 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). Other uses are, at best, nonportable. This But I'm not sure without analysing the code. 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. @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). 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. Have fun! In practice I use reinterpret_cast because it's more descriptive of the intent of the cast operation. (Nearly -1) I don't think this is right, can you provide a reference for this? What properties should my fictional HEAT rounds have to punch through heavy armor and ERA? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Japanese girlfriend visiting me in Canada - questions at border control? Would salt mines, lakes or flats be reasonably found in high, snowy elevations? Losing bytes like this is called 'truncation', and that's what the first warning is telling you. Which one to use when static_cast and reinterpret_cast have the same effect? defines a function, not a pointer to function. Why should I use a pointer rather than the object itself? Why use static_cast(x) instead of (int)x? static\u castv[0] int reinterpret\u cast reinterpret_cast It is efficient because it does not copy the value. Using explicit C++ style static_cast casts, this looks much more complicated, because you have to take the constness into account. (unsigned*)&x therefore reduces to reinterpret_cast<unsigned*>(&x) and doesn't work. They are bad. is it better than static_cast? are member pointers fixed in size and reinterpret_cast? Some POSIX functions need the conversion to be well useful. 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. @anon Apparently you've never worked with POSIX threads before then. char *charPtr1=reinterpret_cast<char*> (intPtr1); cout<<"address="<<charPtr1<<";c="<<*charPtr1<<endl; There's an overload of operator<< (char*), which assumes the pointer points to a NUL-terminated string, and prints that string. From that point on, you are dealing with 32 bits. void is not included here because you can never dereference a void *. reinterpret_cast is a tricky beast. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Not the answer you're looking for? reinterpret_cast in C#. I want to reinterpret cast a function pointer into a void* variable. You don't need to use reinterpret_cast, though. However, you are also casting the result of this operation to (void*). All you need is a single static_cast: We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Connect and share knowledge within a single location that is structured and easy to search. qualifiers, I read the explanation in Casting a function pointer to another type. Otherwise you should reinterpret_cast to exactly the same type of the original pointer (no bases or such). When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? An object pointer can be explicitly converted to an object pointer of a different type. casting a void Dialog *dialog = const_cast<Dialog*>(reinterpret_cast<const Dialog *>(data)); Solution 2 You need to also use a const_castto remove constqualifiers. Why was USB 1.0 incredibly slow even for its time? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. 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). Leaving a link here: @cpplearner The types are in the Type Aliasing section. This has already saved me from bugs where I accidentally tried to coerce one pointer type into another. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Should I use static_cast or reinterpret_cast when casting a void* to whatever. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. like int to pointer and pointer to int etc. Can virent/viret mean "green" in an adjectival sense? See the answer at the link. This cast operator can also convert variable into totally incompatible type too. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Using reinterpret_cast to do this with pointer conversions completely bypasses the compile-time safety check. confusion between a half wave and a centre tapped full wave rectifier. This rule bans (T)expression only when used to perform an unsafe cast. How do I arrange multiple quotations (each with multiple lines) vertically (with a line through the center) so that they're side-by-side? shouldn't Test* *p(void **a); be Test* (*p)(void **a) ? I have used reinterpret_cast for interpret a class object as a char*. In C++11 through C++17, it is implementation defined if conversions between function pointers and void * are allowed. 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. To learn more, see our tips on writing great answers. Can several CRTs be wired in parallel to one oscilloscope circuit? reinterpret_cast from double to unsigned char*, std::cout not properly printing std::string created by reinterpret_cast of unsigned char array. You likely obtained that void* with implicit conversion, so you should use static_cast because it is closest to the implicit conversion. 3.3 void* , static_cast reinterpret_cast , void* . reinterpret_cast,"". 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. 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. 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. - reinterpret_cast is a keyword. Needless to say that like with all techniques that try to work around this limitation, this is undefined behavior. I don't see any rule in the standard allowing this conversion. object pointer type is converted to the object pointer type pointer to cv T, the result is static_cast(static_cast(v)). Case 2: Casting to related classes 1. Find centralized, trusted content and collaborate around the technologies you use most. 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. Below C++ program demonstrates the use of reinterpret_cast to reinterpret the bit pattern. @BenVoigt the "entire expression" isn't a cast though. Is this an at-all realistic configuration for a DHC-2 Beaver? 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). Once a pointer has degenerated into a void* you can static_cast it to any type of pointer. .. Can we keep alcoholic beverages indefinitely? Putting a space in ". You should use static_cast so that the pointer is correctly manipulated to point at the correct location. 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. (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: size as regular pointers, since on Also, casting from void * can use static_cast, it does not need to reinterpret. none prevent the C cast, even in the highest conformance mode. Why do we have reinterpret_cast in C++ when two chained static_cast can do its job? 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. target-type is the target of the cast whereas expr is being cast into the new target-type. Here: CGAC2022 Day 10: Help Santa sort presents! 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.) 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. Using reinterpret_cast to cast a function to void*, why isn't it illegal? 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. This cast operator can convert an integer to a pointer and so on. 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) Was the ZX Spectrum used for number crunching? Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. reinterpret_cast<T&>(x)is equivalent to *reinterpret_cast<T*>(&x). Thanks for contributing an answer to Stack Overflow! reinterpret_cast may be used to cast a pointer to a float. Function @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). It does not check if the pointer type and data pointed by the pointer is same or not. to cv void. At what point in the prequels is it revealed that Palpatine is Darth Sidious? To learn more, see our tips on writing great answers. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. reinterpret_cast casts away const qualifier? I usually do the following, which is doing a type pun, and is the recommended way to do it, according to the manpage of dlopen (which is about doing the converse - casting from void* to a function pointer). Is this an at-all realistic configuration for a DHC-2 Beaver? 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. Find centralized, trusted content and collaborate around the technologies you use most. 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. How can I cast "const void*" to the function pointer in C++11? remember that it's undefined behavior. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Only that if you reinterpret_cast from one type to another. Why is processing a sorted array faster than processing an unsorted array? Is it possible to hide or delete the new Toolbar in 13.1? 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. Find centralized, trusted content and collaborate around the technologies you use most. Are can all casting operations be covered by the other 3 cast operators? 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. I'm just forgetting like an idiot. static_cast will not prevent this from happening. When a prvalue v of static_cast is intended to be used here). [expr.reinterpret.cast]/7:. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. How to do such conversions from void (*)(void*) -> void* effectively so that atleast it compiles almost the same in most of the compilers ? 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 . Is there a good reason to favor one over the other? What properties should my fictional HEAT rounds have to punch through heavy armor and ERA? reinterpret_cast, on the other hand, is a casting operator designed to do conversions that are fundamentally not safe or not portable. CDerived* pD = new CDerived (); It pretends it's pointing to a void* (instead of a function pointer), and then reads it. Asking for help, clarification, or responding to other answers. The reinterpret_cast<> must be used carefully. In C the cast is allowed, but it's behavior isn't defined (i.e. How do I arrange multiple quotations (each with multiple lines) vertically (with a line through the center) so that they're side-by-side? Why is Singapore currently considered to be a dictatorial regime and a multi-party democracy by different publications? What properties should my fictional HEAT rounds have to punch through heavy armor and ERA? The rubber protection cover does not pass through the hole in the rim. It's recently that I needed to properly understand reinterpret_cast, which is a method of converting between data types. And I will eventually cast from the void* back to an int**. 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. 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. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. Why do some airports shuffle connecting passengers through security again. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. 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. For ex, you could typecast an myclass* to void* and then use reinterpret_cast to convert it to yourclass* which may have a completely different layout. Which cast to use; static_cast or reinterpret_cast? The relevant section from cppreference on reinterpret_cast : (Any object pointer type T1* can be converted to another object pointer type cv T2*. (*)(void) to a void*). What happens if you score more than 99 points in volleyball? Did neanderthals need vitamin C from the diet? So why have reinterpret_cast<>? Just be so kind as to only use it for comparison, as key in a hash map or similarly innocent things. (175) QT0-5qimageqpainter . casting from pointer to an integer type and vice versa. Syntax : reinterpret_cast <type> (Expression) reinterpret_cast performs a low level reinterpretation of the bit pattern of its operands. Hidden reinterpret_cast s Here's a fun little puzzle, courtesy of Richard Hodges on Slack. 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 . example how; so does the tinyxml project. Does balls to the wall mean full speed ahead or full speed ahead and nosedive? How many transistors at minimum do you need to build a general-purpose computer? Thanks for contributing an answer to Stack Overflow! How do I put three reasons together in a sentence? 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. in most cases the 2 casts do the same thing but static_cast is far more restrictive than reinterpret_cast. Can several CRTs be wired in parallel to one oscilloscope circuit? [basic.compound]/3:. (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.) 1. static _ cas t static _ cas t static _ca. Syntax : Mathematica cannot find square roots of some matrices? Was the ZX Spectrum used for number crunching? 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). Find centralized, trusted content and collaborate around the technologies you use most. reinterpret_cast means like telling the compiler that I know better than you here, just carry out what I am saying. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. 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. Is energy "equal" to the curvature of spacetime? The Boost Serialization library has a good. Why does the distance from light to subject affect exposure (inverse square law) while from subject to lens does not? reinterpret_cast is very much standard C++. This will allow you to cast it to void*. Allowed static casts and their results are described in 5.2.9 (expr.static.cast). However, I think the spec wants you to use static_cast over reinterpret_cast. @FranoisAndrieux Ugh, I knew that. This means in particular that a cast from a pointer to function to void * is not possible, but you can cast it to void(*)(). Making statements based on opinion; back them up with references or personal experience. 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). Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. As an analogy, a page number in a book's . I use reinterpret_cast to convert integer memory addresses to pointers to overlay structs. CGAC2022 Day 10: Help Santa sort presents! From that point on, you are dealing with 32 bits. Does illicit payments qualify as transaction costs? When is casting void pointer needed in C? Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Should teachers encourage good students to help weaker ones? Connect and share knowledge within a single location that is structured and easy to search. How do I arrange multiple quotations (each with multiple lines) vertically (with a line through the center) so that they're side-by-side? 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. (void*) &d); } Since the arrays are reference types and hold their own metadata about their type you cannot reinterpret them without overwriting the . Others pointed out you cannot do that cast (strongly speaking, casting to void* anything using reinterpret_cast is also not allowed - but silently tolerated by the compilers. Does aliquot matter for final concentration? Cast from Void* to TYPE* using C++ style cast: static_cast or reinterpret_cast. reinterpret_cast to void* not working with function pointers, Casting a function pointer to another type. To clarify: what the author means here by ", @curiousguy Not true according to the standard. 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:. Accessing the result of this cast violates strict aliasing and causes undefined behavior, as usual. Really not sure what else you could do to access registers at memory mapped locations with specific addresses, except C-casts. Are the S&P 500 and Dow Jones Industrial Average securities? reinterpret_cast, on the other hand, is a casting operator designed to do conversions that are fundamentally not safe or not portable. 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. It does not mean they are the only type you can use with reinterpret_cast. rev2022.12.11.43106. regular pointers (e.g. C++. Irreducible representations of a product of two groups, What is this fallacy: Perfection is impossible, therefore imperfection should be overlooked. reinterpret_cast static_cast static_cast reinterpret_cast 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. The type of a pointer to cv void or a pointer to an object type is called an object pointer type. . But usually static_cast is preferred because it is more narrow and in general (but not in this specific case) more safe conversion. static _ cas t const _ cas tre interp ret_ cas t dynamic _ cas t. kingsfar . 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. The type of the function pointer will be of type Class* (*)(void*). In case of char*, I'd use c-style cast, until we have some reinterpret_pointer_cast, because it's weaker and nothing else is sufficient. This is illustrated in the following example: class A {int a; public: A ();}; Excellent observation :-). 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. #include<iostream> float fastInvSqrt( float x ) { const int INV_SQRT_N = 1597292357; const float MULT = 1.000363245811462f; float const mx = 0.5f * MULT * x; Is it legal to use function with no definition c++? Use static_cast on both sides for this, and save reinterpret_cast for when no other casting operation will do. Asking for help, clarification, or responding to other answers. Some give a warning depending on the warning and conformance level, others gave no warning whatever I tried. Ready to optimize your JavaScript with Rust? Asking for help, clarification, or responding to other answers. Ready to optimize your JavaScript with Rust? Du kann nicht werfen einen Zeiger-auf-member void * oder zu jedem anderen "normalen" Zeiger-Typ. The solution may be to replace *static_cast<const T*>(value) to reinterpret_cast<const T*>(value). Error: #694: reinterpret_cast cannot Counterexamples to differentiation under integral sign, revisited, i2c_arm bus initialization and device-tree overlay. For example, when using a C-style cast, as in. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Also, casting from void *can use static_cast, it does not need to reinterpret. Would salt mines, lakes or flats be reasonably found in high, snowy elevations? Don't know why. Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs. However, this doesnt actually describe the effect of a reinterpret_cast. ( reinterpret_cast does not perform the adjustments that might be needed.) 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]); } } confusion between a half wave and a centre tapped full wave rectifier. 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. Ready to optimize your JavaScript with Rust? 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. MOSFET is getting very hot at high frequency PWM. c++ reinterpret_cast unique_ptr * unique_ptr * c++ Using reinterpret_cast to cast unique_ptr * to unique_ptr * for creating a transformable tree structure The above code works well with Visual Studio/x86 compilers. CGAC2022 Day 10: Help Santa sort presents! 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. I suggest using the weakest possible cast always. Use static_cast: it is the narrowest cast that exactly describes what conversion is made here. 10 QGuiApplication::allWindows () 11 QSharedPointer. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. 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. How to cast void pointers to function pointers, i2c_arm bus initialization and device-tree overlay. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. 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. extra contextual information. To learn more, see our tips on writing great answers. Those types are exempt from strict aliasing rules. The full source code is listed as follows: Copy /* IBM_PROLOG_BEGIN_TAG */ /* This is an automatically generated prolog. reinterpret_cast method pointer to different class, is this UB? Counterexamples to differentiation under integral sign, revisited, QGIS expression not working in categorized symbology. 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? by two? 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 . By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? Generic Method Pointer. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. 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. See cppreference.com for the full list of conversions allowed. Japanese girlfriend visiting me in Canada - questions at border control? CbDrawIndexed *drawCmd = reinterpret_cast<CbDrawIndexed*>(mSwIndirectBufferPtr + (size_t)cmd->indirectBufferOffset ); bufferCONST_SLOT_STARTVES_POSITION Both static_cast and reinterpret_cast seem to work fine for casting void* to another pointer type. reinterpret_cast does NOT guarantee that the same address is used. pointers aren't necessarily the same On the other hand, if you're casting between pointer types (as is fairly common when indexing in memory via a char*, for example), static_cast will generate a compiler error and you'll be forced to use reinterpret_cast anyway. rev2022.12.11.43106. To learn more, see our tips on writing great answers. In other words, reinterpret_cast<void(*&)()>(x)performs type punning on the pointer itself. Making statements based on opinion; back them up with references or personal experience. 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 mean besides the standard? There is no need for a cast of any sort when converting to a void* pointer. However it crashes on Windows server 2012 and Windows 10 when built in 64-bit mode using VS2012. 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. Taking the address of the function pointer will give you a data-pointer: Pointer to a function pointer. Thanks for contributing an answer to Stack Overflow! Can several CRTs be wired in parallel to one oscilloscope circuit? Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? Reinterpret-cast: The reinterpret cast operator changes a pointer to any other type of pointer. The purpose of reinterpret_cast is to reinterpret the bits of one value as the bits of another 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. At what point in the prequels is it revealed that Palpatine is Darth Sidious? Why does the USA not have a constitutional court? You should use static_cast so that the pointer is correctly manipulated to point at the correct location. 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. In C++11, the cast to -P.S. However, you are also casting the result of this operation 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. Keyboard shortcuts: Use 'j/k' keys for keyboard navigation; Use 'Shift+S' to show/hide relevant lines; Use '?' to toggle this window int ProgressBar (const uint64_t data_sent, const uint64_t data_total, void const *const data) { Dialog *dialog = reinterpret_cast<Dialog*> (data); dialog->setValue ( (data_sent *100) / data_total); } the reinterpret_cast seems not allowed and say reinterpret_cast from 'const void *) to Dialog * casts away qualifiers Any idea c++ casting 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"? Why can't pointer fit variable of different type, even though sizeof is same? There are two caveats to be made: 1. How should I cast the result of malloc in C++? int* a = new int (); void* b = reinterpret_cast<void*> (a); int* c = reinterpret_cast<int*> (b); a and c contain the same value, but the value of b is unspecified. C++ static _ cas t dynamic _ cas t const _ cas tre interp ret_ cas t. Also, does the direction of the conversion matter. How could my characters be tricked into thinking they are on Mars? It also allows. reinterpret_cast can't be used to cast a pointer to function to a void*. 8 vscodewindows. rev2022.12.11.43106. The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. 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. No there don't (or rather, they might, but a C++ implementation for such a processor wouldn't be standard conformant). @user470379 Wowthat's the very reason I landed on this question at SO! In current C++, you can't use reinterpret_cast like in that code. #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 [] 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 . Why is Singapore currently considered to be a dictatorial regime and a multi-party democracy by different publications? For a conversion of void* to int* you can only use static_cast (or the equivalent C-style cast). some architectures they might contain And I will eventually cast from the void* back to an int**. The static_cast is more appropriate for converting a void* to a pointer of some other type. reinterpret_cast void* C castconst C () POSIX C reinterpret_cast C ++ 0X reinterpret_cast But with ARM compiler, it gives compilation error. 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]. Should teachers encourage good students to help weaker ones? If he had met some scary fish, he would immediately return to the surface. [] 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 . For a conversion between different function type pointers or between different object type pointers you need to use reinterpret_cast. MOSFET is getting very hot at high frequency PWM. 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. Understanding reinterpret_cast. The compiler can then check that the cast is between related types, reporting a compile-time error if this isn't the case. The more structure-breaking the cast is, the more attention using it requires. I'd suggest rethinking the interface to ProgressBar to avoid needing this cast. 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. That page does not say anything similar to that. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, You should at least put in an assert that, Note that p actually has type Test**(void**), and is a function, not a function pointer. As you can see from the last line of the question I'm trying to cast from an. text and read text. Casting between function pointers and Therefore, if it's not possible to do this using reinterpret_cast then it is not possible with a C-style cast either. I'm casting from an int** to a void*. Use static_cast for this. 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. 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. 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. Is this an oversight? Why does Cauchy's equation for refractive index contain only even power terms? Tim Roberts [MVP] wrote: There do exist processors on which a "char *" and an "int *" are different sizes. Don't write a binary file "because it's faster". That brings us to our final answer: auto fptr = &f; void *a = reinterpret_cast<void *&>(fptr); This works. Thanks for contributing an answer to Stack Overflow! Is it illegal to use resources in a University lab to prove a concept could work (to ultimately use to create a startup). Something can be done or not a fit? 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. Thanks for a really detailed answer unfortunately this is going a little different direction from my misunderstanding. reinterpret_cast will forcefully convert the void* to the target data type. How do I arrange multiple quotations (each with multiple lines) vertically (with a line through the center) so that they're side-by-side? So, when you cast a (void*) to (long), you are losing 32 bits of data in the conversion. 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". What reinterpret_cast convention is this? Der C++ FAQ Lite dies erklrt in einigen Details. 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. even round trip isn't guaranteed to work). 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. Are there any situations where reinterpre_cast<> should be used. In any case, the resulting pointer may only be dereferenced safely if allowed by the type aliasing rules). 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. So, when you cast a (void*) to (long), you are losing 32 bits of data in the conversion. Connect and share knowledge within a single location that is structured and easy to search. - Expression is a pointer to be reinterpreted. Ready to optimize your JavaScript with Rust? Image Processing: Algorithm Improvement for 'Coca-Cola Can' Recognition. Increment void pointer by one byte? For example: Thanks for contributing an answer to Stack Overflow! Share Improve this answer 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? Is it cheating if the proctor gives a student the answer key by mistake and the student doesn't report it? Not the answer you're looking for? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. 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. Logan Capaldo 2010-08-13 13:17:58 @Logan Capaldo: Thanks. Something can be done or not a fit? Not the answer you're looking for? So you can use reinterpret_cast and const_cast together. In C++0x, reinterpret_cast<int*> (p) will be . . i.e. Zeiger-zu-member werden nicht korrigiert, wie regelmige Zeiger sind. Do not dereference it or you will enter the myth-enshrouded lands of undefined behavior. Does illicit payments qualify as transaction costs? It doesn't guarantee any safety and your program might crash as the underlying object could be anything. 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. It's in the language for a reason. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? Das Hauptproblem ist, dass die Daten bentigt um einen Zeiger-auf . However, you should only do this if you used static cast to cast the pointer to void* in the first place. The reinterpret_cast operator can be used for conversions such as char* to int*, or One_class* to Unrelated_class*, which are inherently unsafe. How can I use a VPN to access a Russian website that is banned in the EU? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. The rubber protection cover does not pass through the hole in the rim. 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). Making statements based on opinion; back them up with references or personal experience. When casting from a void* there is not type information for the cast to work with. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. You should always avoid reinterpret_cast, and in this case static_cast will do the job. 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. 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." The rubber protection cover does not pass through the hole in the rim. This is a tough question. Does illicit payments qualify as transaction costs? The pointer value (6.9.2) is unchanged by this conversion. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. 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. It's just a quick idea. Write. The expression consists of a dereference applied to a cast. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. 6 QWindowsForeignWindow::setParent. 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). "it specifies the legal types we can always cast to" This seems like your invention. The C style cast automatically dealed with that. 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). The " reinterpret_cast " operator can convert any type of variable to fundamentally different type. Jul 22 '05 # 3 hack_tick hi there 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. Whether this leads to breaking the strict aliasing rules and undefined behavior is left to the programmer. rev2022.12.11.43106. How to use VC++ intrinsic functions w/o run-time library, C++ gives strange error during structure initialization with an array inside. 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. Does balls to the wall mean full speed ahead or full speed ahead and nosedive? int cell = 20; outFile.write (reinterpret_cast<const char *> (&cell),sizeof (cell)); You are asking whether binary files are good. 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. reinterpret_cast is a type of casting operator used in C++. 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. 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. 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