Share Improve this answer The short answer: If you don't know what reinterpret_cast stands for, don't use it. (protobuf) codec . , . For example: That's even pretty easy to generalize to any size with a couple of loops (that will almost certainly be unrolled). Is the first method OK, or would you consider it to be poor code in comparison to the second method? For one, you can limit what cast you actually want, for another it's far easier to do a textual search for the new casts than for C casts. Connect and share knowledge within a single location that is structured and easy to search. TIFFs for example can be either way, and the header describes the endianess. . reinterpret_cast is the most aggressive, insecure and (hopefully) least used of the four C++ cast operators. Now this is not really a cast any more but just a way to tell the compiler to throw away type information and treat the data differently. If that's the caseYou should never ever do it (and you may want to double check your second solution), Which one is best might well depend on in which context it's used. In the absence of a C++ book (which maybe you should read given your latest questions) the MSDN documentation contains quite a few details about those operators: http://msdn.microsoft.com/en-us/library/c36yw7x9.aspx, http://msdn.microsoft.com/en-us/library/e0w9f63b.aspx. like they are from a test. any can describe what is the main difference between static_cast and reinterpret_cast? It's used primarily for things like turning a raw data bit stream into actual data or storing data in the low bits of an aligned pointer. Can several CRTs be wired in parallel to one oscilloscope circuit? As for the builtin bswap, I believe if you write out the shifts on an int manually it gets optimized to a single, reinterpret_cast vs bit shifts for extracting 32-bit integers from bytes, with either endianness [closed], concrete code from a project, with sufficient context. In the original code, the data is serialized little endian, and, I like this solution a lot actually. The biggest advantage with this method is probably performance, readability is debatable. The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. 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. static_cast(expression) - To an extent C style but still respects some incompatibilities between types and do not allow. The C++ static_cast is defined as the operator which has to convert the variable from one data type into another data type mainly it transform into float data type the compiler only done this conversion in the static_cast because it constantly focus on the const types like const_cast, reinterpret_cast it also casting from one type into another type same like . By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Is it illegal to use resources in a University lab to prove a concept could work (to ultimately use to create a startup). "Please explain about dynamic_cast and static_cast. first one is to remove constness from a type and the other is to give its code explicitness. Did neanderthals need vitamin C from the diet? The first method reads a 32 bit value using the computer's natural byte ordering, and then swaps the bytes if bswap is true. where can i use dynamic_cast. How do we know the true value of a parameter, in order to check estimator properties? It can typecast any pointer to any other data type. No. Central limit theorem replacing radical n with n, Irreducible representations of a product of two groups, PSE Advent Calendar 2022 (Day 11): The other side of Christmas, Arbitrary shape cut into triangles and packed into rectangle of the same area. Good question! Or maybe there's just a better method all together (std::byte?). To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Your compiler may be different, so use #ifdef and #if compiler preprocessor directives to identify the compiler and use its builtin. Context which is currently missing. dynamic_cast(expression) - Allows casting between proper class hierarchic. C-casts within a class hierarchy (base to derived, or derived to base) will do a static_cast (which can change the pointer value in all implementations fathomable) , a C-cast between unrelated classes will do a reinterpret_cast. 'reinterpret_cast' is used to convert pointers to objects to integral values (and back), if there is a type that can hold the entire value; between pointers of different functions; between pointers and references of unrelated object types. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, Should I use static_cast or reinterpret_cast when casting a void* to whatever. For one, you can limit what cast you actually want, for another it's far easier to do a textual search for the new casts than for C casts. Be aware that modifiyng objects that actually are declared as const is undefined behaviour. That is, you'd need to calculate the high byte as something like buf[offset + 3] & 0x7Fu, and multiply the result by 1 if buf[offset + 3] & 0x80u or you could dump the whole thing into an unsigned value, and then deal with the sign bit. Share This basically compiles down to a single instruction, two if you need to swap the bytes. Full answer: Let's consider basic number types. It only takes a minute to sign up. Why was USB 1.0 incredibly slow even for its time? Is it to swap bytes from a known endianness to host? , reinterpret_cast , . This cast operator can also convert variable into totally incompatible type too. Introduction to C++ static_cast. If so, I suggest you review Converts between types by reinterpreting the underlying bit pattern. Most compilers have some fast way to perform high-performance 32-bit byte swap. A value of integral or enumeration type to a pointer. Understanding reinterpret_cast. reinterpret_cast. Between two arithmetic types 2. The only legitimate use for reinterpret_cast<> seems to be converting between integral types and back, for instance, from a char * to an int and back to a char *. Keywords. Why is the federal judiciary of the United States divided into circuits? PSE Advent Calendar 2022 (Day 11): The other side of Christmas. This type of cast reinterprets the value of a variable of one type as another variable of a different type. The swap is in the case of binary files where the byte order may be either way. Well, assuming you have a bug where a Foo* is broken (i.e. As i mentioned, this should be the last option, and in the case above, the usage is correct. >->,Son,CDialogEX . Unless you make sure that your compiler turns the other cheek (gcc, for example, does so with -fno-strict-aliasing), don't use that variant, ever. So it will be easy to find it in large code base. . That being said, it's not improbable that your compiler will recognize the undefined behavior and optimize based on it, either. const auto val = * reinterpret_cast<const int32_t *> (&buf [offset]); The first and second methods actually can do slightly different things. They go into a lot of detail as to the differences between the two. A C-style cast will always try to fall back on the crudest possible cast if necessary, while the C++-style cast only compiles if it is possible as intended: a static cast only succeeds if either the values are convertible or the pointers/references are compatible, and a const-cast only works if source and target are cv-qualified versions of one another. Disconnect vertical tab connector from PCB. For e.g. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? In the given situation, the C-style cast is precisely a "reinterpret"-cast. In C there is only a single cast, but it performs many different conversions: 1. (175) QT0-5qimageqpainter . They will be expected to reinterpret classic snacks and duplicate some of the world's most popular snacks. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? Pseudocode, stub code, hypothetical code, obfuscated code, and generic best practices are outside the scope of this site. There is no difference. Very simple bit shifting, about double the instructions as the first method, and can get a bit messy with larger data types. The reinterpret_cast operator, as well as the other named cast operators, is more easily spotted than C-style casts, and highlights the paradox of a strongly typed language that allows explicit casts. No. Why do some airports shuffle connecting passengers through security again. Connect and share knowledge within a single location that is structured and easy to search. Can I use B* pointer after this to access b member? l Wir haben die gesamte Online-Welt untersucht, um Ihnen die Antwort auf Ihre Zweifel zu geben. rev2022.12.11.43106. What properties should my fictional HEAT rounds have to punch through heavy armor and ERA? On a little endian machine, they are equivalent. It can be used only on integral types, enums, all kinds of pointers including function and member pointers and nullpointer constants like std::nullptr. Windows XP Netbook Backup Options. Note htonl and ntohl are platform-dependent, but most platforms provide them. (Granted, if your data has a sign bit, you really need to handle it specifically in any case. See here for a detailed explanation: http://anteru.net/2007/12/18/200/. In most casesthe 2 casts do the same thing but static_cast is far more restrictive than reinterpret_cast. Contribute to libcy/inv.cu development by creating an account on GitHub. Now mentioning reinterpret_cast will give the reader the impression that intentionally the writer have chosen not to care type safety. Actually, this is the key to understanding why C++ has four different casts. Could be bit 0, for example, meaning this value is actually 1342177280. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. More technically, the machine may not always store integers the way you expect, and I'm not referring to endianness by that. Stack Exchange network consists of 181 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. However, in memory manipulation . Here is a compiler explorer link with both examples. This cast operator can convert an integer to a pointer and so on. The problems arise when you shift by more than 8 bits. It is important to remember that even though a program compiles, its . Demonstrates some uses of reinterpret_cast: // pointer to function to another and back, //int &iref = reinterpret_cast(const_iref); //compiler error - can't get rid of const. A tag already exists with the provided branch name. Asking for help, clarification, or responding to other answers. In C++0x, reinterpret_cast<int*> (p) will be equivalent to static_cast<int*> (p). 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. Though I am curious how, Yes, if the buffer is const, swap won't help. For example, if your system is little-endian, and you know the representation is the same as what's in the buffer, you could just memcpy(). rev2022.12.11.43106. While we are at it, we can replace the C-cast with the proper C++ cast, which in this case is reinterpret_cast: constexpr auto FOO = reinterpret_cast<uint8*> (0xBAD50BAD . static_cast only allows conversions like int to float or base class pointer to derived class pointer. Reinterpret_cast VS const_cast. Books that explain fundamental chess concepts, PSE Advent Calendar 2022 (Day 11): The other side of Christmas, MOSFET is getting very hot at high frequency PWM, Name of poem: dangers of nuclear war/energy, referencing music of philharmonic orchestra/trio/cricket. displays) - Mark Jeronimus This is known as the strict aliasing rule and applies to both C++ and C programming languages. Can I assume that reinterpret_cast is not safe and should not be used? That's not a portable solution for a number of technical reasons. static_cast: This is used for the normal/ordinary type conversion. 8 vscodewindows. By contrast, bit_castis not. This is illustrated in the following example: class A {int a; public: A ();}; By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. memcpy accesses the array as a character array, and any type of array can be accessed as character array using the aliasing rules. [] 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 . And yeah, the. Your current code is almost certainly UB. Why was USB 1.0 incredibly slow even for its time? Regards, Paul McKenzie. And of course, static_cast<T>(static_cast<T>(anything))is equivalent to static_cast<T>(anything)because the outer cast is always an identity conversion. Can we keep alcoholic beverages indefinitely? Making statements based on opinion; back them up with references or personal experience. @curiousguy: I've done it a few times. I presume this is the safer solution though, and perhaps more readable. I wouldn't be surprised if you ended up with a function that returned 0 in all cases, or that returns some uninitialized value, or similar. Any form of bit-shifting and bitwise OR, although correct, is terribly slow. C++ static_cast vs dynamic_cast - YouTube Shows the differences between C++ static_cast and dynamic_cast Shows the differences between C++ static_cast and dynamic_cast. 17. ( reinterpret_cast does not perform the adjustments that might be needed.) @juhist describes another option: using memcpy(). (Note that the only valid reinterpret-casts are usually those to void- or char-pointer, unless they're part of some larger trickery.). The problem with C-Style casts is that they do a lot under the hood. You should try to always use the C++-casts, makes life easier in the long run. reinterpret_cast can't cast away cv-qualifiers So you can use reinterpret_castand const_casttogether. However, usually data is in big-endian and you want to access it in host byte order whether it's little- or big-endian. The reinterpret_cast operator produces a value of a new type that has the same bit pattern as its argument. Should I use a C++ reinterpret_cast over a C-style cast? Snack vs. Does illicit payments qualify as transaction costs? Would salt mines, lakes or flats be reasonably found in high, snowy elevations? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. 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. pls provide some example with descript". The form and content of these questions looks suspiciously There may be padding bits and other weirdness going on that ends up meaning that a 32 bit value of 0xFFFFFFFF may not necessarily be represented in memory as 4 bytes of 0xFF. Is it possible to convert an integer pointer to the actual integer located at that memory location? Using it to replace a C cast is encouraged. 7 QDebug<<. reinterpret_cast: Casts anything which has the same size, for example, int to FancyClass* on x86. Is it cheating if the proctor gives a student the answer key by mistake and the student doesn't report it? Zorn's lemma: old friend or historical relic? C-Style casting, using the (type)variable syntax. One winner will get the $50,000 prize. Pointer Type Pointer conversion is a bit complicated, and we'll use the following classes for the rest of this article: class CBaseX { public: int x; So the correct thing to do would probably be to cast each, When I load the file I actually have a min and max set (> 8 < 10mb), but I should still be doing that anyways. Let's have a look from the memory perspective. reinterpret_cast<const char*> Int . The other two is sometimes confusing. But the point about the sign bit not being where you expect it still stands: 0x01 0x00 0x00 0x00, if memcpy()ed on a little-endian machine, could produce 2147483648 rather than 1 or anything else really.). The first method reads a 32 bit value using the computer's natural byte ordering, and then swaps the bytes if bswap is true. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. . So C++ categorical divided the casting to below types based on typical usage. 6 QWindowsForeignWindow::setParent. It is used when we want to work with bits. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? Someone's going to punch me for saying this, but if you, You don't even need a full-blown regex for, Args, yes. Reinterpret_cast cannot remove the const modifier as const . Solution 1. reinterpret_cast changes the interpretation of the data within the object.const_cast adds or removes the const qualifier. This rule bans (T)expression only when used to perform an unsafe cast. As mentioned by @AJNeufeld you need to fix up your out-of-bound checks. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Instead of the reinterpret_cast I could also use a C-style (const char*). static_cast VS reinterpret_cast when casting pointers to pointers. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. How is Jesus God when he sits at the right hand of the true God? The new casts have benefits over C-style casts. The above code is legal because in every arithmetic operation, signed chars and shorts are always promoted to int. When casting from a void* there is not type information for the cast to work with. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. -1. . Why would Henry want to close the breach? I hardly ever care that I got something from a cast, but I often care that I took a particular step that I happen to know involves a cast operation. C style casting but with a name. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. C++ has no compatible types and does not allow access through a pointer or reference to a layout-compatible type if it doesn't satisfy any of the rules listed above (although access to its member may be allowed), C++98 did not allow conversion between function pointers and void*, this was corrected by DR CWG195. The reason you should prefer C++-style casts is that they are explicit about what they are casting. In other words "reinterpret_cast" is needed means, most-likely something is wrong in the design. a static_cast followed by const_cast a reinterpret_cast a reinterpret_cast followed by const_cast So in certain situations, a C-style cast will have the same effect as reinterpret_cast but they are not equivalent. 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 . The main problem with C-style casts in this case is that you could have written (char*)(&v) while with reinterpret_cast, you would need an additional const_cast, so it's a bit safer. Since a C-style cast is basically a "oh, just cast it however you can" cast, it's better to prefer the more specific casts. First, you'll get different behaviour depending on the endianness of the machine. Will reinterpret_cast be used in some cases? It generates UB if buf[offset] doesn't happen to be at the right alignment boundary for a 32-bit value. Also, I'm not concerned with system endianess or portability here. The reinterpret_cast operator can be used for conversions such as char* to int*, or One_class* to Unrelated_class*, which are inherently unsafe. Or if your input data is "0x00 0x00 0x00 0xA0" which if memcpy()ed on a little endian machine will give you 0xA0000000 (that's its value, not its memory representation), which you assume is 2147483648 but that's assuming bit 31 is the sign bit, which is not necessarily true. Ready to optimize your JavaScript with Rust? Bjarne Stroustrup, in his guidelines, recommended reinterpret_cast in another context: if you want to type-pun in a way that the language does not define by a static_cast, he suggested that you do it with something like reinterpret_cast<double&>(uint64) rather than the other methods. Update: This means that when you use it to convert from, say, an int* to a float*, then you have no guarantee that the resulting pointer will point to the same address. Why use static_cast(x) instead of (int)x? Thanks for contributing an answer to Stack Overflow! This operator can also be applied to pointers of any type. reinterpret_cast has nothing to do with 'const'. Is there any difference between these two operators? Other uses are, at best, nonportable. At no point does any const get added or removed. If you don't care about portability and you want speed, well, then the "right" answer depends entirely on 1) the data representation, and 2) your machine. If performance really matters - and, I'll be blunt, if you're using this part of a serialization routine, it almost certainly doesn't; whatever time you spend decoding integers is going to be dwarfed by the I/O - you could write system-specific code. And is suggested to use it using proper data type i.e., (pointer data type should be same as original data type). Honestly, what I'd do if - like you say - you don't care about portability is just copy the buffer bytes into a 32-bit variable like you do in the return line, then conditionally use an intrinsic like. These were covered in the links I provided in your To learn more, see our tips on writing great answers. Now mentioning reinterpret_cast will give the reader the impression that intentionally the writer have chosen not to care type safety. Why do we have reinterpret_cast in C++ when two chained static_cast can do its job? When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? How were sailing warships maneuvered in battle -- who coordinated the actions of all the sailors? class pointer. Any form of reinterpret_cast will be undefined behavior due to strict aliasing rules and possibly due to alignment constraints. it is definitely different. Now that std::bit_cast is coming in C++20, the valid use cases for reinterpret_cast are slim to none in the majority of applications. It is used for reinterpreting bit patterns and is extremely low level. EDIT: As @hoffmale notes in the comments, the return line is still possibly UB. To answer the other part of your question, yes, reinterpret_cast is implementation-defined. . Did neanderthals need vitamin C from the diet? Normal syntax to do reinterpret_cast is as follows: reinterpret_cast <target-type> (expr) Indeed, there were no compiler warnings with pretty pedantic settings, but the sanitizers complained about alignment a few times during runtime. One difference that comes to mind is that a C-style cast can be used to cast away const-ness, but reinterpret_cast cannot (const_cast would have to be used). casts really are mostly avoidable in modern C++ While strictly true, it's often not achievable in practice, where you have to mix C and C++ (for example, even in 2021, lots of libraries for microcontroller and embeddded are written in C, like Arduino, FreeRTOS, ESP-IDF and many peripheral drivers for e.g. Example Also note that this set of rules is more strict than the equivalent rules in the C programming language: C allows access through a pointer to any compatible type. Unlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. In short, static_cast<> will try to convert, for example, float-to-integer, while reinterpret_cast<> simply changing the compiler's intent to reconsider that object as another type. If it's big-endian, maybe memcpy() and a swap intrinsic, or a loop that's basically reverse memcpy() (create the integer, then copy the bytes in reverse order). - ? So if I add constness using reinterpret_cast and if you reinterpret_cast the result back to the original type, it should result back to the original type and . Ready to optimize your JavaScript with Rust? Thanks, the eventual goal is to generalize it for sure. reinterpret_cast is frowned upon when it's used to replace a static_cast or dynamic_cast. The const was there in the c-cast, obviously, Where exactly do you think a const cast was happening there? However, I might want to replace the [0] array look-up with a straight dereference. Can we keep alcoholic beverages indefinitely? If you want a portable high-performance way to access a byte array as 32-bit integers, you're looking for memcpy: An optimizing compiler will notice this is a 32-bit load on a possibly unaligned location, and will generate code accordingly. Arbitrary shape cut into triangles and packed into rectangle of the same area. One difference is that static_cast will perform conversions of one type to another using the standard conversion operators for the type being converted to, while reinterpret_cast does not. The worst ever invented. This should be the last option, and in the case above, the usage is correct. How does alignment work? protobuf . It is a compile time cast .It does things like implicit conversions between types (such as int to float, or pointer to void*), and it can also call explicit conversion functions (or implicit ones). Thanks for contributing an answer to Stack Overflow! But the above usage is legal and cannot be replaced with anything else, right? const_cast(expression) - Casts away const-ness. Does casting with static_cast and with reinterpret_cast via void* give the same result? 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. reninterpret_cast does not check if the pointer type and data pointed by the pointer is same or not. If we go ahead and replace the macro with a constant expression, we should get the warning at the exact location where the C-cast is written, not where the macros are expanded. //Must use const_cast instead: int &iref = const_cast(const_iref); http://en.cppreference.com/mwiki/index.php?title=cpp/language/reinterpret_cast&oldid=91995, implicit conversions from one type to another. Use the reinterpret_cast<> to highlight these dangerous areas in the code. Asking for help, clarification, or responding to other answers. The best answers are voted up and rise to the top, Not the answer you're looking for? Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs. But using c style casting will not give that impression. A reinterpret-cast states explicitly that you wish to examine an underlying binary representation. Help us identify new roles for community members, Array-like container for uints shorter than 8 bits (Rev 1), Sieve of Eratosthenes - segmented to increase speed and range, C++ idiom for selecting integers from a vector according to predetermined properties, Concatenate three 16-bit integers in one 64-bit integer with C++, Place integers into a vector, sum each adjacent pair, refill vector with only the sums of each pair i.e remove all the original data from the vector, isLessThan function for structure with 7 integers, Breaking a 32-bit value into bytes for an ArrayList. Example. The reinterpret_cast operator is designed to convert one type to another, even a non-compatible type. This forum has migrated to Microsoft Q&A. The second method reads a 32 bit value in little endian format (if bswap is false) or big endian format (if bswap is true). You're dealing with signed values, but the type of signedness of the data may not be the same as the machine (for example, the data may be twos-complement, the machine may be signed magnitude). Does aliquot matter for final concentration. Data representation and constness are orthogonal. What is the highest level 1 persuasion bonus you can have? reinterpret_cast allows anything, that's usually a dangerous thing and normally reinterpret_cast is rarely used, tipically to convert pointers to/from integers or to allow some kind of low level memory manipulation. Note:- Most "reinterpret_cast" can be eliminated with proper design. Then htonl and ntohl help you. 4. C++ .reinterpret_cast:reinpreter_cast<type-id> (expression) reinterpret_cast,,.: int n=9; double d= reinterpret_cast< double > (n); . The problem I have with this is casting like this in modern C++ is frowned upon, and I am unsure how safe it actually is despite bounds checking first. Are defenders behind an arrow slit attackable? Any decent optimizing compiler will hoist the invariant conditional and offset out of the loop and unroll the loop (and eliminate the unnecessary shift). The new casts have benefits over C-style casts. And of course, static_cast(static_cast(anything)) is equivalent to static_cast(anything) because the outer cast is always an identity conversion. static_cast<> should be used for virtually anything that dynamic_cast<> and const_cast<> cannot do. It's a misconception that reinterpret_cast<T*> (p) would interpret the bits of . reinterpret_castis generally considered an inherently dangerous operation, indicative of "cheating" the type system in some way. Reinterpret Cast Static Cast: This is the simplest type of cast which can be used. 9 windows. What is the difference between static_cast and reinterpret_cast? Also, casting from void *can use static_cast, it does not need to reinterpret. the documentation for these subjects carefully and craft Shifting and OR-ing is the only portable way to get the little-endian data in buf into an integer for all machines. The bottom line is that shifting and OR-ing is the only portable solution (and you'd need to handle the sign bit properly). Answer (1 of 6): > Why are they really needed? I've written some simple helper functions that read data types from a vector of bytes (binary files) and having a tough time deciding which route to go. Received a 'behavior reminder' from manager. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. Browse other questions tagged, Start here for a quick overview of the site, Detailed answers to any questions you might have, Discuss the workings and policies of this site, Learn more about Stack Overflow the company, I wonder why do you need bswap at all. Code Review Stack Exchange is a question and answer site for peer programmer code reviews. This is because Static_cast calculates the offset of the parent-child class pointer conversion, converts it to the correct address (C has m_a,m_b in it, translates to a b* pointer and points to M_b), and reinterpret_cast does not do this layer conversion. C-casting a float to int is a static cast, C-casting a float * to int * is reinterpret. If it's other-endian or non-twos-complement or has some other quirck well, you'd know how to deal with that in that case. 10 QGuiApplication::allWindows () 11 QSharedPointer. Why do quantum objects slow down when volume increases? Lacks concrete context: Code Review requires concrete code from a project, with sufficient context for reviewers to understand how that code is used. Does integrating PDOS give total charge of a system? I believe there should be a rule heavily discouraging reinterpret_cast in favor of std::bit_cast or another named cast.. Often times switching from reinterpret_cast to std::bit_cast would be the difference between invoking undefined behavior or not, due to type . You cannot cast away a const or volatile qualification. Even if your compiler does not implement any optimizations based on that case of undefined behavior, your program could still crash: Assuming that unaligned accesses work on x86 is a common misbelief, and has been the cause of quite a few bugs since compilers have added SIMD code generation to their backends. earlier thread "Dynamic_cast issue", where you asked Not the answer you're looking for? You should neither be fine with it, nor endorse it without pointing that out first and explaining how to a) avoid it or b) make the compiler guarantee reasonable behavior in that particular instance. Is there any particular reason to use reinterpret_cast? And you can even template the loop on the size and reuse it. Ready to optimize your JavaScript with Rust? How do I iterate over the words of a string? C style casting is very very dangerous. Should I exit and re-enter EU with my EU passport or is it ok? I.e is there any difference between the following expressions? Once that's done, you could simplify the function quite a bit by using std::swap(). Counterexamples to differentiation under integral sign, revisited, Save wifi networks and passwords to recover them after reinstall OS, If he had met some scary fish, he would immediately return to the surface. In C++, reinterpret_cast, static_cast and const_cast is very common. You are correct to be concerned about the first method. Please take a look at the. That part is implementation-defined. static_cast only allows conversions like int to float or base classpointertoderived Plus you can easily find reinterpret_cast with a regex, which is not possible for the C-style casts. integral, enumeration, pointer, pointer-to-member ; cast . reinterpret_cast is frowned upon when it's used to replace a static_cast or dynamic_cast. Where does the idea of selling dragon parts come from? It's a low-level tool, but it's not a tool that messes with the type system. Connect and share knowledge within a single location that is structured and easy to search. Is it possible to hide or delete the new Toolbar in 13.1? You can explicitly perform the following conversions: A pointer to any integral type large enough to hold it. It is efficient because it does not copy the value. The first and second methods actually can do slightly different things. This can cast related type classes. But using c style casting will not give that impression. in most cases the 2 casts do the same thing but static_cast is far more restrictive than reinterpret_cast. - Wayne. reinterpret_cast constness (5.2.11). This solution still contains undefined behavior: @hoffmale That is true. It is generating a new object as if by copying its value representation from an existing one. The answer is not because that is where Unreal reflection system comes into play. reinterpret_cast allows anything, that's usually a dangerous thing and normally reinterpret_cast is rarely used, tipically to convert Notably some uses of reinterpret_cast could be filled by other casts. You are "reinterpreting" 4 bytes as a single 32-bit value. Also, memcpy is not alignment-dependent. What it does is simply stores an additional information about the class in its CDO (Class Default Object). It is meant to be used to convert types that are otherwise not compatible, i.e . Are the following expressions the same? The C++ compiler detects and quietly fixes most but not all violations. 2.. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, It might be worth mentioning that the cast from, I think it is worth mentioning the standard on this for completeness: "8.5.1.10 Reinterpret cast: 7 An object pointer can be explicitly converted to an object pointer of a different type. You should use it in cases like converting float to int, char to int, etc. Note that many C++ compilers relax this rule, as a non-standard language extension, to allow wrong-type access through the inactive member of a union (such access is not undefined in C). They're all undefined behavior, but that makes it very . We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. 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). If you originally serialized the data on the same machine (using, You're making assumptions about the data that don't really match what's in the original code (method 2, because method 1 is broken). This is exclusively to be used in inheritence when you cast from base class to derived class. 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. If the types are not same it will generate some error. When a prvalue v of object pointer type is converted to the object pointer type "pointer to cv T", the result is static_cast<cv T*> (static_cast<cv void*> (v))." reinterpret_cast<T*>(ptr)is specified to behave exactly the same as static_cast<T*>(static_cast<void*>(ptr))(I've left out cv qualifiers for simplicity). Why is the federal judiciary of the United States divided into circuits? What is important is that the starting point of the read plus the size of the value cannot extend beyond the end of the buffer. I have the following template function used to dump data of any standard type into a binary output stream. The second method is the way to go, even if you don't care about portability (but as a bonus, it's portable, except for the use of int32_t). . On a big endian machine, they perform opposite. I'm not looking for the cast. Let's discuss the premiere date and cast of Season 1 of Snack vs. Why is there an extra peak in the Lomb-Scargle periodogram? When you convert for example int (12) to unsigned float (12.0f) your processor needs to invoke some calculations as both numbers has different bit representation. Possible Duplicate: c-style cast vs reinterpret_cast : A* pA = new B; B* p1 = (B*)pA; B* p2 = reinterpret_cast<B*>(pA); ? So b is promoted to int, which must be at least 16 bits, and the shift is cool, and r is int. That means if you did: To solve it, you need to make sure the value being shifted is at least 32 bits before the shift: So you'd need to add this cast before each shift in the return line. Making statements based on opinion; back them up with references or personal experience. reinterpret_cast(expression) - If still the requirement is not met, this is available. Japanese girlfriend visiting me in Canada - questions at border control? The reason it technical. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. How were sailing warships maneuvered in battle -- who coordinated the actions of all the sailors? Between a pointer type and an integer type. reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, C++ gives strange error during structure initialization with an array inside, Pointers to the same address are different from each other. This is also the cast responsible for implicit type coersion and can also be called explicitly. The " reinterpret_cast " operator can convert any type of variable to fundamentally different type. Dialog *dialog = const_cast<Dialog*>(reinterpret_cast<const Dialog *>(data)); Pranit Kothari9461 score:11 You need to also use a const_castto remove constqualifiers. I'm quite fine with the first approach. You reinterpret cast one mutable pointer to another. 2D GPU full waveform inversion package. Because you can use cast it using C-style cast, but this is not explicit so that is not recommended. Why should I use a pointer rather than the object itself? The types will always be promoted to int, but int is only guaranteed to be 16 bits. UB probably not worth the couple nanoseconds! is there any difference between the following expressions? #include <iostream> using namespace std; int main () { I suppose you know the consequence of doing that (that the resulting file can only be read back by the program compiled with the same options with the same compiler on the same architecture). Would like to stay longer than 90 days. 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Casesthe 2 casts do the same size, for example, meaning this value is actually 1342177280 because in arithmetic. Form of bit-shifting and bitwise or, although correct, is terribly slow is undefined behaviour juhist describes option! The same area estimator properties the C++-casts, makes life easier in the case,. Class to derived class pointer, pointer-to-member ; cast different, so use # ifdef and # compiler! Can not be reinterpret_cast vs cast for the cast to work with bits has a sign,! ( int ) x basically compiles down to a pointer rather than the object?. The value of a different type concerned about the first method through heavy armor ERA. In that case reinterpret_cast over a C-style ( const char * ): friend! With this method is probably performance, readability is debatable historical relic rise to the second method ]... Used of the four C++ cast operators the United States divided into circuits in your to learn more, our. In parallel to one oscilloscope circuit and perhaps more readable I presume this is not met this. X ) instead of ( int ) x std::byte?.... Level 1 persuasion bonus you can even template the loop on the size and it.::swap ( ) not to care type safety well, you need... Historical relic always use the C++-casts, makes life easier in the links provided! Old friend or historical relic zorn 's lemma: old friend or historical?! Detects and quietly fixes most but not all violations with that in that case @ hoffmale that structured... 64-Bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs you 'll get different behaviour depending the. Is frowned upon when it 's little- or big-endian, i.e only guaranteed be! Several CRTs be wired in parallel to one oscilloscope circuit, lakes or flats be reasonably found in,. Memory perspective converting float to int is only a single location that is and... Option, and in the comments, the C-style cast changes the interpretation of the data the! Store integers the way you expect, and perhaps more readable and cookie policy applied to pointers of type... Reinterpreting '' 4 bytes as a single location that is where Unreal reflection comes! And with reinterpret_cast via void * can use static_cast < int > ( )! The cast responsible for implicit type reinterpret_cast vs cast and can not remove the const.... A big endian reinterpret_cast vs cast, they perform opposite oscilloscope circuit template function to... Personal experience the hood your RSS reader same area sign bit, 'll! This should be the last option, and in the c-cast, obviously, you. Accessed as character array, and perhaps more readable of all the sailors gives a student the you. Parameter, in order to check estimator properties for a number of technical reasons how,,. To find it in cases like converting float to int, reinterpret_cast vs cast that makes very. Convert an integer to a single 32-bit value browse other questions tagged, where exactly do you think const! Non-Compatible type respects some incompatibilities between types and do not allow the pointer type and data by... Several CRTs be wired in parallel to one oscilloscope circuit not safely be used to perform 32-bit! The reader the impression that intentionally the writer have chosen not to care type.. ( n ) ; we do not currently allow content pasted from on! There is technically no `` opposition '' in parliament a known endianness to?! Any other data type ) above code is legal and can get a bit by using std: (. Reasonably found in high, snowy elevations the new Toolbar in 13.1 link with both.. Rule bans ( t ) expression only when used to replace the [ 0 ] array with! Single location that is where Unreal reflection system comes into play some of world... ) instead of ( int ) x an account on GitHub different things types by reinterpreting the underlying pattern. Buffer is const, swap wo n't help I 've done it a few.! Together ( std::byte? ) Stack Overflow ; read our policy here and re-enter with! Maneuvered in battle -- who coordinated the actions of all the sailors used inheritence... Types by reinterpreting the underlying bit pattern in large code base to perform high-performance 32-bit byte swap goal to. The [ 0 ] array look-up with a straight dereference that makes it.! Produces a value of type new-type rather than the object itself - to an extent C style still... Into a lot actually not concerned with system endianess or portability here total. Type-Id & gt ; ( expression reinterpret_cast vs cast - to an extent C casting... As character array, and perhaps more readable zu geben, const_cast, the reinterpret_cast I could also use C-style! Pointer rather than the object itself not give that impression it a few times eventual goal to... On x86 reasonably found in high, snowy elevations & a I iterate over the words of a,! The United States divided into circuits CRTs be wired in parallel to one oscilloscope circuit ; to highlight dangerous! Upon when it 's other-endian or non-twos-complement or has some other quirck,! That your compiler may be either way of array can be eliminated with proper design upon it. United States divided into circuits undefined behavior, but that makes it very from class. Second methods actually can do its job away constness or volatility I iterate the... Enumeration, pointer, pointer-to-member ; cast creating this branch may cause unexpected behavior structured easy! Exactly do you think a const or volatile qualification has migrated to Microsoft reinterpret_cast vs cast &.... Underlying binary representation remember that even though a program compiles, its they & x27. Deal with that in that case underlying bit pattern can typecast any pointer to any CPU instructions header! Maneuvered in battle -- who coordinated the actions of all the sailors cheating if the proctor gives a the! A tag already exists with the provided branch name rise to the differences between C++ static_cast and reinterpret_cast used! Always use the C++-casts, makes life easier in the given situation, the return line is still UB! C there is technically no `` opposition '' in parliament policy here fast way perform. Answer: Let & # x27 ; s consider basic number types solution a... A little endian machine, they are equivalent question, Yes, if the buffer is const, wo! Store integers the way you expect, and can also be applied to pointers of standard! If so, I suggest you review Converts between types by reinterpreting the underlying bit pattern as its.... Compiler and use its builtin but like const_cast, and reinterpret_cast be used to perform 32-bit. Right hand of the United States divided into circuits method is probably performance, is. ; to highlight these dangerous areas in the comments, the machine only following! Order whether it 's other-endian or non-twos-complement or has some other quirck,... And branch names, so creating this branch may cause unexpected behavior as character array using the aliasing rules of! Cast operator can also be applied to pointers of any standard type a. It OK for example, meaning this value is actually 1342177280 but like const_cast, the reinterpret_cast expression does check... Converting float to int, char to int, etc is true identify the and! If the proctor gives a student the answer key by mistake and the other is to generalize for..., lakes or flats be reasonably found in high, snowy elevations for the normal/ordinary type.! Memory location exclusively to be 16 bits in most cases the 2 casts do the size... Even template the loop on the size and reuse it check estimator properties and bitwise,! Report it `` reinterpret_cast '' is needed means, most-likely something is wrong in the case of binary where! Of cast reinterprets the value of type new-type the bytes, lakes or be. The eventual goal is to give its code explicitness point does any const get added or removed UB! And second methods actually can do its job copy and paste this URL your. Canada - questions at border control branch names, so creating this branch may cause unexpected behavior following conversions 1!, see our tips on writing great answers technically, the return line is still possibly UB std:byte. Not compile to any integral type large enough to hold it new object as if by copying value... Return line is still possibly UB the [ 0 ] array reinterpret_cast vs cast with a straight dereference be done with via. Between C++ static_cast vs dynamic_cast - YouTube Shows the differences between C++ static_cast and reinterpret_cast! And dynamic_cast Shows the reinterpret_cast vs cast between C++ static_cast and with reinterpret_cast via void * there not... Use # ifdef and # if compiler preprocessor directives to identify the compiler and use its builtin well, 'd... The code airports shuffle connecting passengers through security again border control that might be needed. found in high snowy... Note htonl and ntohl are platform-dependent, but most platforms provide them by... And paste this URL into your RSS reader Intel CPUs is structured and easy to.... If your data has a sign bit, you 'd know how to deal that...