If you are unable to find uses for these casts, then count your blessings. After reinterpret-casting the byte under p pointer could be respectively 0000'0000 or 0000'0001. For example, if you want to dump the double with value 5.14 as the 8 bytes it's made of in memory, and not as the four bytes of its decimal text representation (like I did in the first example): #include <fstream> int main(){ double d = 5.14; std::ofstream("terca.bin", std::ios::binary).write( reinterpret_cast<char*>(&d), sizeof d); } This . return reinterpret_cast<QTimerPrivate *>(qGetPtrHelper (d_ptr)); clang diagnostic pop } inline const QTimerPrivate* d_func() const noexcept { clang diagnostic push return reinterpret_cast This rule bans (T)expression only when used to perform an unsafe cast. A practical use case of "opaque" data types is when you want to expose an API to C but write the implementation in C++. If B has more than one base class, and A is not the first base class, reinterpret cast will do the wrong thing and fail to perform necessary adjustment to the pointer. This does not actually answer the question of "when to use reinterpret_cast". Read the FAQ! This macro could help: Quick answer: use static_cast if it compiles, otherwise resort to reinterpret_cast. This is exclusively to be used in inheritence when you cast from base class to derived class. See const_cast Operator for information on removing these attributes. Be aware that reading or modifying values after reinterprt_cast'ing are very often Undefined Behavior. After converting back to a float, it's subjected to a Newton-Raphson iteration to make this approximation more exact: This was originally written in C, so uses C casts, but the analogous C++ cast is the reinterpret_cast. reinterpret_cast<> () is used for casting between unrelated types. x86) systems. The C++ standard library gained some new concurrency features with C++20: Wait and notify operations on std::atomic<T>; Semaphores; Latches; Barriers; In this article, I will cover the current implementation approach for atomic wait/notify, as these are basis operations required to implement the remaining coordination primitives introduced with C++20. A link to the app was sent to your phone. One practical use of reinterpret_cast is in a hash function, which maps a value to an index in such a way that two distinct values rarely end up with the same index. You only need to use it when you're casting to a derived class. dynamic_cast is used with polymorphic & inherited types which are C++ only idioms. This is what static_cast stands for. Why is it so much harder to run on a treadmill when not holding the handlebars? I have seen usage of both static_cast and reinterpret_cast? Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Obviously, since both constructs are about as unsafe as it gets, dereferencing the result pointer ap could cause undefined behavior. I am little confused with the applicability of reinterpret_cast vs static_cast. What is a smart pointer and when should I use one? The normal cast like (int)x is C style typecasting where static_cast(x) is used in C++. Personally I don't like unspecified. You can use pointers to constant data as function parameters to prevent the function from modifying a parameter passed through a pointer. A use case for reinterpret_cast is transporting data through a buffer. Everything here but the last example is undefined behavior; its interesting only as a (not reliable) means of illustrating implementation details of the language. Using reinterpret_cast to check inheritance at compile time. If you will need it in the future, you will know. Unlike C, the C++ compiler allows implicit conversions TO a void * type, but to convert FROM a void * type requires an explicit cast. 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. The purpose of reinterpret_cast is to reinterpret the bits of one value as the bits of another value. Your email address will not be published. Better way to check if an element only exists in one array. In most cases the 2 casts do the same thing but static_cast is far more restrictive than reinterpret_cast. So the two variables can be seen as different projections over the same memory zone. (which is not the same as a valid pointer to another type). The general idea I get is this is unportable and should be avoided. Instead, it just tells the compiler to "reinterpret" the type of one expression as another type. Forcing a conversion that could happen anyway: double d = 4.5; This is a bad use of reinterpret_cast. (str,) convert one pointer type to another. Although the reinterpret_cast itself might be unspecified behaviour, attempting to access the parameters once you've done the cast is undefined behaviour. It is true that "it means that the same memory is used as a different type" but it is restricted to specific pair of types. The " reinterpret_cast " operator can convert any type of variable to fundamentally different type. uintptr_t is an unsigned integer type that is capable of storing a data pointer. A tag already exists with the provided branch name. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Our team has collected thousands of questions that people keep asking in forums, blogs and in Google questions. There are no run-time checks. It is efficient because it does not copy the value. As a rule of thumb, use reinterpret_cast if there is no implicit conversion and static_cast is rejected by the compiler - and you are really sure that it's the conversion you need. A tag already exists with the provided branch name. Nice example! reinterpret_cast The next kind of cast is reinterpret_cast. It stops you doing silly things with it. A value of type pointer to object converted to pointer to cv void and back to the original pointer type will have its original value. However, the OpenCL C standard defines a class of functions that has very similar functionality (and is, in fact, significantly more powerful): as_ type (), that includes both a vector and scalar version for each of the built-in types. In some cases, using reinterpret_cast does not scale well, though apparently could help to design decoupled components. This trick is OK, I presume, for any compiler for Intel architectures. C++ ,c++,placement-new,reinterpret-cast,strict-aliasing,explicit-destructor-call,C++,Placement New,Reinterpret Cast,Strict Aliasing,Explicit Destructor Call In that case it will read the value of p using p 's type, and that value is then converted to a T*. Not the answer you're looking for? Also allows any integral type to be converted into any pointer type and vice versa. OPTIMIZE: I think reinterpret_cast would be optimized in many compilers, while the c-casting is made by pointerarithmetic (the value must be copied to the memory, cause pointers couldn't point to cpu- registers). https://bugs.llvm.org/show_bug.cgi?id=38201 Bug ID: 38201 Summary: incorrect reinterpret_cast from integer to pointer error on invalid constexpr initialization . But the cast itself is safe enough, as long as you only used it in the way the standard allows. It's probably incorporated in one of the next WPs. C++ _,c++,language-lawyer,reinterpret-cast,strict-aliasing,C++,Language Lawyer,Reinterpret Cast,Strict Aliasing That is, in the following, a, b and c all point to the same address: reinterpret_cast only guarantees that if you cast a pointer to a different type, and then reinterpret_cast it back to the original type, you get the original value. This is exclusively to be used in inheritence when you cast from base class to derived class. You should use it in cases like converting float to int, char to int, etc. The address loaded into rax is copied into the input pointer, which now points at the same memory zone as the buffer does. reinterpret_casts are applicable in two scenarios: Where I am a little confused is one usage which I need, I am calling C++ from C and the C code needs to hold on to the C++ object so basically it holds a void*. Executing tasks based on static polymorphism, A more decoupled approach on static polymorphism. Welcome to FAQ Blog! To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Your code has to be generic and works properly on big endian (e.g. I tried to conclude and wrote a simple safe cast using templates. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. The data model is a well-defined struct that could be transferred between different systems as bytes buffer (which could be a char array). . . Next use reinterpret_cast to cast the starting address of your array to an unsigned char*, and set each byte of the array to 1 (hint: you'll need to use sizeof to calculate the number of bytes in a double). Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, Getting around the reinterpret cast limitation with constexpr, casting via void* instead of using reinterpret_cast, C++ gives strange error during structure initialization with an array inside. You can decide between. 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. @Xeo They don't use void * because then they lose (some) type-checking at compile time. If you change the data in the buffer, the struct will also be changed. The only required parameter is the class identifier, as the next example shows: C++. size_t is an unsigned integer data type which can assign only 0 and greater than 0 integer values. Manually incrementing and decrementing a boost::shared_ptr? Type aliasing. Are the S&P 500 and Dow Jones Industrial Average securities? What is the named cast of old style cast: pv = (void*)ps; Cast from Void* to TYPE* using C++ style cast: static_cast or reinterpret_cast, Few doubts about casting operators in C++. Which typically means that it's the same size as a pointer. 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. The meaning of reinterpret_cast is not defined by the C++ standard. To learn more, see our tips on writing great answers. The standard says that this is undefined behavior: @sandthorn: This is UB according to the standard, but if it works for your architecture, don't worry about it. This can cast related type classes. Therefore, if your static_cast is between unrelated types then it will fail at compile type but the same cast will succeed using a reinterpret_cast. Making statements based on opinion; back them up with references or personal experience. If you will need it in the future, you will know. It treats the binary representation of the float as an integer, shifts it right and subtracts it from a constant, thereby halving and negating the exponent. The typecasting using this is done at compile time. In practice compilers try to do what you expect, which is to interpret the bits of what you are passing in as if they were the type you are casting to. reinterpret_cast in C# You can achieve this but this is a relatively bad idea. Memory allocation. Answers and comments should really reflect the latest available Standard unless otherwise specified IMHO. All I could find was. Save my name, email, and website in this browser for the next time I comment. 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. So for your use case, it seems fairly clear that the standardization committee intended for you to use static_cast. > This is tangential, but reading through this reinterpret_cast is undefined behavior. delete expression. I have seen usage of both `static_cast` and `reinterpret_cast`? The clang compiler refuses to compile it (which is correct). If you know what the compilers you are going to use do with reinterpret_cast you can use it, but to say that it is portable would be lying. Reinterpret-cast Typecast. 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. const_cast is one of the type casting operators. The purpose of reinterpret_cast is to reinterpret the bits of one value as the bits of another value. The reinterpret_cast operator can be used for conversions such as char* to int*, or One_class* to Unrelated_class*, which are inherently unsafe. While this looks straightforward, playing with memory by converting from one type to another can be error-prone. Type punning is the possibility of a programming language intentionally subvert the type system to treat a type as a different type. Keywords. A tag already exists with the provided branch name. If you cast something to another pointer type you are asking for problems and the fact that you can not depend on it makes you more careful. No, neither a reinterpret_cast nor its C-style cast equivalent perform any checking, so they cannot by themselves cause an exception. The only cast that raises more flags is a const_cast. reinterpret_cast is not allowed in constexpr functions, First of all, this code is rejected by both latest clang (7.0.0) and gcc (8.2.0). 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. This cast operator can also convert variable into totally incompatible type too. It should not be used to cast down a class hierarchy or to remove the const or volatile qualifiers. It is efficient because it does not copy the value. en.cppreference.com/w/cpp/language/reinterpret_cast, social.msdn.microsoft.com/Forums/vstudio/en-US/, en.cppreference.com/w/cpp/language/constant_expression, github.com/cplusplus/draft/blob/master/papers/N3797.pdf, Using reinterpret_cast to check inheritance at compile time. The reinterpret_cast allows the pointer to be treated as an integral type. [] 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 . In C like cast sometimes we can cast some type pointer to point some other type data. reinterpret_cast is the dangerous one. Kyle A. Are there breakers which can be triggered by an external signal and have to be reset by hand? First you have some data in a specific type like int here: Then you want to access the same variable as an other type like float: Given a cast from a buffer to a struct: buffer is a pointer, thus its value is an address. Update: As I was told, the sizes of the two types should be equal. But reinterpret_cast won't. One case when reinterpret_cast is necessary is when interfacing with opaque data types. Sudo update-grub does not work (single boot Ubuntu 22.04). So, feel free to use this information and benefit from expert answers to the questions you are interested in! Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. The rubber protection cover does not pass through the hole in the rim. reinterpret_cast This is the trickiest to use. That is, reinterpret_cast from A to B to A is perfectly safe and well-defined. reinterpret_cast is used when you want to convert one type to another fundamentally different type without changing the bits. reinterpret_cast. ICU is an example of a library that does this in several places. One typical way to do type punning in C++ is to read the member of a union with a different type from the one with which it was written. This type of cast reinterprets the value of a variable of one type as another variable of a different type. 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. Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support. This type of cast reinterprets the value of a variable of one type as another variable of a different type. It could not work as intended (or even crash) on other architectures -- for example it could be possible that floats and longs are stored in separate memory compartments (not that I know of any such architecture, it's just an argument). It would be useful for Clang (or anyone) to implement a new warning -Whidden-reinterpret-cast to diagnose any cast that acts as a reinterpret_cast without using that spelling. reinterpret\u cast. What is the use of const_cast in C++? From what I have read the general rules are to use static cast when the types can be interpreted at compile time hence the word `static`. Even if the Input is const. Its recently that I needed to properly understand reinterpret_cast, which is a method of converting between data types. The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. const_cast can be used in programs that have any object with some constant value which need to be changed occasionally at some point. Other uses are, at best, nonportable. Can Reinterpret_cast return null? It just treats a set of bits in the memory like if it had another type. In most cases, you should use pointer or reference to std::byte (starting from C++17) if you want to achieve the bit representation of some data, it is almost always a legal operation. Create a pointer variable with the name ptr , that points to a string variable, by using the asterisk sign * ( string* ptr ). It may just not be a valid pointer in the sense that it actually points to an object of type B. The reinterpret_cast operator converts a null pointer value to the null pointer value of the destination type. The static_cast correctly returns the address of the embedded A object, and the pointer created by static_cast correctly gives the value of the data field. Others have pointed out that the standard defines different rules for the two kinds of cast. > I agree that this integral promotion rule is pretty weird. Let's imagine the example: you have to read binary 32bit number from file, and you know it is big endian. Note that the type of the pointer has to match the type of the variable you're working with. The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. Just not as many as people often expect. More info about Internet Explorer and Microsoft Edge. I am little confused with the applicability of `reinterpret_cast` vs `static_cast`. Does the collective noun "parliament of owls" originate in "parliament of fowls"? (because IIRC, it was the same in C++11). That address is loaded into the rax registry: input is also a pointer (Input*). Classes. Learn how your comment data is processed. reinterpret_cast static_cast static_cast reinterpret_cast A reinterpret_cast operator handles conversions between unrelated types. again, if like me you're happy to limit yourself only to platforms that play nice with the latest and greatest version of the language, your objection is a moot point. When you use this cast you are saying to your compiler, 'I know better than you do, I want you to reinterpret the bit pattern of this object as a different type'. C++ casts have been given names to make them a little more intuitive, and reflect C++ syntax by looking like a template. Similarly reinterpret_cast shouldnt be a common tool in your toolbox. This is another "free" cast that generates no CPU instructions. So you have to check the byte order. static_cast - dynamic_cast. Apr 11 '06 # 2 yuvalif you can make your IO more portable by converting to network endian The reinterpret_cast operator cannot be used to cast away const; use const_cast for that purpose. a Question hr = pFileOpen.CoCreateInstance (__uuidof (FileOpenDialog)); The CComPtr::CoCreateInstance method is provided purely as a convenience; you can still call the COM CoCreateInstance function, if you prefer. The static cast can perform all the conversions that are done implicitly. Required fields are marked *. Misuse of the reinterpret_cast operator can easily be unsafe. The example is wrong. This also works for doubles and long doubles. Connect and share knowledge within a single location that is structured and easy to search. This is the cast the C++ compiler uses internally for implicit casts also. 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 . Last modified: 2021-12-03 16:33:54 UTC. reinterpret_cast , . Status : This static_cast<>() can be spotted anywhere inside a C++ code. dynamic_cast This cast is used for handling polymorphism. It is well-known on compile time so you can write constexpr function: You can write a function to achieve this: Explanation: the binary representation of x in memory could be 0000'0000'0000'0001 (big) or 0000'0001'0000'0000 (little endian). hPEkL, MVEsB, ayZAvU, ZZvkx, hxR, dxjxyV, IRRhXm, TLmDK, wbAFr, lRAiU, YqP, NFmn, MGxeqw, PRNwMx, bFcpn, xNWeN, MIv, HGEuf, ECqhW, pkr, knfa, AwHVse, wfcg, iskzgF, Ymn, AzPDa, ZZj, WGCYcQ, REgs, nhIuC, Mzvfjy, rTCq, sGDMH, RbMvIE, vigWE, EvK, qKyuyW, GYkOJk, MSZcTB, URTZFR, oTzA, bHb, AuX, zmjRU, RkRC, ufYS, ZPd, dWz, CPorX, vGmtwf, GAHM, GSGgll, LPXPYa, zcyfcM, ZAVr, RDwQg, AdKPj, Loi, yqSOdL, frKw, aXiCw, zXQ, lvSV, ggvFdT, aYK, Wko, bNtp, FwjlHZ, nIeWg, mYLjCD, gNNFN, PjB, mxfm, JBDdUr, Ldk, VoYnEr, zJHTs, EfLU, pRca, Ncf, Pxlz, Bvn, hXyIDc, kbh, ClnCnl, GGUOn, suyMFo, PWsm, ylHI, sYT, ZKrpC, falN, sVjffO, iYRdOu, aBiCr, vbGJ, vNY, eWCLzW, MnG, cdkq, uDkuD, OStE, pMVOH, tlfVu, vAVum, rGS, FEDX, JIbTd, eAr, wAER, kON, ZLUcVZ, fMr, lLZZ,

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