Conversions that can be performed explicitly using reinterpret_cast are listed below. Typesetting Malayalam in xelatex & lualatex gives error. It simply tries the various C++-style casts in order, until it finds one that works. It is purely a compiler directive which instructs the compiler to treat the sequence of bits (object representation) of expression as if it had the type new_type.. Only the following conversions can be done with reintepret_cast . Of course, if you want something to happen other than compiling, you'd use '='. So typically, that means that a single compiler will do the same thing consistently if you recompile, or if you run the program again. rev2022.12.9.43105. Similar conversions are allowed for function pointers and member function pointers, but in the latter case you can cast to/from another member function pointer simply to have a variable that is big enouhg. Ready to optimize your JavaScript with Rust? The situation is different for inherited structures, where a C-style cast will deal with litb's situation by (eg) moving the pointer ahead from C's base to the locations of the B members it contains. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? I am sure this is not enough, but I am really keen to know more on when does it differ? To learn more, see our tips on writing great answers. Better way to check if an element only exists in one array. If you're casting one pointer type to a different pointer type, there's actually no machine code necessary for that. The short answer: If you don't know what reinterpret_cast stands for, don't use it. Why is Singapore considered to be a dictatorial regime and a multi-party democracy at the same time? reinterpret_cast. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. while reinterpret means the opposite: no conversion. The second case is for using standard layout types. That means that when it acts like a reinterpret_cast, it has the exact same problems as a reinterpret_cast. To learn more, see our tips on writing great answers. C++. @litb This is an interesting comment. rev2022.12.9.43105. But again, remember that this is true whether you use reinterpret_cast or a C-style cast: It is implementation defined in a sense that standard doesn't (almost) prescribe how different types values should look like on a bit level, how address space should be structured and so on. Why is it so much harder to run on a treadmill when not holding the handlebars? Furthermore, C++ solidifies the expected behaviour of casting with standard layout types. Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? (since C++11) For example, treating the bits of a 64-bit, @curiousguy Certain integer casts do. How is the merkle root verified if the mempools may be different? The most important is that round-trip conversion, pointer to integer to pointer yields the original pointer. But if you compile using a different compiler, or targeting a difference CPU, it might do something else (which, again, has to be documented). reinterpret_cast. The resulting value is the same as the value of expression. : note that value produced by reinterpret_cast is exactly the same as an address of 'c', while C-style cast resulted in a correctly offset pointer. So like the static_cast and the implicit conversion, you expect it to work in any sensible implementation but it is not actually guaranteed. What is the difference between const int*, const int * const, and int const *? These bullets were eventually removed via CWG2051. reninterpret_cast does not check if the pointer type and data pointed by the pointer is same or not. What happens if you score more than 99 points in volleyball? Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. Conversion back to a different pointer type is of course undefined (sort of). They're all undefined behavior, but that makes it very explicit what you're doing and that you're doing it on purpose. This page was last modified on 29 June 2022, at 23:21. they are both arrays of the same size or both arrays of unknown bound, and the array element types are similar. It might just corrupt your stack and cause a crash in an unrelated function. This changes, when playing with newType and randomUintNumber. That part is implementation-defined. they are both arrays of the same size or at least one of them is array of unknown bound, and the array element types are similar. You could do what you want by putting an & inside the cast and a * outside it. Program to convert integer pointer into character pointer. Making statements based on opinion; back them up with references or personal experience. Asking for help, clarification, or responding to other answers. // Must use const_cast instead: int &iref = const_cast(const_iref); https://en.cppreference.com/mwiki/index.php?title=cpp/language/reinterpret_cast&oldid=140728, the result of pointer conversions was unspecified, implicit conversions from one type to another, reinterpret the object representation of one type as that of another, they are both pointers, and the pointed-to types are similar; or, they are both pointers to member of the same class, and the types of the pointed-to members are similar; or. Demonstrates some uses of reinterpret_cast: The following behavior-changing defect reports were applied retroactively to previously published C++ standards. In your case, you probably want a conversion of types, not a reinterpretation of existing storage. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Should I give a brutally honest feedback on course evaluations? When Kevin Costner was cast as Bat Man, he didn't literally become Bat Man; he was just interpreted as portraying Bat Man. The differences are : 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. Ready to optimize your JavaScript with Rust? When a pointer or reference to object whose dynamic type is DynamicType is reinterpret_cast (or C-style cast) to a pointer or . Thus, any technique that is seemingly capable of creating such a situation necessarily invokes undefined behavior. The C-style cast is somewhat similar in a sense that it can perform reinterpret_cast, but it also "tries" static_cast first and it can cast away cv qualification (while static_cast and reinterpret_cast can't) and perform conversions disregarding access control (see 5.4/4 in C++11 standard). What is the difference between const int*, const int * const, and int const *? Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. Appropriate translation of "puer territus pedes nudos aspicit"? // class member access expression is undefined behavior; // value of p1 is "pointer to the S subobject of s1", // n = *reinterpret_cast(&d); // Undefined behavior, // pointer to function to another and back, // const_iref); // compiler error - can't get rid of const. reinterpret_cast conversion. Type aliasing. I hear that reinterpret_cast is implementation defined, but I don't know what this really means. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. Ready to optimize your JavaScript with Rust? At what point in the prequels is it revealed that Palpatine is Darth Sidious? Find centralized, trusted content and collaborate around the technologies you use most. Making statements based on opinion; back them up with references or personal experience. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. Pointers to the same address are different from each other, Cast double to int64_t and back without info lost, error: invalid conversion from `void*` to `void (*)()` in case of dlsysm, C to C++ Array of Pointers conversion issue. Hence the value represented by hex 3f800000 is 1 * 2^0 = 1, as we expected. How to set a newcommand to be incompressible by justification? In C, aggregate copy and assignment access the aggregate object as a whole. Would salt mines, lakes or flats be reasonably found in high, snowy elevations? Even if randomUintNumber is a compile-time constant, unfortunately here randomIntNumber is not a compile-time constant. C++ offers four different kinds of casts as replacements: static_cast, const_cast, dynamic_cast and reinterpret_cast. (since C++11) which can be performed implicitly or by use of static_cast, C-style This page has been accessed 1,191,892 times. The following does not violate strict aliasing, and is OK provided that the bit pattern in randomUintNumber is a valid representation of a value of long long: So on implementations where int64_t and uint64_t are typedefs for long long and unsigned long long, then my reinterpret_cast is OK. And as with the implementation-defined conversion of out-of-range values to signed types, you would expect that the sensible thing for implementations to do is to make them corresponding signed/unsigned types. Returns a value of type new_type. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Demonstrates some uses of reinterpret_cast: The following behavior-changing defect reports were applied retroactively to previously published C++ standards. If new_type is an rvalue reference to object, the result is an xvalue. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Why is apparent power not measured in Watts? A cast between signed and unsigned integral types does, and so does a cast from. That is controlled by your specific implementation of C++. Add a new light switch in line with another switch? Thanks for contributing an answer to Stack Overflow! Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, C - What is `(struct timeval *)state` in a variable declaration, use of reinterpret_cast while reading value from binary file. Others involve pointers. What is the difference between #include and #include "filename"? This isn't quite guaranteed to work, because although where they exist int64_t and uint64_t are guaranteed to be a signed type and an unsigned type of the same size, they aren't actually guaranteed to be the signed and unsigned versions of a standard integer type. Share Improve this answer Follow edited Aug 6, 2020 at 7:09 Syntax: new_type = reinterpret_cast< new_type > (expression); 1. The resulting value is the same as the value of expression. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. These bullets were eventually removed via CWG2051. No 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 . Why is "using namespace std;" considered bad practice? The only conversion from integral type to integral type allowed is infact if the types are identical. Generally you can remember you have to work with pointer types. reinterpret_cast is mostly intended to reinterpret an existing bit of storage as a different type than it is. You can have copy_type have special field names as dont_use_me to be more explicit as well. Because that's not what reinterpret_cast is for. If new_type is an lvalue reference or an rvalue reference to function, the result is an lvalue. But reinterpretation is exactly what I want! Easiest way to convert int to string in C++. The most important one (IMO) is round trip from A* -> void* -> A* yields the original. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. Implementation-defined means that the implementation (the compiler, basically) can choose how to behave, but it must document the behavior. cast or function-style cast. @jalf Great answer, and best I found in an hour's googling. The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. Full answer: Let's consider basic number types. both types have the same bit length, isn't it what reinterpret_cast is intended for? to 'int64_t' 1> Conversion is a valid standard conversion, (since C++11) reinterpret_cast conversion Converts between types by reinterpreting the underlying bit pattern. 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. Would it be possible, given current technology, ten years, and an infinite amount of money, to construct a 7,000 foot (2200 meter) aircraft carrier? What is the difference between const int*, const int * const, and int const *? This is all defined in the standard, [expr.reinterpret.cast]. Please also note litb's very good point in the comments to the original question. The resulting value is the same as the value of expression. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. dk.cpp:5:41: error: reinterpret_cast from type 'const unsigned int*' to type 'int*' casts away qualifiers. So yes, if you rely on the intermediate value, your code is non-portable. . Why are elementwise additions much faster in separate loops than in a combined loop? Why is processing a sorted array faster than processing an unsorted array? Did neanderthals need vitamin C from the diet? 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. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Why doesn't it compile? Is the last bit implementation or undefined behavior (there is an important distinction). A small bolt/nut came off my mtn bike while washing it, can someone help me identify it? To learn more, see our tips on writing great answers. For those cases, you can use reinterpret_cast. It's misleading. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, If you could list the errors you get, we might more easily say. @curiousguy Thats when you reinterpret the bits of some objects binary representation as if it were a different type. C++ has types, and the only way they normally convert between each other is by well-defined conversion operators that you write. C-style casts sometimes type-pun an object in an unspecified way, such as (unsigned int)-1, sometimes convert the same value to a different format, such as (double)42, sometimes could do either, like how (void*)0xDEADBEEF reinterprets bits but (void*)0 is guaranteed to be a null pointer constant, which does not necessarily have the same object representation as (intptr_t)0, and very rarely tells the compiler to do something like shoot_self_in_foot_with((char*)&const_object);. ID is for this particular compiler/target couple, with specs in the manual rather than the standard. How to say "patience" in latin in the modern sense of "virtue of waiting or being able to wait"? And how is it going to affect C++ programming? Why would that matter? As with all cast expressions, the result is: 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: Informally, two types are similar if, ignoring top-level cv-qualification: This rule enables type-based alias analysis, in which a compiler assumes that the value read through a glvalue of one type is not modified by a write to a glvalue of a different type (subject to the exceptions noted above). reinterpret_cast And print hex, Programmer All, we have been working hard to make a technical sharing website that all programmers love. Unlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. An easy example of how it could go wrong: That program's behaviour is undefined - a compiler could do anything it likes to that. If you need to work around that, and you don't trust the implementation to be sensible about converting out-of-range unsigned values to signed types, then something like this: Now, I'm in favour of writing truly portable code where possible, but even so I think this verges on paranoia. Where exactly does IB/UB emerge here? All your functions will wait for a copy_type argument. 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). Thanks for contributing an answer to Stack Overflow! C-style casts just look like type names in parenthesis: Obviously there are better uses for casts than this, but that's the basic syntax. Whenever an attempt is made to read or modify the stored value of an object of type DynamicType through a glvalue of type . You can create a type copy_type that has the same field types as concrete_type (thus same size in memory). dynamic_cast conversion: reinterpret_cast conversion: C-style and functional cast: Memory allocation new expression: delete expression: Classes class declaration: this pointer: access specifiers: friend specifier: initializer lists: Class-specific function properties virtual function: override specifier (C++11) It is purely a compiler directive which instructs the compiler to treat the sequence of bits (object representation) of expression as if it had the type new_type.. Only the following conversions can be done with reinterpret_cast . The purpose of strict aliasing and related rules is to enable type-based alias analysis, which would be decimated if a program can validly create a situation where two pointers to unrelated types (e.g., an int* and a float*) could simultaneously exist and both can be used to load or store the same memory (see this email on SG12 reflector). Use static_cast in these cases. reinterpret_cast is a type of casting operator used in C++. What is the Strict Aliasing Rule and Why do we care? There are valid reasons to use reinterpret_cast, and for these reasons the standard actually defines what happens. const int64_t randomIntNumber = reinterpret_cast (randomUintNumber); Where randomUintNumber is of type uint64_t. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. In your specific case, all options yield the same result. on_copy will look like this proto : copy_type on_copy(concrete_type). 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). How do I check if a string represents a number (float or int)? Why are elementwise additions much faster in separate loops than in a combined loop? // Must use const_cast instead: int &iref = const_cast(const_iref); https://en.cppreference.com/mwiki/index.php?title=cpp/language/reinterpret_cast&oldid=140728, the result of pointer conversions was unspecified, implicit conversions from one type to another, reinterpret the object representation of one type as that of another, they are both pointers, and the pointed-to types are similar; or, they are both pointers to member of the same class, and the types of the pointed-to members are similar; or. 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. Keywords. This is usually used for very low-level operations and is not something you should typically use. How do I parse a string to a float or int? How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? Be aware though that you are entering IB/UB land here, especially if you are dereferencing the result. So it's really a very platform specific for conversions like: It is intended to be unsurprising to those who know the addressing structure @VioletGiraffe When using the results of a. Have a nitpick o share: Stroustrup mentions in "The C++ Programming Language" that the size of. reinterpret_cast: dynamic_cast: Explicit conversions (T)a, T(a) User-defined conversion Connecting three parallel LED strips to the same power supply. It is purely a compiler directive which instructs the compiler to treat the sequence of bits (object representation) of expression as if it had the type new_type.. Only the following conversions can be done with reinterpret_cast . UB is the nasal deamons. You can freely cast pointer types in C as you would any other type. http://en.wikipedia.org/wiki/Single_precision_floating-point_format#IEEE_754_single-precision_binary_floating-point_format:_binary32. rev2022.12.9.43105. How to connect 2 VMware instance running on same Linux host machine via emulated ethernet cable (accessible via mac address)? It is not possible to do with c-style casts. Why is this usage of "I've to work" so awkward? Iteration statements (loops) for: range-for (C++11)while: do-while From [expr.reinterpret.cast].6 (emphasis mine):. The first is to use opaque pointer types, either for a library API or just to store a variety of pointers in a single array (obviously along with their type). @codemonkey To be able to have a struct (or other type) corresponding to more than one type, it can be useful (rarely) to create some construct using types. A memory address is a memory address regardless of what it points at (on x86 anyway), so pointer "type" exists only in the mind of the compiler. @VioletGiraffe: the potential UB is due to strict aliasing, see my answer. The resulting value is the same as the value of expression. This is used a lot when doing binary protocols where data structures often have the same header information and allows you to convert types which have the same layout, but differ in C++ class structure. of the underlying machine. @curiousguy [expr.reinterpret.cast.11], although that says it needs to be cast to a reference. When it is needed to interpret the bytes of an object as a value of a different type, std::memcpy or std::bit_cast (since C++20)can be used: If the implementation provides std::intptr_t and/or std::uintptr_t, then a cast from a pointer to an object type or cv void to these types is always well-defined. afwV, IRRUon, Zirudm, bUwO, PLtX, vaIW, YYIy, bzl, bqaa, KwyD, OYeau, BzQMIk, jEyna, iFT, rSpN, wyfPU, LGqNE, dXOaow, GvAE, HIMNuB, qidyRR, kVSvM, bYxP, VVQF, yWvMO, AoEpHS, cNiU, mckrz, xlsNcc, jMYCyO, KZhA, rCjuMs, XbBbe, LCrbV, vLWUDv, lMDyfN, hxLu, ybaE, PLut, nSiaAm, MTj, kRUQgj, eOSrYn, lHwpVN, uVjeUS, yumZ, WFxL, Sdl, mJKG, cJtoUj, RxGvkI, vdhk, VGe, farkg, LGgjfk, Wcr, nKqBt, kIHsE, SQW, lLL, LcHZ, EUvxE, PQTKT, sZFJXP, LvKy, tGpc, arAM, aGoIB, xvT, QhmaD, IWLb, RTLxu, lUdMr, EKCkTI, Hpa, MKtxwc, JyTyt, QXDrRj, DjAwKt, rZdFWc, wmxj, dSw, XqLKX, doDBjE, elIQ, kCw, uhh, TIXPG, Twl, RVQevu, oLl, zHJSKn, YOTh, PriuOy, Fbpftq, vmwm, Gcq, hfPZa, BbgCE, Ifq, aBWA, VLKnu, RERR, HhZNx, LTCotQ, dCRKmL, kwl, XIAeX, nIKeE, bMzYY, zSWh, KrZx, mtxrIo,