For example : 2. When a pointer or reference to object whose dynamic type is DynamicType is reinterpret_cast (or C-style cast) to a pointer or . How to print and pipe log file at the same time? What's the difference between undefined and implementation defined? Appropriate translation of "puer territus pedes nudos aspicit"? Connect and share knowledge within a single location that is structured and easy to search. What is the difference between #include and #include "filename"? which can be performed implicitly or by use of static_cast, C-style But then, how "random" is a compile-time constant? Returns a value of type new_type.. [] ExplanatioUnlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. error C2440: 'reinterpret_cast' : cannot convert from 'const uint64_t' I suppose the language designers in all their wisdom decided that it's not considered "unsafe enough" to warrant a reinterpret_cast. From cppreference.com . http://en.wikipedia.org/wiki/Single_precision_floating-point_format#IEEE_754_single-precision_binary_floating-point_format:_binary32. reinterpret_cast And print hex, Programmer All, we have been working hard to make a technical sharing website that all programmers love. This page was last modified on 29 June 2022, at 23:21. This changes, when playing with newType and randomUintNumber. How to say "patience" in latin in the modern sense of "virtue of waiting or being able to wait"? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. This is something that was de factor supported prior to C++11 and has now been specified in the standard. No And how is it going to affect C++ programming? All your functions will wait for a copy_type argument. rev2022.12.9.43105. reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . Converts between types by reinterpreting the underlying bit pattern. But it's not actually guaranteed in the standard AFAIK. 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);. When would I give a checkpoint to my D&D party that they can return to if they die? Find centralized, trusted content and collaborate around the technologies you use most. There are valid reasons to use reinterpret_cast, and for these reasons the standard actually defines what happens. The resulting value is the same as the value of expression. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. reinterpret_cast is a type of casting operator used in C++. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. reinterpret_cast evaluates expression and converts its value to the type new_type. Reading a different member of a union than you last wrote to does not. If you can take the address of the value, one way is to cast a pointer to it to a pointer to a different type, and then dereference the pointer. 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. 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. 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). Making statements based on opinion; back them up with references or personal experience. You can freely cast any type, it just might not do what you hope it does, and might not be guaranteed to do anything at all. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? Thanks for contributing an answer to Stack Overflow! Let's confirm the output above makes sense, anyway. What is the C equivalent for reinterpret_cast? I'm not sure static_cast (or C-cast) will produce the desired result, have to refresh my memory on how it works first. I'm not certain what you're trying to achieve here, but if you want randomIntNumber to have the same value as randomUintNumber, then do. Cast doesn't necessarily mean conversion. Only the following conversions can be done with reintepret_cast, except when such conversions would cast away constnessor volatility. Note that this is probably not guaranteed to work by the C standard. Iteration statements (loops) for: range-for (C++11)while: do-while But reinterpretation is exactly what I want! I am sure this is not enough, but I am really keen to know more on when does it differ? A function pointer can be explicitly converted to a function pointer of a different type. Where do I find the current C or C++ standard documents? Because of this, the behaviour of reinterpret_cast depends upon how your compiler lays structures out in memory and how it implements reinterpret_cast. 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. How could I forget That's exactly how I've solved this problem the last time I've encountered it (casting a pointer to target type and dereferencing it). 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. This is IEEE-754 floating point format: a sign bit of 0, followed by an 8-bit exponent (011 1111 1), followed by a 23 bit mantissa (all zeroes). If they were pointers, maybe it would matter that they pointed to things of the same size, but they're not pointers. 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). Returns a value of type new_type. The conversion from const char* as returned by c_str() to std . To learn more, see our tips on writing great answers. 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. The only conversion from integral type to integral type allowed is infact if the types are identical. 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 . If you will need it in the future, you will know. How do I check if a string represents a number (float or int)? 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. reinterpret_cast conversion. Typesetting Malayalam in xelatex & lualatex gives error. 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. C-style casts are quite similar to reinterpret_casts, but they have much less syntax and are not recommended. 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). Ready to optimize your JavaScript with Rust? Whenever an attempt is made to read or modify the stored value of an object of type DynamicType through a glvalue of type . In C, aggregate copy and assignment access the aggregate object as a whole. Asking for help, clarification, or responding to other answers. How do I convert a String to an int in Java? In general, that's all you both need and should use to write your programs. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. A cast between signed and unsigned integral types does, and so does a cast from. 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. Ready to optimize your JavaScript with Rust? If you know C, you know which one the C-style cast does, but it's nicer in some ways to have different syntax for both. My Question is now: is it save using reinterpret_cast in this way, as long i don't break the inheritance hierarchy like mentioned above? From C++11 Standard(N3376) 5.2.10.1 this document, page 101: Conversions that can be performed explicitly using reinterpret_cast are listed below. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? (since C++11) Thanks for contributing an answer to Stack Overflow! 1980s short story - disease of self absorption. 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. Why would that matter? reinterpret_cast: dynamic_cast: Explicit conversions (T)a, T(a) User-defined conversion Better way to check if an element only exists in one array. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Find centralized, trusted content and collaborate around the technologies you use most. Type aliasing . Asking for help, clarification, or responding to other answers. reinterpret_cast is used for reinterpreting the storage of the object as a different object. That is controlled by your specific implementation of C++. 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. reinterpret_cast is a type of casting operator used in C++. Is it correct to say "The glue on the back of the sticker is dying down so I can not stick the sticker to the wall"? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. What is wrong with removing constness using const_cast? C++. Hence the value represented by hex 3f800000 is 1 * 2^0 = 1, as we expected. how about some abstract types? rev2022.12.9.43105. (since C++11) What is the difference between ++i and i++? reinterpret_cast. The thinking goes that casting is inherently an ugly operation and it requires ugly syntax to inform the programmer that something dubious is happening. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. Improve INSERT-per-second performance of SQLite, Obtain closed paths using Tikz random decoration on circles. 1. reinterpret_cast can not remove constness. Should I give a brutally honest feedback on course evaluations? How to convert unique_ptr<derived>* to unique_ptr<base>*? A lot of these interpretations is implementation dependent, and the standard lists a specific (rather long to quote here) list of things that can be done with a reinterpret_cast (mostly casting between different pointer/reference types), but says: No other conversion can be performed explicitly using By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. It can cast integer and floating point types into each other. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. 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 . Allow non-GPL plugins in a GPL main program. What is the difference between const int*, const int * const, and int const *? From [expr.reinterpret.cast].6 (emphasis mine):. Except that converting a prvalue of type "pointer to T1" to the type "pointer to T2" (where T1 and T2 are function types) and back to its original type yields the original pointer value, the result of such a pointer conversion is unspecified. Why is "using namespace std;" considered bad practice? 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). Share Improve this answer Follow edited Aug 6, 2020 at 7:09 Where does the idea of selling dragon parts come from? If new_type is an lvalue reference or an rvalue reference to function, the result is an lvalue. static_cast - dynamic_cast: const_cast - reinterpret_cast: Memory allocation: new expression: delete expression: Classes: Class declaration: Initializer lists: this pointer: Access specifiers: friend specifier: Class-specific function properties: It simply tries the various C++-style casts in order, until it finds one that works. What is the difference between const int*, const int * const, and int const *? while reinterpret means the opposite: no conversion. So typically, that means that a single compiler will do the same thing consistently if you recompile, or if you run the program again. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . Interactive code here. Code that violates strict aliasing has undefined behavior. This is usually used for very low-level operations and is not something you should typically use. Otherwise, the result is a prvalue and lvalue-to-rvalue, array-to-pointer, or function-to . 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. You can create a type copy_type that has the same field types as concrete_type (thus same size in memory). But if you take the resulting float* and reinterpret_cast it back into an int*, then you will get the original pointer. 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. This page has been accessed 1,191,892 times. // value of p1 is "pointer to s1.a" because, // s1.a and s1 are pointer-interconvertible, // value of p2 is unchanged by reinterpret_cast, // u.a and u are pointer-interconvertible, // value of p4 is "pointer to u.b": u.a and, // u.b are pointer-interconvertible because, // both are pointer-interconvertible with u, // value of p5 is unchanged by reinterpret_cast. 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). cast or function-style cast. Why is this usage of "I've to work" so awkward? Atlas 300 3010-VPC:1. C++11 introduced a standardized memory model. Conversions that can be performed explicitly using reinterpret_cast are listed below. did anything serious ever run on the speccy? 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). Did neanderthals need vitamin C from the diet? It is purely a compile-time directive which instructs the compiler to treat expression as if it had the type new-type. Others involve pointers. How to use a VPN to access a Russian website that is banned in the EU? 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. Not the answer you're looking for? Please also note litb's very good point in the comments to the original question. That's what reinterpret cast is for. 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. Making statements based on opinion; back them up with references or personal experience. Is there any reason on passenger airliners not to have a physical lock between throttles? @codemonkey 2/2 in case you passed concrete_type directly to functions, compiler will trigger an error. UB is the nasal deamons. Conversely, a C-style cast (as in (int)42.0) is much harder to find reliably by searching To answer the other part of your question, yes, reinterpret_cast is implementation-defined. 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. Why is Singapore considered to be a dictatorial regime and a multi-party democracy at the same time? 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). Better way to check if an element only exists in one array. As mentioned in the previous section, implicit type conversion is safe, and explicit type conversion is risky. For example, dk.cpp: In function 'int main()': Of course, if you want something to happen other than compiling, you'd use '='. Add a new light switch in line with another switch? Returns a value of type new_type. Be aware though that you are entering IB/UB land here, especially if you are dereferencing the result. These bullets were eventually removed via CWG2051. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. [.] Otherwise the result is implementation-defined, but I expect all known implementations that have int64_t will define it to do the obvious thing: the result is equivalent to the input modulo 264. The only conversion from integral type to integral type allowed is infact if the types are identical. 4 base base_collectionderived::base derived_collection::base_collection base_collection . The short answer: If you don't know what reinterpret_cast stands for, don't use it. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. reinterpret_cast. Would salt mines, lakes or flats be reasonably found in high, snowy elevations? Thus, any technique that is seemingly capable of creating such a situation necessarily invokes undefined behavior. Even if randomUintNumber is a compile-time constant, unfortunately here randomIntNumber is not a compile-time constant. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. dk.cpp:5:41: error: reinterpret_cast from type 'const unsigned int*' to type 'int*' casts away qualifiers. Where exactly does IB/UB emerge here? Because that's not what reinterpret_cast is for. reninterpret_cast does not check if the pointer type and data pointed by the pointer is same or not. If you use reinterpret_cast, it is easy to find the places where you did it. If you don't want to go through standard wording, you can find all that reinterpret_cast can do here. To interpret the mantissa, write it after 1 followed by a decimal point: 1.00000000000000000000000 (23 zeroes). Why is apparent power not measured in Watts? A small bolt/nut came off my mtn bike while washing it, can someone help me identify it? In your specific case, all options yield the same result. Why are elementwise additions much faster in separate loops than in a combined loop? The resulting value is the same as the value of expression. @jalf Great answer, and best I found in an hour's googling. Why 'most likely'? In C, aggregate copy and assignment access the aggregate object as a whole. You can have copy_type have special field names as dont_use_me to be more explicit as well. So if you know what you do and how it all looks like on a low-level nothing can go wrong. But there's no inheritance in C anyway. Is the last bit implementation or undefined behavior (there is an important distinction). on_copy will look like this proto : copy_type on_copy(concrete_type). @edA-qamort-ora-y: Yes it is 5.2.10/4 "A pointer can be explicitly converted to any integral type. Other uses are, at best, nonportable. Converts between types by reinterpreting the underlying bit pattern. In both of these cases you should use the explicit reinterpret_cast operator rather than the C-Style. Have a nitpick o share: Stroustrup mentions in "The C++ Programming Language" that the size of. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. A small bolt/nut came off my mtn bike while washing it, can someone help me identify it? What does it mean? For example: In this code c is guaranteed to point to the object b as you'd expected. All the permitted conversions with reinterpret_cast involve pointers or references, with the exception that an integer or enum type can be reinterpret_cast to itself. To learn more, see our tips on writing great answers. Easiest way to convert int to string in C++. 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. What would be the exception here? 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). It's misleading. To learn more, see our tips on writing great answers. When Kevin Costner was cast as Bat Man, he didn't literally become Bat Man; he was just interpreted as portraying Bat Man. // 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. What is the Strict Aliasing Rule and Why do we care? . Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? 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). That part is implementation-defined. @curiousguy [expr.reinterpret.cast.11], although that says it needs to be cast to a reference. That means that when it acts like a reinterpret_cast, it has the exact same problems as a reinterpret_cast. The most important is that round-trip conversion, pointer to integer to pointer yields the original pointer. Both are valid statements, and both will compile only because of the cast. Type Conversion Operators: static_cast, dynamic_cast, const_cast and reinterpret_cast in C++. It can do many different things, and it's not always clear from reading the code which type of cast will be invoked (it might behave like a, Consequently, changing the surrounding code might change the behaviour of the cast, It's hard to find when reading or searching the code -. Why are elementwise additions much faster in separate loops than in a combined loop? When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? If new_type is an rvalue reference to object, the result is an xvalue. // value of p1 is "pointer to s1.a" because, // s1.a and s1 are pointer-interconvertible, // value of p2 is unchanged by reinterpret_cast, // u.a and u are pointer-interconvertible, // value of p4 is "pointer to u.b": u.a and, // u.b are pointer-interconvertible because, // both are pointer-interconvertible with u, // value of p5 is unchanged by reinterpret_cast. (since C++11) Why is apparent power not measured in Watts? In this case the standard treats reinterpret_cast as a static_cast to void* first and then a static_cast to the desination type. // 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. What happens if you score more than 99 points in volleyball? The reinterpret_cast operator can be used for conversions such as char* to int*, or One_class* to Unrelated_class*, which are inherently unsafe. It recommends memcpy and argues, that compiler optimization will skip the copying anyway. Why does the USA not have a constitutional court? So it's really a very platform specific for conversions like: It is intended to be unsurprising to those who know the addressing structure : 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. Find centralized, trusted content and collaborate around the technologies you use most. But if you compile using a different compiler, or targeting a difference CPU, it might do something else (which, again, has to be documented). I see some conversions you showed are guaranteed, does it depend on the type of objects we are interpreting e.g float* to int* or is it because reinterpret_cast works that way. Say we have a struct concrete_type we always want to call a function on_copy onto when passing as an arg to some functions. Why is Singapore considered to be a dictatorial regime and a multi-party democracy at the same time? It might just corrupt your stack and cause a crash in an unrelated function. Conversion back to a different pointer type is of course undefined (sort of). 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 . Most probably, you would get a crash when the string's destructor is called, but who knows! So yes, if you rely on the intermediate value, your code is non-portable. For example, treating the bits of a 64-bit, @curiousguy Certain integer casts do. If you're casting one pointer type to a different pointer type, there's actually no machine code necessary for that. No, reinterpret_cast is for pointer casts. It is not possible to do with c-style casts. Reinterpret_cast in c++ allows any pointer to be converted into any other pointer type. Furthermore, C++ solidifies the expected behaviour of casting with standard layout types. @VioletGiraffe: If you want to reinterpret a certain storage as a different type, then you should reinterpret cast a pointer to that storage. Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. @VioletGiraffe When using the results of a. Can you provide an example of how it can go wrong, and it goes wrong, is it better to use C-Style cast? How did muzzle-loaded rifled artillery solve the problems of the hand-held rifle? This page was last modified on 29 June 2022, at 23:21. @JoachimPileborg: my bad, expanded the question. The exponent is 0. To interpret the exponent, subtract 127: 01111111b = 127, and 127 - 127 = 0. 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. 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 . @edA-qa mort-ora-y: Assuming the integer is large enough to hold the pointer. The paragraph defining the strict aliasing rule in the standard used to contain two additional bullets partially inherited from C: These bullets describe situations that cannot arise in C++ and therefore are omitted from the discussion above. reinterpret_cast conversion Converts between types by reinterpreting the underlying bit pattern. How to set a newcommand to be incompressible by justification? @litb This is an interesting comment. Sed based on 2 words, then replace whole line with variable. You cannot use reinterpret_cast to cast from float to int anyway, but it would be similar for a type that was supported (for example, between different pointer types). Keywords. rev2022.12.9.43105. Syntax : The resulting value is the same as the value of expression. E.g. both types have the same bit length, isn't it what reinterpret_cast is intended for? The paragraph defining the strict aliasing rule in the standard used to contain two additional bullets partially inherited from C: These bullets describe situations that cannot arise in C++ and therefore are omitted from the discussion above. The reinterpret_cast operator cannot be used to cast away const; use const_cast for that purpose. ID is for this particular compiler/target couple, with specs in the manual rather than the standard. Iteration statements (loops) for: range-for (C++11)while: do-while Both reinterpret_cast and c-style casts are implementation defined and they do almost the same thing. Why doesn't it compile? What is the C equivalent for the reinterpret_cast from C++? is a great article, describing the problem and its solution (maybe skip the lengthy part "Now, to the Rule-Book" ;) ). You are allowed to convert a pointer to a suitably sized integer and then back to a pointer and it will be the exact same pointer. Use static_cast in these cases. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. . They're all undefined behavior, but that makes it very explicit what you're doing and that you're doing it on purpose. For those cases, you can use reinterpret_cast. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. At what point in the prequels is it revealed that Palpatine is Darth Sidious? Don't conclude from the answers that (type)expression in C++ is equivalent to a reinterpret_cast. @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. This is bad, as it ends up with undefined behavior (maybe you code malfunctions with a new compiler version.) ReInterpret Cast ( reinterpret_cast) is a cast operator that converts a pointer of some data type into a pointer of another data type, even if the the data types before and after conversion are different. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. 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 . 1)An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. No other conversion can be performed explicitly using reinterpret_cast. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . How is the merkle root verified if the mempools may be different? Assuming that alignment requirements are met, a reinterpret_cast does not change the value of a pointer outside of a few limited cases dealing with pointer-interconvertible objects: Performing a class member access that designates a non-static data member or a non-static member function on a glvalue that does not actually designate an object of the appropriate type - such as one obtained through a reinterpret_cast - results in undefined behavior: Many compilers issue "strict aliasing" warnings in such cases, even though technically such constructs run afoul of something other than the paragraph commonly known as the "strict aliasing rule". By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. @LokiAstari: when A is an object type :) Anyhow, if one needs a full and precise picture it's better to take a look at the standard. 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. I don't like to say "reinterpret_cast", because cast means conversion (in c), Though the C-style would normally do the same thing, it has the danger of being subjected to overloaded conversion operators. @curiousguy Thats when you reinterpret the bits of some objects binary representation as if it were a different type. I checked the assembly code produced by VC2008 and it omits the same code. 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. 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(uint64) rather than the other methods. These bullets were eventually removed via CWG2051. 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). Keywords. // 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. 1980s short story - disease of self absorption. Demonstrates some uses of reinterpret_cast: The following behavior-changing defect reports were applied retroactively to previously published C++ standards. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Assuming that alignment requirements are met, a reinterpret_cast does not change the value of a pointer outside of a few limited cases dealing with pointer-interconvertible objects: Performing a class member access that designates a non-static data member or a non-static member function on a glvalue that does not actually designate an object of the appropriate type - such as one obtained through a reinterpret_cast - results in undefined behavior: Many compilers issue "strict aliasing" warnings in such cases, even though technically such constructs run afoul of something other than the paragraph commonly known as the "strict aliasing rule". If you want to be portable, you have to do the full round-trip. Use static_cast for this purpose. It does not check if the pointer type and data pointed by the pointer is same or not. The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. Does balls to the wall mean full speed ahead or full speed ahead and nosedive? If that results in a compiler warning, or if you just want to be more explicit, then: The result of the cast has the same value as the input if randomUintNumber is less than 263. But in C++ such actions are always performed through a member function call, which accesses the individual subobjects rather than the entire object (or, in the case of unions, copies the object representation, i.e., via unsigned char). Full answer: Let's consider basic number types. So, if you really want to reinterpret the bits used for your uint64_t as int64_t, then do this: However if you just want to convert the object, preserving its value if possible just do what the compiler suggests and use static_cast instead. Why is it so much harder to run on a treadmill when not holding the handlebars? mathk 28 Note that the (type)exression equivalent of C can do much more than reinterpret_cast in C++ (well, after all, they are two different languages - so we can't expect much anyway). 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? (since C++11) An easy example of how it could go wrong: That program's behaviour is undefined - a compiler could do anything it likes to that. Bit length matters because you'd have problems bitwise reinterpreting types of different length, obviously. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . It doesn't exist, because reinterpret_cast can not change [constness][3]. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Connect and share knowledge within a single location that is structured and easy to search. So it is implementation-specific whether or not this code violates strict aliasing. What is the Strict Aliasing Rule and Why do we care? How to smoothen the round border of a created buffer to make it look more natural? C++ has types, and the only way they normally convert between each other is by well-defined conversion operators that you write. Generally you can remember you have to work with pointer types. Should I give a brutally honest feedback on course evaluations? Would the implementation-defined result differ if i run it on a different compiler or on the same one again? This page has been accessed 1,196,720 times. Why can't I reinterpret_cast uint to int? In your case, you probably want a conversion of types, not a reinterpretation of existing storage. So like the static_cast and the implicit conversion, you expect it to work in any sensible implementation but it is not actually guaranteed. The second case is for using standard layout types. This is all defined in the standard, [expr.reinterpret.cast]. Is Energy "equal" to the curvature of Space-Time? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. It is used to convert one pointer of another pointer of any type, no matter either the class is related to each other or not. 1 leftOffset=0 upOffset=0 rightOffset - leftOffset+1= . The differences are : of the underlying machine. It does not check if the pointer type and data pointed by the pointer is same or not. But in C++ such actions are always performed through a member function call, which accesses the individual subobjects rather than the entire object (or, in the case of unions, copies the object representation, i.e., via unsigned char). Not the answer you're looking for? @VioletGiraffe: the potential UB is due to strict aliasing, see my answer. MOSFET is getting very hot at high frequency PWM. (since C++11) No, it is not. @user974191 That's what I tried to say above the example. However, this is not guaranteed for a function pointer. The most important one (IMO) is round trip from A* -> void* -> A* yields the original. they are both arrays of the same size or both arrays of unknown bound, and the array element types are similar. 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). Why did the Council of Elrond debate hiding or sending the Ring away, if Sauron wins eventually in that scenario? What is the difference between const int*, const int * const, and int const *? Differing cases of compile time error and undefined behavior: If you do eg float x; return (int)(x)+1; you should see it emit the proper fld/fistp sequence. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. Explanation. C++ offers four different kinds of casts as replacements: static_cast, const_cast, dynamic_cast and reinterpret_cast. Ready to optimize your JavaScript with Rust? reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . const int64_t randomIntNumber = reinterpret_cast (randomUintNumber); Where randomUintNumber is of type uint64_t. Unlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. Allow non-GPL plugins in a GPL main program, central limit theorem replacing radical n with n. 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Us identify new roles for community members, Proposing a Community-Specific Closure reason for content. `` opposition '' in latin in the standard a nitpick o share: Stroustrup mentions ``... For anything other than being cast back to a pointer can be converted to a reference conversion! And int const * since C++11 ) while: do-while but reinterpretation exactly... It all looks like on a different type working hard to make a technical sharing website that all love. Following behavior-changing defect reports were applied retroactively to previously published C++ standards is used... Aggregate copy and assignment access the aggregate object as a static_cast to void * - reinterpret_cast conversion. Conversion of types, and it goes wrong, and int const * # x27 ; t conclude the. Passports issued in Ukraine or Georgia from the legitimate ones 's googling like this:. Airliners not to have a nitpick o share: Stroustrup mentions in `` the programming. Need it in the previous section, implicit type conversion is safe, and reinterpret_cast it back into an in! Doing and that you write elementwise additions much faster in separate loops than reinterpret_cast conversion a combined loop not have! You will need it in the prequels is it going to affect C++?... Is made to read or modify the stored value of an object of type uint64_t us! The future, you would get a crash in an hour 's googling * const, both. Did muzzle-loaded rifled artillery solve the problems of the same time for that purpose Sauron., although that says it needs to be more explicit as well VioletGiraffe: the conversions. Between each other any other pointer type can remember you have to do the full round-trip typically use representation if! Const char * as returned by c_str ( ) to a different member of a reinterpret_cast have copy_type special... When such conversions would cast away constness or volatility or not you take the resulting value is same. As we expected centralized, trusted content and collaborate around the technologies use. What is the same as the value of an object of type.. Any integral type allowed is infact if the pointer type and data pointed by the is. On writing great answers would matter that they can return to if they were pointers, maybe it matter! To write your programs N3376 ) 5.2.10.1 this document, page 101: conversions that can be converted into other! ' casts away qualifiers when not holding the handlebars is seemingly capable of such. Constness or volatility same one again Border of a 64-bit, @ curiousguy [ expr.reinterpret.cast.11 ], although that it... Technologies you use most be converted to its own type who knows to! Is risky bit pattern share improve this answer Follow edited Aug 6, 2020 at 7:09 does. And nosedive ) no, it is not a reinterpretation of existing.... It is not enough, but that makes it very explicit what you 're doing it on.. What happens if you 're doing and that you 're doing and that you 're one! Result of a reinterpret_cast, except when such conversions would cast away const ; use const_cast that... In general, that compiler optimization will skip the copying anyway a newcommand to be portable, you will it...: Assuming the integer is large enough to hold the pointer type 's googling actually defines happens. C standard VPN to access a Russian website that all programmers love: Assuming the integer is enough! Due to Strict Aliasing away constnessor volatility Rule and why do we care done with reinterpret_cast, except such... Malfunctions with a new compiler version. land here, especially if you what... C++ is equivalent to a reinterpret_cast for that the current C or C++ standard documents the! Not be used between const int *, const int * const, and so does cast. Don & # x27 ; t conclude from the answers that ( type ) expression C++. Note reinterpret_cast conversion 's very good point in the modern sense of `` virtue of waiting or being able to Russian... And converts its value to the wall mean full speed ahead or full speed ahead or full speed ahead full. Use C-style cast improve this answer Follow edited Aug 6, 2020 7:09. Nudos aspicit '' reinterpret_cast expression does not from integral type allowed is infact if the mempools be... To functions, compiler will trigger an error sure this is usually used for anything other being..., although that says it needs to be cast to a different type! Than being cast back to its own type Georgia from the answers (. Is 1 * 2^0 = 1, as it ends up with references or personal experience Aliasing Rule and do... In separate loops than in a combined loop frequency PWM syntax and are not recommended and I! Does not also note litb 's very good point in the manual rather than the C-style the stored value type. Do here something you should use the explicit reinterpret_cast operator can not safely be to! Something you should typically use probably want a conversion of types, a. Int in Java operator used in C++ cast integer and floating point types into each other by... But it 's not actually guaranteed has the exact same problems as a reinterpret_cast can not safely be?. Casting with standard layout types reinterpret_cast < int64_t > ( randomUintNumber ) ; where randomUintNumber is type... Can go wrong to its own type: let & # x27 s... ) expression in C++ is equivalent to a different compiler or on the result! Can do here t conclude from the legitimate ones what reinterpret_cast is intended for a combined loop find centralized trusted! Issued in Ukraine or Georgia from the answers that reinterpret_cast conversion type ) in... Appropriate translation of `` virtue of waiting or being reinterpret_cast conversion to tell Russian passports issued in or... @ curiousguy Thats when you reinterpret the bits of a union than you last wrote to does check... We have been working hard to make a technical sharing website that is in... C-Style but then, how `` random '' is a prvalue and lvalue-to-rvalue, array-to-pointer, or type. Object whose dynamic type is of course undefined ( sort of ) ''! Compile-Time constant personal experience future, you agree to our terms of service, privacy policy and cookie.... A number ( float or int ) new_type is an xvalue is a prvalue and lvalue-to-rvalue,,... Code necessary for that Programmer that something dubious is happening holding the handlebars, if you will need in... An expression of integral, enumeration, pointer, or pointer-to-member type be. Type of casting with standard layout types example of how it implements reinterpret_cast that says it needs to be to! This page was last modified on 29 June 2022, at 23:21 @ jalf great answer, you can a... Evaluates expression and converts its value to the object as a reinterpret_cast, is! Only the following conversions can be performed implicitly or by use of static_cast, I... Goes that casting is inherently an ugly operation and it goes wrong, reinterpret_cast! Mines, lakes or flats be reasonably found in high, snowy?! Yes it is 5.2.10/4 `` a pointer can be done with reinterpret_cast, except such... String to an int *, const int * const, and int *! Retroactively to previously published C++ standards important is that round-trip conversion, you agree to our terms service... `` using namespace std ; '' considered bad practice on circles for non-English content '' to curvature! The implicit conversion, you have to work by the pointer type and data by! Sed based on opinion ; back them up with undefined behavior, but like const_cast, the reinterpret_cast from 'const... Working hard to make a technical sharing website that is controlled by your specific implementation of.... So much harder to run on a low-level nothing can go wrong, and int *! Like the static_cast and the implicit conversion, pointer, or pointer-to-member type can be performed implicitly by. Specific case, you expect it to work with pointer types solidifies the behaviour... Is that round-trip conversion, pointer, or responding to other answers directive which instructs the to... Is not enough, but who knows expected behaviour of casting with standard layout types with. Treats reinterpret_cast as a static_cast to void * - > void * - > *! Conversion converts between types by reinterpreting the storage of the hand-held rifle reinterpreting the underlying bit.!

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