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? Note that this is probably not guaranteed to work by the C standard. Why did the Council of Elrond debate hiding or sending the Ring away, if Sauron wins eventually in that scenario? No other conversion can be performed explicitly using reinterpret_cast. 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. For example, treating the bits of a 64-bit, @curiousguy Certain integer casts do. Reinterpret_cast in c++ allows any pointer to be converted into any other pointer type. Sometimes, however, you want to reinterpret the bits that represent a type into something else. 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. Of course, if you want something to happen other than compiling, you'd use '='. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. Converts between types by reinterpreting the underlying bit pattern. Returns a value of type new_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. Iteration statements (loops) for: range-for (C++11)while: do-while http://en.wikipedia.org/wiki/Single_precision_floating-point_format#IEEE_754_single-precision_binary_floating-point_format:_binary32. did anything serious ever run on the speccy? But if you take the resulting float* and reinterpret_cast it back into an int*, then you will get the original pointer. The second case is for using standard layout types. This page was last modified on 29 June 2022, at 23:21. That part is guaranteed. UB is the nasal deamons. A cast between signed and unsigned integral types does, and so does a cast from. Atlas 300 3010-VPC:1. From [expr.reinterpret.cast].6 (emphasis mine):. while reinterpret means the opposite: no conversion. When a pointer or reference to object whose dynamic type is DynamicType is reinterpret_cast (or C-style cast) to a pointer or . [] ExplanatioUnlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? A small bolt/nut came off my mtn bike while washing it, can someone help me identify it? Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . 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 . 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 . Why are elementwise additions much faster in separate loops than in a combined loop? You can freely cast pointer types in C as you would any other type. Other uses are, at best, nonportable. 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 learn more, see our tips on writing great answers. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . What is the difference between const int*, const int * const, and int const *? 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 -. 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). To subscribe to this RSS feed, copy and paste this URL into your RSS reader. What is the C equivalent for the reinterpret_cast from C++? There are valid reasons to use reinterpret_cast, and for these reasons the standard actually defines what happens. So it's really a very platform specific for conversions like: It is intended to be unsurprising to those who know the addressing structure 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. In this case the standard treats reinterpret_cast as a static_cast to void* first and then a static_cast to the desination type. The most important is that round-trip conversion, pointer to integer to pointer yields the original pointer. 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);. 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. It does not check if the pointer type and data pointed by the pointer is same or not. 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. cast or function-style cast. But it's not actually guaranteed in the standard AFAIK. both types have the same bit length, isn't it what reinterpret_cast is intended for? Find centralized, trusted content and collaborate around the technologies you use most. In C, aggregate copy and assignment access the aggregate object as a whole. 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. The reason why C language adds the syntax of mandatory type conversion is to emphasize the risk and make programmers aware of what they are . The reinterpret_cast operator can be used for conversions such as char* to int*, or One_class* to Unrelated_class*, which are inherently unsafe. of the underlying machine. @curiousguy Thats when you reinterpret the bits of some objects binary representation as if it were a different type. [.] Even if randomUintNumber is a compile-time constant, unfortunately here randomIntNumber is not a compile-time constant. 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? 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 is apparent power not measured in Watts? Converts between types by reinterpreting the underlying bit pattern. Reading a different member of a union than you last wrote to does not. Thanks for contributing an answer to Stack Overflow! In general, that's all you both need and should use to write your programs. 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). Why is "using namespace std;" considered bad practice? It is purely a compile-time directive which instructs the compiler to treat expression as if it had the type new-type. So if you know what you do and how it all looks like on a low-level nothing can go wrong. It also permits any integral type to be converted into any pointer type and vice versa. Explanation. A C-style cast could evaluate to any of those automatically, thus it is considered safer if the programmer explicitly states which kind of cast is expected. Because that's not what reinterpret_cast is for. 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: Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Why is processing a sorted array faster than processing an unsorted array? Demonstrates some uses of reinterpret_cast: The following behavior-changing defect reports were applied retroactively to previously published C++ standards. reinterpret_cast. Why is this usage of "I've to work" so awkward? Syntax: new_type = reinterpret_cast< new_type > (expression); 1. ID is for this particular compiler/target couple, with specs in the manual rather than the standard. Should I give a brutally honest feedback on course evaluations? It can cast integer and floating point types into each other. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Thus, any technique that is seemingly capable of creating such a situation necessarily invokes undefined behavior. Is Energy "equal" to the curvature of Space-Time? This is usually used for very low-level operations and is not something you should typically use. C++. However, this is not guaranteed for a function pointer. Type aliasing . You can create a type copy_type that has the same field types as concrete_type (thus same size in memory). This changes, when playing with newType and randomUintNumber. This is bad, as it ends up with undefined behavior (maybe you code malfunctions with a new compiler version.) Let's confirm the output above makes sense, anyway. No, reinterpret_cast is for pointer casts. How to say "patience" in latin in the modern sense of "virtue of waiting or being able to wait"? // 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. 1. reinterpret_cast can not remove constness. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. MOSFET is getting very hot at high frequency PWM. Why should someone do this? Type aliasing. Making statements based on opinion; back them up with references or personal experience. Not the answer you're looking for? The resulting value is the same as the value of expression. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. I am sure this is not enough, but I am really keen to know more on when does it differ? The resulting value is the same as the value of expression. 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). What happens if you score more than 99 points in volleyball? 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 . Conversion back to a different pointer type is of course undefined (sort of). For example, dk.cpp: In function 'int main()': 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. Conversions that can be performed explicitly using reinterpret_cast are listed below. @VioletGiraffe: If you want to reinterpret a certain storage as a different type, then you should reinterpret cast a pointer to that storage. Others involve pointers. Connecting three parallel LED strips to the same power supply. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . error C2440: 'reinterpret_cast' : cannot convert from 'const uint64_t' @user974191 That's what I tried to say above the example. It is implementation defined because the C++ standard does not really say much at all about how things should actually be laid out in memory. And how is it going to affect C++ programming? Why are elementwise additions much faster in separate loops than in a combined loop? 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. 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. 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. I hear that reinterpret_cast is implementation defined, but I don't know what this really means. The resulting value is the same as the value of expression. Appropriate translation of "puer territus pedes nudos aspicit"? reninterpret_cast does not check if the pointer type and data pointed by the pointer is same or not. 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. When Kevin Costner was cast as Bat Man, he didn't literally become Bat Man; he was just interpreted as portraying Bat Man. How do I check if a string represents a number (float or int)? reinterpret_cast is a type of casting operator used in C++. 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). How do I convert a String to an int in Java? 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. Asking for help, clarification, or responding to other answers. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. is a great article, describing the problem and its solution (maybe skip the lengthy part "Now, to the Rule-Book" ;) ). (since C++11) What is the Strict Aliasing Rule and Why do we care? 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. 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. How is the merkle root verified if the mempools may be different? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. 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. If new_type is an rvalue reference to object, the result is an xvalue. reinterpret_cast evaluates expression and converts its value to the type new_type. @JoachimPileborg: my bad, expanded the question. The only conversion from integral type to integral type allowed is infact if the types are identical. 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). When would I give a checkpoint to my D&D party that they can return to if they die? @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. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. The most important one (IMO) is round trip from A* -> void* -> A* yields the original. Find centralized, trusted content and collaborate around the technologies you use most. 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. You could do what you want by putting an & inside the cast and a * outside it. Add a new light switch in line with another switch? on_copy will look like this proto : copy_type on_copy(concrete_type). . 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. If new_type is an lvalue reference or an rvalue reference to function, the result is an lvalue. What does it mean? Sed based on 2 words, then replace whole line with variable. So typically, that means that a single compiler will do the same thing consistently if you recompile, or if you run the program again. Syntax : 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. However, this is not guaranteed for a function pointer. C++ has types, and the only way they normally convert between each other is by well-defined conversion operators that you write. No, it is not. Can you provide an example of how it can go wrong, and it goes wrong, is it better to use C-Style cast? Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Where does the idea of selling dragon parts come from? isn't it what reinterpret_cast is intended for? (since C++11) They're all undefined behavior, but that makes it very explicit what you're doing and that you're doing it on purpose. Program to convert integer pointer into character pointer. Interactive code here. A small bolt/nut came off my mtn bike while washing it, can someone help me identify it? to 'int64_t' 1> Conversion is a valid standard conversion, Why is Singapore considered to be a dictatorial regime and a multi-party democracy at the same time? If they were pointers, maybe it would matter that they pointed to things of the same size, but they're not pointers. Because of this, the behaviour of reinterpret_cast depends upon how your compiler lays structures out in memory and how it implements reinterpret_cast. : 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. Connect and share knowledge within a single location that is structured and easy to search. How to convert unique_ptr<derived>* to unique_ptr<base>*? Bit length matters because you'd have problems bitwise reinterpreting types of different length, obviously. Generally you can remember you have to work with pointer types. 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). Code that violates strict aliasing has undefined behavior. reinterpret_cast is mostly intended to reinterpret an existing bit of storage as a different type than it is. At what point in the prequels is it revealed that Palpatine is Darth Sidious? Does balls to the wall mean full speed ahead or full speed ahead and nosedive? Whenever an attempt is made to read or modify the stored value of an object of type DynamicType through a glvalue of type . These bullets were eventually removed via CWG2051. Don't conclude from the answers that (type)expression in C++ is equivalent to a reinterpret_cast. 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). Improve INSERT-per-second performance of SQLite, Obtain closed paths using Tikz random decoration on circles. What is the difference between #include and #include "filename"? How did muzzle-loaded rifled artillery solve the problems of the hand-held rifle? Ready to optimize your JavaScript with Rust? Thus, any technique that is seemingly capable of creating such a situation necessarily invokes undefined behavior. But then, how "random" is a compile-time constant? 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). It simply tries the various C++-style casts in order, until it finds one that works. other conversion can be performed explicitly using reinterpret_cast. 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. @jalf Great answer, and best I found in an hour's googling. reinterpret_cast And print hex, Programmer All, we have been working hard to make a technical sharing website that all programmers love. 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 1 in decimal. Please also note litb's very good point in the comments to the original question. 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). (since C++11) C-style casts are quite similar to reinterpret_casts, but they have much less syntax and are not recommended. Iteration statements (loops) for: range-for (C++11)while: do-while Not the answer you're looking for? 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. What would be the exception here? 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). If you use reinterpret_cast, it is easy to find the places where you did it. Why can't I reinterpret_cast uint to int? This page has been accessed 1,196,720 times. Why would that matter? The differences are : reinterpret_cast: dynamic_cast: Explicit conversions (T)a, T(a) User-defined conversion Most probably, you would get a crash when the string's destructor is called, but who knows! The resulting value is the same as the value of expression. 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 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. This page was last modified on 29 June 2022, at 23:21. how about some abstract types? reinterpret_cast conversion Converts between types by reinterpreting the underlying bit pattern. 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. 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). Find centralized, trusted content and collaborate around the technologies you use most. 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 . That's what reinterpret cast is for. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. How to connect 2 VMware instance running on same Linux host machine via emulated ethernet cable (accessible via mac address)? It's misleading. they are both arrays of the same size or both arrays of unknown bound, and the array element types are similar. Allow non-GPL plugins in a GPL main program. For those cases, you can use reinterpret_cast. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy 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). But reinterpretation is exactly what I want! What is the difference between const int*, const int * const, and int const *? Implementation-defined means that the implementation (the compiler, basically) can choose how to behave, but it must document the behavior. Why I can't compile this code? 1 leftOffset=0 upOffset=0 rightOffset - leftOffset+1= . The thinking goes that casting is inherently an ugly operation and it requires ugly syntax to inform the programmer that something dubious is happening. @VioletGiraffe When using the results of a. The short answer: If you don't know what reinterpret_cast stands for, don't use it. Asking for help, clarification, or responding to other answers. 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. E.g. In your specific case, all options yield the same result. . For example: In this code c is guaranteed to point to the object b as you'd expected. 4 base base_collectionderived::base derived_collection::base_collection base_collection . If you want to be portable, you have to do the full round-trip. 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. Why 'most likely'? What is the difference between ++i and i++? Both are valid statements, and both will compile only because of the cast. which can be performed implicitly or by use of static_cast, C-style Share Improve this answer Follow edited Aug 6, 2020 at 7:09 Making statements based on opinion; back them up with references or personal experience. // 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. 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. // 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. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. (since C++11) Why is it so much harder to run on a treadmill when not holding the handlebars? 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 resulting value is the same as the value of expression. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. How to use a VPN to access a Russian website that is banned in the EU? // 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. Only the following conversions can be done with reintepret_cast, except when such conversions would cast away constnessor volatility. Explanation Unlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. You can have copy_type have special field names as dont_use_me to be more explicit as well. But in addition, it has these problems: To answer the other part of your question, yes, reinterpret_cast is implementation-defined. That means that when it acts like a reinterpret_cast, it has the exact same problems as a reinterpret_cast. 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. C++ offers four different kinds of casts as replacements: static_cast, const_cast, dynamic_cast and reinterpret_cast. I checked the assembly code produced by VC2008 and it omits the same code. Would the implementation-defined result differ if i run it on a different compiler or on the same one again? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. I don't like to say "reinterpret_cast", because cast means conversion (in c), Better way to check if an element only exists in one array. dk.cpp:5:41: error: reinterpret_cast from type 'const unsigned int*' to type 'int*' casts away qualifiers. Otherwise, the result is a prvalue and lvalue-to-rvalue, array-to-pointer, or function-to . reinterpret_cast is a type of casting operator used in C++. Hence the value represented by hex 3f800000 is 1 * 2^0 = 1, as we expected. rev2022.12.9.43105. Iteration statements (loops) for: range-for (C++11)while: do-while 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. This page has been accessed 1,191,892 times. Returns a value of type new_type. // 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. How do I parse a string to a float or int? (since C++11) How to set a newcommand to be incompressible by justification? What is the C equivalent for reinterpret_cast? I suppose the language designers in all their wisdom decided that it's not considered "unsafe enough" to warrant a reinterpret_cast. 1)An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. That part is implementation-defined. Ready to optimize your JavaScript with Rust? 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). It is not possible to do with c-style casts. Use static_cast for this purpose. In your case, you probably want a conversion of types, not a reinterpretation of existing storage. I'm not sure static_cast (or C-cast) will produce the desired result, have to refresh my memory on how it works first. This is all defined in the standard, [expr.reinterpret.cast]. Be aware though that you are entering IB/UB land here, especially if you are dereferencing the result. 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). 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). No The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. In C, aggregate copy and assignment access the aggregate object as a whole. 12 comments Contributor RossBencina commented on Jan 15, 2013 edited by horenmar Fixes the following MSVC 2005 warning when doing tests like ClassName *p = 0 ; REQUIRE ( p != 0 ); To interpret the exponent, subtract 127: 01111111b = 127, and 127 - 127 = 0. Don't make your code unprovoked complexer. rev2022.12.9.43105. 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. So like the static_cast and the implicit conversion, you expect it to work in any sensible implementation but it is not actually guaranteed. How to smoothen the round border of a created buffer to make it look more natural? Typesetting Malayalam in xelatex & lualatex gives error. 1980s short story - disease of self absorption. 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 . Why doesn't it compile? reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . To learn more, see our tips on writing great answers. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. // 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. These bullets were eventually removed via CWG2051. It recommends memcpy and argues, that compiler optimization will skip the copying anyway. 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. 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. rev2022.12.9.43105. Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? Sed based on 2 words, then replace whole line with variable. Use static_cast in these cases. 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. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . 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). 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". When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? Does it mean the code would be non-portable? 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. @VioletGiraffe: the potential UB is due to strict aliasing, see my answer. So it is implementation-specific whether or not this code violates strict aliasing. reinterpret_cast. they are both arrays of the same size or both arrays of unknown bound, and the array element types are similar. That is controlled by your specific implementation of C++. 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. Syntax reinterpret_cast < new_type > ( expression ) Returns a value of type new_type. Allow non-GPL plugins in a GPL main program, central limit theorem replacing radical n with n. Is there any reason on passenger airliners not to have a physical lock between throttles? But there's no inheritance in C anyway. Connect and share knowledge within a single location that is structured and easy to search. 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). The conversion from const char* as returned by c_str() to std . @edA-qamort-ora-y: Yes it is 5.2.10/4 "A pointer can be explicitly converted to any integral type. The exponent is 0. It is purely a compile-time directive which instructs the compiler to treat expression as if it had the type new-type. If you don't want to go through standard wording, you can find all that reinterpret_cast can do here. Cast doesn't necessarily mean conversion. Connect and share knowledge within a single location that is structured and easy to search. const int64_t randomIntNumber = reinterpret_cast (randomUintNumber); Where randomUintNumber is of type uint64_t. Keywords. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. @codemonkey 2/2 in case you passed concrete_type directly to functions, compiler will trigger an error. C++11 introduced a standardized memory model. reinterpret_cast. Better way to check if an element only exists in one array. [] ExplanatioUnlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. @curiousguy [expr.reinterpret.cast.11], although that says it needs to be cast to a reference. From cppreference.com . To interpret the mantissa, write it after 1 followed by a decimal point: 1.00000000000000000000000 (23 zeroes). Did neanderthals need vitamin C from the diet? The standard provides several guarantees and requirements for reinterpret_cast. How to print and pipe log file at the same time? 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. @edA-qa mort-ora-y: Assuming the integer is large enough to hold the pointer. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? Thanks for contributing an answer to Stack Overflow! Making statements based on opinion; back them up with references or personal experience. All your functions will wait for a copy_type argument. Is this an at-all realistic configuration for a DHC-2 Beaver? 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). 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. Furthermore, C++ solidifies the expected behaviour of casting with standard layout types. 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. Full answer: Let's consider basic number types. 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) Would salt mines, lakes or flats be reasonably found in high, snowy elevations? So yes, if you rely on the intermediate value, your code is non-portable. 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. Should I give a brutally honest feedback on course evaluations? What happens if you score more than 99 points in volleyball? Type Conversion Operators: static_cast, dynamic_cast, const_cast and reinterpret_cast in C++. 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? 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