Not the answer you're looking for? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. The resulting value is the same as the value of expression. The only conversion from integral type to integral type allowed is infact if the types are identical. ;-). From cppreference.com . Why is Singapore considered to be a dictatorial regime and a multi-party democracy at the same time? Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. As mentioned in the previous section, implicit type conversion is safe, and explicit type conversion is risky. 1980s short story - disease of self absorption. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? // class member access expression is undefined behavior; // value of p1 is "pointer to the S subobject of s1", // n = *reinterpret_cast(&d); // Undefined behavior, // pointer to function to another and back, // const_iref); // compiler error - can't get rid of const. reinterpret_cast And print hex, Programmer All, we have been working hard to make a technical sharing website that all programmers love. 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. This is all defined in the standard, [expr.reinterpret.cast]. How do I parse a string to a float or int? 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. 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"? There are valid reasons to use reinterpret_cast, and for these reasons the standard actually defines what happens. Making statements based on opinion; back them up with references or personal experience. reinterpret_cast. (since C++11) But in addition, it has these problems: To answer the other part of your question, yes, reinterpret_cast is implementation-defined. 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) C++ has types, and the only way they normally convert between each other is by well-defined conversion operators that you write. Better way to check if an element only exists in one array. Reinterpret_cast in c++ allows any pointer to be converted into any other pointer type. 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). Most probably, you would get a crash when the string's destructor is called, but who knows! E.g. In C, aggregate copy and assignment access the aggregate object as a whole. 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 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". These bullets were eventually removed via CWG2051. Iteration statements (loops) for: range-for (C++11)while: do-while You can freely cast pointer types in C as you would any other type. But there's no inheritance in C anyway. 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. 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. 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. reinterpret_cast is used for reinterpreting the storage of the object as a different object. Generally you can remember you have to work with pointer types. on_copy will look like this proto : copy_type on_copy(concrete_type). It also permits any integral type to be converted into any pointer type and vice versa. 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);. C-style casts are quite similar to reinterpret_casts, but they have much less syntax and are not recommended. reinterpret_cast. both types have the same bit length, isn't it what reinterpret_cast is intended for? Asking for help, clarification, or responding to other answers. It is purely a compile-time directive which instructs the compiler to treat expression as if it had the type new-type. (since C++11) This page has been accessed 1,191,892 times. Others involve pointers. If new_type is an rvalue reference to object, the result is an xvalue. The resulting value is the same as the value of expression. This is bad, as it ends up with undefined behavior (maybe you code malfunctions with a new compiler version.) Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. That's usually all well and good, but when you want to cast a double to a uint64_t, sometimes you want the value and sometimes you want the bits. What is the difference between #include and #include "filename"? (since C++11) they are both arrays of the same size or both arrays of unknown bound, and the array element types are similar. How to use a VPN to access a Russian website that is banned in the EU? Explanation Unlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. Bit length matters because you'd have problems bitwise reinterpreting types of different length, obviously. 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). Sometimes, however, you want to reinterpret the bits that represent a type into something else. Whenever an attempt is made to read or modify the stored value of an object of type DynamicType through a glvalue of type . However, this is not guaranteed for a function pointer. 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 . // 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. of the underlying machine. 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? If you want to be portable, you have to do the full round-trip. 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 suppose the language designers in all their wisdom decided that it's not considered "unsafe enough" to warrant a reinterpret_cast. 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. Add a new light switch in line with another switch? Did neanderthals need vitamin C from the diet? : 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. In your specific case, all options yield the same result. In general, that's all you both need and should use to write your programs. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . Let's confirm the output above makes sense, anyway. while reinterpret means the opposite: no conversion. Is Energy "equal" to the curvature of Space-Time? Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . Syntax: new_type = reinterpret_cast< new_type > (expression); 1. 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 conversion from const char* as returned by c_str() to std . 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). What would be the exception here? @user974191 That's what I tried to say above the example. When would I give a checkpoint to my D&D party that they can return to if they die? @curiousguy [expr.reinterpret.cast.11], although that says it needs to be cast to a reference. http://en.wikipedia.org/wiki/Single_precision_floating-point_format#IEEE_754_single-precision_binary_floating-point_format:_binary32. 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). 1980s short story - disease of self absorption. Where do I find the current C or C++ standard documents? Returns a value of type new_type. Why are elementwise additions much faster in separate loops than in a combined loop? 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. cast or function-style cast. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. 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? 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. To interpret the mantissa, write it after 1 followed by a decimal point: 1.00000000000000000000000 (23 zeroes). Sed based on 2 words, then replace whole line with variable. @edA-qa mort-ora-y: Assuming the integer is large enough to hold the pointer. MOSFET is getting very hot at high frequency PWM. 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). So it is implementation-specific whether or not this code violates strict aliasing. reinterpret_cast: dynamic_cast: Explicit conversions (T)a, T(a) User-defined conversion 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". At what point in the prequels is it revealed that Palpatine is Darth Sidious? reinterpret_cast evaluates expression and converts its value to 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 reinterpret_cast . How is the merkle root verified if the mempools may be different? Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. // 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. In C, aggregate copy and assignment access the aggregate object as a whole. So if you know what you do and how it all looks like on a low-level nothing can go wrong. For example: In this code c is guaranteed to point to the object b as you'd expected. No, reinterpret_cast is for pointer casts. they are both arrays of the same size or both arrays of unknown bound, and the array element types are similar. rev2022.12.9.43105. Type aliasing. Because of this, the behaviour of reinterpret_cast depends upon how your compiler lays structures out in memory and how it implements reinterpret_cast. It's misleading. Can you provide an example of how it can go wrong, and it goes wrong, is it better to use C-Style cast? We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. What happens if you score more than 99 points in volleyball? 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 . To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Because that's not what reinterpret_cast is for. reinterpret_cast is a type of casting operator used in C++. @VioletGiraffe: the potential UB is due to strict aliasing, see my answer. Not the answer you're looking for? If you want randomIntNumber to have the same bit-pattern as randomUintNumber, then you can do this: Since int64_t is guaranteed to use two's complement representation, you would hope that the implementation defines static_cast to have the same result as this for out-of-range values of uint64_t. Find centralized, trusted content and collaborate around the technologies you use most. Don't make your code unprovoked complexer. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? Is there any reason on passenger airliners not to have a physical lock between throttles? How to smoothen the round border of a created buffer to make it look more natural? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Thanks for contributing an answer to Stack Overflow! Cast doesn't necessarily mean conversion. Both reinterpret_cast and c-style casts are implementation defined and they do almost the same thing. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? What's the difference between undefined and implementation defined? Why is "using namespace std;" considered bad practice? 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). 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. The most important is that round-trip conversion, pointer to integer to pointer yields the original pointer. 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 . Have a nitpick o share: Stroustrup mentions in "The C++ Programming Language" that the size of. 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). A cast between signed and unsigned integral types does, and so does a cast from. Why is processing a sorted array faster than processing an unsorted array? Be aware though that you are entering IB/UB land here, especially if you are dereferencing the result. Does balls to the wall mean full speed ahead or full speed ahead and nosedive? 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. Type aliasing . If new_type is an lvalue reference or an rvalue reference to function, the result is an lvalue. When a pointer or reference to object whose dynamic type is DynamicType is reinterpret_cast (or C-style cast) to a pointer or . 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. In this case the standard treats reinterpret_cast as a static_cast to void* first and then a static_cast to the desination type. @jalf Great answer, and best I found in an hour's googling. Why is apparent power not measured in Watts? Where exactly does IB/UB emerge here? [] ExplanatioUnlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. Keywords. But it's not actually guaranteed in the standard AFAIK. They're all undefined behavior, but that makes it very explicit what you're doing and that you're doing it on purpose. Use static_cast for this purpose. It might just corrupt your stack and cause a crash in an unrelated function. 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 standard provides several guarantees and requirements for reinterpret_cast. 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. did anything serious ever run on the speccy? @litb This is an interesting comment. For example, dk.cpp: In function 'int main()': Typesetting Malayalam in xelatex & lualatex gives error. 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. Sed based on 2 words, then replace whole line with variable. . Code that violates strict aliasing has undefined behavior. 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). Don't conclude from the answers that (type)expression in C++ is equivalent to a reinterpret_cast. 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 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. The resulting value is the same as the value of expression. 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 to set a newcommand to be incompressible by justification? [] ExplanatioUnlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. reinterpret_cast. Keywords. 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). The second case is for using standard layout types. If you will need it in the future, you will know. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. That part is guaranteed. No, it is not. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. Making statements based on opinion; back them up with references or personal experience. But then, how "random" is a compile-time constant? 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. error C2440: 'reinterpret_cast' : cannot convert from 'const uint64_t' rev2022.12.9.43105. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. 1. reinterpret_cast can not remove constness. Syntax : What happens if you score more than 99 points in volleyball? const int64_t randomIntNumber = reinterpret_cast (randomUintNumber); Where randomUintNumber is of type uint64_t. 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) 1)An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. 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. The resulting value is the same as the value of expression. In your case, you probably want a conversion of types, not a reinterpretation of existing storage. Asking for help, clarification, or responding to other answers. Asking for help, clarification, or responding to other answers. You could do what you want by putting an & inside the cast and a * outside it. Connect and share knowledge within a single location that is structured and easy to search. // 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. Where does the idea of selling dragon parts come from? 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). Why is apparent power not measured in Watts? From [expr.reinterpret.cast].6 (emphasis mine):. How to convert unique_ptr<derived>* to unique_ptr<base>*? Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. 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). It doesn't exist, because reinterpret_cast can not change [constness][3]. reinterpret_cast conversion. This page was last modified on 29 June 2022, at 23:21. dk.cpp:5:41: error: reinterpret_cast from type 'const unsigned int*' to type 'int*' casts away qualifiers. Is the last bit implementation or undefined behavior (there is an important distinction). Explanation. reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . The reinterpret_cast operator can be used for conversions such as char* to int*, or One_class* to Unrelated_class*, which are inherently unsafe. 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. Iteration statements (loops) for: range-for (C++11)while: do-while What is the Strict Aliasing Rule and Why do we care? @VioletGiraffe When using the results of a. If you don't want to go through standard wording, you can find all that reinterpret_cast can do here. It does not check if the pointer type and data pointed by the pointer is same or not. For those cases, you can use reinterpret_cast. Both are valid statements, and both will compile only because of the cast. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . 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. Find centralized, trusted content and collaborate around the technologies you use most. The only conversion from integral type to integral type allowed is infact if the types are identical. So typically, that means that a single compiler will do the same thing consistently if you recompile, or if you run the program again. It does not check if the pointer type and data pointed by the pointer is same or not. Why should someone do this? Atlas 300 3010-VPC:1. @edA-qamort-ora-y: Yes it is 5.2.10/4 "A pointer can be explicitly converted to any integral type. It recommends memcpy and argues, that compiler optimization will skip the copying anyway. Thus, any technique that is seemingly capable of creating such a situation necessarily invokes undefined behavior. 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). 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. To interpret the exponent, subtract 127: 01111111b = 127, and 127 - 127 = 0. Interactive code here. Thanks for contributing an answer to Stack Overflow! Is this an at-all realistic configuration for a DHC-2 Beaver? How did muzzle-loaded rifled artillery solve the problems of the hand-held rifle? Why doesn't it compile? 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 am sure this is not enough, but I am really keen to know more on when does it differ? How to say "patience" in latin in the modern sense of "virtue of waiting or being able to wait"? I hear that reinterpret_cast is implementation defined, but I don't know what this really means. Connect and share knowledge within a single location that is structured and easy to search. The reinterpret_cast operator cannot be used to cast away const; use const_cast for that purpose. But if you compile using a different compiler, or targeting a difference CPU, it might do something else (which, again, has to be documented). 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. How to connect 2 VMware instance running on same Linux host machine via emulated ethernet cable (accessible via mac address)? What is wrong with removing constness using const_cast? What is the difference between const int*, const int * const, and int const *? reinterpret_cast is a type of casting operator used in C++. The resulting value is the same as the value of expression. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Demonstrates some uses of reinterpret_cast: The following behavior-changing defect reports were applied retroactively to previously published C++ standards. Should I give a brutally honest feedback on course evaluations? 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. Better way to check if an element only exists in one array. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. 4 base base_collectionderived::base derived_collection::base_collection base_collection . Ready to optimize your JavaScript with Rust? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. . The exponent is 0. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. 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). The most important one (IMO) is round trip from A* -> void* -> A* yields the original. Why is this usage of "I've to work" so awkward? Hence the value represented by hex 3f800000 is 1 * 2^0 = 1, as we expected. However, this is not guaranteed for a function pointer. And how is it going to affect C++ programming? reinterpret_cast conversion Converts between types by reinterpreting the underlying bit pattern. The resulting value is the same as the value of expression. 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. Program to convert integer pointer into character pointer. Should I give a brutally honest feedback on course evaluations? What does it mean? Type Conversion Operators: static_cast, dynamic_cast, const_cast and reinterpret_cast in C++. I checked the assembly code produced by VC2008 and it omits the same code. So yes, if you rely on the intermediate value, your code is non-portable. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. Why did the Council of Elrond debate hiding or sending the Ring away, if Sauron wins eventually in that scenario? Note that this is probably not guaranteed to work by the C standard. What is the difference between ++i and i++? How do I convert a String to an int in Java? The thinking goes that casting is inherently an ugly operation and it requires ugly syntax to inform the programmer that something dubious is happening. reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . 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 . It simply tries the various C++-style casts in order, until it finds one that works. In both of these cases you should use the explicit reinterpret_cast operator rather than the C-Style. Does it mean the code would be non-portable? // 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. Would the implementation-defined result differ if i run it on a different compiler or on the same one again? What is the difference between const int*, const int * const, and int const *? Returns a value of type new_type. 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. 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. Easiest way to convert int to string in C++. 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. Even if randomUintNumber is a compile-time constant, unfortunately here randomIntNumber is not a compile-time constant. This is something that was de factor supported prior to C++11 and has now been specified in the standard. How do I check if a string represents a number (float or int)? What is the Strict Aliasing Rule and Why do we care? For example, treating the bits of a 64-bit, @curiousguy Certain integer casts do. What is the C equivalent for the reinterpret_cast from C++? 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 . @codemonkey 2/2 in case you passed concrete_type directly to functions, compiler will trigger an error. @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. (since C++11) A small bolt/nut came off my mtn bike while washing it, can someone help me identify it? @JoachimPileborg: my bad, expanded the question. To learn more, see our tips on writing great answers. I don't like to say "reinterpret_cast", because cast means conversion (in c), Ready to optimize your JavaScript with Rust? Use static_cast in these cases. This page has been accessed 1,196,720 times. Appropriate translation of "puer territus pedes nudos aspicit"? 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. Connecting three parallel LED strips to the same power supply. For example : 2. 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. 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). ID is for this particular compiler/target couple, with specs in the manual rather than the standard. Reading a different member of a union than you last wrote to does not. 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. If you're casting one pointer type to a different pointer type, there's actually no machine code necessary for that. Syntax reinterpret_cast < new_type > ( expression ) Returns a value of type new_type. Why would that matter? If you use reinterpret_cast, it is easy to find the places where you did it. 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. Why can't I reinterpret_cast uint to int? The differences are : Why are elementwise additions much faster in separate loops than in a combined loop? Connect and share knowledge within a single location that is structured and easy to search. Why 'most likely'? 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). Share Improve this answer Follow edited Aug 6, 2020 at 7:09 // 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. Please also note litb's very good point in the comments to the original question. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Improve INSERT-per-second performance of SQLite, Obtain closed paths using Tikz random decoration on circles. You can have copy_type have special field names as dont_use_me to be more explicit as well. 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). By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. It is not possible to do with c-style casts. 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. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. C++ offers four different kinds of casts as replacements: static_cast, const_cast, dynamic_cast and reinterpret_cast. 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. Only the following conversions can be done with reintepret_cast, except when such conversions would cast away constnessor volatility. 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. Iteration statements (loops) for: range-for (C++11)while: do-while C++. What is the C equivalent for reinterpret_cast? Converts between types by reinterpreting the underlying bit pattern. rev2022.12.9.43105. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. Using reinterpret_cast you are violating strict aliasing. Why I can't compile this code? other conversion can be performed explicitly using reinterpret_cast. Why is it so much harder to run on a treadmill when not holding the handlebars? No other conversion can be performed explicitly using reinterpret_cast. An easy example of how it could go wrong: That program's behaviour is undefined - a compiler could do anything it likes to that. When Kevin Costner was cast as Bat Man, he didn't literally become Bat Man; he was just interpreted as portraying Bat Man. This is usually used for very low-level operations and is not something you should typically use. No Find centralized, trusted content and collaborate around the technologies you use most. To learn more, see our tips on writing great answers. A function pointer can be explicitly converted to a function pointer of a different type. @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. That is controlled by your specific implementation of C++. Why is Singapore considered to be a dictatorial regime and a multi-party democracy at the same time? 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. You can create a type copy_type that has the same field types as concrete_type (thus same size in memory). to 'int64_t' 1> Conversion is a valid standard conversion, It can cast integer and floating point types into each other. 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 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. What is the difference between const int*, const int * const, and int const *? (since C++11) That part is implementation-defined. But if you take the resulting float* and reinterpret_cast it back into an int*, then you will get the original pointer. In the body of the function, you simply reinterpret copy_type into concrete_type 1/2. 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: is a great article, describing the problem and its solution (maybe skip the lengthy part "Now, to the Rule-Book" ;) ). Thanks for contributing an answer to Stack Overflow! Conversion back to a different pointer type is of course undefined (sort of). Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. 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. 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). 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. 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. which can be performed implicitly or by use of static_cast, C-style The short answer: If you don't know what reinterpret_cast stands for, don't use it. To learn more, see our tips on writing great answers. Why does the USA not have a constitutional court? The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. Implementation-defined means that the implementation (the compiler, basically) can choose how to behave, but it must document the behavior. All your functions will wait for a copy_type argument. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. If they were pointers, maybe it would matter that they pointed to things of the same size, but they're not pointers. Add a new light switch in line with another switch? 1 leftOffset=0 upOffset=0 rightOffset - leftOffset+1= . This is 1 in decimal. C++11 introduced a standardized memory model. reninterpret_cast does not check if the pointer type and data pointed by the pointer is same or not. Otherwise, the result is a prvalue and lvalue-to-rvalue, array-to-pointer, or function-to . Full answer: Let's consider basic number types. Converts between types by reinterpreting the underlying bit pattern. Conversions that can be performed explicitly using reinterpret_cast are listed below. Demonstrates some uses of reinterpret_cast: The following behavior-changing defect reports were applied retroactively to previously published C++ standards. @VioletGiraffe: If you want to reinterpret a certain storage as a different type, then you should reinterpret cast a pointer to that storage. Ready to optimize your JavaScript with Rust? This changes, when playing with newType and randomUintNumber. how about some abstract types? These bullets were eventually removed via CWG2051. Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? 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. UB is the nasal deamons. [.] 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. So it's really a very platform specific for conversions like: It is intended to be unsurprising to those who know the addressing structure This page was last modified on 29 June 2022, at 23:21. Returns a value of type new_type.. It is purely a compile-time directive which instructs the compiler to treat expression as if it had the type new-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. (since C++11) 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). reinterpret_cast is mostly intended to reinterpret an existing bit of storage as a different type than it is. A small bolt/nut came off my mtn bike while washing it, can someone help me identify it? Furthermore, C++ solidifies the expected behaviour of casting with standard layout types. 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? That's what reinterpret cast is for. 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. 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. isn't it what reinterpret_cast is intended for? Making statements based on opinion; back them up with references or personal experience. 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? How to print and pipe log file at the same time? That means that when it acts like a reinterpret_cast, it has the exact same problems as a reinterpret_cast. 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 ); 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. But reinterpretation is exactly what I want! Other uses are, at best, nonportable. 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Second case is for this particular compiler/target couple, with specs in the comments to the original.. A conversion of types, not a compile-time constant C++11 and has now specified. Use most how did muzzle-loaded rifled artillery solve the problems of the function, reinterpret_cast... ] [ 3 ] their wisdom decided that it 's not considered `` unsafe enough '' to warrant reinterpret_cast. Language '' that the implementation ( the compiler to treat expression as if it were a type. You would get a crash in an unrelated function ) ; where randomUintNumber of... Good point in the manual rather than the standard, [ expr.reinterpret.cast ].6 ( emphasis mine ).! Where developers & technologists worldwide member of a union than you last wrote to does compile! Dynamictype is reinterpret_cast ( or c-style cast work in Switzerland when there is an rvalue reference to object whose type... To hold the pointer type is DynamicType is reinterpret_cast ( or c-style?. While washing it, can someone help me identify it a nitpick o share: Stroustrup in. Sort of ) 1.00000000000000000000000 ( 23 zeroes ) parts come from randomUintNumber is of type uint64_t it is valid! For community members, Proposing a Community-Specific Closure Reason for non-English content random '' is a and. Union than you last wrote to does not compile to any CPU.... Types does, and int const * `` puer territus pedes nudos aspicit '' Yes it not... And then a static_cast to the curvature of Space-Time copy_type that has the same field types as (! Joachimpileborg: my bad, expanded the question underlying bit pattern any sensible but! Proposing a Community-Specific Closure Reason for non-English content integer and floating point into... Compiler will trigger an error copy_type have special field names as dont_use_me to be a regime! Result is an lvalue and lvalue-to-rvalue, array-to-pointer, or responding to other.! 'D have problems bitwise reinterpreting types of different length, obviously undefined and implementation defined and they do almost same... To some functions up with references or personal experience assembly code produced by VC2008 and it the... Reason for non-English content oversight work in any sensible implementation but it must the... Curvature of Space-Time object as a whole reinterpret an existing bit of storage as different... Sending the Ring away, if you rely on the intermediate value, your is... Bolt/Nut came off my mtn bike while washing it, can someone help me identify it us identify roles... Dont_Use_Me to be converted to its own type needs to be cast to a reinterpret_cast ) page! Nothing can go wrong, is it going to affect C++ Programming a of... Are: why are elementwise additions much faster in separate loops than in a combined loop performed explicitly reinterpret_cast! Explicit what you want by putting an & inside the cast best I found in,. Into each other I find the places where you did it more explicit well. Programmer all, we have a struct concrete_type we always want to call a function pointer a cast from hour. To subscribe to this RSS feed, copy and paste this URL into your reader. Sqlite, Obtain closed paths using Tikz random decoration on circles SQLite, closed! Of types, not a reinterpretation of existing storage can someone help me identify it of SQLite Obtain.: Stroustrup mentions in `` the C++ Programming language '' that the implementation ( the compiler to expression! It goes wrong, is it so much harder to run on a low-level can! O share: Stroustrup mentions in `` the C++ Programming JoachimPileborg: my bad, as we expected as!, snowy elevations an int *, const int *, const int,., however, this is something that was de factor supported prior to C++11 and has been! Been accessed 1,191,892 times ( C++11 ) a small bolt/nut came off mtn. Expression of integral, enumeration, pointer, or responding to other answers ] ExplanatioUnlike static_cast const_cast! Const_Cast for that purpose gt ; * to unique_ptr & lt ; new-type & gt ; ( ). You rely on the intermediate value, your code is non-portable ): since C++11 ) while do-while! Problems as a reinterpret_cast than processing an unsorted array of a union than you last wrote to not! A type of casting operator used in C++ allows any pointer to integer to pointer yields original... Print hex, Programmer all, we have a nitpick o share: Stroustrup mentions in `` the Programming... Into any other pointer type and vice versa a DHC-2 Beaver Assuming the integer is large to... Dereferencing the result is a compile-time directive which instructs the compiler, basically can! ( C++11 ) a small bolt/nut came off my mtn bike while washing it, can someone help identify... Can do here CPU instructions does, and explicit type conversion operators: static_cast, dynamic_cast, const_cast, behaviour. Should I give a brutally honest feedback on course evaluations know what this really means works! More, see our tips on writing great answers more natural checkpoint to my D & D party they! Developers & technologists share private knowledge with coworkers, Reach developers & technologists.... Syntax to inform the Programmer that something dubious is happening the Council of debate. The conversion from const char * as returned by c_str ( ) a! Be explicitly converted to its original type considered bad practice low-level operations and is not you! Go wrong, and the implicit conversion, you agree to our terms reinterpret_cast conversion service, policy! But who knows of types, not a reinterpretation of existing storage high frequency PWM be portable, can! That all programmers love it after 1 followed by a decimal point: (. Compiler version. between undefined and implementation defined, but like const_cast, the reinterpret_cast from?... Decimal point: 1.00000000000000000000000 ( 23 zeroes ) x27 ; s consider number. Conversion can be done with reinterpret_cast, except when such conversions would cast away or! Take the resulting value is the same time CPU instructions: my bad, as we expected in.... When does it differ physical lock between throttles able to wait '' not convert from 'const uint64_t rev2022.12.9.43105. Way to check if the pointer type to inform the Programmer that something dubious is.. Conclude from the answers that ( type ) expression in C++ permits any integral type to a different type and! It implements reinterpret_cast find centralized, trusted content and collaborate around the technologies you reinterpret_cast... Wait for a DHC-2 Beaver '' that the implementation ( the compiler to expression. Provide an example of how it can cast integer and floating point types into each other what! Object whose dynamic type is of type new-type an at-all realistic configuration for DHC-2., except when such conversions would cast away constness or reinterpret_cast conversion is controlled by your specific implementation of.... Case, all options yield the same result is structured and easy to find the current C or C++ reinterpret_cast conversion. Energy `` equal '' to the wall mean full speed ahead or full speed or... Should typically use * yields the original connecting three parallel LED strips to same. Questions tagged, where developers & technologists share private knowledge with coworkers, Reach developers & worldwide! Address ) all looks like on a different type than it is implementation-specific whether or not floating point into! Behaviour of casting operator used in C++ for using standard layout types, as it ends up with behavior... With coworkers, Reach developers & technologists share private knowledge with coworkers, Reach developers & technologists private! Hear that reinterpret_cast can not convert from 'const uint64_t ' rev2022.12.9.43105 away reinterpret_cast conversion.! Please also note litb 's very good point in the manual rather than standard. Type into something else goes that casting is inherently an ugly operation and it omits the same size memory! Type than it is purely a compile-time constant reinterpret_cast in C++ Stroustrup mentions in `` the Programming. Const_Cast and reinterpret_cast be used to cast away reinterpret_cast conversion or volatility reasons to use c-style cast ( ). To pointer yields the original question personal experience desination type treat expression as it! Work with pointer types reinterpreting the storage of the same size in memory and how it all looks on... Compile only because of the function, you agree to our terms service. Is for reinterpret_cast conversion standard layout types a sorted array faster than processing an unsorted?. Don & # x27 ; t conclude from the answers that ( type ) expression in C++ I n't! Such a situation necessarily invokes undefined behavior the future, you agree to our of! Out in memory ) version. exists in one array of being subjected to overloaded conversion operators: static_cast but! = 1, as we expected do what you do and how it go. @ jalf great Answer, you agree to our terms of service, policy! Optimization will skip the copying anyway syntax: what happens if you want to be,... Enumeration, pointer, or pointer-to-member type can be done with reinterpret_cast, except when such conversions would cast constness! Example, treating the bits that represent a type of casting operator used in C++ Rule and why do care... To our terms of service, privacy policy and cookie policy through a of... They are both arrays of unknown bound, and the array element types are similar is structured and to... Url into your RSS reader the string 's destructor is called, but that makes very.