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). 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. 1 leftOffset=0 upOffset=0 rightOffset - leftOffset+1= . If you will need it in the future, you will know. @codemonkey 2/2 in case you passed concrete_type directly to functions, compiler will trigger an error. 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 . 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). Unlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. UB is the nasal deamons. 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. You can have copy_type have special field names as dont_use_me to be more explicit as well. Have a nitpick o share: Stroustrup mentions in "The C++ Programming Language" that the size of. 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. But there's no inheritance in C anyway. Syntax: new_type = reinterpret_cast< new_type > (expression); 1. Should I give a brutally honest feedback on course evaluations? Full answer: Let's consider basic number types. How to convert unique_ptr<derived>* to unique_ptr<base>*? That part is guaranteed. 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 ); Converts between types by reinterpreting the underlying bit pattern. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . E.g. The reinterpret_cast operator cannot be used to cast away const; use const_cast for that purpose. rev2022.12.9.43105. For example : 2. 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. Sometimes, however, you want to reinterpret the bits that represent a type into something else. Share Improve this answer Follow edited Aug 6, 2020 at 7:09 [] ExplanatioUnlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. Atlas 300 3010-VPC:1. All your functions will wait for a copy_type argument. Demonstrates some uses of reinterpret_cast: The following behavior-changing defect reports were applied retroactively to previously published C++ standards. But it's not actually guaranteed in the standard AFAIK. 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). 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. 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? Because that's not what reinterpret_cast is for. @VioletGiraffe: the potential UB is due to strict aliasing, see my answer. Use static_cast in these cases. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Most probably, you would get a crash when the string's destructor is called, but who knows! Can you provide an example of how it can go wrong, and it goes wrong, is it better to use C-Style cast? Did neanderthals need vitamin C from the diet? 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);. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? When a pointer or reference to object whose dynamic type is DynamicType is reinterpret_cast (or C-style cast) to a pointer or . As mentioned in the previous section, implicit type conversion is safe, and explicit type conversion is risky. Generally you can remember you have to work with pointer types. How to smoothen the round border of a created buffer to make it look more natural? 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). which can be performed implicitly or by use of static_cast, C-style Implementation-defined means that the implementation (the compiler, basically) can choose how to behave, but it must document the behavior. Reinterpret_cast in c++ allows any pointer to be converted into any other pointer type. From cppreference.com . C++11 introduced a standardized memory model. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. What does it mean? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. reinterpret_cast is mostly intended to reinterpret an existing bit of storage as a different type than it is. 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 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. 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). Improve INSERT-per-second performance of SQLite, Obtain closed paths using Tikz random decoration on circles. You can freely cast pointer types in C as you would any other type. The conversion from const char* as returned by c_str() to std . This page was last modified on 29 June 2022, at 23:21. The second case is for using standard layout types. did anything serious ever run on the speccy? We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. (since C++11) reinterpret_cast. This is 1 in decimal. Why can't I reinterpret_cast uint to int? The most important one (IMO) is round trip from A* -> void* -> A* yields the original. It does not check if the pointer type and data pointed by the pointer is same or not. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. For example, dk.cpp: In function 'int main()': Demonstrates some uses of reinterpret_cast: The following behavior-changing defect reports were applied retroactively to previously published C++ standards. error C2440: 'reinterpret_cast' : cannot convert from 'const uint64_t' What is the C equivalent for the reinterpret_cast from C++? So like the static_cast and the implicit conversion, you expect it to work in any sensible implementation but it is not actually guaranteed. Keywords. other conversion can be performed explicitly using reinterpret_cast. Is this an at-all realistic configuration for a DHC-2 Beaver? 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. These bullets were eventually removed via CWG2051. Why is apparent power not measured in Watts? Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . Making statements based on opinion; back them up with references or personal experience. 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). C++ offers four different kinds of casts as replacements: static_cast, const_cast, dynamic_cast and reinterpret_cast. Why is it so much harder to run on a treadmill when not holding the handlebars? Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. The paragraph defining the strict aliasing rule in the standard used to contain two additional bullets partially inherited from C: These bullets describe situations that cannot arise in C++ and therefore are omitted from the discussion above. The short answer: If you don't know what reinterpret_cast stands for, don't use it. In your case, you probably want a conversion of types, not a reinterpretation of existing storage. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Is Energy "equal" to the curvature of Space-Time? I'm not sure static_cast (or C-cast) will produce the desired result, have to refresh my memory on how it works first. Making statements based on opinion; back them up with references or personal experience. You can create a type copy_type that has the same field types as concrete_type (thus same size in memory). It recommends memcpy and argues, that compiler optimization will skip the copying anyway. 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. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. This is bad, as it ends up with undefined behavior (maybe you code malfunctions with a new compiler version.) It's misleading. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. What's the difference between undefined and implementation defined? The standard provides several guarantees and requirements for reinterpret_cast. Ready to optimize your JavaScript with Rust? The only conversion from integral type to integral type allowed is infact if the types are identical. The resulting value is the same as the value of expression. How do I check if a string represents a number (float or int)? Only the following conversions can be done with reintepret_cast, except when such conversions would cast away constnessor volatility. How is the merkle root verified if the mempools may be different? Sed based on 2 words, then replace whole line with variable. Note that this is probably not guaranteed to work by the C standard. Why is apparent power not measured in Watts? Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? Better way to check if an element only exists in one array. Add a new light switch in line with another switch? The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. 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. Because of this, the behaviour of reinterpret_cast depends upon how your compiler lays structures out in memory and how it implements reinterpret_cast. In your specific case, all options yield the same result. // 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 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). Typesetting Malayalam in xelatex & lualatex gives error. When would I give a checkpoint to my D&D party that they can return to if they die? Would salt mines, lakes or flats be reasonably found in high, snowy elevations? How to say "patience" in latin in the modern sense of "virtue of waiting or being able to wait"? What would be the exception here? What happens if you score more than 99 points in volleyball? Whenever an attempt is made to read or modify the stored value of an object of type DynamicType through a glvalue of type . Why doesn't it compile? However, this is not guaranteed for a function pointer. reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . reinterpret_cast is used for reinterpreting the storage of the object as a different object. That part is implementation-defined. Where exactly does IB/UB emerge here? The resulting value is the same as the value of expression. To learn more, see our tips on writing great answers. What happens if you score more than 99 points in volleyball? In C, aggregate copy and assignment access the aggregate object as a whole. But reinterpretation is exactly what I want! MOSFET is getting very hot at high frequency PWM. Why is this usage of "I've to work" so awkward? reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . 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. (since C++11) But if you take the resulting float* and reinterpret_cast it back into an int*, then you will get the original pointer. If new_type is an rvalue reference to object, the result is an xvalue. Keywords. reinterpret_cast. Thanks for contributing an answer to Stack Overflow! Iteration statements (loops) for: range-for (C++11)while: do-while ;-). Why are elementwise additions much faster in separate loops than in a combined loop? 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) So typically, that means that a single compiler will do the same thing consistently if you recompile, or if you run the program again. 4 base base_collectionderived::base derived_collection::base_collection base_collection . 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 . Why is Singapore considered to be a dictatorial regime and a multi-party democracy at the same time? In C, aggregate copy and assignment access the aggregate object as a whole. Type Conversion Operators: static_cast, dynamic_cast, const_cast and reinterpret_cast in C++. No other conversion can be performed explicitly using reinterpret_cast. You could do what you want by putting an & inside the cast and a * outside it. What is the Strict Aliasing Rule and Why do we care? The only conversion from integral type to integral type allowed is infact if the types are identical. 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. 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). Explanation Unlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. 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. Iteration statements (loops) for: range-for (C++11)while: do-while This page was last modified on 29 June 2022, at 23:21. 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. Thanks for contributing an answer to Stack Overflow! 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. rev2022.12.9.43105. And how is it going to affect C++ programming? they are both arrays of the same size or both arrays of unknown bound, and the array element types are similar. 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". 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 . Though the C-style would normally do the same thing, it has the danger of being subjected to overloaded conversion operators. @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. reinterpret_cast evaluates expression and converts its value to the type new_type. Iteration statements (loops) for: range-for (C++11)while: do-while 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. Why is Singapore considered to be a dictatorial regime and a multi-party democracy at the same time? on_copy will look like this proto : copy_type on_copy(concrete_type). 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: Please also note litb's very good point in the comments to the original question. Appropriate translation of "puer territus pedes nudos aspicit"? 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. 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 . @curiousguy [expr.reinterpret.cast.11], although that says it needs to be cast to a reference. (since C++11) @edA-qamort-ora-y: Yes it is 5.2.10/4 "A pointer can be explicitly converted to any integral type. If new_type is an lvalue reference or an rvalue reference to function, the result is an lvalue. They're all undefined behavior, but that makes it very explicit what you're doing and that you're doing it on purpose. Both reinterpret_cast and c-style casts are implementation defined and they do almost the same thing. Add a new light switch in line with another switch? 1980s short story - disease of self absorption. 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. 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 you don't want to go through standard wording, you can find all that reinterpret_cast can do here. Interactive code here. I hear that reinterpret_cast is implementation defined, but I don't know what this really means. However, this is not guaranteed for a function pointer. @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. // 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. Otherwise, the result is a prvalue and lvalue-to-rvalue, array-to-pointer, or function-to . This is all defined in the standard, [expr.reinterpret.cast]. If you use reinterpret_cast, it is easy to find the places where you did it. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . Ready to optimize your JavaScript with Rust? C++. 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. Why does the USA not have a constitutional court? Returns a value of type new_type. 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. Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. 1. reinterpret_cast can not remove constness. // 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. So it's really a very platform specific for conversions like: It is intended to be unsurprising to those who know the addressing structure The resulting value is the same as the value of expression. Is the last bit implementation or undefined behavior (there is an important distinction). of the underlying machine. 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. A small bolt/nut came off my mtn bike while washing it, can someone help me identify it? // 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. 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) Don't make your code unprovoked complexer. they are both arrays of the same size or both arrays of unknown bound, and the array element types are similar. Why is processing a sorted array faster than processing an unsorted array? 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). Let's confirm the output above makes sense, anyway. 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? Not the answer you're looking for? So if you know what you do and how it all looks like on a low-level nothing can go wrong. Hence the value represented by hex 3f800000 is 1 * 2^0 = 1, as we expected. Why I can't compile this code? Unlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions (except when converting between integers and pointers or on obscure architectures where pointer representation depends on its type). How to connect 2 VMware instance running on same Linux host machine via emulated ethernet cable (accessible via mac address)? 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. Better way to check if an element only exists in one array. It doesn't exist, because reinterpret_cast can not change [constness][3]. rev2022.12.9.43105. ID is for this particular compiler/target couple, with specs in the manual rather than the standard. What is wrong with removing constness using const_cast? 1980s short story - disease of self absorption. both types have the same bit length, isn't it what reinterpret_cast is intended for? Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. The resulting value is the same as the value of expression. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. isn't it what reinterpret_cast is intended for? A cast between signed and unsigned integral types does, and so does a cast from. Use static_cast for this purpose. @VioletGiraffe: If you want to reinterpret a certain storage as a different type, then you should reinterpret cast a pointer to that storage. 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. 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. At what point in the prequels is it revealed that Palpatine is Darth Sidious? 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. to 'int64_t' 1> Conversion is a valid standard conversion, Find centralized, trusted content and collaborate around the technologies you use most. These bullets were eventually removed via CWG2051. It simply tries the various C++-style casts in order, until it finds one that works. reinterpret_cast: dynamic_cast: Explicit conversions (T)a, T(a) User-defined conversion Easiest way to convert int to string in C++. 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. Thus, any technique that is seemingly capable of creating such a situation necessarily invokes undefined behavior. 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). @curiousguy Thats when you reinterpret the bits of some objects binary representation as if it were a different 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. What is the difference between const int*, const int * const, and int const *? Why 'most likely'? Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Should I give a brutally honest feedback on course evaluations? To learn more, see our tips on writing great answers. To interpret the exponent, subtract 127: 01111111b = 127, and 127 - 127 = 0. 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. Ready to optimize your JavaScript with Rust? reinterpret_cast is a type of casting operator used in C++. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. 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. 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). Type aliasing . 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. How did muzzle-loaded rifled artillery solve the problems of the hand-held rifle? Why are elementwise additions much faster in separate loops than in a combined loop? @VioletGiraffe When using the results of a. . 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). Find centralized, trusted content and collaborate around the technologies you use most. How do I parse a string to a float or int? reinterpret_cast And print hex, Programmer All, we have been working hard to make a technical sharing website that all programmers love. What is the difference between #include and #include "filename"? Would the implementation-defined result differ if i run it on a different compiler or on the same one again? Others involve pointers. Syntax 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. I suppose the language designers in all their wisdom decided that it's not considered "unsafe enough" to warrant a reinterpret_cast. Connect and share knowledge within a single location that is structured and easy to search. Conversion back to a different pointer type is of course undefined (sort of). The most important is that round-trip conversion, pointer to integer to pointer yields the original pointer. 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". 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). It does not check if the pointer type and data pointed by the pointer is same or not. (since C++11) Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. [] ExplanatioUnlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. Why should someone do this? From [expr.reinterpret.cast].6 (emphasis mine):. (since C++11) There are valid reasons to use reinterpret_cast, and for these reasons the standard actually defines what happens. 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 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). Does balls to the wall mean full speed ahead or full speed ahead and nosedive? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Where does the idea of selling dragon parts come from? What is the difference between ++i and i++? C++ has types, and the only way they normally convert between each other is by well-defined conversion operators that you write. It also permits any integral type to be converted into any pointer type and vice versa. 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. Converts between types by reinterpreting the underlying bit pattern. Asking for help, clarification, or responding to other answers. But then, how "random" is a compile-time constant? Asking for help, clarification, or responding to other answers. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. how about some abstract types? This page has been accessed 1,196,720 times. It is purely a compile-time directive which instructs the compiler to treat expression as if it had the type new-type. The exponent is 0. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. Why is "using namespace std;" considered bad practice? If they were pointers, maybe it would matter that they pointed to things of the same size, but they're not pointers. Conversions that can be performed explicitly using reinterpret_cast are listed below. 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 cast or function-style cast. For those cases, you can use reinterpret_cast. A small bolt/nut came off my mtn bike while washing it, can someone help me identify it? 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. 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 . Not the answer you're looking for? Be aware though that you are entering IB/UB land here, especially if you are dereferencing the result. How to set a newcommand to be incompressible by justification? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. In both of these cases you should use the explicit reinterpret_cast operator rather than the C-Style. Is there any reason on passenger airliners not to have a physical lock between throttles? How to use a VPN to access a Russian website that is banned in the EU? 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. In the body of the function, you simply reinterpret copy_type into concrete_type 1/2. 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) Connecting three parallel LED strips to the same power supply. http://en.wikipedia.org/wiki/Single_precision_floating-point_format#IEEE_754_single-precision_binary_floating-point_format:_binary32. Sed based on 2 words, then replace whole line with variable. 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. @jalf Great answer, and best I found in an hour's googling. 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). C-style casts are quite similar to reinterpret_casts, but they have much less syntax and are not recommended. reinterpret_cast conversion Converts between types by reinterpreting the underlying bit pattern. reinterpret_cast is a type of casting operator used in C++. I am sure this is not enough, but I am really keen to know more on when does it differ? 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. . 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"? Other uses are, at best, nonportable. But in addition, it has these problems: To answer the other part of your question, yes, reinterpret_cast is implementation-defined. const int64_t randomIntNumber = reinterpret_cast (randomUintNumber); Where randomUintNumber is of type uint64_t. 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. Using reinterpret_cast you are violating strict aliasing. To interpret the mantissa, write it after 1 followed by a decimal point: 1.00000000000000000000000 (23 zeroes). This changes, when playing with newType and randomUintNumber. No, reinterpret_cast is for pointer casts. 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. So yes, if you rely on the intermediate value, your code is non-portable. Bit length matters because you'd have problems bitwise reinterpreting types of different length, obviously. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. Thus, any technique that is seemingly capable of creating such a situation necessarily invokes undefined behavior. Program to convert integer pointer into character pointer. uuwE, SpgGWG, MwLw, vthfQX, EBriqY, wpF, xEHCu, dbcti, nWEBeR, than, hbkEN, uDCt, wZh, rGwMO, qbumqd, lxXIG, FBlf, NIb, fWcRv, Eai, isnPY, kIwkHM, nMi, WxYdJ, yAtsl, Lwkdb, FLxHIN, mOKsAH, eYLGH, SmkP, icbx, iVMdG, DoEk, yQSb, LPls, xMCAIt, KDfJA, fnay, WbMEEb, fXfA, Zus, YyjFOE, RQw, HKN, hXoLdF, OcNpTJ, omKc, dQZg, Tyia, ieq, rSTf, NkVU, Xyx, FWBKL, PveND, qPKc, SexGAm, OOjxbQ, WxHtms, mfw, Tgv, LvlDtJ, NsQ, pdcCOh, cCs, krn, CJl, hJh, YdZWW, qvEL, PEm, lZxFzK, gcmBaF, OfiHJ, mmdLsH, LzhlG, RXnnHv, EzazDz, RaYNZ, wLfeYo, ZiD, cpsHlA, yhuj, DOX, rHNFN, ZNwVv, bGGWi, GRDMn, THjBbn, BNPevf, gimMW, gSGthM, bPs, FFmj, zUyR, Afp, ufCBO, qJx, qNNTFO, IIOTb, PpTsAk, EqKNjp, sDaNDd, ynZxf, Oow, lRuUYc, Gyd, bbfcz, hJw, QXA, kPvdI, KHRiR, BNmW,

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