1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? which can be performed implicitly or by use of static_cast, C-style These bullets were eventually removed via CWG2051. So yes, if you rely on the intermediate value, your code is non-portable. The short answer: If you don't know what reinterpret_cast stands for, don't use it. Most probably, you would get a crash when the string's destructor is called, but who knows! The most important one (IMO) is round trip from A* -> void* -> A* yields the original. Converts between types by reinterpreting the underlying bit pattern. The resulting value is the same as the value of expression. To interpret the exponent, subtract 127: 01111111b = 127, and 127 - 127 = 0. Why I can't compile this code? So it is implementation-specific whether or not this code violates strict aliasing. 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: This is 1 in decimal. Would it be possible, given current technology, ten years, and an infinite amount of money, to construct a 7,000 foot (2200 meter) aircraft carrier? (since C++11) That part is guaranteed. Explanation. reinterpret_cast conversion Converts between types by reinterpreting the underlying bit pattern. 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 -. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. 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). To learn more, see our tips on writing great answers. You can create a type copy_type that has the same field types as concrete_type (thus same size in memory). Both reinterpret_cast and c-style casts are implementation defined and they do almost the same thing. Why is processing a sorted array faster than processing an unsorted array? (since C++11) Unlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. I checked the assembly code produced by VC2008 and it omits the same code. (since C++11) Use static_cast for this purpose. 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? In C, aggregate copy and assignment access the aggregate object as a whole. @user974191 That's what I tried to say above the example. @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. dk.cpp:5:41: error: reinterpret_cast from type 'const unsigned int*' to type 'int*' casts away qualifiers. The resulting value is the same as the value of expression. Syntax : So typically, that means that a single compiler will do the same thing consistently if you recompile, or if you run the program again. // 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. Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? 1980s short story - disease of self absorption. You could do what you want by putting an & inside the cast and a * outside it. If you're casting one pointer type to a different pointer type, there's actually no machine code necessary for that. 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). I don't like to say "reinterpret_cast", because cast means conversion (in c), 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". This is usually used for very low-level operations and is not something you should typically use. Syntax reinterpret_cast < new_type > ( expression ) Returns a value of type new_type. @edA-qa mort-ora-y: Assuming the integer is large enough to hold the pointer. 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). Should I give a brutally honest feedback on course evaluations? It simply tries the various C++-style casts in order, until it finds one that works. 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 reinterpret_cast, except when such conversions would cast away constness or volatility . Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Thus, any technique that is seemingly capable of creating such a situation necessarily invokes undefined behavior. C++11 introduced a standardized memory model. they are both arrays of the same size or both arrays of unknown bound, and the array element types are similar. No other conversion can be performed explicitly using reinterpret_cast. Why can't I reinterpret_cast uint to int? @curiousguy Thats when you reinterpret the bits of some objects binary representation as if it were a different type. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Hence the value represented by hex 3f800000 is 1 * 2^0 = 1, as we expected. 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"? (since C++11) What is the Strict Aliasing Rule and Why do we care? This is bad, as it ends up with undefined behavior (maybe you code malfunctions with a new compiler version.) Does it mean the code would be non-portable? 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 Is this an at-all realistic configuration for a DHC-2 Beaver? reinterpret_cast. But in C++ such actions are always performed through a member function call, which accesses the individual subobjects rather than the entire object (or, in the case of unions, copies the object representation, i.e., via unsigned char). 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. 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 second case is for using standard layout types. Otherwise, the result is a prvalue and lvalue-to-rvalue, array-to-pointer, or function-to . 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. 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 . rev2022.12.9.43105. other conversion can be performed explicitly using reinterpret_cast. In your case, you probably want a conversion of types, not a reinterpretation of existing storage. When Kevin Costner was cast as Bat Man, he didn't literally become Bat Man; he was just interpreted as portraying Bat Man. (since C++11) That part 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) It is purely a compile-time directive which instructs the compiler to treat 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). Use static_cast in these cases. 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. 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. Because of this, the behaviour of reinterpret_cast depends upon how your compiler lays structures out in memory and how it implements reinterpret_cast. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. The most important is that round-trip conversion, pointer to integer to pointer yields the original pointer. 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. 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 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). Why are elementwise additions much faster in separate loops than in a combined loop? In this case the standard treats reinterpret_cast as a static_cast to void* first and then a static_cast to the desination type. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Syntax: new_type = reinterpret_cast< new_type > (expression); 1. Full answer: Let's consider basic number types. C++ has types, and the only way they normally convert between each other is by well-defined conversion operators that you write. @VioletGiraffe When using the results of a. cast or function-style cast. Improve INSERT-per-second performance of SQLite, Obtain closed paths using Tikz random decoration on circles. Find centralized, trusted content and collaborate around the technologies you use most. It can cast integer and floating point types into each other. Using reinterpret_cast you are violating strict aliasing. This page has been accessed 1,196,720 times. Appropriate translation of "puer territus pedes nudos aspicit"? rev2022.12.9.43105. It doesn't exist, because reinterpret_cast can not change [constness][3]. Implementation-defined means that the implementation (the compiler, basically) can choose how to behave, but it must document the behavior. 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. However, this is not guaranteed for a function pointer. Interactive code here. 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. Connect and share knowledge within a single location that is structured and easy to search. @litb This is an interesting comment. 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. A function pointer can be explicitly converted to a function pointer of a different type. Thanks for contributing an answer to Stack Overflow! Why would that matter? // 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. error C2440: 'reinterpret_cast' : cannot convert from 'const uint64_t' Even if randomUintNumber is a compile-time constant, unfortunately here randomIntNumber is not a compile-time constant. 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. However, this is not guaranteed for a function pointer. As mentioned in the previous section, implicit type conversion is safe, and explicit type conversion is risky. What is the difference between const int*, const int * const, and int const *? All your functions will wait for a copy_type argument. In general, that's all you both need and should use to write your programs. How do I parse a string to a float or int? const int64_t randomIntNumber = reinterpret_cast (randomUintNumber); Where randomUintNumber is of type uint64_t. C++ offers four different kinds of casts as replacements: static_cast, const_cast, dynamic_cast and reinterpret_cast. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. 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);. E.g. Find centralized, trusted content and collaborate around the technologies you use most. What does it mean? No, it is not. This page has been accessed 1,191,892 times. 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. 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. Type aliasing . For example: In this code c is guaranteed to point to the object b as you'd expected. Keywords. reinterpret_cast conversion. It also permits any integral type to be converted into any pointer type and vice versa. Thus, any technique that is seemingly capable of creating such a situation necessarily invokes undefined behavior. Generally you can remember you have to work with pointer types. Demonstrates some uses of reinterpret_cast: The following behavior-changing defect reports were applied retroactively to previously published C++ standards. For those cases, you can use reinterpret_cast. Iteration statements (loops) for: range-for (C++11)while: do-while But if you take the resulting float* and reinterpret_cast it back into an int*, then you will get the original pointer. Better way to check if an element only exists in one array. Returns a value of type new_type. Where do I find the current C or C++ standard documents? It does not check if the pointer type and data pointed by the pointer is same or not. A small bolt/nut came off my mtn bike while washing it, can someone help me identify it? Code that violates strict aliasing has undefined behavior. they are both arrays of the same size or both arrays of unknown bound, and the array element types are similar. If you want to be portable, you have to do the full round-trip. reinterpret_cast. From cppreference.com . ;-). 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. Why is "using namespace std;" considered bad practice? 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). How to print and pipe log file at the same time? 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). 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 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. Can you provide an example of how it can go wrong, and it goes wrong, is it better to use C-Style 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. 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 ); 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). The standard provides several guarantees and requirements for reinterpret_cast. Add a new light switch in line with another switch? 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. 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). Note that many C++ compilers relax this rule, as a non-standard language extension, to allow wrong-type access through the inactive member of a union (such access is not undefined in C). What is the C equivalent for reinterpret_cast? I suppose the language designers in all their wisdom decided that it's not considered "unsafe enough" to warrant a reinterpret_cast. 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. You can freely cast pointer types in C as you would any other type. These bullets were eventually removed via CWG2051. In C, aggregate copy and assignment access the aggregate object as a whole. 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). This changes, when playing with newType and randomUintNumber. 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 . Please also note litb's very good point in the comments to the original question. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. 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. It is purely a compile-time directive which instructs the compiler to treat expression as if it had the type new-type. @edA-qamort-ora-y: Yes it is 5.2.10/4 "A pointer can be explicitly converted to any integral type. (since C++11) Why did the Council of Elrond debate hiding or sending the Ring away, if Sauron wins eventually in that scenario? For example, treating the bits of a 64-bit, @curiousguy Certain integer casts do. Is the last bit implementation or undefined behavior (there is an important distinction). 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. For example, dk.cpp: In function 'int main()': The exponent is 0. The resulting value is the same as the value of expression. I'm not sure static_cast (or C-cast) will produce the desired result, have to refresh my memory on how it works first. how about some abstract types? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Ready to optimize your JavaScript with Rust? 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. If new_type is an rvalue reference to object, the result is an xvalue. Reinterpret_cast in c++ allows any pointer to be converted into any other pointer type. 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. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. 4 base base_collectionderived::base derived_collection::base_collection base_collection . Allow non-GPL plugins in a GPL main program, central limit theorem replacing radical n with n. Is there any reason on passenger airliners not to have a physical lock between throttles? But then, how "random" is a compile-time constant? A cast between signed and unsigned integral types does, and so does a cast from. Does balls to the wall mean full speed ahead or full speed ahead and nosedive? [] ExplanatioUnlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. How is the merkle root verified if the mempools may be different? The differences are : Only the following conversions can be done with reintepret_cast, except when such conversions would cast away constnessor volatility. Sed based on 2 words, then replace whole line with variable. Making statements based on opinion; back them up with references or personal experience. Add a new light switch in line with another switch? 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. reinterpret_cast evaluates expression and converts its value to the type new_type. What's the difference between undefined and implementation defined? is a great article, describing the problem and its solution (maybe skip the lengthy part "Now, to the Rule-Book" ;) ). Not the answer you're looking for? rev2022.12.9.43105. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . 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). In both of these cases you should use the explicit reinterpret_cast operator rather than the C-Style. reinterpret_cast is mostly intended to reinterpret an existing bit of storage as a different type than it is. But in addition, it has these problems: To answer the other part of your question, yes, reinterpret_cast is implementation-defined. Whenever an attempt is made to read or modify the stored value of an object of type DynamicType through a glvalue of type . Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. // 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. Though the C-style would normally do the same thing, it has the danger of being subjected to overloaded conversion operators. 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? Others involve pointers. What is the difference between ++i and i++? This page was last modified on 29 June 2022, at 23:21. Why 'most likely'? 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. 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. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. But there's no inheritance in C anyway. Thanks for contributing an answer to Stack Overflow! 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. If they were pointers, maybe it would matter that they pointed to things of the same size, but they're not pointers. Find centralized, trusted content and collaborate around the technologies you use most. When a pointer or reference to object whose dynamic type is DynamicType is reinterpret_cast (or C-style cast) to a pointer or . Connecting three parallel LED strips to the same power supply. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. 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? That is controlled by your specific implementation of C++. Reading a different member of a union than you last wrote to does not. Iteration statements (loops) for: range-for (C++11)while: do-while Keywords. Why is apparent power not measured in Watts? There are valid reasons to use reinterpret_cast, and for these reasons the standard actually defines what happens. This page was last modified on 29 June 2022, at 23:21. 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). Program to convert integer pointer into character 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 reintepret_cast . Don't make your code unprovoked complexer. // class member access expression is undefined behavior; // value of p1 is "pointer to the S subobject of s1", // n = *reinterpret_cast(&d); // Undefined behavior, // pointer to function to another and back, // const_iref); // compiler error - can't get rid of const. The 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. Don't conclude from the answers that (type)expression in C++ is equivalent to a reinterpret_cast. while reinterpret means the opposite: no conversion. C++. The reinterpret_cast operator cannot be used to cast away const; use const_cast for that purpose. reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . Both are valid statements, and both will compile only because of the cast. http://en.wikipedia.org/wiki/Single_precision_floating-point_format#IEEE_754_single-precision_binary_floating-point_format:_binary32. on_copy will look like this proto : copy_type on_copy(concrete_type). Share Improve this answer Follow edited Aug 6, 2020 at 7:09 The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. Returns a value of type new_type. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . Why should someone do this? reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . They're all undefined behavior, but that makes it very explicit what you're doing and that you're doing it on purpose. But reinterpretation is exactly what I want! Is Energy "equal" to the curvature of Space-Time? Bit length matters because you'd have problems bitwise reinterpreting types of different length, obviously. 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. How do I check if a string represents a number (float or int)? @VioletGiraffe: the potential UB is due to strict aliasing, see my answer. It is not possible to do with c-style casts. That means that when it acts like a reinterpret_cast, it has the exact same problems as a reinterpret_cast. 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. 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) How to set a newcommand to be incompressible by justification? 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. Asking for help, clarification, or responding to other answers. Conversions that can be performed explicitly using reinterpret_cast are listed below. How to smoothen the round border of a created buffer to make it look more natural? At what point in the prequels is it revealed that Palpatine is Darth Sidious? Note that this is probably not guaranteed to work by the C standard. So it's really a very platform specific for conversions like: It is intended to be unsurprising to those who know the addressing structure If you will need it in the future, you will know. It might just corrupt your stack and cause a crash in an unrelated function. What happens if you score more than 99 points in volleyball? I hear that reinterpret_cast is implementation defined, but I don't know what this really means. Allow non-GPL plugins in a GPL main program. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? 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. 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. Why are elementwise additions much faster in separate loops than in a combined loop? Would salt mines, lakes or flats be reasonably found in high, snowy elevations? How to convert unique_ptr<derived>* to unique_ptr<base>*? A memory address is a memory address regardless of what it points at (on x86 anyway), so pointer "type" exists only in the mind of the compiler. The resulting value is the same as the value of expression. . I am sure this is not enough, but I am really keen to know more on when does it differ? It's misleading. No, reinterpret_cast is for pointer casts. Converts between types by reinterpreting the underlying bit pattern. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. What is the difference between #include and #include "filename"? 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). If new_type is an lvalue reference or an rvalue reference to function, the result is an lvalue. Except that converting a prvalue of type "pointer to T1" to the type "pointer to T2" (where T1 and T2 are function types) and back to its original type yields the original pointer value, the result of such a pointer conversion is unspecified. Why is it so much harder to run on a treadmill when not holding the handlebars? The only conversion from integral type to integral type allowed is infact if the types are identical. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. To interpret the mantissa, write it after 1 followed by a decimal point: 1.00000000000000000000000 (23 zeroes). isn't it what reinterpret_cast is intended for? Asking for help, clarification, or responding to other answers. @JoachimPileborg: my bad, expanded the question. So if you know what you do and how it all looks like on a low-level nothing can go wrong. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. Iteration statements (loops) for: range-for (C++11)while: do-while Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility . 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. What is the C equivalent for the reinterpret_cast from C++? Why does the USA not have a constitutional court? 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". In the body of the function, you simply reinterpret copy_type into concrete_type 1/2. Type aliasing. Have a nitpick o share: Stroustrup mentions in "The C++ Programming Language" that the size of. 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. It does not check if the pointer type and data pointed by the pointer is same or not. 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 . By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. [] ExplanatioUnlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. 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). 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. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. 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