How do I arrange multiple quotations (each with multiple lines) vertically (with a line through the center) so that they're side-by-side? Additionally, and arguably more important, is the fact that every use of reinterpret_cast is downright dangerous because it converts anything to anything else really (for pointers), while static_cast is much more restrictive, thus providing a better level of protection. From C++ . Error: #694: reinterpret_cast cannot Should I use static_cast or reinterpret_cast when casting a void* to whatever. I've played with several compilers I've here: In the last available draft for C++0X, the reinterpret_cast between function pointers and objects pointers is conditionally supported. Increment void pointer by one byte? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. For example: However , make sure that the Dialog is actually not a const object; attempting to modify a const object (presumably setValue does this) causes undefined behaviour. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? If he had met some scary fish, he would immediately return to the surface. In the case of casting an object pointer to another object pointer type, failing to meet the requirements of strict aliasing rules means you cannot safely dereference the result. This is the object: Expand | Select | Wrap | Line Numbers template<class T> class Coordinates { public: T *x; T *y; int size; public: Coordinates (); Coordinates (int s, T data); Coordinates (const Coordinates &c); ~Coordinates ();; void zeros (void); }; Asking for help, clarification, or responding to other answers. It's in the language for a reason. Find centralized, trusted content and collaborate around the technologies you use most. Thanks for contributing an answer to Stack Overflow! However, this doesnt actually describe the effect of a reinterpret_cast. Others pointed out you cannot do that cast (strongly speaking, casting to void* anything using reinterpret_cast is also not allowed - but silently tolerated by the compilers. The static_cast is more appropriate for converting a void* to a pointer of some other type. From that point on, you are dealing with 32 bits. Leaving a link here: @cpplearner The types are in the Type Aliasing section. @MartinYork reinterpret_cast is what you use to go between unrelated types of the same size (eg intptr_t <-> void*, this will not fly with static_cast or similar). Once a pointer has degenerated into a void* you can static_cast it to any type of pointer. Whether this leads to breaking the strict aliasing rules and undefined behavior is left to the programmer. int cell = 20; outFile.write (reinterpret_cast<const char *> (&cell),sizeof (cell)); You are asking whether binary files are good. That's unsafe and compiler warns you here that it could be that the destination type is not big enough to hold the source value. [] Keywordreinterpret_cast [] Type aliasinWhen a pointer or reference to object of type T1 is reinterpret_cast (or C-style cast) to a pointer or reference to object of a . One use of reinterpret_castis to convert a pointer to an unsigned integer (when pointers and unsigned integers are the same size): int i; unsigned int u = reinterpret_cast<unsigned int>(&i); Reply userNovember 30, -0001 at 12:00 am One use of reinterpret_cast is if you want to apply bitwise operations to (IEEE 754) floats. Are there any situations where reinterpre_cast<> should be used. How to print function pointers with cout? It is efficient because it does not copy the value. If you want to print the address, cast the pointer to void* first: cout<< "address=" << ( void *)charPtr1; Should teachers encourage good students to help weaker ones? c++ reinterpret_cast unique_ptr * unique_ptr * c++ Using reinterpret_cast to cast unique_ptr * to unique_ptr * for creating a transformable tree structure i.e. will probably work ok on x86, but Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. For a conversion between different function type pointers or between different object type pointers you need to use reinterpret_cast. reinterpret_cast is a tricky beast. Something can be done or not a fit? none prevent the C cast, even in the highest conformance mode. But the function to pointer to function implicit conversion is made for the argument to reinterpret_cast, so what reinterpret_cast get is a Test** (*p)(void** a). Not the answer you're looking for? Also, casting from void * can use static_cast, it does not need to reinterpret. reinterpret_cast to void* not working with function pointers, Casting a function pointer to another type. There is no need for a cast of any sort when converting to a void* pointer. For example, when using a C-style cast, as in. Books that explain fundamental chess concepts. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. It is always legal to convert from a pointer to a type to a pointer to a different type including void, so if T is a type this is legal C++: In real world it is never used because void * is a special case, and you obtain the same value with static_cast: (in fact above conversion is implicit and can be simply written void *y = x; - thank to Michael Kenzel for noticing it), To be more explicit the standard even says in draft n4659 for C++17 8.2.10 Reinterpret cast [expr.reinterpret.cast], 7. Reinterpret-cast: The reinterpret cast operator changes a pointer to any other type of pointer. Share Improve this answer This cast operator can convert an integer to a pointer and so on. Is this an oversight? It's just a quick idea. The effect of calling a function through a pointer to a function type (8.3.5) that is not the same as the type used in the definition of the function is undefined. It does not mean they are the only type you can use with reinterpret_cast. Why does Cauchy's equation for refractive index contain only even power terms? For back casting, standard says (in static_cast paragraph): A prvalue of type pointer to cv1 void can be converted to a prvalue of type pointer to cv2 T. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? So its better and recommended to use static_cast. Only that if you reinterpret_cast from one type to another. Ready to optimize your JavaScript with Rust? Generally reinterpret_cast is much less restrictive than other C++ style casts in that it will allow you to cast most types to most other types which is both it's strength and weakness. [] Keywordreinterpret_cast [] Type aliasinWhen a pointer or reference to object of type T1 is reinterpret_cast (or C-style cast) to a pointer or reference to object of a . . - Expression is a pointer to be reinterpreted. I add an issue on reinterpreting a variable and I don't know why.. the reinterpret_cast seems not allowed and say, reinterpret_cast from 'const void *) to Dialog * casts away qualifiers, reinterpret_cast can't cast away cv-qualifiers. Is this an at-all realistic configuration for a DHC-2 Beaver? For example, you can use reinterpret_cast to convert from a void * to an int, which will work correctly if your system happens to have sizeof (void*) sizeof (int). Don't know why. Something can be done or not a fit? Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, C++ Converting function pointer to unique hash key. Why would I use dynamic_cast to cast TO a void *? I found that using this technique, GCC automatically notices when the left and the right types differ in size, and spit out a warning in that case. In C++0x, reinterpret_cast<int*> (p) will be . [expr.reinterpret.cast]/7:. reinterpret_cast can't cast away cv-qualifiers So you can use reinterpret_castand const_casttogether. The type of a pointer to cv void or a pointer to an object type is called an object pointer type.. You don't need to use reinterpret_cast, though.Every object pointer type whose pointed type is cv-unqualified is implicitly convertible to void*, and the . Cast from Void* to TYPE* using C++ style cast: static_cast or reinterpret_cast. static_cast is intended to be used here). Did neanderthals need vitamin C from the diet? Therefore, if it's not possible to do this using reinterpret_cast then it is not possible with a C-style cast either. How can I cast "const void*" to the function pointer in C++11? When you refer to byte and char being the only legal types, it is just that it is legal to dereference the converted pointer only for those types. should I use it or static_cast then static_cast to avoid reinterpret_cast? Are can all casting operations be covered by the other 3 cast operators? Casting to and from void* using static_cast and using reinterpret_cast is identical. 1. static _ cas t static _ cas t static _ca. Why do we have reinterpret_cast in C++ when two chained static_cast can do its job? Which cast to use; static_cast or reinterpret_cast? Don't write a binary file "because it's faster". ( reinterpret_cast does not perform the adjustments that might be needed.) Das Hauptproblem ist, dass die Daten bentigt um einen Zeiger-auf . This has already saved me from bugs where I accidentally tried to coerce one pointer type into another. The compiler can then check that the cast is between related types, reporting a compile-time error if this isn't the case. 9 windows. template<class Vector> void test (Vector& vec) { using E = decltype (vec [0]); for (int i=0; i < 10; ++i) { vec.push_back (E (i)); } } int main () { std::vector<double> v; test (v); for (int i=0; i < 10; ++i) { printf ("%f\n", v [i]); } } Japanese girlfriend visiting me in Canada - questions at border control? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Use static_cast: it is the narrowest cast that exactly describes what conversion is made here. (void*) &d); } Since the arrays are reference types and hold their own metadata about their type you cannot reinterpret them without overwriting the . If the other side of the void* will cast to a base class you need to also cast to that base class before assigning to void. Your code is not guaranteed to work on any compiler, ever. It's recently that I needed to properly understand reinterpret_cast, which is a method of converting between data types. 3.3 void* , static_cast reinterpret_cast , void* . reinterpret_cast,"". When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? Something can be done or not a fit? reinterpret_cast in C#. @BenVoigt the "entire expression" isn't a cast though. Does aliquot matter for final concentration? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. reinterpret_cast to void* not working with function pointers, error: passing xxx as 'this' argument of xxx discards qualifiers, Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs. Find centralized, trusted content and collaborate around the technologies you use most. How can I use a VPN to access a Russian website that is banned in the EU? extra contextual information. Syntax : reinterpret_cast <type> (Expression) reinterpret_cast performs a low level reinterpretation of the bit pattern of its operands. reinterpret_cast can't be used to cast a pointer to function to a void*. I use reinterpret_cast to convert integer memory addresses to pointers to overlay structs. remember that it's undefined behavior. It does not check if the pointer type and data pointed by the pointer is same or not. Accessing the result of this cast violates strict aliasing and causes undefined behavior, as usual. The solution may be to replace *static_cast<const T*>(value) to reinterpret_cast<const T*>(value). Otherwise you should reinterpret_cast to exactly the same type of the original pointer (no bases or such). #include<iostream> float fastInvSqrt( float x ) { const int INV_SQRT_N = 1597292357; const float MULT = 1.000363245811462f; float const mx = 0.5f * MULT * x; However, you are also casting the result of this operation to (void*). The reinterpret_cast<> must be used carefully. Was the ZX Spectrum used for number crunching? Why is Singapore currently considered to be a dictatorial regime and a multi-party democracy by different publications? To learn more, see our tips on writing great answers. Making statements based on opinion; back them up with references or personal experience. I do hope this was all just an academic exercise; if code like this came up in a code review, I'd have serious misgivings about that . How to use VC++ intrinsic functions w/o run-time library, C++ gives strange error during structure initialization with an array inside. Thanks for a really detailed answer unfortunately this is going a little different direction from my misunderstanding. size as regular pointers, since on To subscribe to this RSS feed, copy and paste this URL into your RSS reader. How many transistors at minimum do you need to build a general-purpose computer? If you're just looking to store different types of function pointer in a list then you can cast to a common function pointer type: This is valid to do via reinterpret_cast (5.2.10/6): A pointer to a function can be explicitly converted to a pointer to a function of a different type. Ready to optimize your JavaScript with Rust? Japanese girlfriend visiting me in Canada - questions at border control? "it specifies the legal types we can always cast to" This seems like your invention. Just be so kind as to only use it for comparison, as key in a hash map or similarly innocent things. What properties should my fictional HEAT rounds have to punch through heavy armor and ERA? Would salt mines, lakes or flats be reasonably found in high, snowy elevations? Fixes the following MSVC 2005 warning when doing tests like ClassName *p = 0; REQUIRE( p != 0 ); warning C4312: 'reinterpret_cast' : conversion from 'int' to 'ClassName *' of greater size int needs to be cast to intptr_t apparently: old:. This cast operator can also convert variable into totally incompatible type too. However, you are also casting the result of this operation to (void*). reinterpret_cast means like telling the compiler that I know better than you here, just carry out what I am saying. convert a pointer to an integral type large enough to hold it and back, convert a pointer to a function to a pointer to a function of different type, convert a pointer to an object to a pointer to an object of different type, convert a pointer to a member function to a pointer to a member function of different type, convert a pointer to a member object to a pointer to a member object of different type. How to do such conversions from void (*)(void*) -> void* effectively so that atleast it compiles almost the same in most of the compilers ? the reinterpret_cast was a error with some compilers even in the more relaxed level while other accepted it in all case without ever giving a warning. reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . Which one to use when static_cast and reinterpret_cast have the same effect? Is it cheating if the proctor gives a student the answer key by mistake and the student doesn't report it? Casting between function pointers and Using reinterpret_cast to cast a function to void*, why isn't it illegal? We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Those types are exempt from strict aliasing rules. Repeater. -P.S. Can several CRTs be wired in parallel to one oscilloscope circuit? Both static_cast and reinterpret_cast seem to work fine for casting void* to another pointer type. Asking for help, clarification, or responding to other answers. The more structure-breaking the cast is, the more attention using it requires. reinterpret_cast<T&>(x)is equivalent to *reinterpret_cast<T*>(&x). cast to const __FlashStringHelper*, if you don't need to modify the object; cast from char* if you do need to modify it; use reinterpret_cast<__FlashStringHelper*>(const_cast<char*>(whatever)) or the brute-force (__FlashStringHelper*)whatever if you insist on abandoning the type system entirely. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Connect and share knowledge within a single location that is structured and easy to search. This . Losing bytes like this is called 'truncation', and that's what the first warning is telling you. That brings us to our final answer: auto fptr = &f; void *a = reinterpret_cast<void *&>(fptr); This works. When is casting void pointer needed in C? Not the answer you're looking for? However, I think the spec wants you to use static_cast over reinterpret_cast. It pretends it's pointing to a void* (instead of a function pointer), and then reads it. Thanks for this the code makes my eyeballs crawl, but it seems to work properly and avoid warnings/errors even with maxed out warning-levels in gcc and clang. But in the particular case of casting from void* to T* the mapping is completely well-defined by the standard; namely, to assign a type to a typeless pointer without changing its address. Use the reinterpret_cast<> to highlight these dangerous areas in the code. As for which one is preferred by the spec, neither is overly mentioned as "the right one to use" (or at least, I don't remember one of them being mentioned this way.) Making statements based on opinion; back them up with references or personal experience. Is it legal to use function with no definition c++? target-type is the target of the cast whereas expr is being cast into the new target-type. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Asking for help, clarification, or responding to other answers. Note that if that make sense or not will depend on the target more than the compiler: a portable compiler like gcc will have a behavior imposed by the target architecture and possibly ABI. cast away const or other type Why do some airports shuffle connecting passengers through security again. As you can see from the last line of the question I'm trying to cast from an. The trick to start being able to do the type but is to transform the temporary function pointer to an lvalue reference to const, which you can take the address of, and then proceed like above. It's my understanding that C-style cast is defined in terms of the new cast styles. Here it is how this can be done, but i don't recommend it if you have problems to read it - you may however use it inside a macro. How do I put three reasons together in a sentence? The type of a pointer to cv void or a pointer to an object type is called an object pointer type. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? Find centralized, trusted content and collaborate around the technologies you use most. a conversion from type cv void * to a pointer-to-object type; Keep in mind that a c-style cast like (char*) is reduced to either static_cast or reinterpret_cast whose limitations are listed above. (in practice it will typically contain the same address as a and c, but that's not specified in the standard, and it may not be true on machines with more complex memory systems.) Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. However it crashes on Windows server 2012 and Windows 10 when built in 64-bit mode using VS2012. The paragraphs about void* are 4 and 10. Rather, reinterpret_cast has a number of meanings, for all of which holds that the mapping performed by reinterpret_cast is implementation-defined. [5.2.10.3]. C++ static _ cas t dynamic _ cas t const _ cas tre interp ret_ cas t. - type is a pointer reinterpreted as. The C style cast automatically dealed with that. The " reinterpret_cast " operator can convert any type of variable to fundamentally different type. rev2022.12.11.43106. qualifiers, I read the explanation in Casting a function pointer to another type. You could certainly make a case for a different operator to designate pointer reinterprets only (which guaranteed the same address returned), but there isn't one in the standard. Is it possible to hide or delete the new Toolbar in 13.1? 7 QDebug<<. 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. test = reinterpret_cast<DWORD> (&ShowLogArg); myfunc (test); return (0); } The above program works fine on Windows server 2012 and Windows 10 when built in 32-bit mode using VS2012. is it better than static_cast? reinterpret_cast casts away const qualifier? char *charPtr1=reinterpret_cast<char*> (intPtr1); cout<<"address="<<charPtr1<<";c="<<*charPtr1<<endl; There's an overload of operator<< (char*), which assumes the pointer points to a NUL-terminated string, and prints that string. Find centralized, trusted content and collaborate around the technologies you use most. When a prvalue v of Ready to optimize your JavaScript with Rust? Logan Capaldo 2010-08-13 13:17:58 @Logan Capaldo: Thanks. to cv void. Making statements based on opinion; back them up with references or personal experience. For example: reinterpret_cast, on the other hand, is a casting operator designed to do conversions that are fundamentally not safe or not portable. In current C++, you can't use reinterpret_cast like in that code. It's sole purpose is to indicate to the compiler that you want to take some bits and pretend that they represent this other type. Du kann nicht werfen einen Zeiger-auf-member void * oder zu jedem anderen "normalen" Zeiger-Typ. 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. Why do some airports shuffle connecting passengers through security again. Why use static_cast(x) instead of (int)x? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Der C++ FAQ Lite dies erklrt in einigen Details. But with ARM compiler, it gives compilation error. Have fun! [basic.compound]/3:. 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. Generic Method Pointer. Write. If he had met some scary fish, he would immediately return to the surface. An object pointer can be explicitly converted to an object pointer of a different type. The standard guarantees that first one is a standard (read implicit) conversion: A prvalue of type pointer to cv T, where T is an object type, can be converted to a prvalue of type pointer Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, casting via void* instead of using reinterpret_cast. C++. 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). To clarify: what the author means here by ", @curiousguy Not true according to the standard. extensible library interface without reinterpret_cast. 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"? Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. [] Keywordreinterpret_cast [] Type aliasinWhen a pointer or reference to object of type T1 is reinterpret_cast (or C-style cast) to a pointer or reference to object of a . Also, does the direction of the conversion matter. Asking for help, clarification, or responding to other answers. Does illicit payments qualify as transaction costs? You need to also use a const_cast to remove const qualifiers. reinterpret_cast allows anything, that's usually a dangerous thing and normally reinterpret_cast is rarely used, tipically to convert Fixed by #403 yurivict commented on Oct 16, 2018 cryos on Oct 18, 2018 #403 cryos completed in #403 on Oct 18, 2018 Sign up for free to join this conversation on GitHub . By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. int* a = new int (); void* b = reinterpret_cast<void*> (a); int* c = reinterpret_cast<int*> (b); a and c contain the same value, but the value of b is unspecified. some architectures they might contain You mean besides the standard? (175) QT0-5qimageqpainter . When convert a void pointer to a specific type pointer, which casting symbol is better, static_cast or reinterpret_cast? reinterpret_cast may be used to cast a pointer to a float. Also, casting from void *can use static_cast, it does not need to reinterpret. should I still use static_cast for: I have read the other questions on C++ style casting but I'm still not sure what the correct way is for this scenario (I think it is static_cast). The full source code is listed as follows: Copy /* IBM_PROLOG_BEGIN_TAG */ /* This is an automatically generated prolog. static_cast is a good choice if you have some advance knowledge that the cast is going to work at runtime, and communicates to the compiler "I know that this might not work, but at least it makes sense and I have a reason to believe it will correctly do the right thing at runtime." CGAC2022 Day 10: Help Santa sort presents! Do not dereference it or you will enter the myth-enshrouded lands of undefined behavior. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. @FranoisAndrieux Ugh, I knew that. Why is Singapore currently considered to be a dictatorial regime and a multi-party democracy by different publications? Image Processing: Algorithm Improvement for 'Coca-Cola Can' Recognition. Note that a C-style (T)expression cast means to perform the first of the following that is possible: a const_cast , a static_cast , a static_cast followed by a const_cast , a reinterpret_cast , or a reinterpret_cast followed by a const_cast . So you are either doing an invalid cast or a casting back to the original type that was previously cast into a void*. I'd suggest rethinking the interface to ProgressBar to avoid needing this cast. But usually static_cast is preferred because it is more narrow and in general (but not in this specific case) more safe conversion. And I will eventually cast from the void* back to an int**. In case of char*, I'd use c-style cast, until we have some reinterpret_pointer_cast, because it's weaker and nothing else is sufficient. are member pointers fixed in size and reinterpret_cast? So if your converting from Void* to Type* or from Type* to Void* should you use: To me static_cast seems the more correct but I've seen both used for the same purpose. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. For a conversion of void* to int* you can only use static_cast (or the equivalent C-style cast). You should always avoid reinterpret_cast, and in this case static_cast will do the job. reinterpret_cast, on the other hand, is a casting operator designed to do conversions that are fundamentally not safe or not portable. A C++ reinterpret cast seems to accomplish this just fine but so far I have had no success in D after trying quite a few different things. Making statements based on opinion; back them up with references or personal experience. However, you should only do this if you used static cast to cast the pointer to void* in the first place. Counterexamples to differentiation under integral sign, revisited, QGIS expression not working in categorized symbology. defines a function, not a pointer to function. In computer science, a pointer is an object in many programming languages that stores a memory address.This can be that of another value located in computer memory, or in some cases, that of memory-mapped computer hardware.A pointer references a location in memory, and obtaining the value stored at that location is known as dereferencing the pointer. This is illustrated in the following example: class A {int a; public: A ();}; Would salt mines, lakes or flats be reasonably found in high, snowy elevations? How do I arrange multiple quotations (each with multiple lines) vertically (with a line through the center) so that they're side-by-side? static_cast is the cast of choice when there is a natural, intuitive conversion between two types that isn't necessarily guaranteed to work at runtime. reinterpret_cast static_cast static_cast reinterpret_cast casting a void You should use static_cast so that the pointer is correctly manipulated to point at the correct location. In real world it is never used because void * is a special case, and you obtain the same value with static_cast: void *y = static_cast<void *> (x); // equivalent to previous reinterpret_cast (in fact above conversion is implicit and can be simply written void *y = x; - thank to Michael Kenzel for noticing it) pointers aren't necessarily the same From that point on, you are dealing with 32 bits. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Did neanderthals need vitamin C from the diet? */ /* */ /* $Source: src/runtime/rt_vfs.C . Function reinterpret_cast void* C castconst C () POSIX C reinterpret_cast C ++ 0X reinterpret_cast I'm just forgetting like an idiot. 6 QWindowsForeignWindow::setParent. Connect and share knowledge within a single location that is structured and easy to search. Can we keep alcoholic beverages indefinitely? Thanks for contributing an answer to Stack Overflow! rev2022.12.11.43106. What is this fallacy: Perfection is impossible, therefore imperfection should be overlooked. Why should I use a pointer rather than the object itself? I have used reinterpret_cast for interpret a class object as a char*. Using C++ Style casts, this looks like a combination of two static_cast's. To learn more, see our tips on writing great answers. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Is it possible to hide or delete the new Toolbar in 13.1? In C++11 through C++17, it is implementation defined if conversions between function pointers and void * are allowed. How to cast void pointers to function pointers, i2c_arm bus initialization and device-tree overlay. What properties should my fictional HEAT rounds have to punch through heavy armor and ERA? Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Is it Legal to reinterpret_cast to a void*, https://en.cppreference.com/w/cpp/language/reinterpret_cast, stackoverflow.com/questions/573294/when-to-use-reinterpret-cast. CDerived* pD = new CDerived (); Why does the USA not have a constitutional court? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. reinterpret_cast will forcefully convert the void* to the target data type. Was the ZX Spectrum used for number crunching? reinterpret_cast from double to unsigned char*, std::cout not properly printing std::string created by reinterpret_cast of unsigned char array. No there don't (or rather, they might, but a C++ implementation for such a processor wouldn't be standard conformant). We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. The Boost Serialization library has a good. Mathematica cannot find square roots of some matrices? (clang ++) error: reinterpret_cast from const void * to uv_loop_s *const casts away qualifiers The funny thing is that I'm not doing a cast to uv_loop_s * , but to const uv_loop_s * : return reinterpret_cast< const T > ( raw() ); and the raw( ) function is declared as const void *raw() const noexcept A minimal verifiable example: MOSFET is getting very hot at high frequency PWM. Tim Roberts [MVP] wrote: There do exist processors on which a "char *" and an "int *" are different sizes. What happens if you score more than 99 points in volleyball? Other uses are, at best, nonportable. Does illicit payments qualify as transaction costs? reinterpret_cast is a type of casting operator used in C++. CGAC2022 Day 10: Help Santa sort presents! Why can't pointer fit variable of different type, even though sizeof is same? by two? Zeiger-zu-member werden nicht korrigiert, wie regelmige Zeiger sind. I want to reinterpret cast a function pointer into a void* variable. For example, you can use static_cast to convert base class pointers to derived class pointers, which is a conversion that makes sense in some cases but can't be verified until runtime. There are two caveats to be made: 1. The type of the function pointer will be of type Class* (*) (void*). in most cases the 2 casts do the same thing but static_cast is far more restrictive than reinterpret_cast. reinterpret_cast method pointer to different class, is this UB? Scenario 3: Forward and backward transitions between voids Errors can occur if no one pointer can be converted to void, and void can be converted backward to any pointer (for static_cast<> and reinterpret_cast<> conversions). This rule bans (T)expression only when used to perform an unsafe cast. Some give a warning depending on the warning and conformance level, others gave no warning whatever I tried. Every object pointer type whose pointed type is cv-unqualified is implicitly convertible to void*, and the inverse can be done by static_cast. How could my characters be tricked into thinking they are on Mars? Allowed static casts and their results are described in 5.2.9 (expr.static.cast). See cppreference.com for the full list of conversions allowed. The above code works well with Visual Studio/x86 compilers. Connect and share knowledge within a single location that is structured and easy to search. How do I arrange multiple quotations (each with multiple lines) vertically (with a line through the center) so that they're side-by-side? Below C++ program demonstrates the use of reinterpret_cast to reinterpret the bit pattern. The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. Find centralized, trusted content and collaborate around the technologies you use most. I was looking at https://en.cppreference.com/w/cpp/language/reinterpret_cast and I noticed that it specifies the legal types we can always cast to: But I did not see void* in the list. Except that converting an rvalue 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. #include <iostream> using namespace std; int main () { int i = 123456 ; // p123456 int * p = reinterpret_cast < int *> ( i ); return 0 ; } reinterpret_castvoid*static_cast static_cast static_cast will not prevent this from happening. As we learnt in the generic types example, static_cast<> will fail if you try to cast an object to another unrelated class, while reinterpret_cast<> will always succeed by "cheating" the compiler to believe that the object is really that unrelated class. What reinterpret_cast convention is this? Losing bytes like this is called 'truncation', and that's what the first warning is telling you. Is energy "equal" to the curvature of spacetime? 10 QGuiApplication::allWindows () 11 QSharedPointer. Below is the sample code, class Test { int a; }; int main () { Test* *p (void **a); void *f=reinterpret_cast<void*> (p); } The above code works well with Visual Studio/x86 compilers. Does balls to the wall mean full speed ahead or full speed ahead and nosedive? Putting a space in ". You can also use reinterpret_cast to convert a float* to an int* or vice-versa, which is platform-specific because the particular representations of floats and ints aren't guaranteed to have anything in common with one another. How to make voltage plus/minus signs bolder? .. - reinterpret_cast is a keyword. Using reinterpret_cast to do this with pointer conversions completely bypasses the compile-time safety check. To learn more, see our tips on writing great answers. even round trip isn't guaranteed to work). 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 illegal to use resources in a University lab to prove a concept could work (to ultimately use to create a startup). Can virent/viret mean "green" in an adjectival sense? Why does the distance from light to subject affect exposure (inverse square law) while from subject to lens does not? @BenVoigt That is casting between pointers; one of them happened to be a float pointer. In any case, the resulting pointer may only be dereferenced safely if allowed by the type aliasing rules). CGAC2022 Day 10: Help Santa sort presents! Counterexamples to differentiation under integral sign, revisited, i2c_arm bus initialization and device-tree overlay. In C the cast is allowed, but it's behavior isn't defined (i.e. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. But you can still cast the resulting pointer back to the original type safely, and use the result as-if it was the original pointer. In short, if you ever find yourself doing a conversion in which the cast is logically meaningful but might not necessarily succeed at runtime, avoid reinterpret_cast. Thanks for contributing an answer to Stack Overflow! Excellent observation :-). 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. What is a smart pointer and when should I use one? Jul 22 '05 # 3 hack_tick hi there There are a few circumstances where you might want to use a dynamic_cast instead of a static_cast, but these mostly involve casts in a class hierarchy and (only rarely) directly concern void*. Thanks for contributing an answer to Stack Overflow! So why have reinterpret_cast<>? Can several CRTs be wired in parallel to one oscilloscope circuit? confusion between a half wave and a centre tapped full wave rectifier. Needless to say that like with all techniques that try to work around this limitation, this is undefined behavior. I suggest using the weakest possible cast always. The rubber protection cover does not pass through the hole in the rim. The rubber protection cover does not pass through the hole in the rim. Is there a good reason to favor one over the other? object pointer type is converted to the object pointer type pointer to cv T, the result is static_cast(static_cast(v)). @user470379 Wowthat's the very reason I landed on this question at SO! reinterpret_cast is very much standard C++. text and read text. When casting from a void* there is not type information for the cast to work with. static _ cas t const _ cas tre interp ret_ cas t dynamic _ cas t. kingsfar . But I'm not sure without analysing the code. I've reopened the question because I didn't see only the top answer applies to this. While there are a few additional things that a C cast can do which aren't allowed by combination of static, reinterpret and const casts, that conversion is not one of them. The purpose of reinterpret_cast is to reinterpret the bits of one value as the bits of another value. (*)(void) to a void*). The type of the function pointer will be of type Class* (*)(void*). Here: Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. Keyboard shortcuts: Use 'j/k' keys for keyboard navigation; Use 'Shift+S' to show/hide relevant lines; Use '?' to toggle this window Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. This means in particular that a cast from a pointer to function to void * is not possible, but you can cast it to void(*)(). Taking the address of the function pointer will give you a data-pointer: Pointer to a function pointer. On the other hand, if you're casting between pointer types (as is fairly common when indexing in memory via a char*, for example), static_cast will generate a compiler error and you'll be forced to use reinterpret_cast anyway. Thanks for contributing an answer to Stack Overflow! Can several CRTs be wired in parallel to one oscilloscope circuit? Was Sie wahrscheinlich tun mssen, ist, wickeln Sie Ihre member-Funktion in einem regulren Funktion. 3.5 int reinterpret_cast. Use static_cast on both sides for this, and save reinterpret_cast for when no other casting operation will do. Syntax : My use case requires a reinterpret_cast because I'm casting from an int** to a void*. You can achieve this but this is a relatively bad idea. (unsigned*)&x therefore reduces to reinterpret_cast<unsigned*>(&x) and doesn't work. The order of casting operators that's tried always tries to use a static_cast before a reinterpret_cast, which is the behavior you want since reinterpret_cast isn't guaranteed to be portable. The expression consists of a dereference applied to a cast. You should use static_cast so that the pointer is correctly manipulated to point at the correct location. In C++11, the cast to This is usually the case on POSIX compatible systems because dlsym() is declared to return void *, and clients are expected to reinterpret_cast it to the correct function pointer type. EDIT (2017): The answer above is only correct for C++03. Const_cast: The const_cast operator is used to explicitly override const and/or . Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs. void(*println_ptr)(Printer*const, const char*) = reinterpret_cast<void(*)(Printer*const, const char*)>(&Printer::println); Last edited on . All you need is a single static_cast: We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. And I will eventually cast from the void* back to an int**. This type of cast reinterprets the value of a variable of one type as another variable of a different type. How can I cast "const void*" to the function pointer in C++11? The relevant section from cppreference on reinterpret_cast : (Any object pointer type T1* can be converted to another object pointer type cv T2*. When would I give a checkpoint to my D&D party that they can return to if they die? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. static_cast only allows conversions like int to float or base class pointer to derived class pointer. This will allow you to cast it to void*. A casting operator for abnormal casting cases. int ProgressBar (const uint64_t data_sent, const uint64_t data_total, void const *const data) { Dialog *dialog = reinterpret_cast<Dialog*> (data); dialog->setValue ( (data_sent *100) / data_total); } the reinterpret_cast seems not allowed and say reinterpret_cast from 'const void *) to Dialog * casts away qualifiers Any idea c++ casting For example, you can use reinterpret_cast to convert from a void * to an int, which will work correctly if your system happens to have sizeof (void*) sizeof (int). Use static_cast for this. The intermediary cast to void* makes it equivalent to using two static_cast's internally, and makes GCC be quiet about warning about a type pun. The reinterpret_cast operator can be used for conversions such as char* to int*, or One_class* to Unrelated_class*, which are inherently unsafe. In other words, reinterpret_cast<void(*&)()>(x)performs type punning on the pointer itself. Should I use static_cast or reinterpret_cast when casting a void* to whatever. regular pointers (e.g. Is this an at-all realistic configuration for a DHC-2 Beaver? They are bad. Some POSIX functions need the conversion to be well useful. Connect and share knowledge within a single location that is structured and easy to search. I was concerned about the below explanation. I'm casting from an int** to a void*. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. You don't need to use reinterpret_cast, though. Not the answer you're looking for? For ex, you could typecast an myclass* to void* and then use reinterpret_cast to convert it to yourclass* which may have a completely different layout. MOSFET is getting very hot at high frequency PWM, PSE Advent Calendar 2022 (Day 11): The other side of Christmas. Why is processing a sorted array faster than processing an unsorted 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. To learn more, see our tips on writing great answers. CGAC2022 Day 10: Help Santa sort presents! Understanding reinterpret_cast. Not the answer you're looking for? So, when you cast a (void*) to (long), you are losing 32 bits of data in the conversion. example how; so does the tinyxml project. For example, you can use reinterpret_cast to convert from a void * to an int, which will work correctly if your system happens to have sizeof (void*) sizeof (int). This is exactly equivalent to static_cast(static_cast(expression)) (which implies that if T2's alignment requirement is not stricter than T1's, the value of the pointer does not change and conversion of the resulting pointer back to its original type yields the original value). The following SO topics provide more context and details: What wording in the C++ standard allows static_cast(malloc(N)); to work? @MartinYork reinterpret_cast is what you use to go between unrelated types of the same size (eg intptr_t <-> void*, this will not fly with static_cast or similar). If you have a function, and want a void* pointing to it, you can do it all in one line, but that's a bit cumbersome on the syntax. Case 2: Casting to related classes 1. We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. How many transistors at minimum do you need to build a general-purpose computer? reinterpret_cast does NOT guarantee that the same address is used. Raw memory access like this is not type-safe and can only be done under a full trust security environment. Does illicit payments qualify as transaction costs? Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? Theres a misconception that using reinterpret_cast would be a better match because it meanscompletely ignore type safety and just cast from A to B. reninterpret_cast does not check if the pointer type and data pointed by the pointer is same or not. At what point in the prequels is it revealed that Palpatine is Darth Sidious? From: Nathan Sidwell <nathan@acm.org> To: Jakub Jelinek <jakub@redhat.com>, Jason Merrill <jason@redhat.com> Cc: gcc-patches@gcc.gnu.org Subject: Re: [PATCH] c++: Only reject reinterpret casts from pointers to integers for manifestly_const_eval evaluation [PR99456] Date: Thu, 11 Mar 2021 08:35:45 -0500 [thread overview] Message-ID: <d6d0ff1a-1cc1-b689-f4e8-8f7ff57fd4ad@acm.org> () In-Reply-To . 8 vscodewindows. 3.4 reinterpret_cast. casting from pointer to an integer type and vice versa. How do I arrange multiple quotations (each with multiple lines) vertically (with a line through the center) so that they're side-by-side? Ready to optimize your JavaScript with Rust? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Irreducible representations of a product of two groups, What is this fallacy: Perfection is impossible, therefore imperfection should be overlooked. I don't see any rule in the standard allowing this conversion. When casting back to the original type, AliasedType and DynamicType are the same, so they are similar, which is the first case listed by the aliasing rules where it is legal to dereference the result of reinterpret_cast : 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: An object pointer can be explicitly converted to an object pointer of a different type. To learn more, see our tips on writing great answers. It also allows. . Is this an at-all realistic configuration for a DHC-2 Beaver? reinterpret_cast reinterpret_cast . So, when you cast a (void*) to (long), you are losing 32 bits of data in the conversion. However, you should only do this if you used static cast to cast the pointer to void* in the first place. You likely obtained that void* with implicit conversion, so you should use static_cast because it is closest to the implicit conversion. Are the S&P 500 and Dow Jones Industrial Average securities? So you can use reinterpret_cast and const_cast together. Ready to optimize your JavaScript with Rust? Really not sure what else you could do to access registers at memory mapped locations with specific addresses, except C-casts. The rubber protection cover does not pass through the hole in the rim. a reinterpret_cast (5.2.10); One simple solution would be to use intptr_t: static constexpr intptr_t ptr = 0x1; and then cast later on when you need to use it: reinterpret_cast<void*> (foo::ptr) ; It may be tempting to leave it at that but this story gets more interesting though. rev2022.12.11.43106. @anon Apparently you've never worked with POSIX threads before then. Can virent/viret mean "green" in an adjectival sense? Should teachers encourage good students to help weaker ones? It doesn't guarantee any safety and your program might crash as the underlying object could be anything. SWIG%rename " "%pythoncodePythonPythonPython " "carraysPython . shouldn't Test* *p(void **a); be Test* (*p)(void **a) ? Connect and share knowledge within a single location that is structured and easy to search. rev2022.12.11.43106. See the answer at the link. Thanks to Richard which makes me revisit the issue more in depth (for the record, I was mistaken in thinking that the pointer to function to pointer to object was one case where the C cast allowed something not authorized by C++ casts combinations). Similarly, you can use static_cast to convert from an int to a char, which is well-defined but may cause a loss of precision when executed. confusion between a half wave and a centre tapped full wave rectifier. What properties should my fictional HEAT rounds have to punch through heavy armor and ERA? How can I use a VPN to access a Russian website that is banned in the EU? Making statements based on opinion; back them up with references or personal experience. MOSFET is getting very hot at high frequency PWM. 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. 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