Use static_cast: it is the narrowest cast that exactly describes what conversion is made here. Those types are exempt from strict aliasing rules. Putting a space in ". Can virent/viret mean "green" in an adjectival sense? This some architectures they might contain Thanks for contributing an answer to Stack Overflow! Asking for help, clarification, or responding to other answers. However, you should only do this if you used static cast to cast the pointer to void* in the first place. Can virent/viret mean "green" in an adjectival sense? In C the cast is allowed, but it's behavior isn't defined (i.e. 3.3 void* , static_cast reinterpret_cast , void* . reinterpret_cast,"". 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 . When convert a void pointer to a specific type pointer, which casting symbol is better, static_cast or reinterpret_cast? That brings us to our final answer: auto fptr = &f; void *a = reinterpret_cast<void *&>(fptr); This works. I'd suggest rethinking the interface to ProgressBar to avoid needing this cast. reninterpret_cast does not check if the pointer type and data pointed by the pointer is same or not. Is this an at-all realistic configuration for a DHC-2 Beaver? 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. What is this fallacy: Perfection is impossible, therefore imperfection should be overlooked. by two? 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. 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. For a conversion of void* to int* you can only use static_cast (or the equivalent C-style cast). 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. This cast operator can also convert variable into totally incompatible type too. 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 The expression consists of a dereference applied to a cast. Syntax : 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). casting a void Books that explain fundamental chess concepts. How many transistors at minimum do you need to build a general-purpose computer? Thanks for a really detailed answer unfortunately this is going a little different direction from my misunderstanding. For example, when using a C-style cast, as in. Is this an at-all realistic configuration for a DHC-2 Beaver? void(*println_ptr)(Printer*const, const char*) = reinterpret_cast<void(*)(Printer*const, const char*)>(&Printer::println); Last edited on . However it crashes on Windows server 2012 and Windows 10 when built in 64-bit mode using VS2012. This means in particular that a cast from a pointer to function to void * is not possible, but you can cast it to void(*)(). Repeater. pointers aren't necessarily the same 7 QDebug<<. Das Hauptproblem ist, dass die Daten bentigt um einen Zeiger-auf . Share Improve this answer The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. Connect and share knowledge within a single location that is structured and easy to search. 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. In other words, reinterpret_cast<void(*&)()>(x)performs type punning on the pointer itself. 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 . The more structure-breaking the cast is, the more attention using it requires. @BenVoigt the "entire expression" isn't a cast though. Ready to optimize your JavaScript with Rust? Find centralized, trusted content and collaborate around the technologies you use most. As an analogy, a page number in a book's . When a prvalue v of Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs. reinterpret_cast, on the other hand, is a casting operator designed to do conversions that are fundamentally not safe or not portable. Use the reinterpret_cast<> to highlight these dangerous areas in the code. The paragraphs about void* are 4 and 10. reinterpret_cast allows anything, that's usually a dangerous thing and normally reinterpret_cast is rarely used, tipically to convert How do I arrange multiple quotations (each with multiple lines) vertically (with a line through the center) so that they're side-by-side? C++. cast away const or other type regular pointers (e.g. Why is Singapore currently considered to be a dictatorial regime and a multi-party democracy by different publications? On the one hand, Konrad makes an excellent point about the spec definition for reinterpret_cast, although in practice it probably does the same thing. Write. 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. The above code works well with Visual Studio/x86 compilers. Also, casting from void * can use static_cast, it does not need to reinterpret. How could my characters be tricked into thinking they are on Mars? Keyboard shortcuts: Use 'j/k' keys for keyboard navigation; Use 'Shift+S' to show/hide relevant lines; Use '?' to toggle this window Does illicit payments qualify as transaction costs? Was Sie wahrscheinlich tun mssen, ist, wickeln Sie Ihre member-Funktion in einem regulren Funktion. Asking for help, clarification, or responding to other answers. If you want to print the address, cast the pointer to void* first: cout<< "address=" << ( void *)charPtr1; 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. The type of a pointer to cv void or a pointer to an object type is called an object pointer type. (unsigned*)&x therefore reduces to reinterpret_cast<unsigned*>(&x) and doesn't work. The pointer value (6.9.2) is unchanged by this conversion. I usually do the following, which is doing a type pun, and is the recommended way to do it, according to the manpage of dlopen (which is about doing the converse - casting from void* to a function pointer). Not the answer you're looking for? Could you just post that as an answer and I'll accept? And I will eventually cast from the void* back to an int**. You should use static_cast so that the pointer is correctly manipulated to point at the correct location. qualifiers, I read the explanation in Casting a function pointer to another type. 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. I use reinterpret_cast to convert integer memory addresses to pointers to overlay structs. How does legislative oversight work in Switzerland when there is technically no "opposition" in parliament? 1. static _ cas t static _ cas t static _ca. How to print function pointers with cout? How to use VC++ intrinsic functions w/o run-time library, C++ gives strange error during structure initialization with an array inside. rev2022.12.11.43106. See cppreference.com for the full list of conversions allowed. The reinterpret_cast operator can be used for conversions such as char* to int*, or One_class* to Unrelated_class*, which are inherently unsafe. Why use static_cast(x) instead of (int)x? - type is a pointer reinterpreted as. Asking for help, clarification, or responding to other answers. The compiler can then check that the cast is between related types, reporting a compile-time error if this isn't the case. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. 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. It's in the language for a reason. Was the ZX Spectrum used for number crunching? @anon Apparently you've never worked with POSIX threads before then. MOSFET is getting very hot at high frequency PWM. Ready to optimize your JavaScript with Rust? 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. remember that it's undefined behavior. int cell = 20; outFile.write (reinterpret_cast<const char *> (&cell),sizeof (cell)); You are asking whether binary files are good. size as regular pointers, since on Is the EU Border Guard Agency able to tell Russian passports issued in Ukraine or Georgia from the legitimate ones? But I'm not sure without analysing the code. reinterpret_cast may be used to cast a pointer to a float. #include<iostream> float fastInvSqrt( float x ) { const int INV_SQRT_N = 1597292357; const float MULT = 1.000363245811462f; float const mx = 0.5f * MULT * x; @FranoisAndrieux Ugh, I knew that. reinterpret_cast means like telling the compiler that I know better than you here, just carry out what I am saying. 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 cast operator can convert an integer to a pointer and so on. So, when you cast a (void*) to (long), you are losing 32 bits of data in the conversion. 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. c++ reinterpret_cast unique_ptr * unique_ptr * c++ Using reinterpret_cast to cast unique_ptr * to unique_ptr * for creating a transformable tree structure Making statements based on opinion; back them up with references or personal experience. 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]. to cv void. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Is this an oversight? Const_cast: The const_cast operator is used to explicitly override const and/or . (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.) 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. 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. rev2022.12.11.43106. 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. 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. rev2022.12.11.43106. From C++ . And I will eventually cast from the void* back to an int**. Why was USB 1.0 incredibly slow even for its time? CGAC2022 Day 10: Help Santa sort presents! Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. The type of the function pointer will be of type Class* (*) (void*). Only in the rarest of rare cases when there is no other way use reinterpret_cast. It does not check if the pointer type and data pointed by the pointer is same or not. extensible library interface without reinterpret_cast. 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. Making statements based on opinion; back them up with references or personal experience. Connect and share knowledge within a single location that is structured and easy to search. 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. reinterpret_cast to void* not working with function pointers, Casting a function pointer to another type. rev2022.12.11.43106. 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. Using explicit C++ style static_cast casts, this looks much more complicated, because you have to take the constness into account. However, I think the spec wants you to use static_cast over reinterpret_cast. As you can see from the last line of the question I'm trying to cast from an. Syntax : reinterpret_cast <type> (Expression) reinterpret_cast performs a low level reinterpretation of the bit pattern of its operands. Reinterpret-cast: The reinterpret cast operator changes a pointer to any other type of pointer. reinterpret_cast method pointer to different class, is this UB? reinterpret_cast in C#. Are can all casting operations be covered by the other 3 cast operators? To learn more, see our tips on writing great answers. However, you are also casting the result of this operation to (void*). What is a smart pointer and when should I use one? reinterpret_cast is a tricky beast. (Nearly -1) I don't think this is right, can you provide a reference for this? Losing bytes like this is called 'truncation', and that's what the first warning is telling you. You need to also use a const_cast to remove const qualifiers. So you can use reinterpret_cast and const_cast together. 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. 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. A casting operator for abnormal casting cases. Otherwise you should reinterpret_cast to exactly the same type of the original pointer (no bases or such). Can several CRTs be wired in parallel to one oscilloscope circuit? In any case, the resulting pointer may only be dereferenced safely if allowed by the type aliasing rules). We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. Error: #694: reinterpret_cast cannot It's just a quick idea. Why is processing a sorted array faster than processing an unsorted array? EDIT (2017): The answer above is only correct for C++03. 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). I don't see any rule in the standard allowing this conversion. "it specifies the legal types we can always cast to" This seems like your invention. static_cast will not prevent this from happening. Find centralized, trusted content and collaborate around the technologies you use most. Raw memory access like this is not type-safe and can only be done under a full trust security environment. Function 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. [expr.reinterpret.cast]/7:. It's my understanding that C-style cast is defined in terms of the new cast styles. 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. Irreducible representations of a product of two groups, What is this fallacy: Perfection is impossible, therefore imperfection should be overlooked. Why does the distance from light to subject affect exposure (inverse square law) while from subject to lens does not? For a conversion between different function type pointers or between different object type pointers you need to use reinterpret_cast. Why do some airports shuffle connecting passengers through security again. Making statements based on opinion; back them up with references or personal experience. 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. Not the answer you're looking for? Zeiger-zu-member werden nicht korrigiert, wie regelmige Zeiger sind. If he had met some scary fish, he would immediately return to the surface. 3.4 reinterpret_cast. The rubber protection cover does not pass through the hole in the rim. [] 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 . will probably work ok on x86, but 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. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. reinterpret_cast is very much standard C++. defines a function, not a pointer to function. 9 windows. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. How can I use a VPN to access a Russian website that is banned in the EU? Is it possible to hide or delete the new Toolbar in 13.1? I suggest using the weakest possible cast always. example how; so does the tinyxml project. void is not included here because you can never dereference a void *. To learn more, see our tips on writing great answers. Asking for help, clarification, or responding to other answers. reinterpret_cast will forcefully convert the void* to the target data type. From: Nathan Sidwell <[email protected]> To: Jakub Jelinek <[email protected]>, Jason Merrill <[email protected]> Cc: [email protected] 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: <[email protected]> () In-Reply-To . 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