Lets look at them side by side. Basically if you have a child class depending on two parent classes, the static_cast from child to parent will work correctly but not reintrepret_cast. It can typecast any pointer to any other data type. In C++0x, reinterpret_cast(p) will be equivalent to static_cast(p). From the semantics of your problem, I'd go with reinterpret, because that's what you actually do. Another point is that the constant pointers seem come in pairs relatively often: A unit8* that seems to be used for very low level reads and writes to memory, and a pointer to the same location that interprets the data as some object like the S above. However, even if it's not a CDerived you can do it. The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. One of the explicit goals of bit_cast is to be able to do these sorts of things at compile-time:. constexpr vs. static const: Which one to prefer? For a conversion between different function type pointers or between different object type pointers you need to use reinterpret_cast. successfully, but pY2 is not CBaseX CBaseY*pY2 s reinterpret_cast< CBaseY*> (pX); So, when you convert CDerived to CBaseY, it adds 4 bytes to the pointer, and when you convert CBaseY to CDerived, it subtracts 4 from the pointer. Should one never use static inline function? C++11 Passing function as lambda parameter, How can cmake activate "Inherit from parent" link option in Visual Studio. So we might not want to bake the cast into the constant expression. It isn't clear what you're having difficulties with, but this clears it up for me: Copyright 2022 www.appsloveworld.com. cast statements ? { is actually meaningless, and as int*pi s/reinterpret_cast (pf); At no point does any const get added or removed. Before you ask: No, we can not go back to the C-cast, because the rules say that in this case, effectively a reinterpret_cast is performed. ---------------------- Output 13. Ref: 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). static_cast is designed to reverse any implicit conversion. // TODO: sign and unsigned integer numbers, float etc. One thing to notice though is that nullptr_t cant be convert to other pointer types such as void* or int* using reinterpret_cast because nullptr_t is an object type not a pointer type. Finding the definitions took a while were using an IDE for embedded development, and its not blessed with working functionality like jump to definition. In C++0x, reinterpret_cast<int*> (p) will be . Which is better option to use for dividing an integer number by 2? a "new CBaseY()" 17. What is the difference between cout, cerr, clog of iostream header in c++? Don't blame programmers for these mistakes! The actual pointer object is a constant expression, but we still have a macro, what about that? 17. A class template that fulfills these requirements could look like this: std::intptr_t is an alias for some integer type that is large enough to hold a pointer value. It is used when we want to work with bits. In that case it will read the value of p using p's type, and that value is then converted to a T*. The conversions to the two pointer types still have to be done in the runtime code, so they are in functions that are not constepr. CDerived s pD2 s 392fbc. . The reinterpret_cast operator can be used for conversions such as char* to int*, or One_class* to Unrelated_class*, which are inherently unsafe. What information does GCC Profile Guided Optimization (PGO) collect and which optimizations use it? As shown in the figure, CDerived's memory layout consists of two objects, CBaseX and CBaseY, as the compiler knows. // Error, type point You converted to void* implicitly, therefore you can (and should) convert back with static_cast if you know that you really are just reversing an earlier conversion. // Error, the type pointed to is irrelevant (i.e. I suggest that reinterpret_cast is "better" for this task, since it makes the intent clearer. compilation, implicit static_cast<> conversion ), the difference between static_cast and reinterpret_cast, Hands teach you how to configure and compile ogre 1.7.0 plus cegui 0.7.1, A comparison of several multithreaded 3D engine architectures, Research on spatial topology relationship judgment methods, http://blog.csdn.net/jiangdf/archive/2009/05/21/4205481.aspx. The worst ever invented. const_cast on the other hand, could change the data. int*pn2 s static_cast (pv); // reinterpret_cast<> What is the use of private static member functions? To use this class in the current code base, without touching any other code, we would need something like the next two lines: Yay, no more casts in our constants. const_cast is pretty easy to understand as it doesnt change the memory layout and just toggle the const flag for the compiler to help you do or avoid some checks. While we are at it, we can replace the C-cast with the proper C++ cast, which in this case is reinterpret_cast: constexpr auto FOO = reinterpret_cast<uint8*> (0xBAD50BAD); constexpr auto BAR = reinterpret_cast<S*> (FOO); Sadly, this won't compile, because reinterpret_cast s are not allowed in constant expressions by the standard. Required fields are marked *. CDerived* pD = new CDerived(); CBaseY* pY = pD; Successful compilation, pY s pD s 4 Possible Duplicates: vs. python, . CBaseY* pY3 = new CBaseY(); // compiled successfully, (The pointer is backward) offset by 4 (bytes) (4 is the number of bytes that the int type occupies in memory). Comment document.getElementById("comment").setAttribute( "id", "ae10f4a28469b8a73cf8745d5899237d" );document.getElementById("a2d4e530cf").setAttribute( "id", "comment" ); Copyright 2022 Simplify C++!. , - . not CBaseX // Compiled How does ENet manage its arriving packets? But wait the all-caps FOO and BAR look suspicious. The C++ standard just doesn't guarantee you anything once you do it. Furthermore, it is currently impossible to implement a constexpr bit-cast function . static const Member Value vs. What comes to mind is that the actual constant here is the address value, i.e. A reinterpret_cast is a cast that represents an unsafe conversion that might reinterpret the bits of one value as the bits of another value. Using boost::optional with constant types - C++, Trying to assign vector of Base* from vector of Derived*, Modifying element of const std::vector via const_cast. In C++, reinterpret_cast, static_cast and const_cast is very common. CDerived* pD1 = static_cast(pY1); Does boost::asio::deadline_timer use a thread for each timer? If we go ahead and replace the macro with a constant expression, we should get the warning at the exact location where the C-cast is written, not where the macros are expanded. system() fork/execvp. printf("CDerived* pD = %x/n", (int)pD); A C-style cast is basically identical to trying out a range of sequences of C++ casts, and taking the first C++ cast that works, without ever considering dynamic_cast. ; . not CBaseY x 11. Code D3, https://kristerw.blogspot.se/2017/07/hard-coded-hardware-addresses-in-cc.html, Whats Ubiquitous Language and Why You Should Care, [fix dev diary] Week 6-7: Description and Issue ID, isValid()? Since a C-style cast is basically a "oh, just cast it however you can" cast, it's better to prefer the more specific casts. This is true for both the union and reinterpret_cast approaches. The other two is sometimes confusing. But really casts are a language smell. 14. ; ( ), . , . Your email address will not be published. When to use which one? When would I use const volatile, register volatile, static volatile in C++? Immediate scheduler , reinterpret_cast, gradlew gradle, undefined reference android NDK. We could just write reinterpret_cast in the macros and live with the fact that we have ugly macros but silenced the warnings. With that assumption, nothing is being reinterpreted - void is an incomplete type, meaning that it has no values, so at no point are you interpreting either a stored int value "as void" or a stored "void value" as int. Most programmers have studied C before they learn C and are accustomed to C-style (type) conversion. (protobuf) codec . protobuf . The thing everybody fails to mention is that reinterpret_cast<> is a means to document your program. Scenario 3: Forward and backward transitions between voids. crash // pD2->bar(); ---------------------- Protobuf . However, in memory manipulation . When should static_cast, dynamic_cast, const_cast and reinterpret_cast be used? int y; Sometimes we may be a little fuzzy when we use static_cast<> and reinterpret_cast<> when we write C.C. The definitions of FOO and BAR then looked like this: Where uint8 is a typedef to some 8-bit unsigned type, and S is a struct. Trying to get away from them was not as easy as I first thought. to un arerelated // 392fbc CDerived s pD1 CBaseX() { x = 10; } s pY printf ("void s pV1 s %x/n", (int)pV1); const_cast means two things. Can I assume that reinterpret_cast is not safe and should not be used? n s 12 int n s static_cast (f); from being converted to CDerived. 5. Learn on the go with our new app. gives some good details. 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). CDerived s pD2 s static_cast (pV1); 8. printf("CDerived* pD1 = %x/n", (int)pD1); It also allows any integral type to be converted into any pointer type and vice versa. the difference between static_cast and reinterpret_cast Most programmers have studied C before they learn C and are accustomed to C-style (type) conversion. In C++, is it safe/portable to use static member function pointer for C API callbacks? For a conversion between different function type pointers or between different object type pointers you need to use reinterpret_cast. Member enum : Which Method is Better & Why? But nullptr_t itself is an object which has different memory layout compared to other pointer types. Similarly, casting an int to a void* is perfectly legal with reinterpret_cast, though it's unsafe. 12. printf("CBaseY* pY2 = %x/n", (int)pY2); constexpr and initialization of a static const void pointer with reinterpret cast, which compiler is right? the 0xBA50BAD, and the reinterpret_casts are only used in the runtime code. We could go ahead and replace our raw function with an implicit conversion operator, but I think that is not what we should do. public: // Compiled successfully, but pY2 is Which to use when? The amount of code broken when it's rejected will be no fun, so there is no motivation for them to forbid it. --------------------------- CDerived*pD s 392fbb8 http://blog.csdn.net/jiangdf/archive/2009/05/21/4205481.aspx, The difference between anSI, unicode, utf-8, DBCS and other character sets and related data types and functions, The OSG implements a camera that follows the node, OSG uses OpenGL Vertex Shaders and Chip Shaders, OSG adds the osgParticle particle effect to the scene, The STL carefully selects the container type, The more perfect point is judged in the multilateral line (c? IS 5.2.10 - Reinterpret cast -1- The result of the expression reinterpret_cast<T> (v) is the result of converting the expression v to type T. . Lets have a look from the memory perspective. // System printf ("CBaseY s pY s %x/n", (int)pY); Unrelated static_cast<> For example, casting an int* to a double* is legal with a reinterpret_cast, though the result is unspecified. public: The reinterpret_cast operator, as well as the other named cast operators, is more easily spotted than C-style casts, and highlights the paradox of a strongly typed language that allows explicit casts. With this in mind, the question is whether we could come up with a class that. 7. Why do both libstdc++ and libc++ not check for pointer and reference type D for the default unique_ptr constructor? 2. printf("CDerived* pD = %x/n", (int)pD); Is it safe to use the address of a static local variable within a function template as a type identifier? fork/execvp . Conversion 1. Nishant Sivakumar, Casting Basics - Use C++ casts in your VC++.NET programs Is it legal to check whether the address of a subobject lies within the bounds of a containing object. s pY2 s 392fb8 Of course, this problem only happens if you do more inheritance. the conversions that can be performed using a reinterpret_cast are limited; the permissible ones are specified by the standard. : double x = 10.3; int y; y = (int) x; // c-like cast notation : double x = 10.3; int y; y = reinterpret_cast(x). 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. Even in the C++20 world where we can allocate memory at compile time, reinterpret_cast is forbidden in constexpr functions. However . Solution 1. Powered by WordPress and Stargazer. //Successful compilation, but the memory of the spn static_cast provided that you know (by design of your program) that the thing pointed to really is an int. To see how static_cast<> actually works, we have to look at the memory layout of CDerived. downcast then C++. Remember in C++, parent class stuff (vtable and attributes) are putting upfront, followed by child class stuff. Can I use partial template specialization for a (non-member) function? Refresh the page, check Medium 's site. void* pV1 = pY; Successful compilation, pV1 10. reinterpret_cast Observable.create(subscriber -> subscriber.onNext(new Object())).subscribeOn(Schedulers.immediate()).subscribe(); //vs Possible Duplicate: c-style cast vs reinterpret_cast : A* pA = new B; B* p1 = (B*)pA; B* p2 = reinterpret_cast(pA); ? While we are at it, we can replace the C-cast with the proper C++ cast, which in this case is reinterpret_cast: Sadly, this wont compile, because reinterpret_casts are not allowed in constant expressions by the standard. reinterpret_cast if you've omitted details that mean you might actually be reading memory using a type other than the type is was written with, and be aware that your code will have limited portability. It simply tries the various C++-style casts in order, until it finds one that works. Scenario 2: Transition to the relevant class. first one is to remove constness from a type and the other is to give its code explicitness. // pY2->bar(); Error, type point is irrelevant // CBaseY s pY1 s static_cast (pX); How to use reinterpret cast for inner template class? int x; It is important to remember that even though a program compiles, its . You reinterpret cast one mutable pointer to another. 9. float f = 12.3; Incorrect cast - is it the cast or the use which is undefined behavior. How do you use CreateThread for functions which are class members? Therefore, I made the conversion to uint8* an explicit function. reinterpret_cast will never change the memory layout. If your C++ code is using some C API then of course you don't have much choice. When and why would you use static with constexpr? compilation of void?pv s static_cast (pf); 3. , : fftw_complex *H_cast; H_cast = (fftw_complex*) fftw_malloc(sizeof(fftw_complex)*M*N); : H_cast= reinterpret_cast (H); int main() { class_name object; object.method(); fstream file(writeobject.dat , ios::out|ios::app); file.write(reinterpret_cast(&object), sizeof(object)); return 0; } 1 , union { T var_1; U var_2; } var_2 = reinterpret_cast (var_1) ? Note: reinterpret_cast is a very special and dangerous type of casting operator. To be fair, a handful of those macros were in actual C headers provided by third parties, but many of them appeared to be written only in the same style in a project that specifically claims to be a C++ project. // Compile OK, but pY2 is and CBaseY// CBaseX and CBaseY? 3. , Immediate scheduler ? That means that when it acts like a reinterpret_cast, it has the exact same problems as a reinterpret_cast.But in addition, it has these problems: s 392fb8 CBasey There wont be any difference between the two, except for multi-inheritance. //int*pn s static_cast (pf); CDerived* pD3 = static_cast(pY3); We probably do only want these pairs, i.e. Needless to say, this is much more powerful as it combines all of const_cast, static_cast and reinterpret_cast, but it's also unsafe, because it does not use dynamic_cast. It can cast integer and floating point types into each other. It only provides some information for the compiler to generate code. We could stop here and give up. Is there a ffs() equivalent for std::bitset? const_cast<NewType> (static_cast<const NewType> (variable)) reinterpret_cast<const NewType> (variable) const_cast<NewType> (reinterpret_cast<const NewType> (variable)) Functional casting is very similar, though as a few restrictions as the result of its syntax: NewType (expression). Note: When you convert CDerived s to CBaseY with an implicit static_cast<> (line 5), the result is (pointing to) CDerived? In the assembly code, you wont see any CPU instructions corresponding to the reinterpret_cast call. printf("CDerived* pD2 = %x/n", (int)pD2); 4. static_cast<> CDerived?->CBaseY' -> CDerived?//Successful Over the last couple days, I've been reading up on the various casting operators in C++. { }; class CDerived : public CBaseX, public CBaseY As a result, only types without spaces can be cast to. https://kristerw.blogspot.se/2017/07/hard-coded-hardware-addresses-in-cc.html, Its almost as if warnings are useful sometimes , Your email address will not be published. class CBaseY As far as I know, all current compilers allow to reinterpret_cast from void* and behave equivalent to the corresponding static_cast, even though it is not allowed in current C++03. If there are two parent classes, then the result of upcasting to different parent class would be different. 392fbc void s pV1 s 392fbc Misuse of the reinterpret_cast operator can easily be unsafe. This wiki page has a good explanation. int z; // Compiled successfully, pD1 now s pD union reinterpret_cast? But if we can't be sure if it's CBaseY or CDerived, then we have to use dynamic_cast<> or typeid. [MSDN] C++ Language Reference -- Casting static_cast and memory layout difference | by Xianbo QIAN | Medium 500 Apologies, but something went wrong on our end. Bjarne Stroustrup, in his guidelines, recommended reinterpret_cast in another context: if you want to type-pun in a way that the language does not define by a static_cast , he suggested that you do it with something like . CBaseY s pY1 s // System crash!! // Successful between Convert between CBaseX 16. printf("CBaseY* pY3 = %x/n", (int)pY3); reinterpret_cast This is the trickiest to use. 1. dynamic_cast<>, on the other hand, prevents a generic CBaseY? } And is suggested to use it using proper data type i.e., (pointer data type should be same as original data type). }; Since the class holds this integer value and not a pointer value, it can be used as a constant expression. Dipping my toes into a new project, I got a bunch of ugly warnings about a ton of C-casts inside a macro definition. Most C++ developers know that #defines are evil because the are simple text replacement and therefore bring problems like lacking type safety and more. reinterpret_cast has nothing to do with 'const'. Love podcasts or audiobooks? C++: reinterpret_cast v.s. reinterpret_cast. 2. 2. dynamic_cast<> needs the class to become polymorphic, i.e. // pD2 s pY, but we expect pD2 s pY - 4 Which STL container should I use for a FIFO? a reinterpret_cast is a conversion operator. Juan Soulie, C++ Language Tutorial: Type Casting, This article is from the CSDN blog and reproduced with the source: Be aware that modifiyng objects that actually are declared as const is undefined behaviour. cmake: target_link_libraries use static library not shared, Returning unique_ptr