reinterpret_cast vs c cast

Don't blame programmers for these mistakes! CBaseY* pY = pD; Successful compilation, pY s pD s 4 How to use reinterpret cast for inner template class? Looking at the code in question it would be something innocent like this: Both lines do not really look like there is a cast going on. ), 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. }; // Error, type point fct( reinterpret_cast< B* >( pC ) );:-) Here is what Microsoft has to say about reinterpret_cast <Quote> The reinterpret_cast operator allows any pointer to be converted into any other pointer type. Refresh the page, check Medium 's site. How do you use CreateThread for functions which are class members? 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. to include the "virtual" function and therefore not to be void. We could just write reinterpret_cast in the macros and live with the fact that we have ugly macros but silenced the warnings. it is definitely different. printf ("CBaseY s pY s %x/n", (int)pY); 15. downcast then C++. float f = 12.3; // System crash!! interpreting the same address as yet something else might not be wanted. 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. If there are two parent classes, then the result of upcasting to different parent class would be different. Therefore, I made the conversion to uint8* an explicit function. Unrelated static_cast<> public: Most C++ developers know that #defines are evil because the are simple text replacement and therefore bring problems like lacking type safety and more. ; . 2. For a conversion between different function type pointers or between different object type pointers you need to use reinterpret_cast. One of the explicit goals of bit_cast is to be able to do these sorts of things at compile-time:. CDerived s pD2 s static_cast (pV1); // Compiled successfully, but pY2 is As a result, only types without spaces can be cast to. Which std::async implementations use thread pools? Even in the C++20 world where we can allocate memory at compile time, reinterpret_cast is forbidden in constexpr functions. static_cast and memory layout difference | by Xianbo QIAN | Medium 500 Apologies, but something went wrong on our end. Pointer conversion is a bit complicated, and we'll use the following classes for the rest of this article: class CBaseX The worst ever invented. CDerived* pD3 = static_cast(pY3); The C++ standard just doesn't guarantee you anything once you do it. reinterpret_cast will never change the memory layout. Can I assume that reinterpret_cast is not safe and should not be used? So in certain situations, a C-style cast will have the same effect as reinterpret_cast but they are not equivalent. 5. int* py; All rights reserved. // System crash!! In this article, I'll explain what static_cast<> actually did, and point out some situations that will lead to errors. Dipping my toes into a new project, I got a bunch of ugly warnings about a ton of C-casts inside a macro definition. Casts are an indication that a programmer has made a mistake and has not bothered to fix that mistake, so they use cast as a cheap-and-nasty, quick-and-dirty workaround. Getting around the reinterpret cast limitation with constexpr. void* is just an ugly way of saying, "I don't know the type, but I'm going to pass the pointer on to someone else who does". They do not function same. It is important to remember that even though a program compiles, its . It can typecast any pointer to any other data type. CBaseY* pY3 = new CBaseY(); The C++ compiler detects and quietly fixes most but not all violations. reinterpret_cast is a very special and dangerous type of casting operator. For your example, it is preferable to use a static_cast since you know the actual type of the derived object. The actual pointer object is a constant expression, but we still have a macro, what about that? CBaseY s pY s In C++, is it safe/portable to use static member function pointer for C API callbacks? What comes to mind is that the actual constant here is the address value, i.e. Because you can use cast it using C-style cast, but this is not explicit so that is not recommended. I understand dynamic_cast and const_cast, but for the life of me, I can't tell the difference between reinterpret_cast and static_cast. Is it legal to check whether the address of a subobject lies within the bounds of a containing object. 10. reinterpret_cast Conversion Don't use static cast for arithmetic conversions (cpp-core-guidelines). It isn't clear what you're having difficulties with, but this clears it up for me: What information does GCC Profile Guided Optimization (PGO) collect and which optimizations use it? 7. [MSDN] C++ Language Reference -- Casting For object pointers. However . 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. Since the class holds this integer value and not a pointer value, it can be used as a constant expression. void foo() { printf("CBaseX::foo() x=%d/n", x); } I suggest that reinterpret_cast is "better" for this task, since it makes the intent clearer. reinterpret_cast C-style cast? }; For example, casting an int* to a double* is legal with a reinterpret_cast, though the result is unspecified. The const was there in the c-cast, obviously, Where exactly do you think a const cast was happening there? We probably do only want these pairs, i.e. Misuse of the reinterpret_cast operator can easily be unsafe. Member enum : Which Method is Better & Why? As we've learned in generic examples, if you try to convert an object to another unrelated class static_cast<> will fail, and reinterpret_cast<> always succeed in "spoofing" the compiler: that object is the unrelated class. static_cast is designed to reverse any implicit conversion. 14. // Compiled successfully, pD1 now s pD 392fbc CDerived s pD1 In current C++, you can't use reinterpret_cast like in that code. However it will affect how other CPU instructions are generated. static_cast provided that you know (by design of your program) that the thing pointed to really is an int. CDerived* pD1 = static_cast(pY1); The most reliable and understandable document that I can find is cppreference.com Here is the link to static_cast and reinterpret_cast But here is some quick explanation for basic and common cases. Lets look at them side by side. (programs). C# 2- ? 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. There wont be any difference between the two, except for multi-inheritance. When to use which one? However, even if it's not a CDerived you can do it. How to create two classes in C++ which use each other as data? the pointer variable pf is a fluat type, which is now to be converted to an int type) Error, type point is irrelevant // CBaseY s pY1 s static_cast (pX); 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. When you convert CDerived to CBaseX static_cast<> and reinterpret_cast<> are no different. How does ENet manage its arriving packets? 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. . C++: reinterpret_cast v.s. 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. successfully, but pY2 is not CBaseX CBaseY*pY2 s reinterpret_cast< CBaseY*> (pX); However, in memory manipulation . s pY printf ("void s pV1 s %x/n", (int)pV1); // Compile OK, but pY2 is Scenario 2: Transition to the relevant class. (programs). But really casts are a language smell. Compile a static binary which code there a function gethostbyname, is there any difference between static cast to rvalue reference and std::move, Which MinGW file to use as a C++ compiler. 3. In current C++, you can't use reinterpret_cast like in that code. The thing everybody fails to mention is that reinterpret_cast<> is a means to document your program. // static cast <> What is the use of private static member functions? 3. 2. dynamic_cast<> needs the class to become polymorphic, i.e. ; ( ), . 18. printf("CDerived* pD3 = %x/n", (int)pD3); compilation, implicit static_cast<> conversion Most programmers have studied C before they learn C and are accustomed to C-style (type) conversion. If you know C, you know which one the C-style cast does, but it's nicer in some ways to have different syntax for both. 'reinterpret_cast' is used to convert pointers to objects to integral values (and back), if there is a type that can hold the entire value; between pointers of different functions; between pointers and references of unrelated object types. Ref: public: For a conversion between different function type pointers or between different object type pointers you need to use reinterpret_cast. 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. to un arerelated // Comment document.getElementById("comment").setAttribute( "id", "ae10f4a28469b8a73cf8745d5899237d" );document.getElementById("a2d4e530cf").setAttribute( "id", "comment" ); Copyright 2022 Simplify C++!. 8. printf("CDerived* pD1 = %x/n", (int)pD1); The amount of code broken when it's rejected will be no fun, so there is no motivation for them to forbid it. 392fbc void s pV1 s 392fbc The C-style cast isn't better. // pD2 s pY, but we expect pD2 s pY - 4 Why do both libstdc++ and libc++ not check for pointer and reference type D for the default unique_ptr constructor? 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. { - security/portablility/performance . Is it safe to use the address of a static local variable within a function template as a type identifier? To see how static_cast<> actually works, we have to look at the memory layout of CDerived. In C++0x, reinterpret_cast<int*> (p) will be . 12. printf("CBaseY* pY2 = %x/n", (int)pY2); const_cast on the other hand, could change the data. 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. Your email address will not be published. By the way, there are not very many good reasons for using a void* pointer in this way in C++. // TODO: sign and unsigned integer numbers, float etc. constexpr vs. static const: Which one to prefer? So we might not want to bake the cast into the constant expression. boost, shared ptr Vs weak ptr? 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. the compiler knows you should call static_cast<> //int i s reinterpret_cast (f); { With this in mind, the question is whether we could come up with a class that. This means that when you use it to convert from, say, an int* to a float*, then you have no guarantee that the resulting pointer will point to the same address. protobuf . void bar() { printf("CBaseY::bar() y=%d, *py=%d/n", y, *py); This is true for both the union and reinterpret_cast approaches. fork/execvp . 13. s pY2 s 392fb8 Where would you use a friend function vs. a static member function? Of course, this problem only happens if you do more inheritance. , - . and CBaseY// CBaseX and CBaseY? 1. When and why would you use static with constexpr? We could stop here and give up. Establish invariants and avoid zombie objects, Modern C++ Features - std::variant and std::visit, Modern C++ Features - Default Initializers for Member Variables, Modern C++ Features std::variant and std::visit, Trailing Return Types, East Const, and Code Style Consistency, std::string is not a Container for Raw Data. , . template _LIBCPP_INLINE_VISIBILITY _LIBCPP_CONSTEXPR operator _Tp* () const {return 0;}, template _LIBCPP_INLINE_VISIBILITY operator _Tp _Up::* () const {return 0;}. Note that the (type)exression equivalent of C can do much more than reinterpret_cast in C++ (well, after all, they are two different languages - so we can't expect much anyway). Same applies to downcasting. 16. printf("CBaseY* pY3 = %x/n", (int)pY3); 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()? Which STL container should I use for a FIFO? reinterpret_cast vs c style cast Possible Duplicate: c-style cast vs reinterpret_cast : A* pA = new B; B* p1 = (B*)pA; B* p2 = reinterpret_cast<B*>(pA); ? Trying to get away from them was not as easy as I first thought. the conversions that can be performed using a reinterpret_cast are limited; the permissible ones are specified by the standard. a reinterpret_cast is a conversion operator. public: C-style callback interfaces can often be replaced with either a template function (for anything that resembles the standard function qsort) or a virtual interface (for anything that resembles a registered listener). 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. Any light to shed here? We could go ahead and replace our raw function with an implicit conversion operator, but I think that is not what we should do. In C++0x, reinterpret_cast(p) will be equivalent to static_cast(p). , Immediate scheduler ? Once we've converted the pointer to void, we can't easily convert it back to the original class. But wait the all-caps FOO and BAR look suspicious. // Successful elasticsearch sink connector (kafka-connect). CDerived s pD2 s 392fbc. C++ .reinterpret_cast:reinpreter_cast<type-id> (expression) reinterpret_cast,,.: int n=9; double d= reinterpret_cast< double > (n); . --------------------------- CDerived*pD s 392fbb8 In this case, the use of macros made the problem seem worse than in actually was: Only a few dozen of those macros can result in hundreds or thousands of warnings because, after the replacement, the compiler sees that C-cast at every location the macro is used. 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). This is exclusively to be used in inheritence when you cast from base class to derived class. This wiki page has a good explanation. From the semantics of your problem, I'd go with reinterpret, because that's what you actually do. No. CDerived* pD = new CDerived(); Most would say it is a program smell and blame the programmer. compilation of void?pv s static_cast (pf); It's used primarily for things like turning a raw data bit stream into actual data or storing data in the low bits of an aligned pointer. But nullptr_t itself is an object which has different memory layout compared to other pointer types. Use the reinterpret_cast<> to highlight these dangerous areas in the code. 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. In the example above, the only way to return CDerived from a void is to convert it to CBaseY and then to CDerived. class CBaseY 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: the 0xBA50BAD, and the reinterpret_casts are only used in the runtime code. between Convert between CBaseX Copyright 2022 www.appsloveworld.com. not CBaseX // Compiled Which to use when? I wrote about a different way of solving this problem: //int*pn s static_cast (pf); At no point does any const get added or removed. n s 12 int n s static_cast (f); And is suggested to use it using proper data type i.e., (pointer data type should be same as original data type). printf("CDerived* pD = %x/n", (int)pD); const_cast means two things. Powered by WordPress and Stargazer. For a conversion of void* to int* you can only use static_cast (or the equivalent C-style cast). // Compiled successfully, although pY3 is just Scenario 1: Conversion between two unrelated classes. 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. } 6. printf("CBaseY* pY1 = %x/n", (int)pY1); Incorrect cast - is it the cast or the use which is undefined behavior. It would make the same constant BAR convertible to both a S* and a uint8*, which can be rather confusing. - , ; , &a, . 17. Is there a ffs() equivalent for std::bitset? Should one never use static inline function? Sometimes we may be a little fuzzy when we use static_cast<> and reinterpret_cast<> when we write C.C. The reason why using reinterpret_cast would be UB over there is e.g alignment questions (the buffer falls on an uneven address, first member is two-byte integral and the architecture requires that these are on even addresses, stuff like that) , or maybe pointer aliasing. // System 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. Sometimes we may be a little fuzzy when we use static_cast<> and reinterpret_cast<> when we write C.C. As shown in the figure, CDerived's memory layout consists of two objects, CBaseX and CBaseY, as the compiler knows. In the assembly code, you wont see any CPU instructions corresponding to the reinterpret_cast call. reinterpret_cast: Casts anything which has the same size, for example, int to FancyClass* on x86. . Furthermore, it is currently impossible to implement a constexpr bit-cast function . 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? Lets have a look from the memory perspective. -O1 on ARM GCC). Don't conclude from the answers that (type)expression in C++ is equivalent to a reinterpret_cast. Compile C files in C++ project which do not use precompiled header? And to not break the C style, the writer of that code used macros instead of constant expressions. Can I use partial template specialization for a (non-member) function? reinterpret_cast This is the trickiest to use. 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. Remember in C++, parent class stuff (vtable and attributes) are putting upfront, followed by child class stuff. 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. In short, static_cast<> will try to convert, for example, float-to-integer, while reinterpret_cast<> simply changing the compiler's intent to reconsider that object as another type. Which is better option to use for dividing an integer number by 2? Protobuf . CDerived* pD = new CDerived(); It simply tries the various C++-style casts in order, until it finds one that works. // Error, the type pointed to is irrelevant (i.e. Other uses are, at best, nonportable. The definition of nullptr_t could be found from LLVM header __nullptr (link), As we can see from the following two lines, it can be converted to pointer type either pointer to an object or a pointer to a class member, using implicit conversion. In that case it will read the value of p using p's type, and that value is then converted to a T*. from being converted to CDerived. system() fork/execvp. (The pointer is backward) offset by 4 (bytes) (4 is the number of bytes that the int type occupies in memory). not CBaseY x 11. int*pn2 s static_cast (pv); // reinterpret_cast<> : double x = 10.3; int y; y = (int) x; // c-like cast notation : double x = 10.3; int y; y = reinterpret_cast(x). Finding the definitions took a while were using an IDE for embedded development, and its not blessed with working functionality like jump to definition. CBaseY* pY2 = reinterpret_cast(pD); An actual type-pun that directly reads the bits of p using the representation of type T* only happens when you cast to a reference type, as in reinterpret_cast(p). Note: reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . { union reinterpret_cast? It is used when we want to work with bits. CBaseY s pY3 s 390ff0 CDerived s pD3 s 390fec void* pV1 = pY; Successful compilation, pV1 However, the indirection we have through the conversion operator and the raw function is minimal and the function calls are inlined at low levels of optimization (e.g. 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. // pY2->bar(); // compiled successfully, 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. When would I use const volatile, register volatile, static volatile in C++? printf("CDerived* pD2 = %x/n", (int)pD2); 2. printf("CDerived* pD = %x/n", (int)pD); Learn on the go with our new app. int x; 1. dynamic_cast<>, on the other hand, prevents a generic CBaseY? What is the difference between cout, cerr, clog of iostream header in c++? Note: When you convert CDerived s to CBaseY with an implicit static_cast<> (line 5), the result is (pointing to) CDerived? It's a misconception that reinterpret_cast(p) would interpret the bits of p as if they were representing a T*. Possible Duplicates: vs. python, . is actually meaningless, and as int*pi s/reinterpret_cast (pf); It only provides some information for the compiler to generate code. It also allows any integral type to be converted into any pointer type and vice versa. Now this is not really a cast any more but just a way to tell the compiler to throw away type information and treat the data differently. It tells somebody reading the code that we had to compromise something and as a result we have ended up with a dangerous cast, and be careful when you mess with this code. 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. When should static_cast, dynamic_cast, const_cast and reinterpret_cast be used? C++11 Passing function as lambda parameter, How can cmake activate "Inherit from parent" link option in Visual Studio. float* pf = &f; Scenario 3: Forward and backward transitions between voids. //Successful compilation, but the sn2 is meaningless memory (rubbish) Be aware that modifiyng objects that actually are declared as const is undefined behaviour. In C++, reinterpret_cast, static_cast and const_cast is very common. Immediate scheduler , reinterpret_cast, gradlew gradle, undefined reference android NDK. reinterpret_cast. int y; a "new CBaseY()" 17. output --------------------------- CDerived*pD s 392fbb8 Why? 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. }; class CDerived : public CBaseX, public CBaseY The result of a reinterpret_cast cannot safely be used for anything other than being cast back to its original type. The other two is sometimes confusing. It can cast integer and floating point types into each other. constexpr and initialization of a static const void pointer with reinterpret cast, which compiler is right? Required fields are marked *. CBaseY() { y = 20; py = &y; } You reinterpret cast one mutable pointer to another. 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. , ( 0x1122) uint64_t void ** (. gradlew gradle ? cast statements ? 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. At that point, it's up to you to understand exactly what your compiler and machine do in this situation. //Successful compilation, but the memory of the spn How does including gtest.h break template argument deduction for a std algorithm? Love podcasts or audiobooks? 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. But if we can't be sure if it's CBaseY or CDerived, then we have to use dynamic_cast<> or typeid. 2. I will require us to replace all the occurrences of FOO with the call to that function, but that is positive for two reasons: Since we are in an embedded project, memory and performance are critical. crash // pD2->bar(); ---------------------- Qt Plugin with OpenMP Support on MinGW: Undefined reference? s 392fb8 CBasey The reinterpret_cast operator can be used for conversions such as char* to int*, or One_class* to Unrelated_class*, which are inherently unsafe. //error, 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. 9. first one is to remove constness from a type and the other is to give its code explicitness. Solution 1. Nishant Sivakumar, Casting Basics - Use C++ casts in your VC++.NET programs When should static_cast, dynamic_cast, const_cast and reinterpret_cast be used? 1. 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). Similarly, casting an int to a void* is perfectly legal with reinterpret_cast, though it's unsafe. There they were, the C-style casts. CBaseX() { x = 10; } What are the basic rules and idioms for operator overloading? 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. reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . Well, there is one obvious reason: because it wouldn't do everything that bit_cast does. cmake: target_link_libraries use static library not shared, Returning unique_ptr