When would I use const volatile, register volatile, static volatile in C++? Does boost::asio::deadline_timer use a thread for each timer? CBaseY* pY1 = pD; So we might not want to bake the cast into the constant expression. Scenario 2: Transition to the relevant class. CBaseY() { y = 20; py = &y; } reinterpret_cast is a very special and dangerous type of casting operator. Well, there is one obvious reason: because it wouldn't do everything that bit_cast does. 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. C++ .reinterpret_cast:reinpreter_cast<type-id> (expression) reinterpret_cast,,.: int n=9; double d= reinterpret_cast< double > (n); . printf ("CBaseY s pY s %x/n", (int)pY); first one is to remove constness from a type and the other is to give its code explicitness. Once we've converted the pointer to void, we can't easily convert it back to the original class. s pY2 s 392fb8 : double x = 10.3; int y; y = (int) x; // c-like cast notation : double x = 10.3; int y; y = reinterpret_cast(x). If your C++ code is using some C API then of course you don't have much choice. (programs). 1. dynamic_cast<>, on the other hand, prevents a generic CBaseY? 2. printf("CDerived* pD = %x/n", (int)pD); s 392fb8 CBasey As a result, only types without spaces can be cast to. // System crash!! Error, type point is irrelevant // CBaseY s pY1 s static_cast (pX); ; . Incorrect cast - is it the cast or the use which is undefined behavior. 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). However, in memory manipulation . successfully, but pY2 is not CBaseX CBaseY*pY2 s reinterpret_cast< CBaseY*> (pX); In C++, reinterpret_cast, static_cast and const_cast is very common. 392fbc CDerived s pD1 The const was there in the c-cast, obviously, Where exactly do you think a const cast was happening there? , : 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) ? Ref: CBaseY s pY1 s 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". // System crash!! The actual pointer object is a constant expression, but we still have a macro, what about that? And is suggested to use it using proper data type i.e., (pointer data type should be same as original data type). // pD2 s pY, but we expect pD2 s pY - 4 At that point, it's up to you to understand exactly what your compiler and machine do in this situation. 2. 14. it is definitely different. 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. 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. For a conversion between different function type pointers or between different object type pointers you need to use reinterpret_cast. Which STL container should I use for a FIFO? But really casts are a language smell. Why do both libstdc++ and libc++ not check for pointer and reference type D for the default unique_ptr constructor? 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? // Error, the type pointed to is irrelevant (i.e. When should static_cast, dynamic_cast, const_cast and reinterpret_cast be used? In current C++, you can't use reinterpret_cast like in that code. Because you can use cast it using C-style cast, but this is not explicit so that is not recommended. 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. We could just write reinterpret_cast in the macros and live with the fact that we have ugly macros but silenced the warnings. 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. 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. CBaseX() { x = 10; } ), 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. CBaseY* pY3 = new CBaseY(); Powered by WordPress and Stargazer. }; class CDerived : public CBaseX, public CBaseY - security/portablility/performance . Compile C files in C++ project which do not use precompiled header? compilation, implicit static_cast<> conversion CDerived* pD3 = static_cast(pY3); Unrelated static_cast<> 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. Is it safe to use the address of a static local variable within a function template as a type identifier? CBaseY s pY3 s 390ff0 CDerived s pD3 s 390fec 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. C# 2- ? Furthermore, it is currently impossible to implement a constexpr bit-cast function . // pY2->bar(); Most C++ developers know that #defines are evil because the are simple text replacement and therefore bring problems like lacking type safety and more. Love podcasts or audiobooks? constexpr and initialization of a static const void pointer with reinterpret cast, which compiler is right? When should static_cast, dynamic_cast, const_cast and reinterpret_cast be used? What is the difference between cout, cerr, clog of iostream header in c++? 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. What information does GCC Profile Guided Optimization (PGO) collect and which optimizations use it? // compiled successfully, not CBaseX // Compiled 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. 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. Can I use partial template specialization for a (non-member) function? -O1 on ARM GCC). Which to use when? 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. ---------------------- Output { 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: }; const_cast means two things. CBaseY* pY = pD; Successful compilation, pY s pD s 4 is actually meaningless, and as int*pi s/reinterpret_cast (pf); public: class CBaseY from being converted to CDerived. 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). gives some good details. elasticsearch sink connector (kafka-connect). int z; 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. a "new CBaseY()" 17. 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 . In C++0x, reinterpret_cast<int*> (p) will be . reinterpret_cast This is the trickiest to use. 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. C++11 Passing function as lambda parameter, How can cmake activate "Inherit from parent" link option in Visual Studio. 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()? 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); ? When and why would you use static with constexpr? 3. constexpr vs. static const: Which one to prefer? Dipping my toes into a new project, I got a bunch of ugly warnings about a ton of C-casts inside a macro definition. // static cast <> Qt Plugin with OpenMP Support on MinGW: Undefined reference? However it will affect how other CPU instructions are generated. Even in the C++20 world where we can allocate memory at compile time, reinterpret_cast is forbidden in constexpr functions. From the semantics of your problem, I'd go with reinterpret, because that's what you actually do. 4. static_cast<> CDerived?->CBaseY' -> CDerived?//Successful 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. . public: [MSDN] C++ Language Reference -- Casting Immediate scheduler , reinterpret_cast, gradlew gradle, undefined reference android NDK. The C-style cast isn't better. To see how static_cast<> actually works, we have to look at the memory layout of CDerived. Lets have a look from the memory perspective. fork/execvp . int*pn2 s static_cast (pv); // reinterpret_cast<> Don't use static cast for arithmetic conversions (cpp-core-guidelines). 5. 10. reinterpret_cast output --------------------------- CDerived*pD s 392fbb8 not CBaseY x 11. 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. reinterpret_cast. 2. dynamic_cast<> needs the class to become polymorphic, i.e. CDerived* pD = new CDerived(); Finding the definitions took a while were using an IDE for embedded development, and its not blessed with working functionality like jump to definition. 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. // System 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); ? 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. // Error, type point 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). Most programmers have studied C before they learn C and are accustomed to C-style (type) conversion. There they were, the C-style casts. 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. Refresh the page, check Medium 's site. But wait the all-caps FOO and BAR look suspicious. It is used when we want to work with bits. The reinterpret_cast operator can be used for conversions such as char* to int*, or One_class* to Unrelated_class*, which are inherently unsafe. interpreting the same address as yet something else might not be wanted. , . But thats not too satisfying, is it? The compiler emitted a little over 1000 warnings or, more precisely, the same warning 1000 times. static const Member Value vs. 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. For a conversion of void* to int* you can only use static_cast (or the equivalent C-style cast). float* pf = &f; // Compile OK, but pY2 is Note: Since the class holds this integer value and not a pointer value, it can be used as a constant expression. In the assembly code, you wont see any CPU instructions corresponding to the reinterpret_cast call. // Compiled successfully, but pY2 is Juan Soulie, C++ Language Tutorial: Type Casting, This article is from the CSDN blog and reproduced with the source: So in certain situations, a C-style cast will have the same effect as reinterpret_cast but they are not equivalent. 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. 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. In the example above, the only way to return CDerived from a void is to convert it to CBaseY and then to CDerived. The thing everybody fails to mention is that reinterpret_cast<> is a means to document your program. cast statements ? All rights reserved. With this in mind, the question is whether we could come up with a class that. 2. In current C++, you can't use reinterpret_cast like in that code. Sometimes we may be a little fuzzy when we use static_cast<> and reinterpret_cast<> when we write C.C. 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. { 16. printf("CBaseY* pY3 = %x/n", (int)pY3); - , ; , &a, . (programs). How to use reinterpret cast for inner template class? Copyright 2022 www.appsloveworld.com. '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. // TODO: sign and unsigned integer numbers, float etc. //Successful compilation, but the sn2 is meaningless memory (rubbish) Which std::async implementations use thread pools? How to create two classes in C++ which use each other as data? 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. 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. Most would say it is a program smell and blame the programmer. 13. 392fbc void s pV1 s 392fbc It can typecast any pointer to any other data type. 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. When you convert CDerived to CBaseX static_cast<> and reinterpret_cast<> are no different. int* py; I suggest that reinterpret_cast is "better" for this task, since it makes the intent clearer. There wont be any difference between the two, except for multi-inheritance. boost, shared ptr Vs weak ptr? . It is important to remember that even though a program compiles, its . Is it legal to check whether the address of a subobject lies within the bounds of a containing object. It isn't clear what you're having difficulties with, but this clears it up for me: It only provides some information for the compiler to generate code. ; ( ), . 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. Trying to get away from them was not as easy as I first thought. 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. reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . Other uses are, at best, nonportable. Nishant Sivakumar, Casting Basics - Use C++ casts in your VC++.NET programs Learn on the go with our new app. In this article, I'll explain what static_cast<> actually did, and point out some situations that will lead to errors. The other two is sometimes confusing. 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* pD1 = static_cast(pY1); (The pointer is backward) offset by 4 (bytes) (4 is the number of bytes that the int type occupies in memory). to un arerelated // But if we can't be sure if it's CBaseY or CDerived, then we have to use dynamic_cast<> or typeid. 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. and CBaseY// CBaseX and CBaseY? Why? system() fork/execvp. I wrote about a different way of solving this problem: The amount of code broken when it's rejected will be no fun, so there is no motivation for them to forbid it. In that case it will read the value of p using p's type, and that value is then converted to a T*. Therefore, I made the conversion to uint8* an explicit function. cmake: target_link_libraries use static library not shared, Returning unique_ptr