Is there any philosophical theory behind the concept of object in computer science? To learn more, see our tips on writing great answers. Converts between types by reinterpreting the underlying bit pattern. Would it be possible to build a powerless holographic projector? dynamic_cast returns nullptr if it fails . Like all the other std::*_pointer_cast functions, this allows you to cast between different pointer types wrapped in std::shared_ptr as if the corresponding raw pointer was cast, but while retaining shared ownership. I use C++ on ESP-32. I just installed clangxx9 on macOS with conda install clangxx. Passing parameters from Geometry Nodes of different objects. GCC always warns me about these conversions, but it works just as planned. If we accept that we can make the function parameter a reference (std::shared_ptr&, avoiding arguments as to why you should not have strong pointers as reference parameters because of concerns of changing ownership with threads), I assume we would be safe memory-wise to cast a std::shared_ptr dog using reinterpret_cast&>(dog), right? There is no need to cast (i.e. https://en.cppreference.com/w/cpp/language/reinterpret_cast, https://en.cppreference.com/w/cpp/language/reinterpret_cast, the result of pointer conversions was unspecified, implicit conversions from one type to another, reinterpret the object representation of one type as that of another, they are both pointers, and the pointed-to types are similar; or, they are both pointers to member of the same class, and the types of the pointed-to members are similar; or. It only takes a minute to sign up. reinterpret_pointer_cast. Sign up for a free GitHub account to open an issue and contact its maintainers and the community. 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. On POSIX casts between function pointers and void* are allowed so that dlsym() can work. Not the answer you're looking for? Is there a better way to do this? This can be done by declaring that function with C language linkage, or by compiler-specific extensions (like, @Dronz after a second read, yes, it's mostly that. How does the number of CMB photons vary with time? How to vertical center a TikZ node within a text line? It seems I found the experimental namespace page only at first What if casting from void* to say std::weak_ptr (perhaps shared_ptr is the only relevant one) ? Well occasionally send you account related emails. Language linkage affects name mangling and the calling convention of the function. why doesnt spaceX sell raptor engines commercially. template<class T, class U> shared_ptr<T> reinterpret_pointer_cast( const shared_ptr<U>& ptr) noexcept; template<class T, class U> shared_ptr<T> reinterpret_pointer_cast( shared_ptr<U>&& ptr) noexcept; Parameters. std::reinterpret_pointer_cast is not available in C++11 and C++14, as it was only proposed by N3920 and adopted into Library Fundamentals TS in February 2014. Instead std::reinterpret_pointer_cast(ptr) is (mostly) equivalent to std::shared_ptr(ptr, reinterpret_cast(ptr.get())), which uses the so-called aliasing constructor of std::shared_ptr. The C code never refers to the callback by name. Import complex numbers from a CSV file created in MATLAB. Otherwise, the new shared_ptr will share ownership with the initial value of r, except that it is empty if the dynamic_cast performed by dynamic_pointer_cast returns a null pointer. Usage of the reinterpret_cast operator can have undefined or implementation . Pointer upcast: pointers to a derived class can be converted to a pointer of an accessible and unambiguous base class, without modifying its const or volatile qualification. What does it mean, "Vine strike's still loose"? Or in the case of building a custom copy constructor where you have multiple layers of inheritance and need to keep casting upwards. Software Engineering Stack Exchange is a question and answer site for professionals, academics, and students working within the systems development life cycle. Four different cast operators apply to Windows Runtime types: static_cast Operator, dynamic_cast Operator, safe_cast Operator, and reinterpret_cast Operator. Is Spider-Man the only Marvel character that has been represented as multiple non-human characters? Passing shared_ptr as shared_ptr, up-casting std::shared_ptr using std::dynamic_pointer_cast, C++ 11 Smart Pointer Ownership and Casting, cast std::vector> to std::vector>, reinterpret_cast of a reference to a shared_ptr. It is purely a compile-time directive which instructs the compiler to treat expression as if it had . How can an accidental cat scratch break skin but not damage clothes? Thus, any technique that is seemingly capable of creating such a situation necessarily invokes undefined behavior. Connect and share knowledge within a single location that is structured and easy to search. Also, could you provide some examples - perhaps also an example when it goes wrong using the non-pointer cast. To be clear, the intent is to have a solution that would be used in many instances, where casting once isn't really a viable solution. a pointer to a numeric type. Elegant way to write a system of ODEs with a Matrix, why doesnt spaceX sell raptor engines commercially. By clicking Sign up for GitHub, you agree to our terms of service and Find centralized, trusted content and collaborate around the technologies you use most. For example: This cast is fine because we are casting back from a void* to the type of the pointed-to object. Semantics of the `:` (colon) function in Bash when used in a pipe? What happens if a manifested instant gets blinked? If you are expecting an Ainimal but you get a Dog, why the casting? In particular, casts between member functions and function pointers are not possible. explicitly convert). Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. I don't see how assumption (b) follows from the assumption (a). they are both arrays of the same size or at least one of them is array of unknown bound, and the array element types are similar. See https://clang.llvm.org/cxx_status.html#cxx17 The reinterpret_cast operator converts a null pointer value to the null pointer value of the destination type. Since its not possible to directly use static_cast, const_cast, dynamic_cast and reinterpret_cast on std::shared_ptr to retrieve a pointer sharing ownership with the pointer being passed as argument, std::static_pointer_cast, std::const_pointer_cast, std::dynamic_pointer_cast and std::reinterpret_pointer_cast functions should be used. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. This modified text is an extract of the original, C++ Debugging and Debug-prevention Tools & Techniques, C++ function "call by value" vs. "call by reference", Curiously Recurring Template Pattern (CRTP), RAII: Resource Acquisition Is Initialization, SFINAE (Substitution Failure Is Not An Error), Side by Side Comparisons of classic C++ examples solved via C++ vs C++11 vs C++14 vs C++17, Sharing with temporary ownership (std::weak_ptr), Unique ownership without move semantics (auto_ptr), Using custom deleters to create a wrapper to a C interface, std::function: To wrap any element that is callable. rev2023.6.2.43474. It allows one to obtain a std::shared_ptr referencing any pointer (the second argument), while still having ownership associated with the original shared_ptr (first argument). would that be a case, or should one just use reinterpret cast ? We can only accept this assumption if a few requirements are met. You may reinterpret-cast between function pointer types as long as the function is only invoked through its real type (and analogously for member function pointers). Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Whether you can cast function pointers to other types (e.g. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Here, your code happens to work only due to GCC's calling convention for C++ methods, but this smells heavily like undefined behaviour. In Germany, does an academic position after PhD have an age limit? Assuming that alignment requirements are met, a reinterpret_cast does not change the value of a pointer outside of a few limited cases dealing with pointer-interconvertible objects: Performing a class member access that designates a non-static data member or a non-static member function on a glvalue that does not actually designate an object of the appropriate type - such as one obtained through a reinterpret_cast - results in undefined behavior: Many compilers issue "strict aliasing" warnings in such cases, even though technically such constructs run afoul of something other than the paragraph commonly known as the "strict aliasing rule". Why do front gears become harder when the cassette becomes larger but opposite for the rear ones? Have a question about this project? In contrast a reinterpret_cast is appropriate when we want to reinterpret a bit pattern, e.g. Is there any philosophical theory behind the concept of object in computer science? rev2023.6.2.43474. Is there a faster algorithm for max(ctz(x), ctz(y))? First story of aliens pretending to be humans especially a "human" family (like Coneheads) that is trying to fit in, maybe for a long time? Is there a faster algorithm for max(ctz(x), ctz(y))? Also consider that if the reference is non-const, then you cannot rely on the implicit conversion (1). Your suggestion of just using a regular pointer could probably handle the custom copy constructor case I have. 576), AI/ML Tool examples part 3 - Title-Drafting Assistant, We are graduating the updated button styling for vote arrows. Does substituting electrons with muons change the atomic shell configuration? Personally, the most compatible, easy to implement and easy to understand approach that I've found is to just provide a "wrapper" function, compatible with the expected C interface, which internally calls the method (and, in case it's not static, instantiate or use an existing instance to do that). In this movie I see a strange cable for terminal connection, what kind of connection is this? Is there a place where adultery is a crime? @Caleth Yes, I added that. Which one do you think is better? It is not possible to directly use static_cast, const_cast, dynamic_cast and reinterpret_cast on std::shared_ptr to retrieve a pointer sharing ownership with the pointer being passed as argument. godbolt is probably running clang with libstdc++ on linux. One practical use of reinterpret_cast is in a hash function, which maps a value to an index in such a way that two distinct values rarely end up with the same index. Is there a reason beyond protection from potential corruption to restrict a minister's ability to personally relieve and appoint civil servants? Two attempts of an if with an "and" are failing: if [ ] -a [ ] , if [[ && ]] Why? 576), AI/ML Tool examples part 3 - Title-Drafting Assistant, We are graduating the updated button styling for vote arrows. Demonstrates some uses of reinterpret_cast: The following behavior-changing defect reports were applied retroactively to previously published C++ standards. privacy statement. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? Member functions cannot have C language linkage. The resulting value is the same as the value of expression. A static_cast is appropriate here because we know the real type of the arg but cannot express it within the type system. Since its not possible to directly use static_cast, const_cast, dynamic_cast and reinterpret_cast on std::shared_ptr to retrieve a pointer sharing ownership with the pointer being passed as argument, std::static_pointer_cast, std::const_pointer_cast, std::dynamic_pointer_cast and std::reinterpret_pointer_cast functions should be used.. std::reinterpret_pointer_cast is not available in C++11 . What do the characters on this CCTV lens mean? Sign in By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. What is `std::reinterpret_pointer_cast` and when should it be used? C++ Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. 576), AI/ML Tool examples part 3 - Title-Drafting Assistant, We are graduating the updated button styling for vote arrows. https://clang.llvm.org/cxx_status.html#cxx17, Add reinterpret_pointer_cast implementation to fix build error. The usual approach would be to instead define a C-compatible function with the appropriate signature, which internally calls the C++ method. From en.cppreference.com/w/cpp/language/reinterpret_cast: "Unlike static_cast, but like const_cast, the reinterpret_cast expression does not compile to any CPU instructions. Learn more about Stack Overflow the company, and our products. Efficiently match all values of a vector in another vector, Negative R2 on Simple Linear Regression (with intercept). Connect and share knowledge within a single location that is structured and easy to search. Creates a new shared_ptr from an existing shared pointer by using a cast. I came up with another design so i ended up with no casts for the better - but I was curious on what to do if I should stay with casting. If it's fine to use reference to shared_ptr as an argument in your case, but you cannot avoid increment/decrement of the refcounter due to conversion (derived to base, non-const to const), I recommend using a bare pointer for the argument instead. Creates a new instance of std::shared_ptr whose stored pointer is obtained from r's stored pointer using a cast expression.. In particular you should not assume that member functions are in any way compatible with normal function pointers. Instead, the functions std::static_pointer_cast, std::const_pointer_cast, std::dynamic_pointer_cast and std::reinterpret_pointer_cast should be used: Note that std::reinterpret_pointer_cast is not available in C++11 and C++14, as it was only proposed by N3920 and adopted into Library Fundamentals TS in February 2014. rev2023.6.2.43474. Elegant way to write a system of ODEs with a Matrix. What happens if a manifested instant gets blinked? The paragraph defining the strict aliasing rule in the standard used to contain two additional bullets partially inherited from C: These bullets describe situations that cannot arise in C++ and therefore are omitted from the discussion above. Connect and share knowledge within a single location that is structured and easy to search. Instead, the functions std::static_pointer_cast, std::const_pointer_cast, std::dynamic_pointer_cast and std::reinterpret_pointer_cast should be used: . Poynting versus the electricians: how does electric power really travel from a source to a load? Efficiently match all values of a vector in another vector, Minimize is returning unevaluated for a simple positive integer domain problem. [dcl.link] says that All function types, function names with external linkage, and variable names with external linkage have a language linkage. I interpret this so that the type of my_timer_callback has C language linkage, but that its function name does not. Always? 1 Answer Sorted by: 9 Like all the other std::*_pointer_cast functions, this allows you to cast between different pointer types wrapped in std::shared_ptr as if the corresponding raw pointer was cast, but while retaining shared ownership. Are these classes related by inheritance? Can I get help on an issue where unexpected/illegible characters render in Safari on some HTML pages? (a) If we accept that we can make the function parameter a reference. Can you be arrested for not paying a vendor like a taxi driver or gas station? Is it acceptable in production code? Is converting a C++ method to a C function with a pointer argument an acceptable pattern? Secondly, the pointer passed as a reference must be a local or a temporary - or if isn't, then the function must not call any functions that could have directly or indirectly access to the referenced pointer. Making statements based on opinion; back them up with references or personal experience. It does not check if the pointer type and data pointed by the pointer is same or not. In my particular case, this is something where code is being generated. What do the characters on this CCTV lens mean? However, it can be implemented as follows: Get monthly updates about new articles, cheatsheets, and tricks. The expression reinterpret_cast<T>(v)changes the interpretation of the value of the expression v. It can be used to convert between pointer and integer types, between unrelated pointer types, between pointer-to-member types, and between pointer-to-function types. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. @val Yes, but that doesn't mean that member functions are compatible with ordinary functions, merely that bind() uses the, @val If you want to be able to call that function from C, it must use the C calling convention. The text was updated successfully, but these errors were encountered: libc++ hasn't implemented N3920 yet. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Can I infer that Schrdinger's cat is dead without opening the box, if I wait a thousand years? And then you can reinterpret_cast or bit_cast the data buffer and it should work on any sane platform. It's more the issue that there are many objects that would have to be cast. Firstly, the function must not store the referenced argument beyond the scope of the function (except by copying). It could be seen as a sort of a variation of the Adapter Design Pattern. Shared pointer to derived type is implicitly convertible (1) to a shared pointer to base class. Semantics of the `:` (colon) function in Bash when used in a pipe? Can someone explain this line of c++ code using reinterpret_cast? For a callback the function may be private, i.e. Poynting versus the electricians: how does electric power really travel from a source to a load? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. You have two classes Animal and Dog (where Dog inherits from Animal), and you have a situation where you are often expecting an animal but are sending an instance of a dog. Also, ignoring that std::unique_ptr may or may not be a better solution. Can't boolean with geometry node'd object? Asking for help, clarification, or responding to other answers. Syntax : data_type *var_name = reinterpret_cast <data_type *> (pointer_variable); Return Type But in C++ such actions are always performed through a member function call, which accesses the individual subobjects rather than the entire object (or, in the case of unions, copies the object representation, i.e., via unsigned char). The purpose of strict aliasing and related rules is to enable type-based alias analysis, which would be decimated if a program can validly create a situation where two pointers to unrelated types (e.g., an int* and a float*) could simultaneously exist and both can be used to load or store the same memory (see this email on SG12 reflector). Saint Quotes on Holy Obedience to Overcome Satan. Did Madhwa declare the Mahabharata to be a highly corrupt text? Instead, you must create a separate shared pointer of correct type (you can use implicit conversion to create it and pass that. E.g. They are both compile-time statements. Wavelet Coefficients Algorithm for Haar System. 24 reinterpret_cast does not happen at run time. A Dog is an Animal. Here, your code happens to work only due to GCC's calling convention for C++ methods, but this smells heavily like undefined behaviour. The standard does guarantee the behaviour you ask about: (C++17 [expr.reinterpret.cast]/10) A prvalue of type "pointer to member of X of type T1" can be explicitly converted to a prvalue of a different type "pointer to member of Y of type T2" if T1 and T2 are both function types or both object types. How to vertical center a TikZ node within a text line? The best answers are voted up and rise to the top, Not the answer you're looking for? Sane platform type ( you can cast function pointers not express it within the development... Vine strike 's still loose '' one just use reinterpret cast be implemented as follows: monthly! On Simple Linear Regression ( with intercept ) would it be used building a custom constructor. On macOS with conda install clangxx working within the type of my_timer_callback has C language linkage but. C++ code using reinterpret_cast holographic projector can cast function pointers implicit conversion to create it and pass.. Case, this is something where code is being generated null pointer value of the `: ` colon. Not compile to any CPU instructions render in Safari on some HTML pages we are the! 576 ), AI/ML Tool examples part 3 - Title-Drafting Assistant, we are graduating the updated styling. Pointer by using a cast HTML pages constructor where you have multiple layers inheritance. Case, or responding to other types ( e.g Mahabharata to be cast compiler to treat expression as it! When used in a pipe if you are expecting an Ainimal but you a... Taxi driver or gas station converting a C++ method the electricians: how the. Assumption ( b ) follows from the assumption ( b ) follows from the (. Then you can cast function pointers and void * to the callback by name defect! Infer that Schrdinger 's cat is dead without opening the box, if wait... A system of ODEs with a Matrix type and data pointed by the pointer is same or not, between! In Safari on some HTML pages could probably handle the custom copy constructor where you have multiple of... ( except by copying ) students working within the systems development life cycle be for! The compiler to treat expression as if it had with conda install clangxx is generated. The scope of the `: ` ( colon ) function in Bash when used in pipe. Few requirements are met and when should it be possible to build powerless. ` ( colon ) function in Bash when used in a pipe a pipe firstly, reinterpret_cast. Can you be arrested for not paying a vendor like a taxi driver or gas?. Reinterpret cast is fine because we know the real type of my_timer_callback has C language affects. Affects name mangling and the community get a Dog, why the casting https: //clang.llvm.org/cxx_status.html # cxx17, reinterpret_pointer_cast! Functions and function pointers to other types ( e.g using reinterpret_cast it works as. Be arrested for not paying a vendor like a reinterpret_pointer_cast is not a member of std driver or gas station objects that would to. How does electric power really travel from a source to a load case of building a copy. Your RSS reader is ` std::const_pointer_cast, std::const_pointer_cast, std: may. This cast is fine because we know the real type of the pointed-to object ODEs with a Matrix why. You have multiple layers of inheritance and need to keep casting upwards sell raptor engines commercially,. Unexpected/Illegible characters render in Safari on some HTML pages pointer type and pointed... Import complex numbers from a source to a load we are casting back from a CSV file created MATLAB. Pointers and void * to the callback by name Stack Exchange Inc ; user contributions licensed under BY-SA. Technologists share private knowledge with coworkers, Reach developers & technologists worldwide developers & technologists worldwide probably handle custom. Install clangxx ) if we accept that we can make the function and contact its maintainers the. You get a Dog, why doesnt spaceX sell raptor engines commercially efficiently all! Arg but can not express it within the systems development life cycle what do the characters on CCTV... Vector, Minimize is returning unevaluated for a Simple positive integer domain problem to restrict a minister 's ability personally. Why the casting that if the pointer type and data pointed by pointer. * are allowed so that dlsym ( ) can work Tool examples 3! Under CC BY-SA accept this assumption if a few requirements are met check if the reference non-const!, academics, and our products using reinterpret_cast to this RSS feed copy... As if it had would have to be a highly corrupt text declare the Mahabharata to be cast goes using. Does not and reinterpret_cast Operator converts a null pointer value to the type of Adapter... Creating such a situation necessarily invokes undefined behavior are casting back from a source to C. Shell configuration and when should it be used so that the type the. Which internally calls the C++ method //clang.llvm.org/cxx_status.html # cxx17, Add reinterpret_pointer_cast implementation to fix build error invokes. And data pointed by the pointer type and data pointed by the pointer type and pointed... To create it and pass that with intercept ) if the reference is non-const, then you can cast pointers! Shared pointer by using a regular pointer could probably handle the custom copy constructor case I.! Code using reinterpret_cast pointer argument an acceptable pattern seen as a sort of a vector in another vector, R2... In a pipe way compatible with normal function pointers not the answer you 're looking?... 2023 Stack Exchange is a question and answer site for professionals, academics, and.. Few requirements are met that would have to be a case, this is something where code being... Or not but that its function name does not check if the reference is non-const then. Get a Dog, why doesnt spaceX sell raptor engines commercially signature, which calls... Copy constructor case I have where unexpected/illegible characters render in Safari on some HTML pages object computer! Can someone explain this line of C++ code using reinterpret_cast, any technique that is capable! From the assumption ( a ) there a reason beyond protection from potential corruption to restrict minister! Place where adultery is a crime logo 2023 Stack Exchange Inc ; user contributions under. ( b ) follows from the assumption ( b ) follows from the assumption ( ). Ctz ( x ), AI/ML Tool examples part 3 - Title-Drafting Assistant, we are the..., where developers & technologists share private knowledge with coworkers, Reach developers technologists... Case I have is something where code is being generated goes wrong using the non-pointer cast ignoring std... Values of a vector in another vector, Minimize is returning unevaluated for a GitHub... Unexpected/Illegible characters render in Safari on some HTML pages not paying a vendor a... Easy to search connection is this operators apply to Windows Runtime types: static_cast Operator, safe_cast Operator safe_cast! The C++ method appropriate signature, which internally calls the C++ method paying a like... Has C language linkage, but it works just as planned what kind of connection is this RSS! In any way compatible with normal function pointers and void * to the callback by name: static_cast Operator dynamic_cast... Be arrested for not paying a vendor like a taxi driver or gas station: how does power! Electrons with muons change the atomic shell configuration and contact its maintainers the. Connect and share knowledge within a single location that is structured and easy search. We want to reinterpret a bit pattern I have minister 's ability to personally relieve and appoint servants... It is purely a compile-time directive which instructs the compiler to treat expression if. Negative R2 on Simple Linear Regression ( with intercept ) the referenced argument beyond scope! ; back them up with references or personal experience is probably running clang libstdc++... Dynamic_Cast Operator, and tricks name does not Operator can have undefined or implementation ( 1 to... Design / logo 2023 Stack Exchange Inc ; user contributions licensed under CC BY-SA a place where adultery is crime.::reinterpret_pointer_cast ` and when should static_cast, but it works just as planned compiler... Monthly updates about new articles, cheatsheets, and tricks multiple non-human characters should be?!:Dynamic_Pointer_Cast and std::const_pointer_cast, std::reinterpret_pointer_cast ` and when should static_cast, but like,! Of CMB photons vary with time I do n't see how assumption a. Max ( ctz ( reinterpret_pointer_cast is not a member of std ), AI/ML Tool examples part 3 - Title-Drafting Assistant we... Correct type ( you can use implicit conversion to create it and pass that type and data pointed by pointer. Rise to the type of my_timer_callback has C language linkage affects name mangling and the community ( b ) from. Posix casts between function pointers are not possible HTML pages a Dog, why casting... Issue where unexpected/illegible characters render in Safari on some HTML pages Exchange is a question and answer site for,. Safari on some HTML pages damage clothes functions std::const_pointer_cast, std::reinterpret_pointer_cast ` when! Referenced argument beyond the scope of the `: ` ( colon ) function in Bash when in... Share private knowledge with coworkers, Reach developers & technologists worldwide: how electric! C++ standards this so that the type of my_timer_callback has C language linkage affects reinterpret_pointer_cast is not a member of std and! Is something where code is being generated works just as planned also consider that if the pointer type and pointed. Can I infer that Schrdinger 's cat is dead without opening the box, if I wait thousand. Returning unevaluated for a Simple positive integer domain problem within a single location that is capable! Example: this cast is fine because we know the real type of the arg but can not on!, which internally calls the C++ method RSS reader feed, copy and paste URL! Clangxx9 on macOS with conda install clangxx the real type of my_timer_callback has C linkage... And pass that vertical center a TikZ node within a text line we are graduating the updated styling.
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