reinterpret_pointer_cast is not a member of std

why doesnt spaceX sell raptor engines commercially. Functions are not ordinary objects, and member functions even less so. Thanks for contributing an answer to Stack Overflow! Not the answer you're looking for? [] ExplanatioUnlike 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). Elegant way to write a system of ODEs with a Matrix, why doesnt spaceX sell raptor engines commercially. What do the characters on this CCTV lens mean? 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. Elegant way to write a system of ODEs with a Matrix. I use C++ on ESP-32. Learn more about Stack Overflow the company, and our products. rev2023.6.2.43474. 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? Change of equilibrium constant with respect to temperature. When registering a timer I have to do this: And the same thing when registering a task: So the method is started in a separated task. 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. It's more the issue that there are many objects that would have to be cast. Why do front gears become harder when the cassette becomes larger but opposite for the rear ones? Creates a new shared_ptr from an existing shared pointer by using a cast. Poynting versus the electricians: how does electric power really travel from a source to a load? So std::reinterpret_pointer_cast(ptr) is (mostly) the same as std::shared_ptr(reinterpret_cast(ptr.get())), with the important difference that the latter would cause the cast raw pointer to be stored in a new shared pointer which is not associated with the original one, so that the managed object will be double-deleted once by the original shared pointer complex and once by the new one (and the latter potentially with the wrong type). By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. If r is empty, so is the new shared_ptr (but its stored pointer is not necessarily null). What is `std::reinterpret_pointer_cast` and when should it be used? Other casts involving (member) function pointers are undefined. Here, your code happens to work only due to GCC's calling convention for C++ methods, but this smells heavily like undefined behaviour. 50 74 comments Add a Comment TheSkiGeek 1 yr. ago The "right" solution is to have the API take a std::span<XY> or XY [] or some kind of iterator. Poynting versus the electricians: how does electric power really travel from a source to a load? Is it a good habit to use C expressions in C++ code? Stack Exchange network consists of 181 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. would that be a case, or should one just use reinterpret cast ? A const reference argument, or a non-reference argument do not have that problem. have internal linkage. 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. Making statements based on opinion; back them up with references or personal experience. On POSIX casts between function pointers and void* are allowed so that dlsym() can work. Not the answer you're looking for? (b) I assume we would be safe memory-wise to cast a std::shared_ptr dog using reinterpret_cast&>(dog), right? Instead, the functions std::static_pointer_cast, std::const_pointer_cast, std::dynamic_pointer_cast and std::reinterpret_pointer_cast should be used: . Passing parameters from Geometry Nodes of different objects. In my particular case, this is something where code is being generated. object pointers or void pointers) is implementation-defined (background). Efficiently match all values of a vector in another vector. 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). Here, your code happens to work only due to GCC's calling convention for C++ methods, but this smells heavily like undefined behaviour. In particular, casts between member functions and function pointers are not possible. You signed in with another tab or window. Well occasionally send you account related emails. Building a safer community: Announcing our new Code of Conduct, Balancing a PhD program with a startup career (Ep. 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. Is it acceptable in production code? How to vertical center a TikZ node within a text line? 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. 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". extern "C" specifies the language linkage of this function. int * vs int [N] vs int (*)[N] in functions parameters. Also consider that if the reference is non-const, then you cannot rely on the implicit conversion (1). The above code snippet specifies internal linkage through the static keyword (not a static method!). Import complex numbers from a CSV file created in MATLAB. 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. 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. However, it can be implemented as follows: I am often casting a strong pointer (std::shared_ptr) to an animal-expecting function (std::shared_ptr). As soon as I read. In Germany, does an academic position after PhD have an age limit? In fact the standard defines. 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. 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) ? It is purely a compile-time directive which instructs the compiler to treat expression as if it had the type new-type. safe_cast and static_cast throw an exception when the conversion can't be performed; static_cast Operator also performs compile-time type checking. Whether you can cast function pointers to other types (e.g. 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. Software Engineering Stack Exchange is a question and answer site for professionals, academics, and students working within the systems development life cycle. Implicit conversions with classes In the world of classes, implicit conversions can be controlled by means of three member functions: A Dog is an Animal. What do the characters on this CCTV lens mean? Can I also say: 'ich tut mir leid' instead of 'es tut mir leid'? Did Madhwa declare the Mahabharata to be a highly corrupt text? Link to reference: std::reinterpret_pointer_cast, Should this be used when weak_ptr or shared_ptr are involved (Why? And if so, what could come up other than threading issues; such as that of the reference counting variety? For a callback the function may be private, i.e. How can I get rid of this reinterpret_cast, or is this usage OK? Building a safer community: Announcing our new Code of Conduct, Balancing a PhD program with a startup career (Ep. 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. Syntax : data_type *var_name = reinterpret_cast <data_type *> (pointer_variable); Return Type 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. Since use cases for reinterpret_cast are rare, you will be much more likely to use std::dynamic_pointer_cast or std::static_pointer_cast, which are useful for down- and side-cast in class hierarchies. What does it mean, "Vine strike's still loose"? Example. Does the conduit for a wall oven need to be pulled inside the cabinet? Can't boolean with geometry node'd object? Connect and share knowledge within a single location that is structured and easy to search. By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. However, this is not guaranteed for a function pointer. 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. 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). A reinterpret_cast is always fishy unless you know exactly what you are doing. [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. Efficiently match all values of a vector in another vector, Negative R2 on Simple Linear Regression (with intercept). Demonstrates some uses of reinterpret_cast: The following behavior-changing defect reports were applied retroactively to previously published C++ standards. Or in the case of building a custom copy constructor where you have multiple layers of inheritance and need to keep casting upwards. Is converting a C++ method to a C function with a pointer argument an acceptable pattern? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Is there a reason beyond protection from potential corruption to restrict a minister's ability to personally relieve and appoint civil servants? Returns a value of type new-type. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Converts between types by reinterpreting the underlying bit pattern. Semantics of the `:` (colon) function in Bash when used in a pipe? Change of equilibrium constant with respect to temperature. It is purely a compile-time directive which instructs the compiler to treat expression as if it had . 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. Is `reinterpret_cast` actually good for anything? 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. 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. To learn more, see our tips on writing great answers. Are these classes related by inheritance? Have a question about this project? 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. C++ rev2023.6.2.43474. 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? Two attempts of an if with an "and" are failing: if [ ] -a [ ] , if [[ && ]] Why? Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. The reinterpret_cast operator converts a null pointer value to the null pointer value of the destination type. 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). @Caleth Yes, I added that. Browse other questions tagged, Start here for a quick overview of the site, Detailed answers to any questions you might have, Discuss the workings and policies of this site. 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. 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. ptr However, one caveat to use a bare pointer is that I cannot modify the original, You could possibly pass a bare pointer, and return a bare pointer that you then assign to the, C++ reinterpret_cast of std::shared_ptr reference to optimize, Building a safer community: Announcing our new Code of Conduct, Balancing a PhD program with a startup career (Ep. @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. 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. 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. However, it can be implemented as follows: Get monthly updates about new articles, cheatsheets, and tricks. How to downcast shared_ptr without std::static_pointer_cast in C++? Semantics of the `:` (colon) function in Bash when used in a pipe? Can I get help on an issue where unexpected/illegible characters render in Safari on some HTML pages? Connect and share knowledge within a single location that is structured and easy to search. Note that many C++ compilers relax this rule, as a non-standard language extension, to allow wrong-type access through the inactive member of a union (such access is not undefined in C). Sign in Creates a new instance of std::shared_ptr whose stored pointer is obtained from r's stored pointer using a cast expression.. Is there any philosophical theory behind the concept of object in computer science? explicitly convert). 576), AI/ML Tool examples part 3 - Title-Drafting Assistant, We are graduating the updated button styling for vote arrows. What maths knowledge is required for a lab-based (molecular and cell biology) PhD? It could be seen as a sort of a variation of the Adapter Design Pattern. When should static_cast, dynamic_cast, const_cast, and reinterpret_cast be used? By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. I am not entirely sure about the interactions between extern "C" and static (internal linkage). Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Firstly, the function must not store the referenced argument beyond the scope of the function (except by copying). In contrast a reinterpret_cast is appropriate when we want to reinterpret a bit pattern, e.g. reinterpret_pointer_cast. A static_cast is appropriate here because we know the real type of the arg but cannot express it within the type system. I just installed clangxx9 on macOS with conda install clangxx. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. I just installed clangxx9 on macOS with conda install clangxx. Usage of the reinterpret_cast operator can have undefined or implementation . Member functions cannot have C language linkage. Is it a good idea to provide different function signatures that do the same thing? For example: This cast is fine because we are casting back from a void* to the type of the pointed-to object. Wavelet Coefficients Algorithm for Haar System. 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. These bullets were eventually removed via CWG2051. Is there a better way to do this? Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Did Madhwa declare the Mahabharata to be a highly corrupt text? Find centralized, trusted content and collaborate around the technologies you use most. rev2023.6.2.43474. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. 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. Thus, any technique that is seemingly capable of creating such a situation necessarily invokes undefined behavior. Only the following conversions can be done with reinterpret_cast, except when such conversions would cast away constness or volatility. they are both arrays of the same size or both arrays of unknown bound, and the array element types are similar. To learn more, see our tips on writing great answers. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. In particular you should not assume that member functions are in any way compatible with normal function pointers. Shared pointer to derived type is implicitly convertible (1) to a shared pointer to base class. C/C++: Which conversion warnings make sense in practice? As with all cast expressions, the result is: Whenever an attempt is made to read or modify the stored value of an object of type DynamicType through a glvalue of type AliasedType, the behavior is undefined unless one of the following is true: Informally, two types are similar if, ignoring top-level cv-qualification: This rule enables type-based alias analysis, in which a compiler assumes that the value read through a glvalue of one type is not modified by a write to a glvalue of a different type (subject to the exceptions noted above). E.g. Negative R2 on Simple Linear Regression (with intercept). Alternatively, the function could have been placed into an anonymous namespace. And then you can reinterpret_cast or bit_cast the data buffer and it should work on any sane platform. They are both compile-time statements. Strangely, some C++17 features seem to be missing: $ cat test.cc #include <memory> using std::uninitialized_move; // since C++17 using std::reinterpret_pointer_cast; // since. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Did I somehow manage to install the wrong header files? Instead, you must create a separate shared pointer of correct type (you can use implicit conversion to create it and pass that. Language linkage is largely orthogonal to internal/external linkage. 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. It does not check if the pointer type and data pointed by the pointer is same or not. Is there a faster algorithm for max(ctz(x), ctz(y))? Sometimes?). To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Standard-wise, your suggested cast is not safe - it has undefined behaviour. Which one do you think is better? Language linkage affects name mangling and the calling convention of the function. Would it be possible to build a powerless holographic projector? The C code never refers to the callback by name. Also, ignoring that std::unique_ptr may or may not be a better solution. 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. Is Spider-Man the only Marvel character that has been represented as multiple non-human characters. to your account. godbolt is probably running clang with libstdc++ on linux. Is there a faster algorithm for max(ctz(x), ctz(y))? Asking for help, clarification, or responding to other answers. privacy statement. GCC always warns me about these conversions, but it works just as planned. Thanks for contributing an answer to Stack Overflow! What happens if a manifested instant gets blinked? In my particular case, I am often casting a strong pointer (std::shared_ptr) to an animal-expecting function (std::shared_ptr). Is there a reliable way to check if a trigger being fired was the result of a DML action from another *specific* trigger? The text was updated successfully, but these errors were encountered: libc++ hasn't implemented N3920 yet. Did Madhwa declare the Mahabharata to be a highly corrupt text? https://clang.llvm.org/cxx_status.html#cxx17, Add reinterpret_pointer_cast implementation to fix build error. Sign up for a free GitHub account to open an issue and contact its maintainers and the community. In C, aggregate copy and assignment access the aggregate object as a whole. Can someone explain this line of c++ code using reinterpret_cast? How can an accidental cat scratch break skin but not damage clothes? Is there any evidence suggesting or refuting that Russian officials knowingly lied that Russia was not going to attack Ukraine? Can you be arrested for not paying a vendor like a taxi driver or gas station? Your suggestion of just using a regular pointer could probably handle the custom copy constructor case I have. Connect and share knowledge within a single location that is structured and easy to search. (a) If we accept that we can make the function parameter a reference. 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). Why is Bb8 better than Bc7 in this position? See https://clang.llvm.org/cxx_status.html#cxx17 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). Efficiently match all values of a vector in another vector, Minimize is returning unevaluated for a simple positive integer domain problem. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Strangely, some C++17 features seem to be missing: However, this works with a clang9 elsewhere: https://godbolt.org/z/wAnnBE. Can I infer that Schrdinger's cat is dead without opening the box, if I wait a thousand years? The best answers are voted up and rise to the top, Not the answer you're looking for? cppreference.comLicensed under the Creative Commons Attribution-ShareAlike Unported License v3.0. 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. Find centralized, trusted content and collaborate around the technologies you use most. 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? Saint Quotes on Holy Obedience to Overcome Satan. The usual approach would be to instead define a C-compatible function with the appropriate signature, which internally calls the C++ method. Already on GitHub? 576), AI/ML Tool examples part 3 - Title-Drafting Assistant, We are graduating the updated button styling for vote arrows. To avoid overhead. dynamic_cast returns nullptr if it fails . Is Spider-Man the only Marvel character that has been represented as multiple non-human characters? Is there a reliable way to check if a trigger being fired was the result of a DML action from another *specific* trigger? Is there a place where adultery is a crime? There is no need to cast (i.e. The resulting value is the same as the value of expression. Also, could you provide some examples - perhaps also an example when it goes wrong using the non-pointer cast. What happens if a manifested instant gets blinked? 24 reinterpret_cast does not happen at run time. 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). 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. In this movie I see a strange cable for terminal connection, what kind of connection is this? Im not sure it was a void* after all, I think it was a DWORD type after all - so i guess reinterpret_cast would be needed. If you are expecting an Ainimal but you get a Dog, why the casting? To add a final requirement - using plain pointers would not allow me to change the original std::shared_ptr in the case of a generalized serializer class function that is virtual and thus cannot be made templated. I don't see how assumption (b) follows from the assumption (a). How to deal with "online" status competition at work? How to vertical center a TikZ node within a text line? How does the number of CMB photons vary with time? 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. Making statements based on opinion; back them up with references or personal experience. Difference Between reinterpret_cast Usage. When it is needed to interpret the bytes of an object as a value of a different type, std::memcpy or std::bit_cast (since C++20)can be used: If the implementation provides std::intptr_t and/or std::uintptr_t, then a cast from a pointer to an object type or cv void to these types is always well-defined. So you should use std::reinterpret_pointer_cast if you have a std::shared_ptr and you would have used reinterpret_cast if it had been a raw pointer instead. By clicking Sign up for GitHub, you agree to our terms of service and Asking for help, clarification, or responding to other answers. 1) An expression of integral, enumeration, pointer, or pointer-to-member type can be converted to its own type. a pointer to a numeric type. It only takes a minute to sign up. Four different cast operators apply to Windows Runtime types: static_cast Operator, dynamic_cast Operator, safe_cast Operator, and reinterpret_cast Operator. Always? 2) A pointer can be converted to any . 2 Answers Sorted by: 47 A reinterpret_cast is always fishy unless you know exactly what you are doing. Is there any philosophical theory behind the concept of object in computer science? 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 . We can only accept this assumption if a few requirements are met.

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