Note that a C-style (T)expression cast means to perform the first of the following that is possible: a const_cast , a static_cast , a static_cast followed by a const_cast , a reinterpret_cast , or a reinterpret_cast followed by a const_cast . Are these two types the same types of not? We do not currently allow content pasted from ChatGPT on Stack Overflow; read our policy here. p = reinterpret_cast<char*>(numeric_limits<uintptr_t>::max); C++ intchar,c++,pointers,casting,C++,Pointers,Casting,ifstreamint int myInt = 0; fileStream.read(reinterpret_cast<char*>(&myInt), sizeof(int)); int-myInt=0 readreinterpret_cast&myIntsizeofint reinterpret\u cast reinterpret_cast . To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Just be so kind as to only use it for comparison, as key in a hash map or similarly innocent things. generic member function pointer as a template parameter, Get a pointer to object's member function. The C and C++, specifications are very careful about that. Connecting three parallel LED strips to the same power supply. reinterpret_cast , , , : , . Below C++ program demonstrates the use of reinterpret_cast to reinterpret the bit pattern. Error encrypting string with AES128 using OpenSSL in C++, Evolutionary Algorithm without an objective function, Approximation of the length of the decimal representation of a type, Templates: SFINAE specialization not getting applied, Getting exception when calling std::call_once, how to search the computer for files and folders, Bit trick to detect if any of some integers has a specific value, Overload resolution: assignment of empty braces, Most efficient way in C/C++ to trim extra whitespace except 1 blank for very large data sets, OpenCV Undefined symbols for architecture x86_64: error, Accessing classes from another source in C++, issues initializing the constructor, Change an object passed by pointer to a function, Clang vs. GCC: Friend-ing a global function via qualified/unqualified names. Stack Overflow for Teams is moving to its own domain! . The effect of calling a function through a pointer to a function type that is not the same as the type used in the definition of the function is undefined. Ready to optimize your JavaScript with Rust? Why is declaration order important for passing a member function pointer as a template argument? Dynamic Cast 3. What happens if you score more than 99 points in volleyball? Find centralized, trusted content and collaborate around the technologies you use most. How can I use a VPN to access a Russian website that is banned in the EU? (GCC requires the -pedantic-errors flag to enforce that, though.). How Best to Keep Function From Closing File? . B(const function & f)
GCC Warns About Function Pointer to Object Pointer Cast. Mathematica cannot find square roots of some matrices? 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. A glvalue of type T1, designating an object x, can be cast to the type reference to T2 if an expression of type pointer to T1 can be explicitly converted to the type pointer to T2 using a reinterpret_cast. 2. Does balls to the wall mean full speed ahead or full speed ahead and nosedive? A static_cast may change the value of the pointer in the case of multiple inheritance (or when casting an interface to a concrete type), this offset calculation may involve an extra machine instruction. Fairly sure that anything that can be placed in a void* (without a cast) is the same size. . In that case, the first sizeof (Thing*)bytes of it will absolutely not happen to point at anything sensible. - reinterpret_cast is a keyword. C++ supports four types of casting: 1.Static Cast 2. However, they do have drawbacks -- strong enough ones that I would tend to avoid them if possible. Why was USB 1.0 incredibly slow even for its time? . Except that converting a prvalue of type pointer to T1 to the type pointer to T2 (where T1 and T2 are function types) and back to its original type yields the original pointer value, the result of such a pointer conversion is unspecified. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. The only contexts that prevent function type decay are unary & operator, sizeof operator and some others. Dont do whatever this is. A function type is not the same as a pointer to function type. Why does this work? Is there a way to "reinterpret_cast" function objects? It also allows casting from pointer to an integer type and vice versa. @PeteBecker It doesn't seem to make a difference to the compilers. s. In the last available draft for C++0X, the reinterpret_castbetween function pointers and objects pointers is conditionally supported. ?pointer to T1? to the type ? Cast pointer to member function to intptr_t, Calling C++ member functions via a function pointer, Disambiguate overloaded member function pointer being passed as template parameter, C++ function pointer (class member) to non-static member function. Since reinterpret_cast between pointers to object types (in your case, between pointers to pointers (to [member] functions)) is always allowed, it's also allowed in your case. reinterpret_cast(x) is equivalent to *reinterpret_cast(&x). Welcome to the All-In-One Code Framework. Now the problem is to set the self pointer after constructing a node.self To achieve that i am using reinterpret_cast:reinterpret_cast void createNodeX_children(NodeX *nodex) { // create childY nodex->childY = make_unique<NodeY>(); // . Can Reinterpret_cast return null? I don't see any rule in the standard allowing this conversion. unsigned char* x = reinterpret_cast<unsigned char*> (p); > double *z= reinterpret_cast<double*> (&x); Again - here you takes an address of you local variable x, not the value it contains. Cast pointer to member function to normal pointer. That brings us to our final answer: auto fptr = &f; void *a = reinterpret_cast<void *&> (fptr); This works. 1> Types pointed to are unrelated; conversion requires reinterpret_cast, C-style cast or function-style cast 1>AssemblyInfo.cpp 1>Generating Code. One last thing about C pointers: confused about what this snippet of code will do? which a "char *" and an "int *" are different sizes. Reinterpret Cast Static Cast: This is the simplest type of cast that can be used. Why do function pointer definitions work with any number of ampersands '&' or asterisks '*'? How to make voltage plus/minus signs bolder? reinterpret_cast<Thing*>(it); you are telling the compiler to literally reinterpret the itobject as a pointer. #include <stdio.h> reinterpret_cast is not an exception, which means that every time you specify function name as an operand, it gets implicitly converted to function pointer type. Why is there a performance warning on cast pointer to bool? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. @uneven_mark -- okay, thanks. Is there any need to make an integer literal unsigned? In this case, the program might print a, but in many cases it will print something.. Why can I access a derived private member function via a base class pointer to a derived object? Here's 3 that aren't the same size in VC++ 2010: While it is true that all pointers-to-members are of oddball sizes, you can't really store a pointer-to-member-function in a void*. Does balls to the wall mean full speed ahead or full speed ahead and nosedive? Did neanderthals need vitamin C from the diet? Since reinterpret_cast between pointers to object types (in your case, between pointers to pointers (to [member] functions)) is always allowed, it's also allowed in your case. Check your email for updates. A value of integral or enumeration type to a pointer. This necessitates that all pointers-to-object have the same size. Comparison is always false due to limited range with templates. Perhaps casting the a pointer of the function object to the needed type will work, e.g. Where to implement tolerance to decide when to detect tochscreen press events and move events in Qt? reinterpret_cast < new-type > ( expression ) Returns a value of type new-type . However GCC, Clang and MSVC seem to agree that casting a member function pointer to a reference to function pointer is ok. See the following example: This program compiles on all three compilers, but fails to compile on all (with pedantic flags) when the out-commented line is used instead of the previous one. Should I use std::function or a function pointer in C++? The reinterpret_cast operator produces a value of a new type that has the same bit pattern as its argument. MSVC violates this last requirement unless certain compiler options are chosen. However, if you disable the warnings or errors in any way, for example by using. But the Posix round-trip would presumably work also on such architecture, unless the data address space was much smaller than the instruction address space. If the latter, what is the exact conversion sequence, where does the standard allow it and can the function pointer be converted back to the original type in order to call it or is the use of the reference for initialization of y already undefined behavior? What does"one-past-the-last-element" mean in vectors? For example a simple qsort () function can be used to sort arrays in ascending order or descending or by any other order in case of array of structures. Here: All rights reserved. "Request for member which is of non-class type", assignment statement incorrect? Red herring. There is the code that I've written for experiments with reinterpret_cast. The limitations are that object pointers can be cast only to object pointers with similar or stricter . I don't see any rule in the standard allowing this conversion. This forum has migrated to Microsoft Q&A. I don't see any rule in the standard allowing this conversion. the reinterpret_cast line doesn't compile, the compiler says ISO C++ doesn allow a cast from pointer-to-object to pointer-to-function. rev2022.12.9.43105. It is used to convert one pointer of another pointer of any type, no matter either the class is related to each other or not. All rights reserved. class B : private A
Calling base class definition of virtual member function with function pointer. It is most often used to cast a pointer to a different pointer type. Asking for help, clarification, or responding to other answers. So about the worst that can happen when you call via the casted pointer, is that contrary to the compiler's expectations a register has changed value. The content you requested has been removed. In other words, reinterpret_cast(x) performs type punning on the pointer itself. Pass a vector of member function pointer to a function, Move constructor called twice when move-constructing a std::function from a lambda that has by-value captures, C++ std::function is null for all instances of class exept first (only Visual2019 compiler problem). If he had met some scary fish, he would immediately return to the surface, Irreducible representations of a product of two groups. To learn more, see our tips on writing great answers. static_cast (though ignoring access restrictions) static_cast (see above), then const_cast. I have a bunch of function objects, that I need to cast to a coomon type: The case if specializing by size_t does not compile. public:
HolyBlackCat65903 Source: stackoverflow.com In the worst case, the function succeeds and you have a mismatched pointer. How do I get the argument types of a function pointer in a variadic template class? C-style pointer casting detected. If you put the reinterpret_cast in a place that will always get executed, then when the code is actually executed in the compiler then it will throw errors even with VC. Accessing the result of this cast violates strict aliasing and causes undefined behavior, as usual. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. There do exist processors on. A dynamic_cast performs casts on polymorphic types and can cast a A* pointer into a B* pointer only if the object being pointed to actually is a B object. How is that not clear? Not the answer you're looking for? Is it cheating if the proctor gives a student the answer key by mistake and the student doesn't report it? -- with that you can store any type of function objects you like. reinterpret_cast<T&> (x) is equivalent to *reinterpret_cast<T*> (&x). Except that converting a prvalue of type pointer to T1 to the type pointer to T2 (where T1 and T2 are function types) and back to its original type yields the original pointer value, the result of such a pointer conversion is unspecified. When the migration is complete, you will access your Teams at stackoverflowteams.com, and they will no longer appear in the left sidebar on stackoverflow.com.. You can explicitly perform the following conversions: A pointer to any integral type large enough to hold it A value of integral or enumeration type to a pointer A pointer to a function can be explicitly converted to a pointer to a function of a different type. Passing capturing lambda as function pointer, "unpacking" a tuple to call a matching function pointer. It's clear. const_cast const C++ cast const ( reinterpret_cast ) stackoverflow const_cast const const const The only difference is, that we added a virtual function in struct C.This leads to this object containing another pointer.It points to a vtable. I guess I have two questions:- First, there are a number of compilers available for Windows and Linux. Why should I use a pointer rather than the object itself? Calling C++ member functions via a function pointer, C++ lambda with captures as a function pointer, Resolving ambiguous overload on function pointer and std::function for a lambda using + (unary plus), How to make a function return a pointer to a function? Asking for help, clarification, or responding to other answers. Derived::Foo) rather than what you wrote which is ill-formed. I can't even use reinterpret_cast to convert C++ member function pointer to C function pointer, but I have . Pointers to functions and pointers to member functions are not subject to const_cast const_cast makes it possible to form a reference or pointer to non-const type that is actually referring to a const object or a reference or pointer to non-volatile type that is actually referring to a volatile object. Passing a pointer to a member function as a template argument. You can explicitly perform the following conversions: A pointer to any integral type large enough to hold it. For conversions between object pointer types and function pointer types, the C++11 standard says (5.2.10 [expr . 1>Build log was saved at "file://i:\Documents\Client\Debug\BuildLog.htm" 1>Client - 1 error (s), 0 warning (s) ========== Rebuild All: 0 succeeded, 1 failed, 0 skipped ========== Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Is it correct to say "The glue on the back of the sticker is dying down so I can not stick the sticker to the wall"? In practice you're casting away a function result of type int, that would be returned in a register.So about the worst that can happen when you call via the casted pointer, is that contrary to the compiler's expectations a register has changed value. which a "char *" and an "int *" are different sizes. Is void *function() a pointer to function or a function returning a void*? [PATCH] D40720: No -fsanitize=function warning when calling noexcept function through non-noexcept pointer in C++17. A C-style cast is defined as the first of the following which succeeds: const_cast. Sander's answer. rev2022.12.9.43105. Solution 3. It can be found in section 6.7.5.3, paragraph 15: For two function types to be compatible, both shall specify compatible return types 127. : A( []()->void* { return reinterpret_cast(f()); } ). {
why can't I cast a pointer to Derived class member function to the same but of class Base? Another practical consideration: C++ does not support casting between function and data pointers, but Posix effectively requires cast to void* and back to work OK. This is one of the most complex and dangerous casting operators available in C++. The " reinterpret_cast " operator can convert any type of variable to fundamentally different type. How to judge whether a project is a c or c++ project? However, function type, @St.Antario From 4.3/1: "An lvalue of function type T can be converted to a prvalue of type pointer to T. The result is a pointer to the function.". Is the program ill-formed and the compilers fail to diagnose it or is the program well-formed? Note that if that make sense or not will depend on the target more than the compiler: a portable compiler like gcc will have a behavior imposed by the target architecture and possibly ABI. Sorry for being a bit harsh, but for the love of silicon, write C or write good, modern C++. Accessing the result of this cast violates strict aliasing and causes undefined behavior, as usual. Visit Microsoft Q&A to post new questions. (int*)p is a C-style cast, which is in this case resolved to reinterpret_cast<int*>(const_cast<char*>(p)). So in other words, you can cast a function pointer to a different function pointer type, cast it back again, and call it, and things will work. Ready to optimize your JavaScript with Rust? Const Cast 4. I.e. What does actually "conversion between pointers" mean?"" If I have something like type_1* x, type_2* y and call dynamic_cast<type_1*>(y) (the question is not about dynamic cast, I used it because it is a function that works well with pointers casting as far as I know) what will it return?type_1* x, type_2* ydynamic . If so please share the solution with us and mark the right answers. My example did show ways you could store a bunch of std::function objects of different type in a single collection. Help us identify new roles for community members, Proposing a Community-Specific Closure Reason for non-English content, Calling derived class through base class function pointer. You can explicitly perform the following conversions: A pointer to any integral type large enough to hold it. Casting between void * and a pointer to member function, Cast pointer to member function to normal pointer. std::tr1::function -> std::tr1::function. This seems like one of them. Accessing the result of this cast violates strict aliasing and causes undefined behavior, as usual. . The effect of calling a function through a pointer to a function type that is not the same as the type used in the definition of the function is undefined. It can therefore be used as a replacement for other casts in some instances, but can be extremely dangerous because . What is a smart pointer and when should I use one? p . The reinterpret_cast operator produces a value of a new type that has the same bit pattern as its argument. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. 1. From [expr.reinterpret.cast].6 (emphasis mine):. C++ may have a sharp edge, or two. The reinterpret_cast operator performs potentially unsafe type casts. Why should I use a pointer rather than the object itself? When I read that quote earlier, for some reason I thought that the member function pointer wasn't an object, but of course it is. reinterpret_cast(x) is equivalent to *reinterpret_cast(&x). Pick one. How to check if a string has a valid file path or directory path format in unmanaged C++ code? In practice you're casting away a function result of type int, that would be returned in a register. In C++, is it safe/portable to use static member function pointer for C API callbacks? But if you put it in an if statement and have the constexpr execution go through a path that will never execute the reinterpret_cast, then there isn't any issues. Can I legally cast a member function pointer to a function pointer? What is a smart pointer and when should I use one? The reinterpret_cast operator (C++ only) The const_cast operator (C++ only) The dynamic_cast operator (C++ only) . I cast a, @St.Antario: That's true.. No other conversion could be performed explicitly using reinterpret_cast. What do you mean "not all pointers are the same size"? Do not dereference it or you will enter the myth-enshrouded lands of undefined behavior. reinterpret_cast is a type of casting operator used in C++. Use unsafe code to reinterpret pointers. boost::any
ReInterpret Cast ( reinterpret_cast) is a cast operator that converts a pointer of some data type into a pointer of another data type, even if the the data types before and after conversion are different. Copyright 2022 www.appsloveworld.com. (That vtable itself is globally accessible at runtime and contains pointers to all virtual functions which that class contains. Can Windows run on one of those processors? constexpr and initialization of a static const void pointer with reinterpret cast, which compiler is right? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Course Hero uses AI to attempt to automatically extract content from documents to surface to you and others so you can study better, e.g., in search results, to enrich docs, and more. Passing a pointer to a class member function as a parameter, Cannot use pointer to public member function that comes from a private base, Explicit void pointer as function parameter, Using condition variable in a producer-consumer situation. I feel stupid for asking, but I suppose I should not delete an upvoted question. Function multiple definition linker error building on GCC - Makefile issue. I don't see any rule in the standard allowing this conversion. In practice you're casting away a function result of type int, that would be returned in a register. This C function uses the void pointer to cast into a C++ instance pointer, from which a member function call can be made. You must form the pointer to member with the qulaified name (i.e. Give me an. Your syntax for specialization is a bit off, try. You cannot cast away a const or volatile qualification. How to pass a constant array literal to a function that takes a pointer without using a variable C/C++? set of function types and won't require the cast). The fact that the implementations you have seen happen to have pointers of, the same size does not make that universally true. Not only this, with function pointers and void pointers, it is possible to use qsort for any data type. I thought such conversion is ill-formed. A tag already exists with the provided branch name. forward declaration of class using templates, Characters extracted by istream >> double. Concentration bounds for martingales with adaptive Gaussian steps. How long does it take to fill up the tank? Casting a pointer to a different pointer and back is usually safe and yields the original value. This cast operator can convert an integer to a pointer and so on. Formally calling via the pointer casted to different function type is Undefined Behavior (by C++11 5.2.10/6). 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. . C++ logging library that allows HTML/CSS output? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. The reinterpret_cast expression circumvents the C++ type system, so the compiler will not catch this error. : A(f), I imagine you could do something along the lines of, B(const function & f)
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