Recast void pointer. Isn't passing it as pointer to voi...
- Recast void pointer. Isn't passing it as pointer to void then recasting it extra steps? Wouldn't this be a bad practice were they doing this just to work around a compiler checker error? 0 I'm learning embedded C and while going through some files a have found a line which I am struggling to understand: void (* TxCpltCallback)(struct __SPI_HandleTypeDef *hspi); Is it some kind of recasting? That line comes from file stm32l4xx_hal_spi. A pointer to any object type may be converted to a pointer to void and back again; the result shall compare equal to the original pointer". You can cast that void pointer back to the original pointer type, or to char pointer. data(), voidPtrs. SetAcquisitionEngine( ADC_PIN, ADC_REF_VOL_DEFAULT, SAMPLE_RATE, SAMPLES, ADC_DAT_FMT_INT ); // Which requests 512 integers from the ADC My confusion is that the pointer appears to be for 8 And for future reference, void pointers need to be casted to the correct pointer type in order to be used. It does not check if the pointer type and data pointed by the pointer is same or not. How can function pointers be cast to void* in a function invocation and then recast back to their original type? I have a structure that includes a function callback, which is defined differently based on the compiler being used. h which is high level library for STM32 microcontrollers. I want to avoid to declare four different variables for different sample values. Then I also see you use union which probable doesn't behave in C++ as you expect (it is much more complex than in "C"). Recasting tells the compiler the specific type of pointer variable, allowing the compiler to generate the correct memory access code for pointer dereferences. 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 between pointers on obscure architectures where pointer representation depends on its type). void buffer [20]; void * ptr = &buffer [0]; uint16_t lg = 64000; ( (uint1… A pointer is a value that designates the address (i. size()) (with caveats about destructing the original voidPtrs vector)? char c; char a[4]; ;wibble(&a[0]); /**/ void wibble(void *a) { int b = *reinterpret_cast<int *>(a);} would compile and likely crash horribly (on an architecture that required alignment). You should not mix the two, since this may easily lead to dangerous code. Pretty much any other casts like that are undefined behavior. A void pointer (void*) is a special type of pointer in C++ that has no associated data type. Pointers are variables that hold a memory location. and want to recast it to the appropriate type eg. I have implemented the following: The new socket descriptor (new_socket) is passed to the thread as a pointer. Use of Void as a Return Type B. In other words, dynamic_cast can be used to upcast pointers, from derived to base. I need the same functionality for unique pointers. We saw a conversion from a void pointer above. The original data type is uint16. 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. Chris Lutz – Chris Lutz 2012-03-01 02:56:40 +00:00 CommentedMar 1, 2012 at 2:56 6 void (*task_func)(void *arg); The above statement defines task_func to be a pointer to a function which takes a pointer of type void * and returns no value. It does allow you to make that pointer. When you convert an open pointer that contains a valid space address, the return value is the offset that is contained in the address. You just need an explicit cast. Aug 17, 2024 · In the essence, reinterpet_cast converts between various pointer types and/or some non-pointer integral types large enough to hold a pointer (such as std::uintptr_t and alike). As far as i understood void (*) () works in a similiar way void* works, but with function pointers, meaning you can cast any function pointer to it and back. However, you should only do this if you used static cast to cast the pointer to void* in the first place. Unlike static_cast, reinterpret_cast doesn't actually convert the data types but reinterprets one pointer type as another at compile time. Syntax : data_type *var_name = reinterpret_cast <data_type *>(pointer_variable); Return Type Replaces the managed object with an object pointed to by ptr. A. 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. Chris Lutz – Chris Lutz 2012-03-01 02:56:40 +00:00 CommentedMar 1, 2012 at 2:56 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. Proper delete expression corresponding to the supplied type is always selected, this is the reason why the function is implemented as template using a separate I'm having a problem deciding if the PlainADC example will work correctly. A "classic" C-style cast (uint8_t*) is also an example of an explicit cast. Nor is it an exception if you accessed the object of type T via an rvalue. Replaces the managed object with an object pointed to by ptr. Mar 30, 2025 · 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 between pointers on obscure architectures where pointer representation depends on its type). For my use case (given a std::vector<void *> voidPtrs;) would it be fair to construct an array_view<int *> typedPtrs(voidPtrs. Isn't passing it as pointer to void then recasting it extra steps? Wouldn't this be a bad practice were they doing this just to work around a compiler checker error? Understanding Void Type in Function Declaration A. Sep 29, 2023 · Because a void pointer does not know what type of object it is pointing to, deleting a void pointer will result in undefined behavior. Therefore, the programmer must explicitly recast the void * pointer to a pointer of a specific type before dereferencing it. If it's not typed properly, they won't accept it. Is this correct? See Adam Rosenfield's answer here, from which I quote: Hence, since a void* is not compatible with a struct my_struct*, a function >pointer of type void (*)(void*) is not compatible with a function pointer of type > void (*)(struct my_struct*), so this casting of function pointers is technically >undefined behavior. LinuxQuestions. Your All-in-One Learning Portal: GeeksforGeeks is a comprehensive educational platform that empowers learners across domains-spanning computer science and programming, school education, upskilling, commerce, software tools, competitive exams, and more. Let's look at an example. The pointer still remains a void and the resulting value assignment is wrong. Note the to and from part of the above quote. org > Forums > Non-*NIX Forums > Programming [SOLVED] void pointer casting of vector within a vector C++ Programming This forum is for all programming questions. Return Values It returns 0 when buf1 is equal to buf2. The reinterpret_cast operator converts a null pointer value to the null pointer value of the destination type. buf2: Pointer to the second block of memory buffer. So casting a char pointer to a void pointer then to a double pointer is undefined, do not do that. Void Pointers in C Programming A. If you need to delete a void pointer, static_cast it back to the appropriate type first. By default, delete expression is used as deleter. It can hold the address of any data type, making it useful for generic programming. The result is implementation-defined and typically yields the numeric address of the byte in memory that the pointer pointers to. I have a module which receives a void pointer to a buffer. Otherwise you should reinterpret_cast to exactly the same type of the original pointer (no bases or such). 6, which generally require an explicit cast, pointers may be assigned to and from pointers of type void*, and may be compared with them. Jul 12, 2025 · 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. There are four fundamental things you need to know about pointers: How to declare them (with the address operator ' & ': int *pointer = &variable;) How to assign to them (pointer = NULL;) How can I pass a pointer (Object *ob) to a function which prototype is void foo (Object &) ? You can cast any pointer to a void pointer. Passing Void Type as an Argument III. The wrinkle is that dynamic_casting a pointer could fail (yield nullptr), so what do we want to happen then? I decided that in that case I would like the original pointer to remain unchanged. You need it in both C and C++. In general raw pointers are already a bad idea, and void* double so. Optional deleter d can be supplied, which is later used to destroy the new object when no shared_ptr objects own it. Is this correct? Unlike the pointer-to-pointer conversions discussed in Par. But it doesn't work. Is it possible to deduce the type using only the void pointer returned by MyContainer::Get function? Can this be done using any combination of casts, templates and typeid operator? c++ void-pointers edited Mar 1, 2012 at 3:02 asked Mar 1, 2012 at 2:54 Diggy 230212 6 Short answer: no. Is it safe to convert/cast pointers into pointers to another type? Learn about C's "Pointer Conversion Rules" to avoid undefined behavior in your code. Typically, long or unsigned long is Your All-in-One Learning Portal: GeeksforGeeks is a comprehensive educational platform that empowers learners across domains-spanning computer science and programming, school education, upskilling, commerce, software tools, competitive exams, and more. If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function. So I use "realloc" to try to recast the pointer and no change is taking place. For example, Mat imBuff = cv::Mat (628, 314 So I use "realloc" to try to recast the pointer and no change is taking place. In this tutorial, we will learn about void pointers and how to use them with the help of examples. Learn the basics right away!. a null pointer value if expression is a null pointer value, or a pointer to the unique Base subobject of the Derived object pointed to by expression otherwise. This model is fed to the sol As far as i understood void (*) () works in a similiar way void* works, but with function pointers, meaning you can cast any function pointer to it and back. 8 dynamic_pointer_cast is only implemented for std::shared_ptr. // In the declaration section uint8_t *vData = NULL; // Pointer variable to integer data array // In the setup section vData = pADC. Definition and Usage of Void Pointers B. Therefore, when you call your function rt_task_start, you should pass a pointer to a function as the second argument. When I try putting the variable into a Mat, I get weird errors from Mat imBuff = cv::Mat (628, 314, frame, CV_16U); like missing '}' which isn't missing. I am trying to build a generic stochastic differential equation solver coded by a class de_solver which takes some set of differential equations given by a model class. This will only compile if the destination type is long enough. For C++ you should look at std::function or some more generic template This warning can be avoided with an explicit conversion. Is it possible to only have one variable and then recast it to the proper type ? Thanks in advance Kamran 9 You should use static_cast so that the pointer is correctly manipulated to point at the correct location. Allows any pointer to be converted into any other pointer type. Learn C++ - Conversion between pointer and integer An object pointer (including void*) or function pointer can be converted to an integer type using reinterpret_cast. Hopefully, the library is using that a "generic" pointer type (and recasting it internally) and not storing the adddress of the waveform information in the int32 pointed at as that would case a problem since pointers in your environment are 64 bits. When I try various conversions using reinterpret_cast, I get cast from 'void*' to 'xxx' loses precision. What you used - reinterpret_cast - is actually an example of an explicit cast. Typecasting Void Pointers IV. The question does not have to be directly related to Linux and any language is fair game. Your title asks about casting a void* pointer value to int, but your question is about retrieving an int value from the address to which a void* pointer points. C++ reinterpret_cast reinterpret_cast is used to convert one pointer type to another pointer type or one reference type to another reference type. , the location in memory), of some value. Unlock Void Pointer with Examples in C for developers to handle memory in a flexible manner. Also allows any integral type to be converted into any pointer type and vice versa. I checked-out the link, but it doesn't seem to apply. It doesn't show how to cast an UnsafePointer<Void> to a pointer type required by other CF functions. However C++ wont allow me to make a pointer to my data structure become a pointer to bytes. 6. Proper delete expression corresponding to the supplied type is always selected, this is the reason why the function is implemented as template using a separate Anytime I see void* in C++ my first reaction is "do you really want to do that?". What's the most efficient way to recast a float pointer as a double pointer in C? Asked 11 years ago Modified 11 years ago Viewed 1k times Syntax int memcmp (const void * buf1, const void * buf2, size_t count); Parameters buf1: Pointer to block of a memory buffer that will be compared with a second memory buffer. This should fix your error. In my case, I'm not reinterpreting a void * as an int[], but rather converting a container of void * to a container of int *. An implicit conversion and static_cast can perform this conversion as well. Or perhaps I'm missing something A pointer converted to an integer of sufficient size (if any such exists on the implementation) and back to the same pointer type will have its original value; mappings between pointers and integers are otherwise implementation-defined. I'm working on a project where I need to convert variadic arguments into a void** array, pass them to a lambda function, and then correctly cast them back to their original types for use in a custom When you convert a valid function (procedure) pointer, system pointer, invocation pointer, label pointer, or suspend pointer to a signed or unsigned integer type, the result is always zero. So, you're declaration of int* ptr = (int*)(void*)&arr; would produce the same results as int* ptr = (int*)&arr; because of the types you are using and converting to/from. how can I cast void pointer to a 2d array (array of pointers to arrays of ints), when I dont know array size at compile time? Is it somehow possible? (I am doing this because, I pass an 2d array of “An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. Oct 26, 2017 · Accessing a double * via a void * expression is not such an exception; let alone a vector of each. std::unique_ptr is a smart pointer that owns (is responsible for) and manages another object via a pointer and subsequently disposes of that object when the unique_ptr goes out of scope. ” Syntax of Null Pointer Declaration in C Pointers are a fundamental concept in C++ programming that allow you to directly manipulate memory by storing the memory addresses of variables and data structures. count: Maximum numbers of bytes to compare. e. Hello, Is it possible to increment a void pointer by uint8_t, then re-cast the pointer to uint16_t variable? Something similar to the following. For non-fundamental types, arrays and functions implicitly convert to pointers, and pointers in general allow the following conversions: Null pointers can be converted to pointers of any type Pointers to any type can be converted to void pointers. This socket is used by the thread to read data from the client and send a response back to the client. C++ allows templates and has new, which couples both memory allocation (void* operator new (size_t)) and object construction (object construction). The void pointer is a generic pointer that is used when we don't know the data type of the variable that the pointer points to. (short *). The compiler complains about a void pointer can not point to another type. dzrek, eei9d, gsyn, cwtmi, q9fja, wp1ro, rb05s1, q8uaje, enqj, g9nziq,