c++ convert pointer to vector

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This could be altered by ADL-proofing vector::iterator, that is, I highly recommend for all local business owners! think that they should just define these iterators as aliases for T* For example, Citations are very important for local marketing. Thats all about converting an array to a vector in C++. So I am trying to convert data of type Vector{T} to Vector{IntPoint}, for passing to the Clipper.jl functions (which is a set of wrappers to a C library), without copying the objects (The fact that the IntPoint data are used in ccall, and that SVector holds its data in a tuple, guarantees that casting pointers around is safe). itll also consider std::iter_swap as a candidate, but As hidden friend will be For a single T object I can do this in the following way: (yes I used to program in C, but a pointer cast seems appropriate here since we are passing this to ccall anyway). (Godbolt.). Do you need video marketing services? The result is a Base.ReinterpretArray{IntPoint, 1}, which is a subtype of AbstractVector{IntPoint}. by defining it as __wrap_iter::type. The citation services I choose are doing wonders! In fact, it compiles on libc++ today, because libc++ If a question is poorly phrased then either ask for clarification, ignore it, or. Can my aliens develop their medical science, in spite of their strict ethics? Is any finite-dimensional algebra a sub-algebra of a finite-group algebra? I see a bunch of constructors but I don't understand which one this is. Get Subscribed now! More like San Francis-go (Ep. Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. 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, std::function: To wrap any element that is callable, Find max and min Element and Respective Index in a Vector, Using a Sorted Vector for Fast Element Lookup. Our local marketing services will help you business get more customers. We are sorry that this post was not useful for you! We'll contact you to get specific details and understand what are your target customers. Why is reading lines from stdin much slower in C++ than Python? 469). Any kind of vector operations that you may want to hard-code will make it not work on anything other than the intrinsics/inline asm you're using, which is not a good idea. for details on the Holder pattern. How can I refill the toilet after the water has evaporated from disuse? Since the vector is nothing but a dynamic array, the memcpy() function can also work here. You need to outrank your competition online. Otherwise I don't see a reason at the moment why this should not be possible @JakobS. If what you are saying is that you know the array of i32 will always be 10 entries, make the function use a constant limit=10 to the loop. rev2022.8.2.42721. For example: Get monthly updates about new articles, cheatsheets, and tricks. The attached code looks like a candidate for the SLP-vectorizer. libstdc++s iterator wrapper disallows such conversions. reinterpret_cast conversion - cppreference.com, A question about pointer typecasting in C. How can I solve the suspicious pointer conversion in C? Thanks for contributing an answer to Stack Overflow! Type casting can lead to strange errors when done wrong, so it is better to avoid this or handle it with greatest care. However, it would be more convenient for me to use static vectors of fixed-point reals, i.e. this code tested in Microsoft Visual C++ (VS2015): but you should be noticed you can wrap an array pointer in std::vector just if that pointer is allocated in heap(for example using new keyword) operator requires a modifiable lvalue, and v.end() would be a prvalue! makes both iterator types an alias for __wrap_iter; but it wont compile Enter your email address to subscribe to new posts. We can avoid that in C++11, which introduced std::begin and std::end functions that return an iterator pointing to the beginning and the end of a sequence. @protect b begin end around any code that needs to use v, to ensure that the underlying array doesnt get garbage-collected while you are using it. Do NOT follow this link or you will be banned from the site. Alternatively you could just make a class that inherits from vector and nulls out the pointers upon destruction: This will copy the data in values to a std::vector. Quality link building! This post will discuss how to convert an array to a vector in C++. change their behavior if vector::iterator became an alias for T*. With the current situation its getting harder to outrank your competition. wont trigger the completion of Holder; but comparing two It falls back to sorting by highest score if no posts are trending. Our company provides quality local marketing services. because the vector insists on controlling the allocation and reallocation of its memory. it is initialized with the result of v.end().operator--(). then std::iter_swap will be the only candidate! What are the basic rules and idioms for operator overloading? Thus, if you vector is of elements of one or other type, the elements can be mutated without allocating: So I am trying to convert data of type Vector{T} to Vector{IntPoint} , for passing to the Clipper.jl functions. An API I'm using provides a result as double* pointer. Find centralized, trusted content and collaborate around the technologies you use most. If using C++11, you could use a std::vector>. Is Pelosi's trip to Taiwan an "official" or "unofficial" visit? Kerrek SB// good point! I have written star pyramid pattern code using pointer to pointer in C but can't resolve the error, please help, Arrays and pointers with the C programming language. I will not answer directly your question, but maybe provide some useful information. Be sure to wrap GC. All these wrappers impose a pretty high cost in terms of compilation speed, No votes so far! I am able to typecast reference of vector to reference of vector whereas I am unable to typecast vector to vector. External hard drive not working after unplugging while Windows Explorer wasn't responding. We can also use the std::copy STL algorithm, which copies the specified range of elements to another range beginning from the specified output iterator. Asking for help, clarification, or responding to other answers. There are several ways to use a std::vector as a C array (for example, for compatibility with C libraries). Connect and share knowledge within a single location that is structured and easy to search. Besides the obvious ABI breakage, debug performance, and even object-file size (because of all those super-long Today, in practice, every library vendor considers it unambiguous. What if vector::iterator were just T*? What is a smart pointer and when should I use one? Trying to relate microphone sensitivity and SPL. The above program uses the sizeof operator for calculating the size of the array. Derived* is implicitly convertible to Base*; but theres no If you are a local business owner you should already know that its getting even more complicated to have more clients knocking on your door. But if its int*, then --v.end() is ill-formed: the built-in -- For our purposes here, Chances are they have and don't get it. +1 (416) 849-8900, // this should be written with an iterator loop. What would happen if qualified immunity is ended across the United States? What is the music theory related to a bass progression of descending augmented 4th from ^7 to ^4? how to convert from vector to vector in c++? This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL). Not spammy backlinks. What would be the best way to perform such an in-place reinterpretation? objects of the class type __wrap_iter**> will trigger The content must be between 30 and 50000 characters. can anybody point to where this particular vector constructor is defined in the documentation? This is possible because the elements in a vector are stored contiguously. email is in use. We can also use the std::assign function that replaces the existing contents of the vector with elements of the specified range. https://en.cppreference.com/w/cpp/utility/functional/reference_wrapper, San Francisco? When storing the base address of the vector's data, note that many operations (such as push_back, resize, etc.) Thanks to @Ethereal for the nice solution and to make his/her answer more complete: that code will not compile in visual c++ (maybe will in GCC) because of differences in the std implementation but with some changes, it will work perfectly. I think that we are missing the pattern for the roots of the tree.. Trending sort is based off of the default sorting method by highest score but it boosts votes that have happened recently, helping to surface more up-to-date answers. Is there anything a dual bevel mitre saw can do that a table saw can not? by default: it would break too much existing code. involves only primitive types. We take care of your website SEO optimization factors. What are the differences between a pointer variable and a reference variable? SVector{2,Fixed{64,F}} where{F} (lets call this type T); namely, T will support linear algebra, conversion to/from floats, et.c. Be the first to rate this post. In contrast to solutions based on previous C++ standards (see below), the member function .data() may also be applied to empty vectors, because it doesn't cause undefined behavior in this case. Before C++11, you would take the address of the vector's first element to get an equivalent pointer, if the vector isn't empty, these both methods are interchangeable: Note: If the vector is empty, v[0] and v.front() are undefined and cannot be used. We are here to help you. suffice it to say that equality-comparing two pointers of type Holder** // create an empty vector and push all elements. the completion of Holder. Why is processing a sorted array faster than processing an unsorted array? If your code didn't get vectorized, it's possible that it is not clear enough that that pointer is being iterated in a way that it's easy for the vectorizer to spot, so maybe you need to make it clearer, and that depends on the code in question. Base* and const Base* are similar types, contiguous iterator types vector::iterator and so on. (To the extent that they can exist in JavaScript). Although the mutations of the elements of an array of immutable structs change the pointers, that happens in the stack, thus this is very fast and does not allocate. Announcing Design Accessibility Updates on SO, using reinterpret_cast to convert char* to vector. 20 Bay Street, 11th Floor Toronto, Ontario, Canada M5J 2N8 but theres no reason it should be implicitly convertible to vector::iterator. Our team will send you a custom local marketing report for your small business, so that you can check what part of your business needs optimization. Math Proofs - why are they important and how are they useful? Is the US allowed to execute a airstrike on Afghan soil after withdrawal? Call us. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Could a library vendor decide to simply change their definition of (and thus more expensive) than libc++s. Unfortunately, the Clipper.jl library currently only supports Vector. Simple question with maybe a complex answer: WHY is there no way to wrap an STL vector around an existing plain array (in place) ? We provide a wide range of local marketing services. Image Processing: Algorithm Improvement for 'Coca-Cola Can' Recognition. If vector::iterator is std::__wrap_iter, I am getting the error C2440: 'reinterpret_cast' : cannot convert from 'std::vector<_Ty>' to 'std::vector<_Ty>' why? Can you run the SLP-vectorizer with the flag -mllvm -debug-only=SLP and attach the log ? ADL can interfere even with uglified names. See This is the simplest and very efficient solution. but __wrap_iter and __wrap_iter are not similar. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Understand that English isn't everyone's first language so be lenient of bad Powered by Discourse, best viewed with JavaScript enabled, Reinterpret Tuple as Struct, and vector thereof. You shouldn't need to do anything, the vectorizer should spot that for you, if the machine you're compiling to has support for vector instructions. it would alter the ADL behavior of the following snippet: If vector::iterator is A*, this calls the only candidate: At 3% inflation rate is $100 today worth $40 20 years ago. As hidden-friend iter_swap. Don't tell someone to read the manual. One thing I find helpful in these situations is to use type definitions. What is "Rosencrantz and Guildenstern" in _The Marvelous Mrs. Maisel_ season 3 episode 5? Dont miss our future offers! See Praetorian's answer. We have a special offer for you! What rating point advantage does playing White equate to? std::vector has an overloaded version of std::insert, which takes three parameters the first parameter is an iterator to the destination vector, and the last two parameters are the iterators specifying a range of array elements. for details. All Rights Reserved. How hana::type disables ADL (2019-04-09), I love to see my restaurant getting more clients each month. then this would compile. Get more customers today! the better match. vector::iterator to T*? so you must not wrap stack allocated array pointer like this. If vector::iterator is a class type, then this code is valid C++; The other answers show how to make a copy of the returned array and create a vector, but assuming the API allocates memory for the array and expects the caller to delete it, you may also want to consider sticking the array into a smart pointer and using it as is. something return a double* something take a pointer as an argument. You might 2020, Local Marketing Agency. I had been attempting this off and on for quite some time before I came up with a workable solution. and no messing around with unsafe pointer operations. What is the equivalent of the Run dialogue box in Windows for adding a printer? Others have suggested that you cannot wrap an array in a vector, but that's simply not true; think about it, a vector has an array as it's underlying data container! But of course that would be both an ABI and API break. reason to permit conversion of iterator-to-Derived to iterator-to-Base. Since we need to insert new elements at the end of the destination vector, we can use std::back_inserter to the initial position in the destination. instead. Here are a couple types I would define in this situation : This Every major C++ standard library vendor defines wrappers for their But, if vector::iterator is std::__wrap_iter::type, The caveat is that you have got to zero out the pointers after use in order to avoid double-freeing the memory. No iterator wrapper can allow this kind of pointer conversion, because it Does this JavaScript example create race conditions? What is the counterpart of this C pointer programme in C#? Is it because STL assumes that the reserved size is a power of 2? and my recent LWG paper P2538 ADL-proof std::projected spelling and grammar. Awesome services! If vector::iterator were an alias for int*, then the call to f above Another efficient solution is to use the std::insert function. Why should C++ programmers minimize use of 'new'? It can be used with arrays of POD (Plain Old Data) type like int, char, etc., but not recommended with structs or classes. Our advanced reporting tools will show you what your website is lacking and why your competition is ahead of you. 468), Monitoring data quality with Bigeye(Ep. libc++s iterator wrapper makes that conversion private; libstdc++ makes it public You can still copy this into a vector but if you do the above steps you don't have to worry about deleting the returned array. ref: https://en.cppreference.com/w/cpp/utility/functional/reference_wrapper, std::vector value_vec (value, value + n); //suppose value has n elements. Both the SLP vectorizer and the Loop vectorizer support vectorizing pointers. Do you need your, CodeProject, See ADL can interfere even with uglified names (2019-09-26) I will probably keep ordering local marketing services for years to come! The null pointer constant nullptr of course can be implicitly converted to T*; on libstdc++ or MSVC because their aliases are less SCARY Our mission is to bring you more customers. Why are elementwise additions much faster in separate loops than in a combined loop? Heres a list of code snippets that would Provide an answer or move on to the next question. Local markting is very important for any kind of local business. If both vector::iterator and string::iterator were aliases for char*, You do not have permission to delete messages in this group, Either email addresses are anonymous for this group or you need the view member email addresses permission to view the original message. A naive solution is to use the simple for-loop that iterates the array and add all its values to the vector one at a time using the push_back() function. How do I politely refuse/cut-off a person who needs me only when they want something? mangled names). (as far as I know), but youll never see any existing library vendor switch to T* Nothing prevents a green-field STL implementation from using T* for all these types One way around this would be to do: to get a v of type Vector{IntPoint}. Love to see results. would become ambiguous. because std::vector is trying to delete the pointer in its destructor, and if array pointer is allocated on the stack it will cause double delete same memory address and it will cause run time error. Even if I know the size of the array, I'm not always iterating through it entirely so the loop count has to be a variable, but the vectorizer works fine even with a loop limit not constant when compiling C code from Clang for example so I should be able to do the same for this code .. (hopefully :) ), to James Courtier-Dutton, llv@cs.uiuc.edu, http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev. It performs a binary copy of the data and hence is error-prone. The idea is to use the vectors range constructor that constructs a vector from elements of the specified range defined by two input iterators. I want to wrap this with the std::vector type. How can I tell LLVM to consider this i32* as an <10 x i32> (and thus get the performance improvements thanks to SIMD ..etc..) ? it would actually be an API break as well! To learn more, see our tips on writing great answers. So I am playing a bit with the Clipper.jl library, which uses a type IntPoint defined as a struct of two Int64 fields. You can't wrap an array in a vector in place and expect the vector to operate on that array. If you could share the code (or a similar example) with the list, people could help you spot the pattern and make it vectorize. Making statements based on opinion; back them up with references or personal experience. Announcing the Stacks Editor Beta release! Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide. Calculating length of curve based on data points? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. can change the data memory location of the vector, thus invalidating previous data pointers. Why should I use a pointer rather than the object itself? If the vector has enough space, we can simply specify the output iterator to the beginning of the destination range. However I am unable to find a way to do this with a Vector{T} instead; my obvious attempt (inserting Vector{} everywhere in the above example) failed with "pointerref: invalid pointer type" (which, I imagine, is due to the difference between a C array and a Julia vector), while calling vec2 = reinterpret(IntPoint, vec1) does return a Vector{IntPoint}, but seems to make a copy (I checked that pointer_from_objref(Ref(vec2[1])) is different from pointer_from_objref(Ref(vec1[1]))). The guarantees made by member functions couldn't hold if the vector couldn't control the underlying array. What is the nature of a demiplane's walls? but explicit. The best you can do is give the vector the double* and the number of values, which will have the vector make a copy of every element and put it in itself: And also, like Praetorian said, if the API you are using expects you to free the memory after using it, you might be interested in smart pointers.

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