vector of objects vs vector of pointers

Can it contain duplicates? There are probably some smart pointers or references in boost or other libraries that can be used and make the code much safer than the second proposed solution. There are many convenience functions to refer to the elements of the span. If your objects are in CPU cache, then it can be two orders of magnitude faster than when they need to be fetched from the main memory. It shows how much more expensive it is to sort a vector of large objects that are stored by value, than it is when they're stored by pointer [3]. "Does the call to delete affect the pointer in the vector?". 0}. Safety and Robustness are also more important. Storing copies of objects themselves in a std::vector is inefficient and probably requires a copy assignment operator. Not consenting or withdrawing consent, may adversely affect certain features and functions. As you may expect, the from a std::vector created mySpan1 (1) and the from a pointer and a size created mySpan (2) are equal (3). 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Dynamic dispatch (virtual method calls) work only on pointers and references (and you can't store references in a std::vector). particles example I just wanted to test with 1k particles, 2k. Thank you for your understanding. C++ Core Guidelines: Better Specific or Generic? It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions. We can use the vector of pointers to manage values that are not stored in continuous memory. Heres a great summary that explains the problem: The picture comes from the book: Systems Performance: Enterprise and the Cloud. Heres the corresponding graph (this time I am using mean value of of Load data for the second particle. What i was missing was the std::move() function and I wasnt able to find it for months now. In this blog post, youll see why there might be a perf difference of almost 2.5x (in both directions!) How do you know? You haven't provided nearly enough information. * Kurtosis Before we can update any fields of the first particle, it has to be fetched from the main memory into cache/registers. It Thank you! Note about C++11: In C++11 shared_ptr became part of the standard as std::shared_ptr, so Boost is no longer required for this approach. As pointed out in Maciej Hs answer, your first approach results in object slicing. If the objects are in dynamic memory, the memory must be initialized first (allocated). 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How to erase & delete pointers to objects stored in a vector? A Computer Science portal for geeks. If you have objects that take a lot of space, you can save some of this space by using COW pointers. Containers of pointers let you avoid the slicing problem. For the rest it is a balance between "simple and maintainable" vs. "the least CPU cycles ever". WebYou should use a vector of objects whenever possible; but in your case it isn't possible. Load data for the first particle. Dynamic Polymorphism and Dynamic Memory Allocation. How to use boost lambda to populate a vector of pointers with new objects, C++ vector of objects vs. vector of pointers to objects. First, let's create a synthetic "large" object that has well defined ordering properties by some numeric ID: struct SomeLargeData { SomeLargeData ( int id_) : id (id_) {} int id; int arr [ 100 ]; }; Calling a destructor on a pointer value does nothing. However its also good to remember that when the object inside a container is heavy it might be better to leave them in the same place, but use some kind of indexing when you sort or perform other algorithms that move elements around. On the diagram above, you can see that all elements of the vector are next to each other in the memory block. 1. In your example, the vector is created when the object is created, and it is destroyed when the object is destroyed. This is exactly the behavior y With the Celero https://www.youtube.com/watch?v=YQs6IC-vgmo, https://www.youtube.com/watch?v=WDIkqP4JbkE, Performance of container of objects vs performance of container of pointers. Consenting to these technologies will allow us to process data such as browsing behavior or unique IDs on this site. As you may expect, the from a std::vector created mySpan1 (1) and the from a pointer and a size created mySpan (2) are equal (3). This can affect the performance and be totally different than a regular use case when objects are allocated in random order at a random time and then added to a container. With pointers to a base class and also with virtual methods you can achieve runtime polymorphism, but thats a story for some other experiment. This is a bad design at any rate, because the vector can internally make copies of the stored objects, so pointers to those objects will be invalidated on a regular basis. we can not copy them, only move them. Pointers. In C++, should different game entities have different classes? Flexible particle system - OpenGL Renderer, Flexible particle system - The Container 2. This can simulate, for example, references in C#. Accessing the objects takes a performance hit. This will "slice" d, and the vector will only contain the 'Base' parts of the object. Additionally, the hardware Prefetcher cannot figure out the pattern - it is random - so there will be a lot of cache misses and stalls. This effect can be achieved in few ways: use the std::pair of bool and Object, add the bool member to Object structure or handle with pointers to Object, where nullptr will stand for not existing value. We can also ask another question: are pointers in a container always a bad thing? This email address is being protected from spambots. Also, you probably don't need a pointer to a vector in the first place, but I won't judge you since I don't know your situation. With this post I wanted to confirm that having a good benchmarking To provide the best experiences, we use technologies like cookies to store and/or access device information. Vector of pointers In In Re Man. It affects the behavior invoked by using this pointer since the object it points to no longer exists. 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Back in main the data type receives this vector pointer by a necessary data type. Assuming an array of 'bool', can 'a[n] == (-1)' ever be true? C++: Vector of objects vs. vector of pointers to new objects? And as usual with those kinds of experiments: pleas measure, measure and measure - according to your needs and requirements. 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For example, if the difference between the worst performing data structure and the best is 10 nanoseconds, that means that you will need to perform at least 1E+6 times in order for the savings to be significant. The table presents the functions to refer to the elements of a span. We can also push std::thread without specifically specifying std::move(), if we pass them as rvalue i.e. Using vectors of pointers #include #include using namespace std; static const int NUM_OBJECTS = 10; In Nonius we can use a bit more advanced approach Copying pointers is much faster than a copy of a large object. * Baseline us/Iteration << Notes on C++ SFINAE, Modern C++ and C++20 Concepts, Revisiting An Old Benchmark - Vector of objects or pointers. The difference to the first approach is, that here your objects get destroyed when the vector gets destroyed, whereas above they may live longer than the container, if other shared_ptrs referencing them exist. Our particle has the size of 72bytes, so we need two cache line loads (cache line is usually 64 byte): first will load 64 bytes, then another 64 bytes. If it is something complex, or very time-consuming to construct and destruct, you might prefer to do that work only once each and pass pointers into the vector. Which pdf bundle should I provide? These are all my posts to then ranges library: category ranges library. Why do we need Guidelines for Modern C++? This can be used to operate over to create an array containing multiple pointers. the variance is also only a little disturbed. You can also have a look and join discussions in those places: I've prepared a valuable bonus if you're interested in Modern C++! For the unique_ptr and shared_ptr examples, is it still covariant, because they all return the "How is the appropriate overloaded output operator for std::string found?" Return a const vector of const shared pointers to const objects, A vector of pointers to objects that may or may not exist. But, since recently Im * Samples The sharing is implemented using some garbage So, why it is so important to care about iterating over continuous block of memory? 2023 ITCodar.com. My understanding of the dangers of vectors is opposite to this, if you have a vector of pointers, vector as you resize (reduce in size) the vector the You have to manually iterate the vector and delete the pointers yourself when you know they're dynamically allocated, or better, use std::unique_ptr and you never need to call delete on anything. If you know that copying is a blocker for the elements in the container, then it might be good to even replace the sorting algorithm into selection sort - which has a worse complexity than quicksort, but it has the lowest number of writes. WebA vector of pointers is useful in cases of polymorphic objects, but there are alternatives you should consider: If the vector owns the objects (that means their lifetime is bounded by that of the vector), you could use a boost::ptr_vector. Which pdf bundle should I provide? If a second is significant, expect to access the data structures more times (1E+9). Assignment of read-only location while using set_union to merge two sets, Can't create recursive type `using T = vector`. A subreddit for all questions related to programming in any language. In C++ we can declare vector pointers using 3 methods: Using vectors to create vector pointers is the easiest and most effective method as it provides extra functionality of STL. A std::span stands for an object that can refer to a contiguous sequence of objects. Yes, it is possible - benchmark it. Now lets create a std::function<> object that we will pass to thread object as thread function i.e. * Standard Deviation I think it would be interesting the discussion and I would like , Jake, GS, Lawton Shoemake, Animus24, Jozo Leko, John Breland. Particles vector of pointers but not randomized: mean is 90ms and Check out this lecture about linked lists by Bjarne Stroustrup: WebThe difference to the first approach is, that here your objects get destroyed when the vector gets destroyed, whereas above they may live longer than the container, if other Vector * Max (us) Thanks in particular to Jon Hess, Lakshman, Christian Wittenhorst, Sherhy Pyton, Dendi Suhubdy, Sudhakar Belagurusamy, Richard Sargeant, Rusty Fleming, Ralf Abramowitsch, John Nebel, Mipko, and Alicja Kaminska. The safest version is to have copies in the vector, but has performance hits depending on the size of the object and the frequency of reallocating the reserved memory area. However, unless you really need shared ownership, it is recommended you use std::unique_ptr, which was newly introduced in C++11. You still need to do the delete yourself as, again, the vector is only managing the pointer, not the YourType. Maybe std::vector would be more reasonable way to go. Why inbuilt sort is not able to sort map of vectors? By a different container, are you talking about a list? In our If speed of insertion and removal is your concern, use a different container. Now, as std::thread objects are move only i.e. Nonius), but it can easily output csv data. I think it has something to do with push_back and the capacity of the vector and if the capacity is reached a new vector that uses new contiguous addresses that don't contain the right objects is created. * Iterations/sec boost::optional. What to do when So they not only read the data but also perform a copy (when the algorithm decides to swap items or move to a correct place according to the order). measurements/samples) and only one iteration (in Nonius there was 100 std::vector Returns pointer to the underlying array serving as element storage. Bounds-Safe Views for Sequences of Objects All data and information provided on this site is for informational purposes only. That means the pointer you are saving is not a pointer to the object inside the vector. WebYou use a vector of pointers when you need a heterogeneous container of polymorphic objects, or your objects need to persist against operations performed on the vector, for Libraries like If I gradually build up from one to a hundred strings in an array, is that enough information to tell which is better? You wont get what You want with this code. Yes and no. comparator for sorting a vector contatining pointers to objects of custom class, GDB & C++: Printing vector of pointers to objects. A vector of pointers takes performance hits because of the double dereferencing, but doesn't incur extra performance hits when copying because pointers are a consistent size. Learn how your comment data is processed. The algorithmstd::iota fills myVec with thesequentially increasing values, starting with 0. of objects vs Cirrus advanced automation frees up personnel to manage strategic initiatives and provides the ability to work from anywhere, on any device, with the highest level of security available. Vector Nonius performs some statistic analysis on the gathered data. Most of the time its better to have objects in a single memory block. Binary search with returned index in STL? Class members that are objects - Pointers or not? Will it need to have elements added and removed frequently? Overloading, variadic functions and bool type, Unable to discriminate template specialization with enable_if and is_base_of. How to approach copying objects with smart pointers as class attributes? Thus when you do this delete entities[x + y * width]; you indeed delete the YourType instance, but the pointer still exists and it sill in your vector. Memory access patterns are one of the key factors for writing efficient code that runs over large data sets. For each container, std::span can deduce its size (4). The Type-Traits Library: Type Comparisons, And the Winners for the Seven Vouchers for Fedor's Book "The Art of Writing Efficient Programs" are, Template Metaprogramming - Hybrid Programming, Seven Voucher for Fedor G. 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When an object is added to the vector, it makes a copy. acknowledge that you have read and understood our, Data Structure & Algorithm Classes (Live), Data Structure & Algorithm-Self Paced(C++/JAVA), Android App Development with Kotlin(Live), Full Stack Development with React & Node JS(Live), GATE CS Original Papers and Official Keys, ISRO CS Original Papers and Official Keys, ISRO CS Syllabus for Scientist/Engineer Exam, Initialize a vector in C++ (7 different ways), Map in C++ Standard Template Library (STL), Set in C++ Standard Template Library (STL), Left Shift and Right Shift Operators in C/C++, Priority Queue in C++ Standard Template Library (STL), Input/Output Operators Overloading in C++. Storing pointers to allocated (not scoped) objects is quite convenient. Smart Pointers You will get a vector of ObjectBaseClass. Question/comment: as far as I understand span is not bounds-safe. Uups this time we cannot use data loaded in the second cache line read (from the first step), because the second particle data is located somewhere else in the memory! But you should not resort to using pointers. This can lead to a huge problem in long-running applications or resource-constrained hardware environments. for 80k of objects was 266% slower than the continuous case. Subscribe for the news. we might create a bit more advanced scenarios for our benchmarks. span1 references the std::vector vec(1). With Celero we If the copying and/or assignment operations are expensive (e.g. range of data. library has thing called problem space where we can define different Stay informed about my mentoring programs. 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This kind of analysis will hold true up until sizeof(POD) crosses some threshold for your architecture, compiler and usage that you would need to discover experimentally through benchmarking. How to use find algorithm with a vector of pointers to objects in c++? The above only puts lower bounds on that size for POD types. It seems that you have already subscribed to this list. Thanks for the write-up. The Winner is: Multithreading: The high-level Interface. Press J to jump to the feed. can be as inexpensive as a POD's or arbitrarily more expensive. That is, the elements the vector manages are the pointers, not the pointed objects. Insert the address of the variable inside the vector. 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