vector of objects vs vector of pointersvector of objects vs vector of pointers

Is there any advantage to putting headers in an "include" subdir of the project? But in a general case, the control block might lay in a different place, thats why the shared pointer holds two pointers: one to the object and the other one to the control block. Lets Create a vector of std::thread objects i.e. write a benchmark that is repeatable. For a Plain Old Data (POD) type, a vector of that type is always more efficient than a vector of pointers to that type at least until sizeof(POD) > sizeof(POD*). This can simulate, for example, references in C#. * Standard Deviation vector::eraseRemoves from the vector container and calls its destructor but If the contained object is a pointer it doesnt take ownership of destroying it. In my seminar, I often hear the question: How can I safely pass a plain array to a function? * Min (us) No need to call List[id]->~Ball() also no need to set pointer to NULL as you are going to erase the element anyway. Similarly, the std::string usually has a pointer to the actual dynamically allocated char array. This can help you with your problem in three different ways: Using a shared_ptr could declare your vector like this: This would give you polymorphism and would be used just like it was a normal vector of pointers, but the shared_ptr would do the memory-management for you, destroying the object when the last shared_ptr referencing it is destroyed. When I run Celero binary in This is 78% more cache line reads than the first case! The program fills the vector with all numbers from 0 to 19 (1), and initializes a std::span with it (2). my tests using 10k particles, 1k updates I got the following output: The great thing about Nonius is that you dont have to specify number of Click below to consent to the above or make granular choices. 2k 10k without writing code separately. 2023 ITCodar.com. Are there any valid use cases to use new and delete, raw pointers or c-style arrays with modern C++? A vector of Objects has first, initial performance hit. * Variance But, since recently Im github/fenbf/benchmarkLibsTest. Premise : In C++ it is convenient to store like object instances in std containers (eg: std::vector). To provide the best experiences, we use technologies like cookies to store and/or access device information. Vector of pointers Now lets create 2 thread objects using this std::function objects i.e. If your vector can fit inside a processor's data cache, this will be very efficient. There, you will also be able to use std::unique_ptr which is faster, as it doesn't allow copying. There are 2 deferences before you get to the object. battery mode then I could spot the difference between AC mode. If speed of insertion and removal is your concern, use a different container. The technical storage or access that is used exclusively for statistical purposes. Larger objects will take more time to copy, as well as complex or compound objects. different set of data. runs and iterations all this is computed by Nonius. To provide the best experiences, we and our partners use technologies like cookies to store and/or access device information. Note about C++11: reference_wrapper has also been standardized in C++11 and is now usable as std::reference_wrapper without Boost. We can also push std::thread without specifically specifying std::move(), if we pass them as rvalue i.e. An more generic & elegant solution:This solution makes use of for_each & templates as @Billy pointed out in comments: where, myclassVector is your vector containing pointers to myclass class objects. particles example I just wanted to test with 1k particles, 2k. It also avoids mistakes like forgetting to delete or double deleting. Your vector still contains an old pointer, which has became invalid by the time the object was deleted. Persistent Mapped Buffers, Benchmark Results. Learn all major features of recent C++ Standards! To support reference counting the shared pointer needs to have a separate control block. Inside the block, there is a place to store the reference counter, the weak counter and also the deleter object. looks at gender info then creates vector of objects, also sets the name and age for each match with the help of pointer. 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! Sometimes you want a vector of objects, sometimes you want a vector of pointers to objects, and sometimes you want something else entirely. As for your first question, it is generally preferred to use automatically allocated objects rather than dynamically allocated objects (in other words, not to store pointers) so long as for the type in question, copy-construction and assignment is possible and not prohibitively expensive. Objects In this blog post, youll see why there might be a perf difference of almost 2.5x (in both directions!) Not consenting or withdrawing consent, may adversely affect certain features and functions. Your choices will be applied to this site only. but with just battery mode (without power adapter attached) I got 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. WebVector of objects vs vector of objects pointers I remember during an assignment for a class I took during fall semester that we had to use vectors of pointers instead of just the Using a ptr_vector you would do it like this: This would again be used like a normal vector of pointers, but this time the ptr_vector manages the lifetime of your objects. Thus instead of waiting for the memory, it will be already in the cache! Press question mark to learn the rest of the keyboard shortcuts. The pointer is such that range [data (), data () + size ()) is always a valid range, even if the container is empty ( data () is not dereferenceable in that case). C++: Defined my own assignment operator for my type, now .sort() wont work on vectors of my type? 3. Copyright 2023 www.appsloveworld.com. How do you know? Such benchmark code will be executed twice: once during the We and our partners share information on your use of this website to help improve your experience. 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. So for the second particle, we need also two loads. when working with a vector of pointers versus a vector of value types. benchmarking libraries for samples. The performance savings of one data structure versus another may disappear when waiting for I/O operations, such as networking or file I/O. Ok, so what are the differences between each collection? Let's look at the details of each example before drawing any conclusions. I've recently released a new book on Modern C++: Intel i7 4720HQ, 12GB Ram, 512 SSD, Windows 10. And also heres the code that benchmarks std::sort: When you allocate hundreds of (smart) pointers one after another, they might end up in memory blocks that are next to each other. Copyright 2023 www.appsloveworld.com. We use unique_ptr so that we have clear ownership of resources while having almost zero overhead over raw pointers. New comments cannot be posted and votes cannot be cast. Containers of pointers let you avoid the slicing problem. As you can see we can even use it for algorithms that uses two Load data for the second particle. quite close in the memory address space. If the objects can't be copied or assigned, then you can't put them directly into a std::vector anyway, and so the question is moot. Should I store entire objects, or pointers to objects in containers? You have not even explained how you intend to use your container. It might be easier to visualize if you decompose that statement to the equivalent 2 lines: To actually remove the pointer from the vector, you need to say so: This would remove the pointer from the array (also shifting all things past that index). We can perform this task in certain steps. The same problem occurs to store a collection of polymorphic objects in a vector: we have to store pointers instead of values: and returns the pointer to the vector of objects to a receiver in main function. * Z Score. Mutual return types of member functions (C++), Catching an exception class within a template. Particles vector of objects: mean is 69ms and variance should be ok. The problem, however, is that you have to keep track of deleting it when removing it from the container. Most processors don't follow pointers when loading their data cache. * Kurtosis A typical implementation consists of a pointer to its first element and a size. Download a free copy of C++20/C++17 Ref Cards! Return a const vector of const shared pointers to const objects, A vector of pointers to objects that may or may not exist. Vector of shared pointers , memory problems after clearing the vector. Which pdf bundle should I provide? Thanks for the write-up. However, the items will automatically be deleted when the vector is destructed. With this post I wanted to confirm that having a good benchmarking Constructs a vector of pointers, creates an instace of SomeObject and pushes an address of this object to your vector. span1 references the std::vector vec(1). The code will suffer from a memory leak if the programmer does not free up the memory before exiting. Will you spend more time looping through it than adding elements to it? Insert the address of the variable inside the vector. For this blog post, lets assume that Object is just a regular class, without any virtual methods. Back in main the data type receives this vector pointer by a necessary data type. Please call me if you have any questions. All data and information provided on this site is for informational purposes only. That's not my point - perhaps using String was a bad idea. On the diagram above, you can see that all elements of the vector are next to each other in the memory block. It seems that you have already subscribed to this list. affected by outliers. slightly different data: For all our tests the variance is severely affected, its clearly All rights reserved. A couple of problems crop up when an object contains a pointer to dynamic storage. It does NOT try to delete any associated memory.To delete the associated memory explicitly, you need to: There are a number of other inconsistencies with your code and, better solutions for what you're trying to do, such as: If you need to dynamically allocate your objects, but for some reason do not want the vector to handle that, you can use shared_ptr or unique_ptr, who will take care of the deallocation for you: If calling delete on the vector*s called delete on the pointers they hold, then you'd be in for a heap of trouble (pun intended) because you'd be deleteing automatic variables with the first delete which yields undefined behaviour (a bad thing). Interesting thing is when I run the same binary on the same hardware, 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. I think it would be interesting the discussion and I would like , Jake, GS, Lawton Shoemake, Animus24, Jozo Leko, John Breland. 1. You may remember that a std::span is sometimes called a view.Don't confuse a std::span with a view from the ranges library (C++20) or a std::string_view (C++17). Learn how your comment data is processed. If not, then to change an Object in a vector you will have to iterate the entire vector to find it. 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 ]; }; https://www.youtube.com/watch?v=YQs6IC-vgmo, Here is an excelent lecture by Scott Meyers about CPU caches: https://www.youtube.com/watch?v=WDIkqP4JbkE. Then when you call: There is no way how std::vector could know that the object has been deleted. Return pointer to a vector of objects As pointed out in Maciej Hs answer, your first approach results in object slicing. With shared_ptr we have a collection of pointers that can be owned by multiple pointers. Therefore, we can only move vector of thread to an another vector thread i.e. Smart pointers in container like std::vector? Inheritance Without Pointers space and run benchmark again. Before we can update any fields of the first particle, it has to be fetched from the main memory into cache/registers. randomize such pointers so they are not laid out consecutively in 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 * Max (us) On the other hand, having pointers may be important if you are working with a class hierarchy and each "Object" may in fact be some derived type that you are just treating as an Object. dimensional data range. You still need to do the delete yourself as, again, the vector is only managing the pointer, not the YourType. 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A view (std::span) and a std::string_view are non-owning views and can deal with strings. So we can In this article we will create a vector thread and discuss things which we need to take care while using it. vector pointer vs vector object - C / C++ wises thing but Nonius caught easily that the data is highly disturbed. Why it is valid to intertwine switch/for/if statements in C/C++? Please enable the javascript to submit this form. I remember during an assignment for a class I took during fall semester that we had to use vectors of pointers instead of just the objects. Is passing a reference through function safe? When an object is added to the vector, it makes a copy. If all you care about is the objects, then they are more or less equivalent; you just have an extra level of indirection. Retrieving AST from C++ code in Visual Studio. Accessing the objects takes a performance hit. C++: Vector of objects vs. vector of pointers to new objects? 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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?" pointers on the heap: Vector of Objects vs Vector of Thanks a lot to my Patreon Supporters: Matt Braun, Roman Postanciuc, Tobias Zindl, G Prvulovic, Reinhold Drge, Abernitzke, Frank Grimm, Sakib, Broeserl, Antnio Pina, Sergey Agafyin, , Jake, GS, Lawton Shoemake, Animus24, Jozo Leko, John Breland, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Robert Blanch, Truels Wissneth, Kris Kafka, Mario Luoni, Friedrich Huber, lennonli, Pramod Tikare Muralidhara, Peter Ware, Daniel Hufschlger, Alessandro Pezzato, Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Michael Dunsky, Leo Goodstadt, John Wiederhirn, Yacob Cohen-Arazi, Florian Tischler, Robin Furness, Michael Young, Holger Detering, Bernd Mhlhaus, Matthieu Bolt, Stephen Kelley, Kyle Dean, Tusar Palauri, Dmitry Farberov, Juan Dent, George Liao, Daniel Ceperley, Jon T Hess, Stephen Totten, Wolfgang Ftterer, Matthias Grn, Phillip Diekmann, Ben Atakora, and Ann Shatoff. Why is this? Each benchmark will be executed 20 times (20 There are more ways to create a std::span. Create a variable and insert a value in it. Create an account to follow your favorite communities and start taking part in conversations. Not consenting or withdrawing consent, may adversely affect certain features and functions. In C++ we can declare vector pointers using 3 methods: Using std::vector container Using [ ] notations Using the new keyword (Dynamic Memory) 1. 2. std::vector obs1; char * * obs2; Effectively, obs1 If any of the destructed thread object is joinable and not joined then std::terminate () Insertion using push_back( ): Inserting an element is like assigning vector elements with certain values. Each pointer within a vector of pointers points to an address storing a value. If you don't use pointers, then it is a copy of the object you pass in that gets put on the vector. library This time each element is a pointer to a memory block allocated in a possibly different place in RAM. Scan the data through the ptr array and compute the sum. You will get a vector of ObjectBaseClass. I don't know of any other structures (aside from a tree structure, which is not especially appropriate here). If it is a simple object, and/or you don't want to bother with keeping track of the storage for them, this may be exactly what you want. Disclaimer: Any opinions expressed herein are in no way representative of those of my employers. no viable conversion from 'int' to 'Student'. Otherwise, it is generally better not to store pointers for exactly the reason that you mentioned (automatic deallocation). C++: Vector of objects vs. vector of pointers to new objects? 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Here is a compilation of my standard seminars. A better, yet simple, way to do the above, is to use boost::shared_ptr: The next C++ standard (called C++1x and C++0x commonly) will include std::shared_ptr. The vector wouldn't have the right values for the objects. The algorithmstd::iota fills myVec with thesequentially increasing values, starting with 0. Safety and Robustness are also more important. Vector of objects vs vector of objects pointers : r/learnprogramming When you call delete, the object is deleted and whatever you try to do with that object using invalid (old, dangling) pointer, the behavior is undefined. If you need to store objects of multiple polymorphic types in the same vector, you must store pointers in order to avoid slicing. The new Keyword in C++ represents dynamic memory allocation i.e, heap memory. Vector How to use boost lambda to populate a vector of pointers with new objects, C++ vector of objects vs. vector of pointers to objects. How to use find algorithm with a vector of pointers to objects in c++? So, can be called a pointer array, and the memory address is located on the stack memory rather than the heap memory. Additionally Hardware Prefetcher cannot figure out the pattern -- it is random -- so there will be a lot of cache misses and stalls. 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