vector of objects vs vector of pointers

In our Free the pointer (Remove address from variable). Learn all major features of recent C++ Standards! A subreddit for all questions related to programming in any language. Deleting all elements in a vector manually is an anti-pattern and violates the RAII idiom in C++. So if you have to store pointers to objects in a Ask your rep for details. You haven't provided nearly enough information. By using our site, you Insert the address of the variable inside the vector. Notice that only the first 8 bytes from the second load are used for the first particle. Will you spend more time looping through it than adding elements to it? wises thing but Nonius caught easily that the data is highly disturbed. 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. Before we can update any fields of the first particle, it has to be fetched from the main memory into cache/registers. it would be good to revisit my old approach and measure the data again. (On the other hand, calling delete on a pointer value runs the destructor for the pointed-to object, and frees the memory.). Persistent Mapped Buffers, Benchmark Results. space and run benchmark again. C++ Core Guidelines: Better Specific or Generic? Figure 4: A Vector object after three values have been added to the vector. Yes and no. As for your second question, yes, that is another valid reason to store pointers. Larger objects will take more time to copy, as well as complex or compound objects. Assuming an array of 'bool', can 'a[n] == (-1)' ever be true? affected by outliers. Additionally, the hardware Prefetcher cannot figure out the pattern - it is random - so there will be a lot of cache misses and stalls. When an object is added to the vector, it makes a copy. Definitely the first! You use vector for its automatic memory management. Using a raw pointer to a vector means you don't get automatic memory mana If any of the destructed thread object is joinable and not joined then std::terminate () Further, thanks to the functions std::erase and std::erase_if, the deletion of the elements of a container works like a charm. As you can see this time, we can see the opposite effect. To provide the best experiences, we and our partners use technologies like cookies to store and/or access device information. vArray is nullptr (represented as X), while vCapacity and vSize are 0. Do you try to use memory-efficient data structures? Why is dereferenced element in const vector of int pointers mutable? 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). different set of data. 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++! Should I store entire objects, or pointers to objects in containers? What operations with temporary object can prevent its lifetime prolongation? * Min (us) code: we can easily test how algorithm performs using 1k of particles, write a benchmark that is repeatable. Your vector still contains an old pointer, which has became invalid by the time the object was deleted. library is probably better that your own simple solution. when working with a vector of pointers versus a vector of value types. Maybe std::vector would be more reasonable way to go. Objects The problem, however, is that you have to keep track of deleting it when removing it from the container. We can also push std::thread without specifically specifying std::move(), if we pass them as rvalue i.e. What std::string? unique_ptr thread_local static class is destroyed at invalid address on program exit. Before we can update any fields of the first particle, it has to be fetched from the main memory into cache/registers. 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. Nonius), but it can easily output csv data. Your time developing the code is worth more than the time that the program runs. You just need to How to erase & delete pointers to objects stored in a vector? Use nullptr for not existing object Instead of the vector of Objects, the Pool will store the vector of pointers to Objects. How to use find algorithm with a vector of pointers to objects in c++? If speed of insertion and removal is your concern, use a different container. vector pointer vs vector object Stay informed about my mentoring programs. 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 Yes, it is possible - benchmark it. There are: From the article: For 1000 particles we need on the average 2000 cache line reads! As vector contains various thread objects, so when this vector object is destructed it will call destructor of all the thread objects in the vector. Objects that cannot be copied/moved do require a pointer approach; it is not a matter of efficiency. You need JavaScript enabled to view it. Lets see You truly do not want to use global variables for anything without extremely good reason. The real truth can be found by profiling the code. 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. It also avoids mistakes like forgetting to delete or double deleting. "Does the call to delete affect the pointer in the vector?". We use unique_ptr so that we have clear ownership of resources while having almost zero overhead over raw pointers. Memory leaks; Shallow copies; Memory Leaks These are all my posts to then ranges library: category ranges library. For 1000 particles we need on the average 2000 cache line reads! the object stores a large amount of data), then you might want to store pointers for efficiency reasons. samples and 1 iteration). Dynamic dispatch (virtual method calls) work only on pointers and references (and you can't store references in a std::vector). I've prepared a valuable bonus if you're interested in Modern C++! Full repository can be found here: github/fenbf/PointerAccessTest but the code is also tested with Quick Bench: Theres also experimental code at https://github.com/fenbf/benchmarkLibsTest where I wrote the same benchmark with a different library: Celero, Google Benchmark, Nonius or Hayai (and see the corresponding blog post: Revisiting An Old Benchmark - Vector of objects or pointers). So, as usual, its best to measure and measure. If you want to delete pointer element, delete will call object destructor. 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. No need to call List[id]->~Ball() also no need to set pointer to NULL as you are going to erase the element anyway. A-143, 9th Floor, Sovereign Corporate Tower, We use cookies to ensure you have the best browsing experience on our website. 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. The technical storage or access is required to create user profiles to send advertising, or to track the user on a website or across several websites for similar marketing purposes. Lets Create a vector of std::thread objects i.e. 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! Why is RTTI needed for non-polymorphic typeid? So, can be called a pointer array, and the memory address is located on the stack memory rather than the heap memory. Thanks for this tutorial, its the first tutorial I could find that resolved my issue. 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. New comments cannot be posted and votes cannot be cast. Windows High Performance Timer for measurement. std::unique_ptr does the deletion for free: I suggest to use it instead. 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An unsafe program will consume more of your time fixing issues than a safe and robust version. Two cache line reads. Will it need to have elements added and removed frequently? To mimic real life case we can WebVector of Objects vs Vector of Pointers Updated. A couple of problems crop up when an object contains a pointer to dynamic storage. Now, as std::thread objects are move only i.e. we might create a bit more advanced scenarios for our benchmarks. Insertion using push_back( ): Inserting an element is like assigning vector elements with certain values. If we use default deleter or stateless deleter, then theres no extra memory use. Inside the block, there is a place to store the reference counter, the weak counter and also the deleter object. For each container, std::span can deduce its size (4). How to initialise a vector of pointers based on the vector of objects in c++ in the most elegant way? C++ difference between reference, objects and pointers, Moving objects from one unordered_map to another container, store many of relation 1:1 between various type of objects : decoupling & high performance, Atomic pointers in c++ and passing objects between threads, Using a base class as a safe container for pointers, STL container assignment and const pointers. Copying pointers is much faster than a copy of a large object. If I gradually build up from one to a hundred strings in an array, is that enough information to tell which is better? This time, however, we have a little more overhead compared to the case with unique_ptr. Click below to consent to the above or make granular choices. The benchmarks was solely done from scratch and theyve used only But then you have to call delete So we can If you really need to store resources that have to be allocated by new, then you should use boost::shared_ptr. Smart pointers in container like std::vector? To have a useful example for the object class I selected the Particle class which can simulate some physical interactions and implements a basic Euler method: The Particle class holds 72 bytes, and theres also some extra array for our further tests (commented out for now). If we will try to change the value of any element in vector of thread directly i.e. Or maybe you have some story to share? Vector of pointers 3. Vector of 20,000 small objects vs vector of 20,000 object pointers to 20,000 heap objects. And pointers come with their lot of constraints: they have their own semantics, they make things harder to copy objects, etc. C++20: Define the Concept Regular and SemiRegular, C++20: Define the Concepts Equal and Ordering, A Brief Overview of the PVS-Studio Static Code Analyzer, C++20: Two Extremes and the Rescue with Concepts, The new pdf bundle is ready: C++ Core Guidelines: Performance, "Concurrency with Modern C++" has a new chapter, C++ Core Guidelines: Naming and Layout Rules, C++ Core Guidelines: Lifetime Safety And Checking the Rules, C++ Core Guidelines: Type Safety by Design. A vector of Objects has first, initial performance hit. For this blog post, lets assume that Object is just a regular class, without any virtual methods. 0}. C++: Defined my own assignment operator for my type, now .sort() wont work on vectors of my type? Idea 4. 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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. 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?" Then we can take it and use * Problem Space I've recently released a new book on Modern C++: Intel i7 4720HQ, 12GB Ram, 512 SSD, Windows 10. Using Then we can define fixture classes for the final benchmarks: and vector of pointers, randomized or not: quite simple right? comparator for sorting a vector contatining pointers to objects of custom class, GDB & C++: Printing vector of pointers to objects. Passing Vector to a Function In other words, for each particle, we will need 1.125 cache line reads. How can I point to a member of a std::set in such a way that I can tell if the element has been removed? How do you know? Thank you for your understanding. when I want to test the same code but with different data set. This may have an initialization performance hit. Why can't `auto&` bind to a volatile rvalue expression? There are more ways to create a std::span. Is there any advantage to putting headers in an "include" subdir of the project? However, you can choose to make such a Bounds-Safe Views for Sequences of Objects Why it is valid to intertwine switch/for/if statements in C/C++? * Samples 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 you don't use pointers, then it is a copy of the object you pass in that gets put on the vector. This time each element is a pointer to a memory block allocated in a possibly different place in RAM. Therefore, we need to move these 2 thread objects in vector i.e. Same as #2, but first sort That would remove your confusion: No delete or new anymore, because the object is directly in the vector. This is 78% more cache line reads than the first case! In the article, weve done several tests that compared adjacent data structures vs a case with pointers inside a container. So both vectors will manage their pointers, but you have to think of how the lifecycle of those two pointers (the one from entities and the one from projectiles) interact with the object itself. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions. Scan the data through the ptr array and compute the sum. Unfortunately I found it hard to create a series of benchmarks: like for 80k of objects was 266% slower than the continuous case. 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. First of all we need to define a fixture class: The code above returns just a vector of pairs {1k, 0}, {2k, 0}, {10k, Contracts did not make it into C++20. To fully understand why we have such performance discrepancies, we need to talk about memory latency. Are function pointers function objects in C++? Is passing a reference through function safe? Lets make a comparison: The memory is allocated on the heap but vector guarantees that the mem block is continuous. That means the pointer you are saving is not a pointer to the object inside the vector. Load data for the second particle. C++: Vector of objects vs. vector of pointers to new objects? These seminars are only meant to give you a first orientation. The algorithmstd::iota fills myVec with thesequentially increasing values, starting with 0. benchmarking libraries for Check out this lecture about linked lists by Bjarne Stroustrup: A view from the ranges library is something that you can apply on a range and performs some operation. When I run Celero binary in On the diagram above, you can see that all elements of the vector are next to each other in the memory block. If all you care about is the objects, then they are more or less equivalent; you just have an extra level of indirection. C++ : Is it bad practice to use a static container in a class to contain pointers to all its objects for ease of access? CPU will detect that we operate on one huge memory block and will prefetch some of the cache lines before we even ask. If you create a shared pointer through make_shared, then the control block will be placed next to the memory block for the object. In the declaration: vector v; the word vector represents the object's base type. The main reason for having a std::span is that a plain array will be decay to a pointer if passed to a function; therefore, the size is lost. Consequently, the mapping of each element to its square (3) only addresses these elements. Some objects are cheaper to construct/copy contruct/move construct/copy/move/destruct than others, regardless of size. Thank you for one more great post! The following program shows how a subspan can be used to modify the referenced objects from a std::vector. << Notes on C++ SFINAE, Modern C++ and C++20 Concepts, Revisiting An Old Benchmark - Vector of objects or pointers. 1. appears that if you create one pointer after another they might end up I've read it, but I didn't find an answer as to which one is faster. This may be a performance savings depending on the object size. With shared_ptr we have a collection of pointers that can be owned by multiple pointers. Or should it be in one class which contains all behaviours? * Experiment, All rights reserved. a spreadsheed to analyze it and produce charts. 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. A Computer Science portal for geeks. Deletion of the element is not as simple as pop_back in the case of pointers. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. In contrast, span2 only references all elements of the underlying vec without the first and the last element (2). Does it need to stay sorted? WebVector of Objects A vector of Objects has first, initial performance hit. See my previous post about those benchmarking libraries: Micro 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). This is a type of array that can store the address rather than the value. The values for a given benchmark execution is actually the min of all 2011-2022, Bartlomiej Filipek Here is a compilation of my standard seminars. Pointers. * Z Score. The same problem occurs to store a collection of polymorphic objects in a vector: we have to store pointers instead of values: Return pointer to a vector of objects span1 references the std::vector vec(1). Does Vector::Erase() on a Vector of Object Pointers Destroy the If a second is significant, expect to access the data structures more times (1E+9). In C++, a variable is the variable that it is representing. Such benchmark code will be executed twice: once during the But you should not resort to using pointers. What is the fastest algorithm to find the point from a set of points, which is closest to a line? Thus instead of waiting for the memory, it will be already in the cache! This can simulate, for example, references in C#. How to use find algorithm with a vector of pointers to objects in c++? The difference is in object lifetime and useability; the speed is insignificant. Binary search with returned index in STL? Operations with the data structures may need to be performed a huge amount of times in order for the savings to be significant. Disclaimer: Any opinions expressed herein are in no way representative of those of my employers. 2. std::vector obs1; char * * obs2; Effectively, obs1 There are 2 deferences before you get to the object. How to approach copying objects with smart pointers as class attributes? Pass By Reference. All data and information provided on this site is for informational purposes only. Return a const vector of const shared pointers to const objects, A vector of pointers to objects that may or may not exist. Thank you for your understanding. Please call me if you have any questions. C++ Vector of Pointers - GeeksforGeeks http://info.prelert.com/blog/stl-container-memory-usage, http://en.cppreference.com/w/cpp/container. range of data. Vector of Objects vs Vector of Pointers The vector will also make copies when it needs to expand the reserved memory. Vector of objects vs vector of objects pointers : r/learnprogramming In this article we will create a vector thread and discuss things which we need to take care while using it. Designed by Colorlib. However, unless you really need shared ownership, it is recommended you use std::unique_ptr, which was newly introduced in C++11. However, the items will automatically be deleted when the vector is destructed. particles example I just wanted to test with 1k particles, 2k. github/fenbf/benchmarkLibsTest. Similarly, the std::string usually has a pointer to the actual dynamically allocated char array. This method will be memory-bound as all operations inside are too simple. Obviously there is very good locality of access to both arrays. Containers of pointers let you avoid the slicing problem. library has thing called problem space where we can define different All Rights Reserved. Be careful with hidden cost of std::vector for user defined, C++11 Multithreading - Part 1 : Three Different ways to, C++11 - Variadic Template Function | Tutorial & Examples, C++11 : Start thread by member function with arguments. 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 Constructs a vector of pointers, creates an instace of SomeObject and pushes an address of this object to your vector. A pointer to a vector is very rarely useful - a vector is cheap to construct and destruct. For elements in the vector , there's no correct ans This time we also get some data of the third particle. This can be used to operate over to create an array containing multiple pointers. When a vector is passed to a function, a copy of the vector is created. Disclaimer: Any opinions expressed herein are in no way representative of those of my employers. 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. 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*). Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Michael Dunsky. Heres a great summary that explains the problem: The picture comes from the book: Systems Performance: Enterprise and the Cloud. 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. Nonius performs some statistic analysis on the gathered data. 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 The When an object is added to the vector, it makes a copy. What is going to happen is called object slicing. allocated in a continuous memory block vs allocated individually as So the vector manages it for you instead of just managing the pointer and letting you deal with the pointed object. distribution or if they were disturbed. Difference between constant pointer, pointers to constant, and constant pointers to constants, vector::front() and vector::back() in C++ STL, vector::empty() and vector::size() in C++ STL, vector::operator= and vector::operator[ ] in C++ STL, vector::at() and vector::swap() in C++ STL, vector::begin() and vector::end() in C++ STL, vector :: cbegin() and vector :: cend() in C++ STL, How to flatten a Vector of Vectors or 2D Vector in C++, vector::crend() & vector::crbegin() with example, vector::push_back() and vector::pop_back() in C++ STL. If the copying and/or assignment operations are expensive (e.g. I don't know of any other structures (aside from a tree structure, which is not especially appropriate here). Why inbuilt sort is not able to sort map of vectors? Check it out here: Examples of Projections from C++20 Ranges, Fun with printing tables with std::format and C++20, std::initializer_list in C++ 2/2 - Caveats and Improvements. C++, C++ vector of objects vs. vector of pointers to objects. 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. visible on the chart below: Of course, running benchmarks having on battery is probably not the doing Java the C++ way), sending lparam as a pointer to class, and use it in WndProc(), C++ last digit of a random sequence of powers, Function return in branches of an `if` vs outside the `if`, in C++, QLineEdit could not set shortcuts when it's in focus, Physical Boost.Units User Defined Literals, Why does std queue not define a swap method specialisation, Linking C++ to static library; undefined reference errors. std::vector Returns pointer to the underlying array serving as element storage. Heres the code for a vector of unique_ptr, the code is almost the same for a vector of shared_ptr. Accessing the objects is very efficient - only one dereference. 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. A vector of smart pointers may take additional performance hits compared to a vector of raw pointers. All data and information provided on this site is for informational purposes only. C++ template function gets erronous default values, Why does C++ accept multiple prefixes but not postfixes for a variable, Prevent derived classes from hiding non virtual functions from base. Having vector of objects is much slower than a vector of pointers. interested in more professional benchmarking