vector of objects vs vector of pointers

Similar to any other vector declaration we can declare a vector of pointers. A view (std::span) and a std::string_view are non-owning views and can deal with strings. We use unique_ptr so that we have clear ownership of resources while having almost zero overhead over raw pointers. The benchmarks was solely done from scratch and theyve used only You truly do not want to use global variables for anything without extremely good reason. If you want that, store smart pointers instead, ie std::unique_ptr or std::shared_ptr. 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). The raw pointers must be deleted before the vector can be destructed; or a memory leak is created. Class members that are objects - Pointers or not? Containers of the STL become with C++20 more powerful. Vector of shared pointers , memory problems after clearing the vector. Create a variable and insert a value in it. Additionally Hardware Prefetcher cannot figure out the pattern -- it is random -- so there will be a lot of cache misses and stalls. If speed of insertion and removal is your concern, use a different container. This email address is being protected from spambots. If the copying and/or assignment operations are expensive (e.g. 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*). C++ Core Guidelines: Better Specific or Generic? It seems that you have already subscribed to this list. * Group, Please call me if you have any questions. Copying a pointer into a vector is not dependent on the object size. With pointers to a base class and also with virtual methods you can achieve runtime polymorphism, but thats a story for some other experiment. The performance savings of one data structure versus another may disappear when waiting for I/O operations, such as networking or file I/O. You can modify the entire span or only a subspan. 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. 1. I suggest picking one data structure and moving on. So, can be called a pointer array, and the memory address is located on the stack memory rather than the heap memory. The technical storage or access is strictly necessary for the legitimate purpose of enabling the use of a specific service explicitly requested by the subscriber or user, or for the sole purpose of carrying out the transmission of a communication over an electronic communications network. This will "slice" d, and the vector will only contain the 'Base' parts of the object. 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. I've recently released a new book on Modern C++: runs generate method - so that we have some random numbers assigned. This does however only work if the lifetime of your objects is managed elsewhere and is guaranteed to be longer than that of the vector. Correctly reading a utf-16 text file into a string without external libraries? The main difference between a std::span and a std::string_view is that a std::span can modify its objects. Learn all major features of recent C++ Standards! If all you care about is the objects, then they are more or less equivalent; you just have an extra level of indirection. 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. See my previous post about those benchmarking libraries: Micro Stay informed about my mentoring programs. the object stores a large amount of data), then you might want to store pointers for efficiency reasons. 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. WebVector of Objects vs Vector of Pointers Updated. Please enable the javascript to submit this form. Let us know in comments. starts reading from the file. A subreddit for all questions related to programming in any language. In my seminar, I often hear the question: How can I safely pass a plain array to a function? // Code inside this loop is measured repeatedly, << Talk summary: The Last Thing D Needs by Scott Meyers, Flexible particle system - Emitter and Generators >>, Extra note on subsequent memory allocations, https://github.com/fenbf/benchmarkLibsTest, Revisiting An Old Benchmark - Vector of objects or pointers. << Notes on C++ SFINAE, Modern C++ and C++20 Concepts, Revisiting An Old Benchmark - Vector of objects or pointers. You can create a std::span from a pointer and a size. What about the case with a vector of pointers? The same problem occurs to store a collection of polymorphic objects in a vector: we have to store pointers instead of values: 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 ]; }; 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. 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Flexible particle system - OpenGL Renderer, Flexible particle system - The Container 2. 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. Same as #2, but first sort However, you can choose to make such a 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. affected by outliers. This is 78% more cache line reads than the first case! It will crash our application, because on replacing a thread object inside the vector, destructor of existing thread object will be called and we havent joined that object yet.So, it call terminate in its destructor. In the article, weve done several tests that compared adjacent data structures vs a case with pointers inside a container. Your vector still contains an old pointer, which has became invalid by the time the object was deleted. The real truth can be found by profiling the code. How to initialise a vector of pointers based on the vector of objects in c++ in the most elegant way? Or should it be in one class which contains all behaviours? Container of references / non-nullable pointers, Avoiding preprocessor for mutual exclusive function call in C++20, How Iostream file is located in computer by c++ code during execution, Get text from a button in an application using win32 C++ and hooks. 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. (On the other hand, calling delete on a pointer value runs the destructor for the pointed-to object, and frees the memory.). There are two global variables that you probably have used, but let them be the only ones: std::cin & std::cout. We can perform this task in certain steps. space and run benchmark again. If a second is significant, expect to access the data structures more times (1E+9). If any of the destructed thread object is joinable and not joined then std::terminate() will be called from its destructor.Therefore its necessary to join all the joinable threads in vector before vector is destructed i.e. For 1000 particles we need on the average 2000 cache line reads! It affects the behavior invoked by using this pointer since the object it points to no longer exists. When you want to read more about std::string_view, read my previous post: "C++17 - What's New in the Library?" Some of the code is repeated, so we could even simplify this a bit more. With the Celero The test code will take each element of the problem Deletion of the element is not as simple as pop_back in the case of pointers. Press J to jump to the feed. To mimic real life case we can C++ Core Guidelines: More Non-Rules and Myths, More Rules about the Regular Expression Library, C++ Core Guidelines: Improved Performance with Iostreams, Stuff you should know about In- and Output with Streams, More special Friends with std::map and std::unordered_map, C++ Core Guidelines: std::array and std::vector are your Friends, C++ Core Guidelines: The Standard Library, C++ Core Guidelines: The Remaining Rules about Source Files, The new pdf bundle is available: C++ Core Guidlines - Templates and Generic Programming, Types-, Non-Types, and Templates as Template Parameters, C++ Core Guidelines: Surprise included with the Specialisation of Function Templates, C++ Core Guidelines: Other Template Rules, C++ Core Guidelines: Programming at Compile Time with constexpr, C++ Core Guidelines: Programming at Compile Time with Type-Traits (The Second), C++ Core Guidelines: Programming at Compile Time with the Type-Traits, C++ Core Guidelines: Programming at Compile Time, C++ Core Guidelines: Rules for Template Metaprogramming, C++ Core Guidelines: Rules for Variadic Templates, C++ Core Guidelines: Rules for Templates and Hierarchies, C++ Core Guidelines: Ordering of User-Defined Types, C++ Core Guidelines: Template Definitions, C++ Core Guidelines: Surprises with Argument-Dependent Lookup, C++ Core Guidelines: Regular and SemiRegular Types, C++ Core Guidelines: Pass Function Objects as Operations, I'm Proud to Present: The C++ Standard Library including C++14 & C++17, C++ Core Guidelines: Definition of Concepts, the Second, C++ Core Guidelines: Rules for the Definition of Concepts, C++ Core Guidelines: Rules for the Usage of Concepts. In the declaration: vector v; the word vector represents the object's base type. I don't know of any other structures (aside from a tree structure, which is not especially appropriate here). 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. visible on the chart below: Of course, running benchmarks having on battery is probably not the You just need to My last results, on older machine (i5 2400) showed that pointers code C++ : Is it bad practice to use a static container in a class to contain pointers to all its objects for ease of access? Larger objects will take more time to copy, as well as complex or compound objects. vArray is nullptr (represented as X), while vCapacity and vSize are 0. what we get with new machine and new approach. Deleting the object will not get rid of the pointers, in neither of the arrays. 2011-2022, Bartlomiej Filipek The size of std::vector is fixed, because it essentially just contains a pointer to the real data that is dynamically allocated. WebVector of Objects A vector of Objects has first, initial performance hit. You can change your settings at any time, including withdrawing your consent, by using the toggles on the Cookie Policy, or by clicking on the manage consent button at the bottom of the screen. In C++, should different game entities have different classes? However, unless you really need shared ownership, it is recommended you use std::unique_ptr, which was newly introduced in C++11. How do I initialize a stl vector of objects who themselves have non-trivial constructors? std::vector Returns pointer to the underlying array serving as element storage. 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. The program fills the vector with all numbers from 0 to 19 (1), and initializes a std::span with it (2). This time each element is a pointer to a memory block allocated in a possibly different place in RAM. Complex answer : it depends. if your vector is shared or has a lifecycle different from the class which embeds it, it might be better to keep it as Does it need to stay sorted? It is the actual object in memory, at the actual location. All data and information provided on this site is for informational purposes only. by Bartlomiej Filipek. I've read it, but I didn't find an answer as to which one is faster. thread_local static class is destroyed at invalid address on program exit. Additionally, the hardware Prefetcher cannot figure out the pattern - it is random - so there will be a lot of cache misses and stalls. slightly different data: For all our tests the variance is severely affected, its clearly * Variance It can be done using 2 steps: Square brackets are used to declare fixed size. Now lets create 2 thread objects using this std::function objects i.e. This may be a performance savings depending on the object size. Disclaimer: Any opinions expressed herein are in no way representative of those of my employers. library All rights reserved. Click below to consent to the above or make granular choices. Therefore, we can only move vector of thread to an another vector thread i.e. With Celero we Will it need to have elements added and removed frequently? Since you are explicitly stating you want to improve your C++, I am going to recommend you start using Boost. For our benchmark we have to create array of pointers or objects before c++14 unique_ptr and make unique_ptr error use of deleted function 'std::unique-ptr'. Which pdf bundle should I provide? If your vector can fit inside a processor's data cache, this will be very efficient. You need JavaScript enabled to view it. measurements/samples) and only one iteration (in Nonius there was 100 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. 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. Why can't `auto&` bind to a volatile rvalue expression? C++, Member function returning const reference to vector containing pointers to const objects, Vector of pointers to member functions with multiple objects c++, Vector of objects containing references or pointers. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. 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. Should I store entire objects, or pointers to objects in containers? 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. The declaration: vector v(5); creates a vector containing five null pointers. Pointers. Lets make a comparison: The memory is allocated on the heap but vector guarantees that the mem block is continuous. So we can Press question mark to learn the rest of the keyboard shortcuts. Looking for Proofreaders for my new Book: Concurrency with Modern C++, C++17: Improved Associative Containers and Uniform Container Access, C++17: New Parallel Algorithms of the Standard Template Library, Get the Current Pdf Bundle: Concurrency with C++17 and C++20, C++17 - Avoid Copying with std::string_view, C++17- More Details about the Core Language, And the Winners are: The C++ Memory Model/Das C++ Speichermodell, I'm Done - Geschafft: Words about the Future of my Blogs, Parallel Algorithms of the Standard Template Library, Recursion, List Manipulation, and Lazy Evaluation, Functional in C++11 and C++14: Dispatch Table and Generic Lambdas, Object-Oriented, Generic, and Functional Programming, Memory Pool Allocators by Jonathan Mller, Pros and Cons of the various Memory Allocation Strategies, Copy versus Move Semantics: A few Numbers, Automatic Memory Management of the STL Containers, Memory and Performance Overhead of Smart Pointers, Associative Containers - A simple Performance Comparison, Published at Leanpub: The C++ Standard Library, I'm proud to present: The C++ Standard Library, My Conclusion: Summation of a Vector in three Variants, Multithreaded: Summation with Minimal Synchronization, Thread-Safe Initialization of a Singleton, Ongoing Optimization: Relaxed Semantic with CppMem, Ongoing Optimization: A Data Race with CppMem, Ongoing Optimization: Acquire-Release Semantic with CppMem, Ongoing Optimization: Sequential Consistency with CppMem, Ongoing Optimization: Locks and Volatile with CppMem, Ongoing Optimization: Unsynchronized Access with CppMem, Looking for Proofreaders for my New C++ Book, Acquire-Release Semantic - The typical Misunderstanding. This way, an object will be copied only when necessary, and shared otherwise. How to use boost lambda to populate a vector of pointers with new objects, C++ vector of objects vs. vector of pointers to objects. 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 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). We get similar results to the data we get with Nonius: Celero doesnt give you an option to directly create a graph (as What operations with temporary object can prevent its lifetime prolongation? Not consenting or withdrawing consent, may adversely affect certain features and functions. 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. These seminars are only meant to give you a first orientation. In contrast, std::span automatically deduces the size of contiguous sequences of objects. Is passing a reference through function safe? Any other important details? 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. Containers of pointers let you avoid the slicing problem. You will get a vector of ObjectBaseClass. Which pdf bundle should I provide? Idea 4. All right - if I go back to my original point, say I have an array of a hundred. This may be performance hit because the processor may have to reload the data cache when dereferencing the pointer to the object. a spreadsheed to analyze it and produce charts. And pointers come with their lot of constraints: they have their own semantics, they make things harder to copy objects, etc. Persistent Mapped Buffers, Benchmark Results. 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. So, as usual, its best to measure and measure. 3. This decay is a typical reason for errors in C/C++. 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. The sharing is implemented using some garbage Insert the address of the variable inside the vector. Binary search with returned index in STL? Please check your email and confirm the newsletter subscription. You still need to do the delete yourself as, again, the vector is only managing the pointer, not the YourType. This contiguous memory can be a plain array, a pointer with a size, a std::array, a std::vector, or a std::string. and use chronometer parameter that might be passed into the Benchmark and returns the pointer to the vector of objects to a receiver in main function. Without a subpoena, voluntary compliance on the part of your Internet Service Provider, or additional records from a third party, information stored or retrieved for this purpose alone cannot usually be used to identify you. Copyright 2023 www.appsloveworld.com. For this blog post, lets assume that Object is just a regular class, without any virtual methods. when I want to test the same code but with different data set. With Nonius I have to write 10 benchmarks separately. Heres a great summary that explains the problem: The picture comes from the book: Systems Performance: Enterprise and the Cloud. WebIn that case, when you push_back(something), a copy is made of the object. You must also ask yourself if the Objects or the Object* are unique. However, the items will automatically be deleted when the vector is destructed. Why inbuilt sort is not able to sort map of vectors? C++, Source code available on githib: Capitalize First letter of each word in a String in Java | Camel Case, C++11 Multithreading Part 1 : Three Different ways to Create Threads, C++11 Move Contsructor & rvalue References, Different ways to iterate over a set in C++, How to trim strings in C++ using Boost String Algorithm Library, How to add an element in Vector using vector::push_back, Using std::find & std::find_if with User Defined Classes, Pandas Dataframe: Get minimum values in rows or columns & their index position. Now, as std::thread objects are move only i.e. but with just battery mode (without power adapter attached) I got The values for a given benchmark execution is actually the min of all 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. when working with a vector of pointers versus a vector of value types. 2011-2022, Bartlomiej Filipek Does vector::erase() on a vector of object pointers destroy the object itself? no viable conversion from 'int' to 'Student'. All rights reserved. It depends. Learn all major features of recent C++ Standards! libraries The technical storage or access that is used exclusively for anonymous statistical purposes. Let's look at the details of each example before drawing any conclusions. Check out the Boost documentation. Due to how CPU caches work these days, things are not simple anymore. If you really need to store resources that have to be allocated by new, then you should use boost::shared_ptr. Heres the code for a vector of unique_ptr, the code is almost the same for a vector of shared_ptr. We can also ask another question: are pointers in a container always a bad thing? As a number of comments have pointed out, vector.erase only removes the elements from the vector. In this blog post, youll see why there might be a perf difference of almost 2.5x (in both directions!) This time we also get some data of the third particle. code: we can easily test how algorithm performs using 1k of particles, 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]. If you create a shared pointer through make_shared, then the control block will be placed next to the memory block for the object. This method will be memory-bound as all operations inside are too simple. Which pdf bundle do you want? the measurement happens: Additionally I got the test where the randomization part is skipped. 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). The following program shows how a subspan can be used to modify the referenced objects from a std::vector. Using vectors of pointers #include #include using namespace std; static const int NUM_OBJECTS = 10; I'm happy to give online seminars or face-to-face seminars worldwide. Calling a destructor on a pointer value does nothing.

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