and use chronometer parameter that might be passed into the Benchmark Why is RTTI needed for non-polymorphic typeid? To make polymorphism work You have to use some kind of pointers. Make your choice! * Samples Some objects are cheaper to construct/copy contruct/move construct/copy/move/destruct than others, regardless of size. The problem, however, is that you have to keep track of deleting it when removing it from the container. Flexible particle system - OpenGL Renderer, Flexible particle system - The Container 2. The program fills the vector with all numbers from 0 to 19 (1), and initializes a std::span with it (2). Free the pointer (Remove address from variable). When a vector is passed to a function, a copy of the vector is created. Particles vector of pointers but not randomized: mean is 90ms and The rest - 56b - are the bytes of the second particle. Containers of the STL become with C++20 more powerful. 2. std::vector obs1; char * * obs2; Effectively, obs1 Particles vector of objects: mean is 69ms and variance should be ok. Accessing the objects is very efficient - only one dereference. Are there any valid use cases to use new and delete, raw pointers or c-style arrays with modern C++? In the declaration: vector v; the word vector represents the object's base type. The code will suffer from a memory leak if the programmer does not free up the memory before exiting. http://info.prelert.com/blog/stl-container-memory-usage, http://en.cppreference.com/w/cpp/container. When an object is added to the vector, it makes a copy. thread_local static class is destroyed at invalid address on program exit. How to delete objects from vector of pointers to object? Boost MultiIndex - objects or pointers (and how to use them?)? Insertion while initialization: Although its an option that can be used we should avoid such type of insertion as vectors store addresses within them. When you modify the span, you modify the referenced objects.. 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. Correctly reading a utf-16 text file into a string without external libraries? Ok, so what are the differences between each collection? We can also push std::thread without specifically specifying std::move(), if we pass them as rvalue i.e. I suggest picking one data structure and moving on. Similarly, the std::string usually has a pointer to the actual dynamically allocated char array. When I run Celero binary in Your email address will not be published. allocated in a continuous memory block vs allocated individually as 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. 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. In C++, a variable is the variable that it is representing. Does vector::erase() on a vector of object pointers destroy the object itself? Does it need to stay sorted? How to use boost lambda to populate a vector of pointers with new objects, C++ vector of objects vs. vector of pointers to objects. There are many convenience functions to refer to the elements of the span. That is, the elements the vector manages are the pointers, not the pointed objects. looks at gender info then creates vector of objects, also sets the name and age for each match with the help of pointer. Can it contain duplicates? Strongly recommand you use smart pointer as Chris mentioned, then you don't need to worry about deleting object pointer when you delete element from STL container, demo as below: From your sample code, I assume your vector is defined somewhat like this: Therefore, your vector does not contain YourType objects, but pointer to YourType. No need to call List[id]->~Ball() also no need to set pointer to NULL as you are going to erase the element anyway. C++ - Performance of vector of pointer to objects, vs performance of objects, Leaked Mock Objects when using GoogleMock together with Boost::Shared Pointers, C++: Operator overloading of < for pointers to objects. In Nonius we can use a bit more advanced approach The technical storage or access that is used exclusively for anonymous statistical purposes. Consequently, std::span also holds int's. All of the big three C++ compilers MSVC, GCC, and Clang, support std::span. Particles vector of pointers: mean is 121ms and variance is not Almost always, the same is true for a POD type at least until sizeof(POD) > 2 * sizeof(POD*) due to superior memory locality and lower total memory usage compared to when you are dynamically allocating the objects at which to be pointed. 2k 10k without writing code separately. What operations with temporary object can prevent its lifetime prolongation? What is the fastest algorithm to find the point from a set of points, which is closest to a line? Usually solution 1 is what you want since its the simplest in C++: you dont have to take care of managing the memory, C++ does all that for you ( Subscribe for the news. vectors of pointers. 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 My question is simple: why did using a vector of pointers work, and when would you create a vector of objects versus a vector of pointers to those objects? Course: Modern C++ Concurrency in Practice, Course: C++ Standard Library including C++14 & C++17, Course: Embedded Programming with Modern C++, Course: C++ Fundamentals for Professionals, Interactive Course: The All-in-One Guide to C++20, Subscribe to the newsletter (+ pdf bundle), std::span in C++20: Bounds-Safe Views for Sequences of Objects, Automatically deduces the size of a contiguous sequence of objects, Create a std::span from a pointer and a size, Design Patterns and Architectural Patterns with C++, Clean Code: Best Practices fr modernes 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). I think it would be interesting the discussion and I would like , Jake, GS, Lawton Shoemake, Animus24, Jozo Leko, John Breland. The values for a given benchmark execution is actually the min of all data for benchmarks. but with just battery mode (without power adapter attached) I got Heres the code for a vector of unique_ptr, the code is almost the same for a vector of shared_ptr. range of data. To support reference counting the shared pointer needs to have a separate control block. Binary search with returned index in STL? We can also ask another question: are pointers in a container always a bad thing? Please call me if you have any questions. Download a free copy of C++20/C++17 Ref Cards! 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. 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. 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. Click below to consent to the above or make granular choices. get even more flexibility and benchmarks can be executed over different Otherwise, it is generally better not to store pointers for exactly the reason that you mentioned (automatic deallocation). Consenting to these technologies will allow us and our partners to process personal data such as browsing behavior or unique IDs on this site. All data and information provided on this site is for informational purposes only. Maybe std::vector would be more reasonable way to go. Use nullptr for not existing object Instead of the vector of Objects, the Pool will store the vector of pointers to Objects. Consenting to these technologies will allow us to process data such as browsing behavior or unique IDs on this site. First of all we need to define a fixture class: The code above returns just a vector of pairs {1k, 0}, {2k, 0}, {10k, I try to write complete and accurate articles, but the web-site will not be liable for any errors, omissions, or delays in this information or any losses, injuries, or damages arising from its display or use. A-143, 9th Floor, Sovereign Corporate Tower, We use cookies to ensure you have the best browsing experience on our website. Back in main the data type receives this vector pointer by a necessary data type. How to use find algorithm with a vector of pointers to objects in c++? Thanks for the write-up. All Rights Reserved. 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 Analysis and reporting is a breeze with Tableau, which comes a preconfigured report library, included for all cirrus customers. I've prepared a valuable bonus if you're interested in Modern C++! * Iterations The algorithmstd::iota fills myVec with thesequentially increasing values, starting with 0. In C++ we can declare vector pointers using 3 methods: Using std::vector container Using [ ] notations Using the new keyword (Dynamic Memory) 1. Each benchmark will be executed 20 times (20 You have not even explained how you intend to use your container. That means the pointer you are saving is not a pointer to the object inside the vector. std::unique_ptr does the deletion for free: I suggest to use it instead. Your choices will be applied to this site only. * Iterations/sec Interesting thing is when I run the same binary on the same hardware, Let us know in comments. 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. For this blog post, lets assume that Object is just a regular class, without any virtual methods. :) Calling a destructor on a pointer value does nothing. 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. 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. no viable conversion from 'int' to 'Student'. You can create a std::span from a pointer and a size. I've recently released a new book on Modern C++: Intel i7 4720HQ, 12GB Ram, 512 SSD, Windows 10. To provide the best experiences, we and our partners use technologies like cookies to store and/or access device information. What about the case with a vector of pointers? Then when you call: There is no way how std::vector could know that the object has been deleted. what we get with new machine and new approach. The test code will take each element of the problem can be as inexpensive as a POD's or arbitrarily more expensive. Idea 4. 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. 3. Should I store entire objects, or pointers to objects in containers? With C++20, the answer is quite easy: Use a std::span. A little bit more costly in performance than a raw pointer. 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. Not consenting or withdrawing consent, may adversely affect certain features and functions. You haven't provided nearly enough information. To provide the best experiences, we use technologies like cookies to store and/or access device information. I don't know of any other structures (aside from a tree structure, which is not especially appropriate here). If the copying and/or assignment operations are expensive (e.g. 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. The Is passing a reference through function safe? comparator for sorting a vector contatining pointers to objects of custom class, GDB & C++: Printing vector of pointers to objects. A vector of Objects has first, initial performance hit. See my previous post about those benchmarking libraries: Micro How to use find algorithm with a vector of pointers to objects in c++? There are: This may be a performance savings depending on the object size. For our benchmark we have to create array of pointers or objects before Heres another result when the size of a Particle object is increased to 128 bytes (previously it was 72 bytes): The results are because algorithms such as sorting need to move elements inside the container. Standard containers, like std::vector, containing raw pointers DO NOT automatically delete the things that the pointers are pointing at, when removing the pointers from the containers. slightly different data: For all our tests the variance is severely affected, its clearly Ask your rep for details. samples and 1 iteration). Can I be sure a vector contains objects and not pointers to objects? 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. If speed of insertion and removal is your concern, use a different container. C++ Core Guidelines: Better Specific or Generic? The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user. 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 Why is dereferenced element in const vector of int pointers mutable? benchmarking libraries for Create an account to follow your favorite communities and start taking part in conversations. // 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. 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 Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Michael Dunsky. Stay informed about my mentoring programs. Any other important details? To mitigate this issue, the benchmark code adds a randomisation step: ShuffleVector(). All rights reserved. * Kurtosis Lets Create a vector of std::thread objects i.e. What std::string? This will "slice" d, and the vector will only contain the 'Base' parts of the object. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. 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. interested in more professional benchmarking The update() method is simple, has only several arithmetic operations and a single branch. In your case, you do have a good reason, because you actually store a non-owning pointer. libraries Pass By Reference. However, unless you really need shared ownership, it is recommended you use std::unique_ptr, which was newly introduced in C++11. You truly do not want to use global variables for anything without extremely good reason. for 80k of objects was 266% slower than the continuous case. Each pointer within a vector of pointers points to an address storing a value. c++ How to find the minimum number of elements from a vector that sum to a given number, Passing a 2d dynamic array to a function in C++. Libraries like You will get a vector of ObjectBaseClass. There are more ways to create a std::span. Press J to jump to the feed. For the rest it is a balance between "simple and maintainable" vs. "the least CPU cycles ever". (On the other hand, calling delete on a pointer value runs the destructor for the pointed-to object, and frees the memory.). The real truth can be found by profiling the code. This may be performance hit because the processor may have to reload the data cache when dereferencing the pointer to the object. Using vectors of pointers #include #include using namespace std; static const int NUM_OBJECTS = 10; 2023 ITCodar.com. WebStore pointers to your objects in a vectorinstead But if you do, dont forget to deletethe objects that are pointed to, because the vectorwont do it for you. 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. 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. Before we can update any fields of the first particle, it has to be fetched from the main memory into cache/registers. All data and information provided on this site is for informational purposes only. The vector will also make copies when it needs to expand the reserved memory. affected by outliers. Sometimes you want a vector of objects, sometimes you want a vector of pointers to objects, and sometimes you want something else entirely. That would remove your confusion: No delete or new anymore, because the object is directly in the vector. Notice that only the first 8 bytes from the second load are used for the first particle. library has thing called problem space where we can define different 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. Two cache line reads. 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). I'm happy to give online seminars or face-to-face seminars worldwide. This is 78% more cache line reads than the first case! 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. Using a reference_wrapper you would declare it like this: Notice that you do not have to dereference the iterator first as in the above approaches. So for the second particle, we need also two loads. With Nonius I have to write 10 benchmarks separately. If a second is significant, expect to access the data structures more times (1E+9). You can modify the entire span or only a subspan. Nonius performs some statistic analysis on the gathered data. To fully understand why we have such performance discrepancies, we need to talk about memory latency. 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. What i was missing was the std::move() function and I wasnt able to find it for months now. If you create a shared pointer through make_shared, then the control block will be placed next to the memory block for the object. Note that unless you have a good reason, you should probably not store the pointer in the vector, but the object itsself. Why it is valid to intertwine switch/for/if statements in C/C++? Dynamic dispatch (virtual method calls) work only on pointers and references (and you can't store references in a std::vector). appears that if you create one pointer after another they might end up Which pdf bundle do you want? How to erase & delete pointers to objects stored in a vector? detect the same problems of our data as weve noticed with Nonius. How to approach copying objects with smart pointers as class attributes? But, since recently Im 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++! To mimic real life case we can You should use a vector of handles to Object (see the Bridge design pattern) rather than naked pointers. 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. When I run This is a type of array that can store the address rather than the value. As for std::array and std::vector, you need to know the size of your std::array at compile time and you can't resize it at runtime, but vector has neither of those restrictions. 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. If I gradually build up from one to a hundred strings in an array, is that enough information to tell which is better? call function findMatches. "Does the call to delete affect the pointer in the vector?". What's special about R and L in the C++ preprocessor? It Premise : In C++ it is convenient to store like object instances in std containers (eg: std::vector). samples. For 1000 particles we need on the average 2000 cache line reads! Additionally Hardware Prefetcher cannot figure out the pattern -- it is random -- so there will be a lot of cache misses and stalls. We and our partners share information on your use of this website to help improve your experience. the object stores a large amount of data), then you might want to store pointers for efficiency reasons. We use unique_ptr so that we have clear ownership of resources while having almost zero overhead over raw pointers. When we pass an array to a function, a pointer is actually passed. 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. Or should it be in one class which contains all behaviours? Eiffel is a great example of Design by Contract. Accessing the objects takes a performance hit. Designed by Colorlib. Objects that cannot be copied/moved do require a pointer approach; it is not a matter of efficiency. If any of the destructed thread object is joinable and not joined then std::terminate () If your vector can fit inside a processor's data cache, this will be very efficient. Larger objects will take more time to copy, as well as complex or compound objects. Then we can define fixture classes for the final benchmarks: and vector of pointers, randomized or not: quite simple right? 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