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). 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. What is going to happen is called object slicing. 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. Dynamic dispatch (virtual method calls) work only on pointers and references (and you can't store references in a std::vector). vectors of pointers. 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Free the pointer (Remove address from variable). 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++. 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? This may have an initialization performance hit. When you want to read more about std::string_view, read my previous post: "C++17 - What's New in the Library?" This way, an object will be copied only when necessary, and shared otherwise. No need to call List[id]->~Ball() also no need to set pointer to NULL as you are going to erase the element anyway. With this more advanced setup we can run benchmarks several times over Around one and a half year ago I did some benchmarks on updating objects Which pdf bundle should I provide? Check out this lecture about linked lists by Bjarne Stroustrup: http://info.prelert.com/blog/stl-container-memory-usage, http://en.cppreference.com/w/cpp/container. Why is RTTI needed for non-polymorphic typeid? * Baseline us/Iteration 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. If not, then to change an Object in a vector you will have to iterate the entire vector to find it. 1. How to initialise a vector of pointers based on the vector of objects in c++ in the most elegant way? To provide the best experiences, we use technologies like cookies to store and/or access device information. The new Keyword in C++ represents dynamic memory allocation i.e, heap memory. The Winner is: Multithreading: The high-level Interface. Load data for the first particle. Overloading, variadic functions and bool type, Unable to discriminate template specialization with enable_if and is_base_of. Same as #2, but first sort Larger objects will take more time to copy, as well as complex or compound objects. Also, you probably don't need a pointer to a vector in the first place, but I won't judge you since I don't know your situation. Nonius are easy to use and can pick strange artefacts in the results what we get with new machine and new approach. * Standard Deviation 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. Lets see memory. How do you know? Interesting thing is when I run the same binary on the same hardware, Memory leaks; Shallow copies; Memory Leaks Accessing the objects takes a performance hit. 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For example, a std::string and std::vector can be created at modified at compile-time. And as usual with those kinds of experiments: pleas measure, measure and measure - according to your needs and requirements. Consequently, std::span also holds int's. C++: Defined my own assignment operator for my type, now .sort() wont work on vectors of my type? Consenting to these technologies will allow us to process data such as browsing behavior or unique IDs on this site. But you should not resort to using pointers. If you create a shared pointer through make_shared, then the control block will be placed next to the memory block for the object. Before we can update any fields of the first particle, it has to be fetched from the main memory into cache/registers. The test code will take each element of the problem slightly different data: For all our tests the variance is severely affected, its clearly Your time developing the code is worth more than the time that the program runs. With the Celero The declaration: vector v(5); creates a vector containing five null pointers. As you can see this time, we can see the opposite effect. It also avoids mistakes like forgetting to delete or double deleting. To mimic real life case we can You have to manually iterate the vector and delete the pointers yourself when you know they're dynamically allocated, or better, use std::unique_ptr and you never need to call delete on anything. Deleting the object will not get rid of the pointers, in neither of the arrays. Question/comment: as far as I understand span is not bounds-safe. New comments cannot be posted and votes cannot be cast. function objects versus function pointers, Proper destruction of pointers to objects, memory mapped files and pointers to volatile objects. To compile the above example in linux use. The main difference between a std::span and a std::string_view is that a std::span can modify its objects. A typical implementation consists of a pointer to its first element and a size. The size of std::vector is fixed, because it essentially just contains a pointer to the real data that is dynamically allocated. visible on the chart below: Of course, running benchmarks having on battery is probably not the gathered samples). (On the other hand, calling delete on a pointer value runs the destructor for the pointed-to object, and frees the memory.). The algorithmstd::iota fills myVec with thesequentially increasing values, starting with 0. Larger objects will take more time to copy, as well as complex or compound objects. 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). Copying a pointer into a vector is not dependent on the object size. However, to pass a vector there are two ways to do so: Pass By value. If you need to store objects of multiple polymorphic types in the same vector, you must store pointers in order to avoid slicing. Heres the corresponding graph (this time I am using mean value of of Note that unless you have a good reason, you should probably not store the pointer in the vector, but the object itsself. With shared_ptr we have a collection of pointers that can be owned by multiple pointers. This time, however, we have a little more overhead compared to the case with unique_ptr. Thanks for the write-up. In one of our experiments, the pointer code for 80k of particles was more 266% slower than the continuous case. Heres a great summary that explains the problem: The picture comes from the book: Systems Performance: Enterprise and the Cloud. Figure 4: A Vector object after three values have been added to the vector. 2. std::vector obs1; char * * obs2; Effectively, obs1 Vector of shared pointers , memory problems after clearing the vector. 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). Not consenting or withdrawing consent, may adversely affect certain features and functions. We can use the vector of pointers to manage values that are not stored in continuous memory. Load data for the second particle. It doesn't affect the pointer. Operations with the data structures may need to be performed a huge amount of times in order for the savings to be significant. C++: Vector of objects vs. vector of pointers to new objects? There are 2 deferences before you get to the object. - default constructor, copy constructors, assignment, etc.) 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. Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Michael Dunsky. 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]. This can lead to a huge problem in long-running applications or resource-constrained hardware environments. There are many convenience functions to refer to the elements of the span. 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. the variance is also only a little disturbed. 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. As pointed out in Maciej Hs answer, your first approach results in object slicing. Create a variable and insert a value in it. Lets make a comparison: The memory is allocated on the heap but vector guarantees that the mem block is continuous. A std::span, sometimes also called a view, is never an owner. There are: The program fills the vector with all numbers from 0 to 19 (1), and initializes a std::span with it (2). If I gradually build up from one to a hundred strings in an array, is that enough information to tell which is better? This works perfectly for particles test Why can't `auto&` bind to a volatile rvalue expression? But then you have to call delete The table presents the functions to refer to the elements of a span. 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Your email address will not be published. Return a const vector of const shared pointers to const objects, A vector of pointers to objects that may or may not exist. 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++! affected by outliers. A vector of smart pointers may take additional performance hits compared to a vector of raw pointers. Boost MultiIndex - objects or pointers (and how to use them?)? 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 Most of the time its better to have objects in a single memory block. The technical storage or access that is used exclusively for anonymous statistical purposes. the object stores a large amount of data), then you might want to store pointers for efficiency reasons. Why it is valid to intertwine switch/for/if statements in C/C++? interested in more professional benchmarking In this blog post, youll see why there might be a perf difference of almost 2.5x (in both directions!) It is difficult to say anything definitive about all non-POD types as their operations (e.g. Thank you for one more great post! Contracts did not make it into C++20. That is, the elements the vector manages are the pointers, not the pointed objects. C++ Vector: push_back Objects vs push_back Pointers performance. With C++20, the answer is quite easy: Use a std::span. call function findMatches. This may be a performance savings depending on the object size. 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. 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. Therefore, we can only move vector of thread to an another vector thread i.e. We use unique_ptr so that we have clear ownership of resources while having almost zero overhead over raw pointers. 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. Click below to consent to the above or make granular choices. Do you try to use memory-efficient data structures? All of the big three C++ compilers MSVC, GCC, and Clang, support std::span. Mutual return types of member functions (C++), Catching an exception class within a template. The same problem occurs to store a collection of polymorphic objects in a vector: we have to store pointers instead of values: That means the pointer you are saving is not a pointer to the object inside the vector. 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 If the objects are in dynamic memory, the memory must be initialized first (allocated). Storing copies of objects themselves in a std::vector is inefficient and probably requires a copy assignment operator. distribution or if they were disturbed. By using our site, you You must also ask yourself if the Objects or the Object* are unique. Analysis and reporting is a breeze with Tableau, which comes a preconfigured report library, included for all cirrus customers. and returns the pointer to the vector of objects to a receiver in main function. There are more ways to create a std::span. runs and iterations all this is computed by Nonius. In the article, weve done several tests that compared adjacent data structures vs a case with pointers inside a container. we can not copy them, only move them. Built on the Hugo Platform! For this blog post, lets assume that Object is just a regular class, without any virtual methods. get even more flexibility and benchmarks can be executed over different Now lets create a std::function<> object that we will pass to thread object as thread function i.e. This can simulate, for example, references in C#. Retrieving AST from C++ code in Visual Studio. Particles vector of pointers but not randomized: mean is 90ms and Particles vector of objects: mean is 69ms and variance should be ok. WebSet ptr [i] to point to data [i]. Lets Create a vector of std::thread objects i.e. You truly do not want to use global variables for anything without extremely good reason. The values for a given benchmark execution is actually the min of all Consenting to these technologies will allow us and our partners to process personal data such as browsing behavior or unique IDs on this site. Additionally, the hardware Prefetcher cannot figure out the pattern - it is random - so there will be a lot of cache misses and stalls. We can also push std::thread without specifically specifying std::move(), if we pass them as rvalue i.e. write a benchmark that is repeatable. If you don't use pointers, then it is a copy of the object you pass in that gets put on the vector. Which pdf bundle do you want? Some of the code is repeated, so we could even simplify this a bit more. Thanks for this tutorial, its the first tutorial I could find that resolved my issue. Thanks to CPU cache prefetchers CPUs can predict the memory access patterns and load memory much faster than when its spread in random chunks. If you want that, store smart pointers instead, ie std::unique_ptr or std::shared_ptr. * Problem Space Download a free copy of C++20/C++17 Ref Cards! In our Is comparing two void pointers to different objects defined in C++? 1. The difference to the first approach is, that here your objects get destroyed when the vector gets destroyed, whereas above they may live longer than the container, if other shared_ptrs referencing them exist. * Samples In your case, you do have a good reason, because you actually store a non-owning pointer. C++, Search a vector of objects by object attribute, Vector of const objects giving compile error. Can it contain duplicates? This time each element is a pointer to a memory block allocated in a possibly different place in RAM. Learn all major features of recent C++ Standards! However, the items will automatically be deleted when the vector is destructed. I suggest picking one data structure and moving on. 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. Further, thanks to the functions std::erase and std::erase_if, the deletion of the elements of a container works like a charm. 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. How to use find algorithm with a vector of pointers to objects in c++? You haven't provided nearly enough information. All data and information provided on this site is for informational purposes only. data for benchmarks. Insertion while initialization: Although its an option that can be used we should avoid such type of insertion as vectors store addresses within them. Please call me if you have any questions. The rest - 56b - are the bytes of the second particle. 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. Obviously there is very good locality of access to both arrays. Notice that only the first 8 Unfortunately I found it hard to create a series of benchmarks: like So it might make sense that entities and projectiles store pointers, so they actually point at the same objects. Accessing the objects is very efficient - only one dereference. 2011-2022, Bartlomiej Filipek It * Min (us) Yes and no. Containers of the STL become with C++20 more powerful. Thank you for your understanding. It affects the behavior invoked by using this pointer since the object it points to no longer exists. By looking at the data you can detect if your samples got a proper So we can Pointers. a spreadsheed to analyze it and produce charts. 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 pointers on the heap: Vector of Objects vs Vector of A subreddit for all questions related to programming in any language. Is there any advantage to putting headers in an "include" subdir of the project? 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? You wont get what You want with this code. 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I've recently released a new book on Modern C++: runs generate method - so that we have some random numbers assigned. In C++ we can declare vector pointers using 3 methods: Using std::vector container Using [ ] notations Using the new keyword (Dynamic Memory) 1. Check out the Boost documentation. Can I be sure a vector contains objects and not pointers to objects? Containers of pointers let you avoid the slicing problem. This method will be memory-bound as all operations inside are too simple. Make your choice! There is something more interesting in this simple example. Binary search with returned index in STL? Why is dereferenced element in const vector of int pointers mutable? by Bartlomiej Filipek. So, as usual, its best to measure and measure. All rights reserved. Cirrus advanced automation frees up personnel to manage strategic initiatives and provides the ability to work from anywhere, on any device, with the highest level of security available. 10k. Windows High Performance Timer for measurement. Pass By Reference. Training or Mentoring: What's the Difference? The vector will also make copies when it needs to expand the reserved memory. What about the case with a vector of pointers? So for the second particle, we need also two loads. Any other important details? Now, as std::thread objects are move only i.e. 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). This may be performance hit because the processor may have to reload the data cache when dereferencing the pointer to the object. Why is this? Due to how CPU caches work these days, things are not simple anymore. Should I store entire objects, or pointers to objects in containers? The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user. In my seminar, I often hear the question: How can I safely pass a plain array to a function? std::vector and other containers will just remove the pointer, they won't free the memory the pointer points to. These seminars are only meant to give you a first orientation. In In Re Man. Objects that cannot be copied/moved do require a pointer approach; it is not a matter of efficiency. Therefore, we need to move these 2 thread objects in vector i.e. If we use default deleter or stateless deleter, then theres no extra memory use. I'm happy to give online seminars or face-to-face seminars worldwide. Most processors don't follow pointers when loading their data cache. 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). 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. I've recently released a new book on Modern C++: Intel i7 4720HQ, 12GB Ram, 512 SSD, Windows 10. You still need to do the delete yourself as, again, the vector is only managing the pointer, not the YourType. So, why it is so important to care about iterating over continuous block of memory? Will you spend more time looping through it than adding elements to it? but with just battery mode (without power adapter attached) I got 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. This is a bad design at any rate, because the vector can internally make copies of the stored objects, so pointers to those objects will be invalidated on a regular basis.