vector of objects vs vector of pointers

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. Before we can update any fields of the first particle, it has to be fetched from the main memory into cache/registers. randomize such pointers so they are not laid out consecutively in Can I be sure a vector contains objects and not pointers to objects? In our Before we can update any fields of the first particle, it has to be fetched from the main memory into cache/registers. Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Michael Dunsky. You haven't provided nearly enough information. There are: To compile the above example in linux use. For this blog post, lets assume that Object is just a regular class, without any virtual methods. Do you optimise for memory access patterns? when I want to test the same code but with different data set. 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. In Nonius we can use a bit more advanced approach Memory leaks; Shallow copies; Memory Leaks This contiguous memory can be a plain array, a pointer with a size, a std::array, a std::vector, or a std::string. Do you try to use memory-efficient data structures? This can affect the performance and be totally different than a regular use case when objects are allocated in random order at a random time and then added to a container. Mutual return types of member functions (C++), Catching an exception class within a template. If you need to store objects of multiple polymorphic types in the same vector, you must store pointers in order to avoid slicing. However, you can choose to make such a 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 In the case of an array of pointers to objects, you must free the objects manually if that's what you want. With Nonius I have to write 10 benchmarks separately. Additionally, the 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. Press question mark to learn the rest of the keyboard shortcuts. Interesting thing is when I run the same binary on the same hardware, Well, it depends on what you are trying to do with your vector. Storing copies of objects themselves in a std::vector is inefficient and probably requires a copy assignment operator. For 1000 particles we need on the average 2000 cache line reads! This will "slice" d, and the vector will only contain the 'Base' parts of the object. When I run Celero binary in measurements/samples) and only one iteration (in Nonius there was 100 no viable conversion from 'int' to 'Student'. To provide the best experiences, we and our partners use technologies like cookies to store and/or access device information. Due to how CPU caches work these days, things are not simple anymore. The code will suffer from a memory leak if the programmer does not free up the memory before exiting. Persistent Mapped Buffers, Benchmark Results. This time, however, we have a little more overhead compared to the case with unique_ptr. can be as inexpensive as a POD's or arbitrarily more expensive. 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. 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. My last results, on older machine (i5 2400) showed that pointers code The pointer is such that range [data (), data () + size ()) is always a valid range, even if the container is empty ( data () is not dereferenceable in that case). C++ - 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. This may be performance hit because the processor may have to reload the data cache when dereferencing the pointer to the object. Obviously there is very good locality of access to both arrays. This may be a performance savings depending on the object size. 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). libraries it would be good to revisit my old approach and measure the data again. We get similar results to the data we get with Nonius: Celero doesnt give you an option to directly create a graph (as Before randomisation, we could get the following pointers addresses: The second table shows large distances between neighbour objects. 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 Thanks a lot to my Patreon Supporters: Matt Braun, Roman Postanciuc, Tobias Zindl, G Prvulovic, Reinhold Drge, Abernitzke, Frank Grimm, Sakib, Broeserl, Antnio Pina, Sergey Agafyin, , Jake, GS, Lawton Shoemake, Animus24, Jozo Leko, John Breland, Venkat Nandam, Jose Francisco, Douglas Tinkham, Kuchlong Kuchlong, Robert Blanch, Truels Wissneth, Kris Kafka, Mario Luoni, Friedrich Huber, lennonli, Pramod Tikare Muralidhara, Peter Ware, Daniel Hufschlger, Alessandro Pezzato, Bob Perry, Satish Vangipuram, Andi Ireland, Richard Ohnemus, Michael Dunsky, Leo Goodstadt, John Wiederhirn, Yacob Cohen-Arazi, Florian Tischler, Robin Furness, Michael Young, Holger Detering, Bernd Mhlhaus, Matthieu Bolt, Stephen Kelley, Kyle Dean, Tusar Palauri, Dmitry Farberov, Juan Dent, George Liao, Daniel Ceperley, Jon T Hess, Stephen Totten, Wolfgang Ftterer, Matthias Grn, Phillip Diekmann, Ben Atakora, and Ann Shatoff. Let us know in comments. If the copying and/or assignment operations are expensive (e.g. You wont get what You want with this code. Thank you! Notice that only the first 8 bytes from the second load are used for the first particle. This decay is a typical reason for errors in C/C++. To mitigate this issue, the benchmark code adds a randomisation step: ShuffleVector(). A view from the ranges library is something that you can apply on a range and performs some operation. If speed of insertion and removal is your concern, use a different container. The following program shows how a subspan can be used to modify the referenced objects from a std::vector. A typical implementation consists of a pointer to its first element and a size. How to erase & delete pointers to objects stored in a vector? Make your choice! 3. Pointers. 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. 10k. method: Only the code marked as //computation (that internal lambda) will be 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. First of all we need to define a fixture class: The code above returns just a vector of pairs {1k, 0}, {2k, 0}, {10k, A vector of smart pointers may take additional performance hits compared to a vector of raw pointers. If not, then to change an Object in a vector