Most Malleable Memory Management Method

Examples for wanting to manage the memory usage of your program can be to reduce the cost of heap allocations, improve locality of reference, or maybe reduce heap fragmentation.

Regardless of reason, PMR, Polymorphic Memory Resource, is available since C++17, and makes your life much easier.

I will show you...

* Tools and techniques for analysing the memory usage of your program.

* How PMR makes memory management easier.

* How to use PMR with the standard library types.

* How to make your own types use PMR.

* Advice, and pitfalls to avoid, on your quest to improving the memory usage of your program.

1. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 1/112
2. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 2/112 Most Malleable Memory Management Method Björn Fahller
3. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 3/112 Most Malleable Memory Management Method
4. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 4/112 Most Malleable Memory Management Method
5. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 5/112 Most Malleable Memory Management Method
6. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 6/112 Most Malleable Memory Management Method
7. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 7/112 Most Malleable Memory Management Method Björn Fahller
8. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 8/112 The heap Allows you to... – Allocate objects of any size and alignment – Allocate and deallocate from any thread • Even allocate in one thread and deallocate in another – Let objects lifetimes vary wildly
9. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 9/112 The heap Allows you to... – Allocate objects of any size and alignment – Allocate and deallocate from any thread • Even allocate in one thread and deallocate in another – Let objects lifetimes vary wildly Complex structure with lookup time and space overhead
10. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 10/112 The heap Allows you to... – Allocate objects of any size and alignment – Allocate and deallocate from any thread • Even allocate in one thread and deallocate in another – Let objects lifetimes vary wildly Poor locality of reference
11. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 11/112 The heap Allows you to... – Allocate objects of any size and alignment – Allocate and deallocate from any thread • Even allocate in one thread and deallocate in another – Let objects lifetimes vary wildly Fragmentation may make everything worse over time
12. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 12/112 The heap Allows you to... – Allocate objects of any size and alignment – Allocate and deallocate from any thread • Even allocate in one thread and deallocate in another – Let objects lifetimes vary wildly Synchronisation overhead
13. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 13/112 So you turn to allocators...
14. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 14/112 So you turn to allocators... https://www.edvardmunch.org/the-scream.jsp
15. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 15/112 So you turn to allocators... https://www.edvardmunch.org/the-scream.jsp But PMR allocators suck less
16. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 16/112 Polymorphic Memory Resource “container” PMR allocator <<memory resource>>
17. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 17/112 Polymorphic Memory Resource “container” PMR allocator <<memory resource>> A “container” has a PMR allocator
18. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 18/112 Polymorphic Memory Resource “container” PMR allocator <<memory resource>> A “container” has a PMR allocator A PMR allocator references a memory resource
19. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 19/112 Polymorphic Memory Resource “container” PMR allocator <<memory resource>> A “container” has a PMR allocator A PMR allocator references a memory resource Standard containers are conveniently available under namespace std::pmr, e.g. std::pmr::vector<int>
20. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 20/112 memory_resource class std::pmr::memory_resource { public:         void* allocate(size_t bytes, size_t alignment);         void deallocate(void* addr, size_t bytes, size_t alignment);         bool is_equal(const memory_resource&) const noexcept; private:         virtual void* do_allocate(size_t bytes, size_t align) = 0;         virtual void do_deallocate(void* addr, size_t bytes, size_t align) = 0;         virtual bool do_is_equal(const memory_resource&) const noexcept = 0; }; <memory_resource>
21. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 21/112 memory_resource class std::pmr::memory_resource { public:         void* allocate(size_t bytes, size_t alignment);         void deallocate(void* addr, size_t bytes, size_t alignment);         bool is_equal(const memory_resource&) const noexcept; private:         virtual void* do_allocate(size_t bytes, size_t align) = 0;         virtual void do_deallocate(void* addr, size_t bytes, size_t align) = 0;         virtual bool do_is_equal(const memory_resource&) const noexcept = 0; }; <memory_resource> Inherit and implement these
22. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 22/112 memory_resource class std::pmr::memory_resource { public:         void* allocate(size_t bytes, size_t alignment);         void deallocate(void* addr, size_t bytes, size_t alignment);         bool is_equal(const memory_resource&) const noexcept; private:         virtual void* do_allocate(size_t bytes, size_t align) = 0;         virtual void do_deallocate(void* addr, size_t bytes, size_t align) = 0;         virtual bool do_is_equal(const memory_resource&) const noexcept = 0; }; <memory_resource> Mildly annoying that alignment uses size_t and not align_val_t Inherit and implement these
23. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 23/112 memory_resource class tracing_resource final : public std::pmr::memory_resource { public:     tracing_resource(std::ostream& os) : os_(os) {} private:     void* do_allocate(size_t bytes, size_t align) override    {         auto addr = operator new (bytes, std::align_val_t(align));         os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n';         return addr;     }     void do_deallocate(void* addr, size_t bytes, size_t align) override {         os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n";         operator delete(addr, bytes, std::align_val_t(align));     }     bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override {         return &other == this;     }     std::ostream& os_; };
24. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 24/112 memory_resource class tracing_resource final : public std::pmr::memory_resource { public:     tracing_resource(std::ostream& os) : os_(os) {} private:     void* do_allocate(size_t bytes, size_t align) override    {         auto addr = operator new (bytes, std::align_val_t(align));         os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n';         return addr;     }     void do_deallocate(void* addr, size_t bytes, size_t align) override {         os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n";         operator delete(addr, bytes, std::align_val_t(align));     }     bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override {         return &other == this;     }     std::ostream& os_; };
25. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 25/112 memory_resource class tracing_resource final : public std::pmr::memory_resource { public:     tracing_resource(std::ostream& os) : os_(os) {} private:     void* do_allocate(size_t bytes, size_t align) override    {         auto addr = operator new (bytes, std::align_val_t(align));         os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n';         return addr;     }     void do_deallocate(void* addr, size_t bytes, size_t align) override {         os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n";         operator delete(addr, bytes, std::align_val_t(align));     }     bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override {         return &other == this;     }     std::ostream& os_; };
26. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 26/112 memory_resource class tracing_resource final : public std::pmr::memory_resource { public:     tracing_resource(std::ostream& os) : os_(os) {} private:     void* do_allocate(size_t bytes, size_t align) override    {         auto addr = operator new (bytes, std::align_val_t(align));         os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n';         return addr;     }     void do_deallocate(void* addr, size_t bytes, size_t align) override {         os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n";         operator delete(addr, bytes, std::align_val_t(align));     }     bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override {         return &other == this;     }     std::ostream& os_; };
27. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 27/112 memory_resource class tracing_resource final : public std::pmr::memory_resource { public:     tracing_resource(std::ostream& os) : os_(os) {} private:     void* do_allocate(size_t bytes, size_t align) override    {         auto addr = operator new (bytes, std::align_val_t(align));         os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n';         return addr;     }     void do_deallocate(void* addr, size_t bytes, size_t align) override {         os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n";         operator delete(addr, bytes, std::align_val_t(align));     }     bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override {         return &other == this;     }     std::ostream& os_; };
28. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 28/112 https://godbolt.org/z/4WvYe1eKj
29. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 29/112 Most Malleable Memory Management Method It is very important that your memory resource instance outlives all containers that use it.
30. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 30/112 Most Malleable Memory Management Method It is very important that your memory resource instance outlives all containers that use it. It’s very easy to miss using a PMR type, and not so easy to detect the mistake
31. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 31/112 Most Malleable Memory Management Method It is very important that your memory resource instance outlives all containers that use it. It’s very easy to miss using a PMR type, and not so easy to detect the mistake There are tools, though
32. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 32/112 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; };
33. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 33/112 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; };
34. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 34/112 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; };
35. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 35/112 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; };
36. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 36/112 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; };
37. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 37/112 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; };
38. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 38/112 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; };
39. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 39/112 string frequency example class histogram { public:     void add(const std::string& word) {         ++words_[word];     }     void print_top(size_t n, std::ostream& os) const {         using count = std::pair<std::string, size_t>;         std::vector<count> popular(words_.begin(), words_.end());         std::ranges::partial_sort(popular,                                                   popular.begin() + n, std::greater{}, &count::second);         for (const auto& stat : popular | std::ranges::views::take(n)) {             os << stat.first << '\t' << stat.second << '\n';         }     } private:     std::unordered_map<std::string, size_t> words_; }; int main() { std::string word; histogram hist; while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); }
40. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 40/112 Live Demo
41. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 41/112 Most Malleable Memory Management Method Nothing surprising here, but good to have a baseline and a tool Let’s PMR it!
42. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 42/112 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; };
43. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 43/112 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; PMR-ify the container and strings
44. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 44/112 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; Get an allocator to use
45. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 45/112 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; In C++17 it was necessary to express a type here, but in C++20 onwards, just use the diamond
46. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 46/112 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; int main() { std::pmr::string word; histogram hist; while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); }
47. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 47/112 Live Demo
48. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 48/112 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; int main() { std::pmr::string word; histogram hist; while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); }
49. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 49/112 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; int main() { std::pmr::string word; histogram hist; while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); } Not much new, because a default constructed polymorphic_allocator uses the default memory_resource, which is new_delete_resource.
50. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 50/112 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; int main() { std::pmr::string word; histogram hist; while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); } But we did see it being used!
51. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 51/112 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource
52. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 52/112 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource You’ve seen this one. Plain operator new and operator delete.
53. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 53/112 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource A resource that always fails to allocate. Can be useful when testing.
54. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 54/112 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource A thread safe pool of memory blocks, in which allocations are simple indexing operations into an available slot in a block.
55. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 55/112 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource Same as above, but without the overhead of thread synchronization. Very fast when you know allocation and deallocation will happen in the same thread.
56. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 56/112 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource Very fast resource, where each allocation is at a monotonically increasing space in the available buffer. Deallocation only happens when the resource is destroyed. Useful in very local scopes.
57. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 57/112 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource You can cascade memory resources. Construct with a pointer to an upstream resource
58. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 58/112 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool);
59. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 59/112 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool); Construct a polymorphic allocator using pool.
60. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 60/112 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool); Constrct a pool resource using monotonic_buffers.
61. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 61/112 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool); Construct a monotonic buffer resource using the heap.
62. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 62/112 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool); Get the new_delete_allocator.
63. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 63/112 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool);
64. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 64/112 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool); The allocator will use the pool to allocate memory.
65. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 65/112 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool); If the pool does not have any free memory, it allocates more via the monotonic buffer.
66. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 66/112 Choosing a memory resource The header <memory_resource> offers: std::pmr::new_delete_resource std::pmr::null_resource std::pmr::synchronized_pool_resource std::pmr::unsynchronized_pool_resource std::pmr::monotonic_buffer_resource void do_work(std::pmr::polymorphic_allocator<> alloc); auto heap = std::pmr::new_delete_resource(); std::pmr::monotonic_buffer_resource monotonic_buffers(heap); std::pmr::unsynchronized_pool_resource pool(&monotonic_buffers); do_work(&pool); If the monotonic buffer doesn’t have any memory, it gets more from the heap
67. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 67/112 A slight detour class tracing_resource final : public std::pmr::memory_resource { public: tracing_resource(std::ostream& os) : os_(os) {} private: void* do_allocate(size_t bytes, size_t align) override    { auto addr = operator new (bytes, std::align_val_t(align)); os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n'; return addr; } void do_deallocate(void* addr, size_t bytes, size_t align) override { os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n"; operator delete(addr, bytes, std::align_val_t(align)); } bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override { return &other == this; } std::ostream& os_; };
68. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 68/112 A slight detour class tracing_resource final : public std::pmr::memory_resource { public: tracing_resource(std::ostream& os) : os_(os) {} private: void* do_allocate(size_t bytes, size_t align) override    { auto addr = operator new (bytes, std::align_val_t(align)); os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n'; return addr; } void do_deallocate(void* addr, size_t bytes, size_t align) override { os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n"; operator delete(addr, bytes, std::align_val_t(align)); } bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override { return &other == this; } std::ostream& os_; }; Maybe going directly to the heap isn’t such a great idea?
69. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 69/112 Improving the tracing resource class tracing_resource final : public std::pmr::memory_resource { public: tracing_resource(std::ostream& os, std::pmr::memory_resource* next = std::pmr::get_default_resource()) : os_(os), next_(next) {} private: void* do_allocate(size_t bytes, size_t align) override    {   auto* addr = next_->allocate(bytes, align); os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n'; return addr; } void do_deallocate(void* addr, size_t bytes, size_t align) override { os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n"; next_->deallocate(addr, bytes, align); } bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override { auto p = dynamic_cast<const tracing_resource*>(&other); return p && next_ == p->next_; } std::ostream& os_; std::pmr::memory_resource* next_; };
70. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 70/112 Improving the tracing resource class tracing_resource final : public std::pmr::memory_resource { public: tracing_resource(std::ostream& os, std::pmr::memory_resource* next = std::pmr::get_default_resource()) : os_(os), next_(next) {} private: void* do_allocate(size_t bytes, size_t align) override    {   auto* addr = next_->allocate(bytes, align); os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n'; return addr; } void do_deallocate(void* addr, size_t bytes, size_t align) override { os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n"; next_->deallocate(addr, bytes, align); } bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override { auto p = dynamic_cast<const tracing_resource*>(&other); return p && next_ == p->next_; } std::ostream& os_; std::pmr::memory_resource* next_; };
71. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 71/112 Improving the tracing resource class tracing_resource final : public std::pmr::memory_resource { public: tracing_resource(std::ostream& os, std::pmr::memory_resource* next = std::pmr::get_default_resource()) : os_(os), next_(next) {} private: void* do_allocate(size_t bytes, size_t align) override    {   auto* addr = next_->allocate(bytes, align); os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n'; return addr; } void do_deallocate(void* addr, size_t bytes, size_t align) override { os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n"; next_->deallocate(addr, bytes, align); } bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override { auto p = dynamic_cast<const tracing_resource*>(&other); return p && next_ == p->next_; } std::ostream& os_; std::pmr::memory_resource* next_; }; Get an underlying memory resource
72. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 72/112 Improving the tracing resource class tracing_resource final : public std::pmr::memory_resource { public: tracing_resource(std::ostream& os, std::pmr::memory_resource* next = std::pmr::get_default_resource()) : os_(os), next_(next) {} private: void* do_allocate(size_t bytes, size_t align) override    {   auto* addr = next_->allocate(bytes, align); os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n'; return addr; } void do_deallocate(void* addr, size_t bytes, size_t align) override { os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n"; next_->deallocate(addr, bytes, align); } bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override { auto p = dynamic_cast<const tracing_resource*>(&other); return p && next_ == p->next_; } std::ostream& os_; std::pmr::memory_resource* next_; }; Use it to allocate and deallocate memory
73. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 73/112 Improving the tracing resource class tracing_resource final : public std::pmr::memory_resource { public: tracing_resource(std::ostream& os, std::pmr::memory_resource* next = std::pmr::get_default_resource()) : os_(os), next_(next) {} private: void* do_allocate(size_t bytes, size_t align) override    {   auto* addr = next_->allocate(bytes, align); os_ << "allocate(" << bytes << ", " << align << ") -> " << addr << '\n'; return addr; } void do_deallocate(void* addr, size_t bytes, size_t align) override { os_ << "deallocate(" << addr << ", " << bytes << ", " << align << ")\n"; next_->deallocate(addr, bytes, align); } bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override { auto p = dynamic_cast<const tracing_resource*>(&other); return p && next_ == p->next_; } std::ostream& os_; std::pmr::memory_resource* next_; }; Use the default memory resource if none is given
74. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 74/112 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; };
75. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 75/112 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; Back to chosing an allocator for the word frequency histogram
76. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 76/112 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; int main() { std::pmr::string word; std::pmr::unsynchronized_pool_resource r; histogram hist(&r); while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); }
77. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 77/112 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; int main() { std::pmr::string word; std::pmr::unsynchronized_pool_resource r; histogram hist(&r); while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); } I choose an unsynchronized pool resource.
78. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 78/112 Live Demo
79. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 79/112 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; };
80. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 80/112 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; int main() { std::pmr::unsynchronized_pool_resource r; std::pmr::string word(&r); histogram hist(&r); while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); }
81. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 81/112 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; int main() { std::pmr::unsynchronized_pool_resource r; std::pmr::string word(&r); histogram hist(&r); while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); } 84 allocations down from 133000
82. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 82/112 PMR histogram class histogram { public: histogram(const std::pmr::polymorphic_allocator<>& alloc = {}): words_(alloc){} void add(const std::pmr::string& word) { ++words_[word]; } void print_top(size_t n, std::ostream& os) const { using count = std::pair<std::pmr::string, size_t>; std::vector<count> popular(words_.begin(), words_.end()); std::ranges::partial_sort(popular, popular.begin() + n, std::greater{}, &count::second); for (const auto& stat : popular | std::ranges::views::take(n)) { os << stat.first << '\t' << stat.second << '\n'; } } private: std::pmr::unordered_map<std::pmr::string, size_t> words_; }; int main() { std::pmr::unsynchronized_pool_resource r; std::pmr::string word(&r); histogram hist(&r); while (std::cin >> word) { hist.add(word); } hist.print_top(5, std::cout); } But you really need tools like heaptrack to find the places you’ve missed.
83. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 83/112 PMR for your types class counted_string { public: counted_string(size_t count, std::pmr::string str) : str_(std::move(str)) , count_(count)   {} std::string_view str() const { return str_; } size_t count() const { return count_; } friend bool operator<(const counted_string& lh, const counted_string& rh) { return lh.count_ < rh.count_; } protected: std::pmr::string str_; size_t count_; };
84. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 84/112 https://godbolt.org/z/8c4eKcxhh
85. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 85/112 PMR for your types A PMR enabled type needs:
86. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 86/112 PMR for your types A PMR enabled type needs: – A public allocator_type type
87. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 87/112 PMR for your types A PMR enabled type needs: – A public allocator_type type – Constructors with the allocator type as the last argument
88. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 88/112 PMR for your types A PMR enabled type needs: – A public allocator_type type – Constructors with the allocator type as the last argument – Variadic constructors with std::allocator_arg_t and allocator_type as the first parameters
89. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 89/112 PMR for your types A PMR enabled type needs: – A public allocator_type type – Constructors with the allocator type as the last argument – Variadic constructors with std::allocator_arg_t and allocator_type as the first parameters – And a copy-like constructor with allocator
90. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 90/112 PMR for your types A PMR enabled type needs: – A public allocator_type type – Constructors with the allocator type as the last argument – Variadic constructors with std::allocator_arg_t and allocator_type as the first parameters – And a copy-like constructor with allocator – And move-like constructor with allocator
91. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 91/112 PMR for your types class counted_string { public: using allocator_type = std::pmr::polymorphic_allocator<>; counted_string(size_t count,std::pmr::string str, const allocator_type& alloc = {}); template <typename ... Ts> counted_string(std::allocator_arg_t,  const allocator_type& alloc, size_t count, Ts&& ... ts); counted_string(const counted_string& orig); counted_string(const counted_string& orig, const allocator_type& alloc); counted_string(counted_string&& orig) noexcept; counted_string(counted_string&& orig, const allocator_type& alloc) noexcept; std::string_view str() const { return str_; } size_t count() const { return count_; } friend bool operator<(const counted_string& lh, const counted_string& rh); protected: std::pmr::string str_; size_t count_; };
92. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 92/112 PMR for your types class counted_string { public: using allocator_type = std::pmr::polymorphic_allocator<>; counted_string(size_t count,std::pmr::string str, const allocator_type& alloc = {}); template <typename ... Ts> counted_string(std::allocator_arg_t,  const allocator_type& alloc, size_t count, Ts&& ... ts); counted_string(const counted_string& orig); counted_string(const counted_string& orig, const allocator_type& alloc); counted_string(counted_string&& orig) noexcept; counted_string(counted_string&& orig, const allocator_type& alloc) noexcept; std::string_view str() const { return str_; } size_t count() const { return count_; } friend bool operator<(const counted_string& lh, const counted_string& rh); protected: std::pmr::string str_; size_t count_; }; Signal that this type uses a PMR allocator.
93. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 93/112 PMR for your types class counted_string { public: using allocator_type = std::pmr::polymorphic_allocator<>; counted_string(size_t count,std::pmr::string str, const allocator_type& alloc = {}); template <typename ... Ts> counted_string(std::allocator_arg_t,  const allocator_type& alloc, size_t count, Ts&& ... ts); counted_string(const counted_string& orig); counted_string(const counted_string& orig, const allocator_type& alloc); counted_string(counted_string&& orig) noexcept; counted_string(counted_string&& orig, const allocator_type& alloc) noexcept; std::string_view str() const { return str_; } size_t count() const { return count_; } friend bool operator<(const counted_string& lh, const counted_string& rh); protected: std::pmr::string str_; size_t count_; }; Constructor with allocator last
94. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 94/112 PMR for your types class counted_string { public: using allocator_type = std::pmr::polymorphic_allocator<>; counted_string(size_t count,std::pmr::string str, const allocator_type& alloc = {}); template <typename ... Ts> counted_string(std::allocator_arg_t,  const allocator_type& alloc, size_t count, Ts&& ... ts); counted_string(const counted_string& orig); counted_string(const counted_string& orig, const allocator_type& alloc); counted_string(counted_string&& orig) noexcept; counted_string(counted_string&& orig, const allocator_type& alloc) noexcept; std::string_view str() const { return str_; } size_t count() const { return count_; } friend bool operator<(const counted_string& lh, const counted_string& rh); protected: std::pmr::string str_; size_t count_; }; Variadic constructor with non-allocator args at end
95. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 95/112 PMR for your types class counted_string { public: using allocator_type = std::pmr::polymorphic_allocator<>; counted_string(size_t count,std::pmr::string str, const allocator_type& alloc = {}); template <typename ... Ts> counted_string(std::allocator_arg_t,  const allocator_type& alloc, size_t count, Ts&& ... ts); counted_string(const counted_string& orig); counted_string(const counted_string& orig, const allocator_type& alloc); counted_string(counted_string&& orig) noexcept; counted_string(counted_string&& orig, const allocator_type& alloc) noexcept; std::string_view str() const { return str_; } size_t count() const { return count_; } friend bool operator<(const counted_string& lh, const counted_string& rh); protected: std::pmr::string str_; size_t count_; }; Copy-like constructor with allocator, used by containers
96. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 96/112 PMR for your types class counted_string { public: using allocator_type = std::pmr::polymorphic_allocator<>; counted_string(size_t count,std::pmr::string str, const allocator_type& alloc = {}); template <typename ... Ts> counted_string(std::allocator_arg_t,  const allocator_type& alloc, size_t count, Ts&& ... ts); counted_string(const counted_string& orig); counted_string(const counted_string& orig, const allocator_type& alloc); counted_string(counted_string&& orig) noexcept; counted_string(counted_string&& orig, const allocator_type& alloc) noexcept; std::string_view str() const { return str_; } size_t count() const { return count_; } friend bool operator<(const counted_string& lh, const counted_string& rh); protected: std::pmr::string str_; size_t count_; }; Move-like constructor with allocator, used by containers
97. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 97/112 https://godbolt.org/z/aoYPWGxP8
98. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 98/112 In conclusion
99. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 99/112 In conclusion • Making your own types use PMR allocators is not hard, but it’s easy to make mistakes
100. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 100/112 In conclusion • Making your own types use PMR allocators is not hard, but it’s easy to make mistakes • Using PMR types with standard containers is easy
101. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 101/112 In conclusion • Making your own types use PMR allocators is not hard, but it’s easy to make mistakes • Using PMR types with standard containers is easy • PMR is very flexible because the memory resources and the types that uses them are decoupled
102. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 102/112 In conclusion • Making your own types use PMR allocators is not hard, but it’s easy to make mistakes • Using PMR types with standard containers is easy • PMR is very flexible because the memory resources and the types that uses them are decoupled • Memory usage is quite opaque, so it’s not easy to see where you’ve made mistakes
103. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 103/112 In conclusion • Making your own types use PMR allocators is not hard, but it’s easy to make mistakes • Using PMR types with standard containers is easy • PMR is very flexible because the memory resources and the types that uses them are decoupled • Memory usage is quite opaque, so it’s not easy to see where you’ve made mistakes • PMR is still not a panacea for your memory management needs, but it’s often sufficient
104. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 104/112 Just one more thing... http://mojtv.hr/images/fb2de057-6db9-45f8-ba8a-6891149a4a87.jpg
105. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 105/112 Just one more thing... http://mojtv.hr/images/fb2de057-6db9-45f8-ba8a-6891149a4a87.jpg A pmr container never changes its memory resource.
106. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 106/112 Just one more thing... http://mojtv.hr/images/fb2de057-6db9-45f8-ba8a-6891149a4a87.jpg A pmr container never changes its memory resource. Not even when move-assigning.
107. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 107/112 Just one more thing... http://mojtv.hr/images/fb2de057-6db9-45f8-ba8a-6891149a4a87.jpg A pmr container never changes its memory resource. Not even when move-assigning. • If resource operator== says the memory resources are the same, then the internal pointers are taken over, otherwise it’s element-wise move assignment to newly allocated memory.
108. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 108/112 Just one more thing... http://mojtv.hr/images/fb2de057-6db9-45f8-ba8a-6891149a4a87.jpg A pmr container never changes its memory resource. Not even when move-assigning. • If resource operator== says the memory resources are the same, then the internal pointers are taken over, otherwise it’s element-wise move assignment to newly allocated memory.
109. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 109/112 Just one more thing... http://mojtv.hr/images/fb2de057-6db9-45f8-ba8a-6891149a4a87.jpg A pmr container never changes its memory resource. Not even when move-assigning. • If resource operator== says the memory resources are the same, then the internal pointers are taken over, otherwise it’s element-wise move assignment to newly allocated memory.
110. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 110/112 Just one more thing... http://mojtv.hr/images/fb2de057-6db9-45f8-ba8a-6891149a4a87.jpg A pmr container never changes its memory resource. Not even when move-assigning. • If resource operator== says the memory resources are the same, then the internal pointers are taken over, otherwise it’s element-wise move assignment to newly allocated memory.
111. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 111/112 Learn more https://tinyurl.com/cppweekly-on-pmr YouTube playlist with 6 episodes of Jason Turner’s C++Weekly covering PMR https://youtu.be/RLezJuqNcEQ CppConn 2019 – Pablo Halpern and Alisdair Meredith, “Congratulations! You survived C++11 allocators!”
112. Mmmmm – C++OnSea 2023 © Björn Fahller @bjorn_fahller@mastodon.social 112/112 bjorn@fahller.se @bjorn_fahller@mastodon.social @rollbear Björn Fahller Most Malleable Memory Management Method