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std::pmr::n...ry_resource(3) C++ Standard Libary std::pmr::n...ry_resource(3) NAME std::pmr::null_memory_resource - std::pmr::null_memory_resource Synopsis Defined in header <memory_resource> std::pmr::memory_resource* null_memory_resource() noexcept; (since C++17) Returns a pointer to a memory_resource that doesn't perform any al- location. Return value Returns a pointer p to a static storage duration object of a type derived from std::pmr::memory_resource, with the following properties: * its allocate() function always throws std::bad_alloc; * its deallocate() function has no effect; * for any memory_resource r, p->is_equal(r) returns &r == p. The same value is returned every time this function is called. Example The program demos the main usage of null_memory_resouce: ensure that a memory pool which requires memory allocated on the stack will NOT allocate mem- ory on the heap if it needs more memory. // Run this code #include <array> #include <cstddef> #include <iostream> #include <memory_resource> #include <string> #include <unordered_map> int main() { // allocate memory on the stack std::array<std::byte, 20000> buf; // without fallback memory allocation on heap std::pmr::monotonic_buffer_resource pool{ buf.data(), buf.size(), std::pmr::null_mem- ory_resource() }; // allocate too much memory std::pmr::unordered_map<long, std::pmr::string> coll{ &pool }; try { for (std::size_t i = 0; i < buf.size(); ++i) { coll.emplace(i, "just a string with number " + std::to_string(i)); if (i && i % 50 == 0) std::clog << "size: " << i << "...\n"; } } catch(const std::bad_alloc& e) { std::cerr << e.what() << '\n'; } std::cout << "size: " << coll.size() << '\n'; } Possible output: size: 50... size: 100... size: 150... std::bad_alloc size: 183 http://cppreference.com 2022.07.31 std::pmr::n...ry_resource(3)
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