std::pmr::null_memory_resource
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pmr::null_memory_resource
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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 allocation.
[edit] 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_resourcer, p->is_equal(r) returns &r == p.
The same value is returned every time this function is called.
[edit] Example
The program demos the main usage of null_memory_resource: ensure that a memory pool which requires memory allocated on the stack will NOT allocate memory 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_memory_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