std::pmr::n...ry_resource(3)

💅 std::pmr::n...ry_resource(3) 💅

okay bestie, welcome to the C++ Standard Libary manual — yes, that’s literally how the manual name is spelled up top, and no I’m not fixing it, iykyk. this page is all about std::pmr::null_memory_resource, and it’s lowkey iconic because it’s a memory resource that does zero allocation. no cap.

💖 NAME 💖

std::pmr::null_memory_resource — std::pmr::null_memory_resource

the way the name is just the same thing twice? periodt, that’s the official label.

✨ Synopsis ✨

Defined in header <memory_resource>.

std::pmr::memory_resource* null_memory_resource() noexcept;  (since C++17)

this lil function returns a pointer to a memory_resource that doesn’t perform any allocation. yes bestie, that’s the whole vibe: a resource that refuses to allocate. 💅

🫶 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. consistency ate, fr fr. ✨

🌸 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 🌈

Possible output:

        size: 50...
        size: 100...
        size: 150...
        std::bad_alloc
        size: 183

source: http://cppreference.com · version/date: 2024.06.10 — byeee bestie, stay allocated (or don’t, that’s literally the point) 💖🦋✨