330.md

Info

• Did you know that C++17 added std::pmr::polymorphic_allocator?

  • https://eel.is/c++draft/mem.res.syn#header:%3cmemory_resource

Example

#include <memory_resource>
#include <cassert>

int main() {
    // Create a memory resource
    std::pmr::monotonic_buffer_resource resource(1024);

    // Create a polymorphic allocator using the memory resource
    std::pmr::polymorphic_allocator<int> allocator(&resource);

    // Allocate memory for an array of 10 integers using the polymorphic allocator
    int* ptr = allocator.allocate(10);

    // Test that the allocation was successful
    assert(ptr);

    // Deallocate the memory
    allocator.deallocate(ptr, 10);
}

https://godbolt.org/z/1h7voEGo6

Puzzle

• Can you create pmr::vector which will use pmr::monotonic_buffer_resource/pmr::polymorphic_allocator?

#include <memory_resource>
#include <vector>

int main() {
    using namespace boost::ut;
    std::array<char, 1024> buffer{};

    "pmr"_test = [&buffer] {
        std::pmr::vector<char> vec{};  // TODO

        vec.push_back('a');
        vec.push_back('b');
        vec.push_back('c');

        expect(vec[0] == 'a' and vec[1] == 'b' and vec[2] == 'c');
        expect(std::string_view(buffer.data(), buffer.size()).contains("abc"));
    };
}

https://godbolt.org/z/o1hx1fdEn

Solutions

int main() {
    using namespace boost::ut;
    std::array<char, 1024> buffer{};

    "pmr"_test = [&buffer] {
        std::pmr::monotonic_buffer_resource buf{buffer.data(), buffer.size()};
        std::pmr::polymorphic_allocator<int> alloc{&buf};
        std::pmr::vector<char> vec{alloc};

        vec.push_back('a');
        vec.push_back('b');
        vec.push_back('c');

        expect(vec[0] == 'a' and vec[1] == 'b' and vec[2] == 'c');
        expect(std::string_view(buffer.data(), buffer.size()).contains("abc"));
    };
}

https://godbolt.org/z/9xP7odnc6

int main() {
    using namespace boost::ut;

    std::array<char, 1024> buffer{};

    "pmr"_test = [&buffer] {
        std::pmr::monotonic_buffer_resource resource{std::data(buffer),
                                                     std::size(buffer)};
        std::pmr::polymorphic_allocator<char> allocator{&resource};

        std::pmr::vector<char> vec{allocator};

        vec.push_back('a');
        vec.push_back('b');
        vec.push_back('c');

        expect(vec[0] == 'a' and vec[1] == 'b' and vec[2] == 'c');
        expect(std::string_view(buffer.data(), buffer.size()).contains("abc"));
    };
}

https://godbolt.org/z/dK9M6M1fx

int main() {
    using namespace boost::ut;
    std::array<char, 1024> buffer{};
    std::pmr::monotonic_buffer_resource resource{buffer.data(), buffer.size()};

    "pmr"_test = [&] {
        std::pmr::vector<char> vec(&resource);

        vec.push_back('a');
        vec.push_back('b');
        vec.push_back('c');

        expect(vec[0] == 'a' and vec[1] == 'b' and vec[2] == 'c');
        expect(std::string_view(buffer.data(), buffer.size()).contains("abc"));
    };
}

https://godbolt.org/z/obM6vv8Yc

int main() {
    using namespace boost::ut;
    std::array<char, 1024> buffer{};

    "pmr"_test = [&buffer] {
        // Create a memory resource
        std::pmr::monotonic_buffer_resource resource{buffer.data(),
                                                     buffer.size()};

        // Create a polymorphic allocator using the memory resource
        std::pmr::polymorphic_allocator<char> allocator(&resource);

        // std::vector<char, decltype(allocator)> vec{allocator};
        std::pmr::vector<char> vec{allocator};

        vec.push_back('a');
        vec.push_back('b');
        vec.push_back('c');

        expect(vec[0] == 'a' and vec[1] == 'b' and vec[2] == 'c');
        expect(std::string_view(buffer.data(), buffer.size()).contains("abc"));
    };
}

https://godbolt.org/z/f6boajrG4