# std::pmr::set_default_resource

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< cpp‎ | memory

Defined in header `<memory_resource>` |  |
std::pmr::memory_resource* set_default_resource(std::pmr::memory_resource* r) noexcept; |  | (since C++17)

If r is not null, sets the default memory resource pointer to r; otherwise, sets the default memory resource pointer to std::pmr::new_delete_resource().

The default memory resource pointer is used by certain facilities when an explicit memory resource is not supplied. The initial default memory resource pointer is the return value of std::pmr::new_delete_resource.

This function is thread-safe. Every call to `std::pmr::set_default_resource` synchronizes with (see std::memory_order) the subsequent `std::pmr::set_default_resource` and std::pmr::get_default_resource calls.

### Return value

Returns the previous value of the default memory resource pointer.

### Example

Run this code

    #include <array>
    #include <cstddef>
    #include <cstdint>
    #include <iostream>
    #include <iterator>
    #include <memory_resource>
    #include <vector>

    class noisy_allocator : public std::pmr::memory_resource
    {
        void* do_allocate(std::size_t bytes, std::size_t alignment) override
        {
            std::cout << "+ Allocating " << bytes << " bytes @ ";
            void* p = std::pmr::new_delete_resource()->allocate(bytes, alignment);
            std::cout << p << '\n';
            return p;
        }

        void do_deallocate(void* p, std::size_t bytes, std::size_t alignment) override
        {
            std::cout << "- Deallocating " << bytes << " bytes @ " << p << '\n';
            return std::pmr::new_delete_resource()->deallocate(p, bytes, alignment);
        }

        bool do_is_equal(const std::pmr::memory_resource& other) const noexcept override
        {
            return std::pmr::new_delete_resource()->is_equal(other);
        }
    };

    int main()
    {
        constexpr int push_back_limit{16};
        noisy_allocator mem;
        std::pmr::set_default_resource(&mem);

        {
            std::cout << "Entering scope #1 (without buffer on stack)...\n";
            std::cout << "Creating vector v...\n";
            std::pmr::vector<std::uint16_t> v{ {1, 2, 3, 4} };
            std::cout << "v.data() @ " << v.data() << '\n';

            std::cout << "Requesting more...\n";
            for (int i{0}; i != push_back_limit; ++i)
            {
                v.push_back(i);
                std::cout << "v.size(): " << v.size() << '\n';
            }
            std::cout << "Exiting scope #1...\n";
        }

        std::cout << '\n';

        {
            std::cout << "Entering scope #2 (with buffer on stack)...\n";

            std::uint8_t buffer[16];
            std::cout << "Allocating buffer on stack: " << sizeof buffer << " bytes @ "
                      << static_cast<void*>(buffer) << '\n';
            std::pmr::monotonic_buffer_resource mem_res{std::data(buffer), std::size(buffer)};

            std::cout << "Creating vector v...\n";
            std::pmr::vector<std::uint16_t> v{ {1, 2, 3, 4}, &mem_res };
            std::cout << "v.data() @ " << v.data() << '\n'; // equals to `buffer` address

            std::cout << "Requesting more...\n";
            for (int i{0}; i != push_back_limit; ++i)
            {
                v.push_back(i);
                std::cout << "v.size(): " << v.size() << '\n';
            }
            std::cout << "Exiting scope #2...\n";
        }
    }

Possible output:

    Entering scope #1 (without buffer on stack)...
    Creating vector v...
    + Allocating 8 bytes @ 0x1f75c30
    v.data() @ 0x1f75c30
    Requesting more...
    + Allocating 16 bytes @ 0x1f75c50
    - Deallocating 8 bytes @ 0x1f75c30
    v.size(): 5
    v.size(): 6
    v.size(): 7
    v.size(): 8
    + Allocating 32 bytes @ 0x1f75c70
    - Deallocating 16 bytes @ 0x1f75c50
    v.size(): 9
    v.size(): 10
    v.size(): 11
    v.size(): 12
    v.size(): 13
    v.size(): 14
    v.size(): 15
    v.size(): 16
    + Allocating 64 bytes @ 0x1f75ca0
    - Deallocating 32 bytes @ 0x1f75c70
    v.size(): 17
    v.size(): 18
    v.size(): 19
    v.size(): 20
    Exiting scope #1...
    - Deallocating 64 bytes @ 0x1f75ca0

    Entering scope #2 (with buffer on stack)...
    Allocating buffer on stack: 16 bytes @ 0x7fffbe9f8240
    Creating vector v...
    v.data() @ 0x7fffbe9f8240
    Requesting more...
    + Allocating 64 bytes @ 0x1f75ca0
    v.size(): 5
    v.size(): 6
    v.size(): 7
    v.size(): 8
    v.size(): 9
    v.size(): 10
    v.size(): 11
    v.size(): 12
    v.size(): 13
    v.size(): 14
    v.size(): 15
    v.size(): 16
    + Allocating 128 bytes @ 0x1f75cf0
    v.size(): 17
    v.size(): 18
    v.size(): 19
    v.size(): 20
    Exiting scope #2...
    - Deallocating 128 bytes @ 0x1f75cf0
    - Deallocating 64 bytes @ 0x1f75ca0

### See also

get_default_resource (C++17) | gets the default std::pmr::memory_resource (function)
new_delete_resource (C++17) | returns a static program-wide std::pmr::memory_resource that uses the global operator new and operator delete to allocate and deallocate memory (function)

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