std::pmr::set_default_resource(3)

NAME
std::pmr::set_default_resource - std::pmr::set_default_resource

Synopsis
   Defined in header <memory_resource>
   std::pmr::memory_resource* (since C++17)
   set_default_resource(std::pmr::memory_resource* r) noexcept;
   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 gets the default std::pmr::memory_resource
   (C++17) (function)
                        returns a static program-wide std::pmr::memory_resource that
   new_delete_resource uses the global operator new and operator delete to allocate
   (C++17) and deallocate memory
                        (function)