NAME¶

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

Synopsis¶

Defined in header <memory_resource>
std::pmr::memory_resource* set_default_resource( (since C++17)
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)