NAME

std::pmr::null_memory_resource - std::pmr::null_memory_resource

Synopsis

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

Returns a pointer to a memory_resource that doesn't perform any allocation.

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.

Example

The program demos the main usage of null_memory_resouce: 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:

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