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.
Returns a pointer p to a static storage duration object of a type derived from std::pmr::memory_resource, with the following properties:
allocate() function always throws std::bad_alloc; deallocate() function has no effect; memory_resource r, p->is_equal(r) returns &r == p. The same value is returned every time this function is called.
The program demos the main usage of null_memory_resource: ensure that a memory pool which requires memory allocated on the stack will NOT allocate memory on the heap if it needs more memory.
#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
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