std::pmr::null_memory_resource

 

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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_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.

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