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_resourcer, 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 stackstd::array<std::byte, 20000> buf;   // without fallback memory allocation on heapstd::pmr::monotonic_buffer_resource pool{buf.data(), buf.size(), std::pmr::null_memory_resource()};   // allocate too much memorystd::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(conststd::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