C++ - Allocators & pmr

Overview

Estimated time: 60–80 minutes

Control memory behavior with allocators. Use std::pmr polymorphic allocators and memory resources for performance-sensitive or bounded-allocation scenarios.

Learning Objectives

• Understand classic allocator hooks and why pmr simplifies usage.
• Use pmr containers with monotonic or unsynchronized_pool resource.

Prerequisites

• C++ - Smart Pointers & RAII

pmr basics

#include 
#include 
#include 
#include 

int main(){
  std::pmr::monotonic_buffer_resource pool;
  std::pmr::vector v{ &pool };
  v.emplace_back("alpha", &pool);
  v.emplace_back("beta", &pool);
  for (auto& s: v) std::cout << s << "\n";
}

Custom buffer

#include 
#include 
#include 

int main(){
  std::array buf{};
  std::pmr::monotonic_buffer_resource pool(buf.data(), buf.size());
  std::pmr::vector v{ &pool };
  for (int i=0;i<100;++i) v.push_back(i);
}

Common Pitfalls

• pmr containers do not own the resource—ensure resource outlives containers.
• Monotonic resource never frees individual allocations; reset between phases.

Checks for Understanding

1. When use pmr over default allocators?
2. How to avoid dangling resource usage?

Exercises

1. Benchmark std::vector vs std::pmr::vector with a monotonic resource.
2. Use an unsynchronized_pool_resource for many small allocations and compare performance.