1. Problem It Solves
Allocator types normally become part of a container's static type, making runtime allocation-policy changes awkward. C++17 polymorphic allocators route allocation through a runtime memory_resource interface.
This lesson reduces that broad problem to one fixed-input program so the language rule and its observable result can be checked independently.
2. Prerequisites
A C++17 compiler invoked with warnings enabled and the earlier lessons listed in the course order.
Know allocators, alignment, object lifetime, container allocation, and stack-backed buffers.
3. Core Idea
Containers in std::pmr use polymorphic_allocator and point to a resource. monotonic_buffer_resource serves allocations from a buffer and releases them together, trading individual deallocation for cheap bulk lifetime.
Keep the type, object lifetime, ownership, and evaluation boundary visible while reading the example; syntax is useful only when those semantics are understood.
4. Minimal Syntax
std::byte buffer[1024];
std::pmr::monotonic_buffer_resource arena{buffer, sizeof buffer};
std::pmr::vector<int> values{&arena};5. How It Works
A monotonic resource receives a fixed local buffer, and a PMR vector is explicitly associated with that resource.
The vector's dynamic storage comes from the arena when space permits; destruction ends elements before the resource releases storage in bulk.
The program prints
size: 4andsum: 10, giving a small test oracle that can be compared with the prediction made before compilation.
6. Common Mistakes
A container must not outlive its resource, and monotonic growth is unsuitable when long-lived workloads require meaningful individual reclamation.
A successful build is not proof of correct semantics. Recheck lifetimes, invalidation, ordering, error paths, and required headers or link flags for the real program.
7. When to Use It
Use this technique when many related short-lived allocations share a clear arena lifetime and profiling shows allocation overhead.
Choose a simpler C++11/14 form when the C++17 rule does not improve safety, clarity, or measured performance for the supported toolchains.
8. Simple Example
The buffer, resource, and vector are declared in lifetime order so destruction occurs safely in reverse order.
The companion .cpp file has no input or external dependency. Predict the complete output, compile it, run it, then change one constant and explain the new result.
Complete sample code
Source file
cpp17/40_pmr_polymorphic_memory_resources/main.cpp
#include <array>
#include <cstddef>
#include <iostream>
#include <memory_resource>
#include <numeric>
#include <vector>
int main() {
std::array<std::byte, 1024> buffer{};
std::pmr::monotonic_buffer_resource arena{
buffer.data(), buffer.size()};
std::pmr::vector<int> values{
std::pmr::polymorphic_allocator<int>{&arena}};
values.insert(values.end(), {1, 2, 3, 4});
const int total = std::accumulate(values.begin(), values.end(), 0);
std::cout << "size: " << values.size() << '\n';
std::cout << "sum: " << total << '\n';
}
9. Key Takeaways
PMR separates container algorithms from runtime allocation policy, but resource lifetime becomes an explicit dependency.
C++17 mode must be selected explicitly; a newer compiler default can otherwise hide a portability error.
Warnings, deterministic examples, and small assertions turn a remembered rule into evidence.
Document any lifetime, ownership, synchronization, or allocation contract at the API boundary.
10. Self-Check Questions
Easy — What problem does std::pmr and Polymorphic Memory Resources address?
Medium — Which object must outlive the PMR vector?
Hard — What happens when the initial monotonic buffer is exhausted and an upstream resource exists?