C++ std::pmr Guide: Polymorphic Memory Resources & Memory Pools [#39-2]

Introduction: malloc dominates your profile

Pools & polymorphic allocators

Frequent small allocations fragment the heap and burn cycles. Memory pools pre-allocate slabs; C++17 std::pmr lets std::pmr::vector, std::pmr::string, etc. share std::pmr::memory_resource.

Topics: scenarios, monotonic_buffer_resource, synchronized_pool_resource, custom memory_resource, pmr containers, errors (lifetime!), alignment, benchmarks, production patterns (per-request arena, game frame).

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monotonic_buffer_resource

Allocate forward-only; O(1) bump pointer; release/reset at scope end—ideal HTTP request or game frame scratch.

std::pmr::monotonic_buffer_resource pool;
std::pmr::vector<std::pmr::string> parts{&pool};

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pool resources

unsynchronized_pool_resource / synchronized_pool_resource for many fixed-ish sizes with frees.

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Custom memory_resource

Override do_allocate/do_deallocate/do_is_equal to plug jemalloc arenas, GPU pinned memory, etc.

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Pitfalls

• Dangling: pmr containers must not outlive their memory_resource.
• Thread safety: unsynchronized resources require external sync.
• Alignment: respect alignof for over-aligned types.

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Benchmarks

Compare default allocator vs pmr monotonic on representative parsers—expect fewer syscalls and better locality.

Full examples in this series also cover std::pmr::map in a request-handler style allocator setup.

Keywords

std::pmr, memory_resource, monotonic buffer, allocator, performance