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Monday, May 7 • 2:30pm - 4:00pm
An Allocator is a Handle to a Heap

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C++17 introduced the std::pmr framework. In this framework, a std::pmr::polymorphic_allocator<T> holds a pointer to a std::pmr::memory_resource. The memory resource is in charge of managing and organizing the heap itself, and the allocator object is just a thin "handle" pointing to the memory resource.

This is not just a convenient implementation strategy for std::pmr! Rather, this elucidates the true meaning of the Allocator concept which has existed, unchanged, since C++98. An Allocator *is* a handle to a MemoryResource. Even std::allocator can — and should — be viewed as a handle to a global singleton "heap", and not as a MemoryResource in its own right.

From this core insight we derive many corollaries, such as the need for allocator types to be lightweight and efficiently copyable, the fundamental impossibility of implementing an "in-place" std::vector via stupid allocator tricks, and the philosophical underpinnings of "rebinding."

We'll show at least two non-standard examples of types modeling Allocator that act as different kinds of handles to heaps: a `shmem_allocator` that holds a `shmem_ptr` to a memory resource, and a `shutdown_safe_allocator` that holds a `weak_ptr` to a memory resource.

Time permitting, we'll
- discuss what we can expect from a "moved-from" allocator object
- relate the notion of "handle" to neighboring notions such as "façade" and "adaptor"
- suggest similarities between "allocator/heap" and "executor/execution-context"

avatar for Arthur O'Dwyer

Arthur O'Dwyer

Arthur O'Dwyer is the author of "Colossal Cave: The Board Game," "Mastering the C++17 STL" (the book), and "The STL From Scratch" (the training course). He runs professional C++ training courses, is occasionally active on the C++ Standards Committee, and has a blog mostly about C... Read More →

Monday May 7, 2018 2:30pm - 4:00pm
Flug Auditorium

Attendees (45)