Alisdair Meredith and Pablo Halpern
Allocators are great
std::pmr::monotoic_buffer_resource src(buffer, sizeof (buffer); pmr::set. uniq(&src);
…
What if we could reduce the cost
goal: reduce cost of developign an allocator aware
What’s our problem?
Allocators bloat class interfaces
Performance in practice
- Memory hiearchy: Cache; main, virtual
- objects used together should be together
C++ allocatros
allocator_traits simplify allocators; mostly for users. Containers now have problems in their face
What about nested containers
- In a vector<string> vector has one and each string potentiallly different allocator
- scoped allocator model: container passes allocator into each item. Solves memory diffisuion
- scoped_allocator_adapter
- Allocator template policies interfere with vocabulary types
C++17: PMR: a simpler allocator model
- Non-template std::pmr::memoryresource
- The wrapper
- class memory_resource
- polomorphic_allocator
- polymorphic_allocator<byte>
- “One true” allocator vocabulary type
- Only a template to be compatible for C++11
- std::pmr::new_delete_resource()
- Uses new/delete
- always availabel
- thread safe
- unsynchronized_pool_resource
- Pools of similar sized objects
- good for dynamic data strucures
- Single-threaded
- avoids concurrency lock overhead
- monootonic_buffer_resource
- Ultra fast; single threaded, contiguous
- for containers that grow monotonically
- test resource
- GitHub bloomberg/p1160
How to use allocators
class Student {
pmr::string d_name;
pmr::string d_email
pmr::vector<int d_grades;
using allocator_type = pmr::polymovic_allocator<>; // Important
}
- Overload each constructor having default_arguments
- Variadic argument lists: allocator goes at the beginning
Rewriting std::unordered_map to use pmr allocators
- Example with copying constructors. Lots of duplicated code
What about a language solution
- Build on a strong foundation built up to C++17
HashMap<int,int> x {} using myAllocator;
- using myAllocator
Bloomberg
- Bloomberg vision 2020
Pete Ware 