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