[cppcon 2018] An allocator is a handle to a heap - Lessons learned from std::pmr - Arthur O'Dwyer

This talk given by Arthur is a recap from his talk on C++Now 2018.

Define object:

• An object has an address.
  (Unlike pure value, which will be replaced with the value by compiler in the assembly)
• Has state.
• Address, pointer, name, unique identifier, handle --- all synonymous for the purpose, they are value type.

Sequence Container:

• Act as object.
• Act as value.

Define allocator:

For std::vector,

• Where does the memory for v[i] come from?
• From A::allocate(n)
• What is the thing represented by points to the value?
• From A::pointer type.
• Container holds an instance of the allocator type A within itself.
• What can we put inside the allocator instance?
• :-)

C++17, let's use std::pmr::polymorphic_allocator !

• Has a pointer to a std::pmr::memory_resource
• All the shared state goes into the memory_resource

Usage:

// std
namespace pmr {
    template <class T>
    using vector = std::vector<T, std::pmr::polymorphic_allocator<T>>;
}
std::vector<int, std::pmr::polymorphic_allocator<int>> vec(allocator_instance);

Allocator is:
A value represents a handle to a source of memory with some other orthogonal pieces.
Thus:

• Allocator types should be copyable, just like pointers.
• Allocator types should be cheaply copyable, like pointers.
  They need not be trivially copyable
• Memory-resource types should generally be immobile.

Recap, Allocators must be 'copy-only' types

i.e If std::move(allocator) should do a copy instead of move, exactly act like std::move(pointer) should remains the same pointer.

Reference:

http://vsdmars.blogspot.com/2018/06/ccppcon-2018note-allocator-is-handle-to.html

Refers to: LWG issue 2593 https://timsong-cpp.github.io/lwg-issues/2593

An allocator should satisfy allocator completeness requirements.

Allocator completeness requirements

An allocator type X for type T additionally satisfies the allocator completeness requirements if both of the following are true regardless of whether T is a complete type:

• X is a complete type
• Except for value_type, all the member types of std::allocator_traits<X> are complete types.

Take aways:

• Use std::allocator_traits::construct to create instance.
• iterator_facade
  We implement a complete set of primitive functions (and data members); then inherit from iterator_facade to provide the standard zoo of operators.
• iterator_adaptor
  We override some but not all the primitive functions; then inherit from iterator_adaptor<Base> to provide the missing parts.
• Executor coming in C++23
  P0443: std::executor