overview.md

Polymorphic Memory Resources

(PMR)

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Overview

• namespace std::pmr
• polymorphic_allocator
• polymorphic_memory_resources
• Containers

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Why A new Allocator?

std::vector<T, std::allocator<T>>
is a different type from
std::vector<T, MyAlloc<T>>

(or any other std container)

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Polymorphic Allocation

• New in C++17
• Type erasure for allocators
• Virtual call to alloc/dealloc

#include <memory_resource>

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std::polymorphic_allocator<T>

• Wrapper around a memory resource
• Allocates a pointer or contructs object
• Move-only
• Convertable from any T to any U
• Comparable ==, !=

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std::memory_resource

• Abstract base class
• Not templated
• Small interface
  • do_allocate, do_deallocate, is_equal

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std::memory_resource (2)

class memory_resource {
  public:
    virtual memory_resouce() = default;
    void* allocate(std::size_t bytes, std::size_t alignment = /*...*/);
    void deallocate(void* p, std::size_t bytes, std::size_t alignment = /*...*/);
    bool is_equal(const memory_resource& other) const noexcept;
  private:
    virtual void* do_allocate(std::size_t bytes, std::size_t alignment) = 0;
    virtual void do_deallocate(void* p, std::size_t bytes, std::size_t alignment) = 0;
    virtual bool do_is_equal(const std::pmr::memory_resource& other) const noexcept = 0;
}

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predefined memory resources

• new_delete_resource()
• null_memory_resource()
• get_default_resource(), set_default_resource()
• synchronized_pool_resource
• unsynchronized_pool_resource
• monotonic_buffer_resource

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Demo Time

demo1.cpp

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Demo Time (a)

Now change to std::pmr::vector<>

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Containers

• std::pmr:: variant of std containers with allocators
• vector, list, deque, ...
• set, map, unordered_*, ...

namespace std::pmr {
    template <typename T>
    using vector = ::std::vector<T, 
            ::std::pmr::polymorphic_allocator>;
}

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Demo Time (b)

demo1b.cpp

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Automatic propagation

• std containers will automatically propagate allocators
• also std functions allocate_shared()
• see std::uses_allocator
• simplified for user code with C++20 std::make_obj_using_allocator